The current deepest radio surveys detect hundreds of sources per square
degree below 0.1mJy. There is a growing consensus that a large fraction of
these sources are dominated by star formation although the exact proportion has
been debated in the literature. However, the low luminosity of these galaxies
at most other wavelengths makes determining the nature of individual sources
difficult. If future, deeper surveys performed with the next generation of
radio instrumentation are to reap high scientific reward we need to develop
reliable methods of distinguishing between radio emission powered by active
galactic nuclei (AGN) and that powered by star formation. In particular, we
believe that such discriminations should be based on purely radio, or relative
to radio, diagnostics. These diagnostics include radio morphology, radio
spectral index, polarisation, variability, radio luminosity and flux density
ratios with non-radio wavelengths e.g. with different parts of the infrared
(IR) regime. We discuss the advantages and limitations of these various
diagnostics methods with current and future surveys. However, weeding AGN out
of deep radio surveys can already provide several insights into the star
formation at high redshift. As well as reproducing the well known rise with
redshift in the comoving star formation rate density, we also see evidence for
the continued dominance of LIRGs and ULIRGs to the total star forming budget
across redshifts 1-3. Additionally, while we see that the IR-radio relation for
star forming galaxies does hold to high redshifts (z>1) there is a mild
deviation depending on the IR waveband used and the range of IR SEDs found. We
will discuss the possible reasons behind this change in properties.Comment: 6 pages, 6 figures. Panoramic Radio Astronomy: Wide-field 1-2 GHz
  research on galaxy evolution - PRA2009 Groningen, the Netherlands June 02-05,
  200

Seymour, Nick

English

arXiv

University of Groningen

   University of GroningenTracing High Redshift Starformation in the Current and Next Generation of Radio SurveysSeymour, NickPublished in:EPRINTS-BOOK-TITLEIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2009Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Seymour, N. (2009). Tracing High Redshift Starformation in the Current and Next Generation of RadioSurveys. In EPRINTS-BOOK-TITLE University of Groningen.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019arXiv:0911.4599v1  [astro-ph.CO]  24 Nov 2009Tracing High Redshift Starformation in the Currentand Next Generation of Radio SurveysN. Seymour∗University College London, Department of Space & Climate PhysicsMullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UKE-mail: nps@mssl.ucl.ac.ukThe current deepest radio surveys detect hundreds of sources per square degree below 0.1mJy.There is a growing consensus that a large fraction of these sources are dominated by star formationalthough the exact proportion has been debated in the literature. However, the low luminosity ofthese galaxies at most other wavelengths makes determining the nature of individual sourcesdifficult. If future, deeper surveys performed with the next generation of radio instrumentationare to reap high scientific reward we need to develop reliable methods of distinguishing betweenradio emission powered by active galactic nuclei (AGN) and that powered by star formation.In particular, we believe that such discriminations should be based on purely radio, or relative toradio, diagnostics. These diagnostics include radio morphology, radio spectral index, polarisation,variability, radio luminosity and flux density ratios with non-radio wavelengths e.g. with differentparts of the infrared (IR) regime. We discuss the advantages and limitations of these variousdiagnostics methods with current and future surveys. However, weeding AGN out of deep radiosurveys can already provide several insights into the star formation at high redshift. As well asreproducing the well known rise with redshift in the comoving star formation rate density, wealso see evidence for the continued dominance of LIRGs and ULIRGs to the total star formingbudget across redshifts 1−3. Additionally, while we see that the IR-radio relation for star forminggalaxies does hold to high redshifts (z > 1) there is a mild deviation depending on the IR wavebandused and the range of IR SEDs found. We will discuss the possible reasons behind this change inproperties.Panoramic Radio Astronomy: Wide-field 1-2 GHz research on galaxy evolutionJune 2-5 2009Groningen, the Netherlands∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/Tracing High Redshift Starformation in the Current and Next Generation of Radio Surveys N. SeymourFigure 1: Left The 1.4GHz Euclidean normalised source counts from the 13H field divided into SFGsand AGN [2]. Models of the contribution from different populations are overlaid: S04 [3], J08 [4]. RightCompilation from the literature of the fraction of SFGs as a function of 1.4GHz flux density [2,5,6,7,8,9].1. IntroductionTraditionally radio surveys have been dominated by powerful radio-loud active galactic nuclei(AGN) and radio galaxies were the first probe of the distant Universe [1], and references therein.However, surveys by the next generation of radio telescopes we will obtain vast samples of starforming galaxies (SFGs). In fact, as studies of star formation, these surveys will have severaladvantages over current surveys at other wavelengths. Firstly, radio luminosity is a direct tracer ofstar formation rate (SFR) once AGN activity has been accounted for and secondly, the combinationof area and the sensitivity will provide a fast survey speed allowing for an unprecedented census ofstar formation in the distant Universe.The sub-mJy regime of the radio source counts has been studied for several decades now andthe up-turn below ∼ 1mJy has been well characterised albeit with some scatter. Currently thedebate is over the normalisation of the source counts and type of sources that are responsible forthe up-turn. However, the most important question is how do we distinguish between AGN andSFGs in the deepest surveys?We believe that as some AGN have very low emission in the radio, compared to even lowMilky Way rates of star formation, the simple presence of an AGN does not necessarily infer thatthe radio emission of a galaxy is from an accretion driven source. Hence, methods to discriminatebetween whether AGN or star forming activity contribute to the observed radio flux density shouldbe related to the radio emission, and we list some of these methods here:• Radio morphology• Radio spectral index/radio Spectral Energy Distribution (SED)• Radio variability• Radio polarisation• Flux density ratios/full SED modeling2Tracing High Redshift Starformation in the Current and Next Generation of Radio Surveys N. SeymourFigure 2: Left Star formation rate plotted against redshift for a sample of radio selected SFGs from the13H field (coloured symbols indicating host galaxies of different stellar masses). Local radio-selected starforming galaxies are illustrated by black dots. Equivalent IR luminosity is plotted on the right axis. At highredshift we see massive galaxies with star formation rates in excess of that seen in the local Universe. RightDerived comoving star formation rate density (SFRD) as a function of redshift compared with the meanSFRD presented in [11]. The results in the radio agree well with those at other wavelengths which bodeswell for future radio surveys making detailed studies of star formation at high redshift.This list is not exhaustive and several of these methods have already been used in variouscombinations (e.g. [2,5,6,7,8,9]). We have found that a combination of the radio morphology andspectral index is the most effective method if you have data of sufficient quality, but in practice forthe deepest surveys the flux ratio method, based on SEDs of SFGs and AGN, is the most productive.So employing these techniques, and on the assumption that one process dominates we can separatethe faint radio source counts by galaxy type (e.g. Fig. 1 left).We have demonstrated that while AGN dominate the 1.4 GHz source counts around 0.4 mJy,SFGs make up 70−80% below 0.1 mJy [2] with the transition between the two populations occur-ing between 0.1 and 0.2mJy. So while SFGs dominate at the faintest flux densities there remainsa non-negligible contribution from radio quiet AGN around 50 µJy. This result is consistent withdeterminations and constraints in the literature (Fig. 1 right) which are reaching a consensus aboutthe nature of the faint radio source counts. The observed differences in this plot are easily explainedby the different methods used in discriminating between AGN and SFGs as well as sample variancebetween the different deep narrow survey fields used in each study.Our knowledge of the faint radio source population will be greatly improved with surveys fromup-coming radio facilities (see other contributions to these proceedings). At fainter flux densitiesthe modeled evolution of the luminosity function of both populations suggests that the fraction ofSFGs will asymptote towards 100% [4]. The eVLA and eMERLIN will push to such flux densitieswhere star formation completely dominates and provide samples of radio-selecting SFGs out tothe highest redshifts. Other projects, like the Evolutionary Map of the Universe (EMU, see [10]in these proceedings) on the Australian Square Kilometer Array Pathfinder, will probe the starforming regime well below 0.1mJy over a large fraction of the whole sky. However, in preparationfor such surveys we must obtain a better understanding of radio emission from star formation inthe local Universe and a better understanding of the radio luminosity to star formation rate (SFR)conversion.3Tracing High Redshift Starformation in the Current and Next Generation of Radio Surveys N. SeymourFigure 3: The comoving IR luminosity density (IRLD) plotted as a function of redshift. The gray pointsare SFRD data converted to IRLD from [15], the shaded regions (from [14]) are the IRLD separated by IRluminosity as indicated in the plot. The coloured points are based on radio-selected SFGs in the 13H fieldand follow the same division by IR luminosity. We observe that the contribution of LIRGs and ULIRGsremains high across 1≤ z≤ 3.2. Science with Radio Selected Star Forming GalaxiesWith the data we have in hand we can already study the properties of host galaxies of powerfulstarbursts selected at radio wavelengths. We discover above z = 1 galaxies with high SFRs whichare simply not detected in the local Universe (Fig. 2 left), even allowing for volume effects. In thelocal Universe the most extreme galaxies have SFRs of ∼ 300Myr−1 whereas at high redshift wesee galaxies with SFRs up to almost an order of magnitude greater. Furthermore, we find that suchgalaxies are always have high stellar masses implying they have already built up a large populationof stars [11].We can also use such samples to study the comoving star formation rate density (SFRD) asa function of redshift (e.g. [12], [13]). We find that the radio selected sample produce resultsconsistent with those from other wavelengths (Fig. 2 right and [11]). The future radio surveysdiscussed in the previous section will no doubt greatly increase the accuracy with which we cantrace the formation of stars in such a plot.From radio surveys we can not only study when stars formed, but also where. The contri-bution to the comoving IR luminosity density (a proxy for the SFRD) by galaxies of differentIR luminosities changes with redshift [14]. The contribution of luminous IR galaxies (LIRGs,1011 ≤ L/L ≤ 1012) and ultra-luminous IR galaxies (ULIRGs, 1012 ≤ L/L ≤ 1013) increasesfrom z = 0 to z = 1. However, this is not surprising as it is a natural result of integrating a rapidlyevolving luminosity function.By exploiting the IR/radio correlation for SFGs we find that results from radio surveys agreewell those from the IR and can extend such studies to redshifts greater than unity. In Fig. 3 we seethat LIRGs and ULIRGs continue to make a high contribution to the IR luminosity density across1 ≤ z ≤ 3. The extreme starbursts, hyper-luminous IR galaxies (HyLIRGs, L/L ≥ 1013) whichare not seen in the local Universe only contribute a few percent to the total star forming budget atz = 2−3.4Tracing High Redshift Starformation in the Current and Next Generation of Radio Surveys N. SeymourFigure 4: The mean observed value of q70 (= log[S70µm/S1.4GHz]) for radio-selected SFGs plotted as a func-tion of redshift (red squares). The numbers beside each data point indicate the mean total IR luminosityderived from the radio SFR. The black lines represent the tracks from local templates from [16] at lumi-nosities of as indicted in the figure. These lines become thinner at the redshift a given template becomesundetectable in our radio survey. The trend of a decrease in q70 toward higher redshifts is generally consistentwith the tracks with local SEDs.3. The IR/Radio Relation for Star Forming GalaxiesAn important question for future surveys is does the radio luminosity/star formation rate re-lation hold at high redshifts and extreme luminosities? This relation primarily depends on theIR/radio correlation. What do we expect at high redshift from local galaxies? In Fig. 4 we showthe observed 70 µm to 1.4GHz flux density ratio for average SEDs of local galaxies [16]. Weobserve that the flux density ratio increases with IR luminosity which is due to more luminous IRgalaxies being characterised by hotter dust temperatures and hence peaking at shorter wavelengths(i.e. they contribute relatively more to the 70 µm band). However, all the tracks decrease withincreasing redshifts due to k-correction effects.We have stacked 70 µm images at positions of radio-selected SFGs and hence we can obtaina flux density ratio as a function of redshift, [17] and Fig. 4. We note, however, that we areprobing different luminosities at each redshift, hence must be careful to compare each point tothe correct SED track. The data broadly agree with the general trend of decreasing flux densityratio toward higher redshift, but there are differences in the detail. We see that the bin at z ∼ 0.9,which contains SFGs close to ULIRG power (¯L∼ 1011.7 L), lies next to the low luminosity tracks(log(LIR) = 10) and 2σ away from the ULIRG track. Why might the 70um/radio correlationchange at high redshift? Locally the IR SED is luminosity dependent so it is possible that thesehigh redshift luminous starburst may have different IR SEDs compared to local luminous starbursts.If high redshift star forming regions in luminous starbursts are more extended, they may be moreoptically thin and hence have less free-free absorption (and therefore have a higher radio flux).Such SEDs would also be characterised as being cooler and have a lower 70 µm flux density.There is now strong evidence for cold LIRGs and ULIRGs at high redshift. We have found thatsuch galaxies at 0.5 ≤ z ≤ 1.0 span a range of characteristic dust temperatures from the hot localstarbursts to that of higher-z sub-mm selected galaxies [18]. We have also found that luminousgalaxies colder than local hot starbursts dominate the luminosity density at these redshifts [19].5Tracing High Redshift Starformation in the Current and Next Generation of Radio Surveys N. Seymour4. ConclusionsRadio observations of the distant Universe have traditionally been used to study AGN, butwe are now begining to obtain a census of star formation from deep, wide radio surveys. Thereare three crucial issues in exploiting such data: (i) distinguishing between AGN and SFGs, (ii)calibrating the radio luminosity/SFR relation across all redshifts, radio luminosities and type ofgalaxy, and (iii) obtaining redshifts from ancillary data. The radio/IR relation for SFGs appears todepend on IR SED and hence waveband used in the IR. We must better understand this relationshiplocally before applying it to galaxies at high redshift.References[1] G. Miley & C. De Breuck, 2008, A&AR, 15, 67[2] N. Seymour et al., 2008, MNRAS, 386, 1695[3] N. Seymour et al., 2004, MNRAS, 352, 131[4] R. Wilman et al., 2008, MNRAS, 388, 1335[5] T. Muxlow et al., 2005, MNRAS, 358, 1159[6] C. Simpson et al., 2006, MNRAS, 372, 741[7] V. Smolcˇic´ et al., 2008, ApJS, 177, 14[8] E. Ibar et al., 2008, MNRAS, 386, 953[9] P. Padovani et al., 2009, ApJ, 694, 235[10] R. Norris et al., 2009, POS(PRA2009)003[11] A. Hopkins et al., 2007, ASPC, 380, 423[12] S. Lilly et al., 1996, ApJ, 460, L1[13] P. Madau et al., 1996, MNRAS, 283, 1388[14] E. Le Floc’h et al., 2005, ApJ, 632, 169[15] A. Hopkins & J.F. Beacom, 2006, ApJ, 651, 142[16] G. Rieke et al., 2009, ApJ, 692, 556[17] N. Seymour et al., 2009a, MNRAS, 398, 1573[18] M. Symeonidis et al., 2009, MNRAS, 397, 1728[19] N. Seymour et al., 2009b, MNRAS, in press6

Tracing High Redshift Starformation in the Current and Next Generation of Radio Surveys

The current deepest radio surveys detect hundreds of sources per square degree below 0.1mJy. There is a growing consensus that a large fraction of these sources are dominated by star formation although the exact proportion has been debated in the literature. However, the low luminosity of these galaxies at most other wavelengths makes determining the nature of individual sources difficult. If future, deeper surveys performed with the next generation of radio instrumentation are to reap high scientific reward we need to develop reliable methods of distinguishing between radio emission powered by active galactic nuclei (AGN) and that powered by star formation. In particular, we believe that such discriminations should be based on purely radio, or relative to radio, diagnostics. These diagnostics include radiomorphology, radio spectral index, polarisation, variability, radio luminosity and flux density ratios with non-radio wavelengths e.g. with different parts of the infrared (IR) regime. We discuss the advantages and limitations of these various diagnostics methods with current and future surveys. However, weeding AGN out of deep radio surveys can already provide several insights into the star formation at high redshift. As well as reproducing the well known rise with redshift in the comoving star formation rate density, we also see evidence for the continued dominance of LIRGs and ULIRGs to the total star forming budget across redshifts 1−3. Additionally, while we see that the IR-radio relation for star forming galaxies does hold to high redshifts (z>1) there is amild deviation depending on the IR waveband used and the range of IR SEDs found. We will discuss the possible reasons behind this change in properties. Panorami

NARCIS 

The current deepest radio surveys detect hundreds of sources per square degree below 0.1mJy. There is a growing consensus that a large fraction of these sources are dominated by star formation although the exact proportion has been debated in the literature. However, the low luminosity of these galaxies at most other wavelengths makes determining the nature of individual sources difficult. If future, deeper surveys performed with the next generation of radio instrumentation are to reap high scientific reward we need to develop reliable methods of distinguishing between radio emission powered by active galactic nuclei (AGN) and that powered by star formation. In particular, we believe that such discriminations should be based on purely radio, or relative to radio, diagnostics. These diagnostics include radiomorphology, radio spectral index, polarisation, variability, radio luminosity and flux density ratios with non-radio wavelengths e.g. with different parts of the infrared (IR) regime. We discuss the advantages and limitations of these various diagnostics methods with current and future surveys. However, weeding AGN out of deep radio surveys can already provide several insights into the star formation at high redshift. As well as reproducing the well known rise with redshift in the comoving star formation rate density, we also see evidence for the continued dominance of LIRGs and ULIRGs to the total star forming budget across redshifts 1−3. Additionally, while we see that the IR-radio relation for star forming galaxies does hold to high redshifts (z&gt;1) there is amild deviation depending on the IR waveband used and the range of IR SEDs found. We will discuss the possible reasons behind this change in properties. Panorami

ARTS repository - University of Groningen

   University of GroningenTracing High Redshift Starformation in the Current and Next Generation of Radio SurveysSeymour, NickPublished in:EPRINTS-BOOK-TITLEIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2009Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Seymour, N. (2009). Tracing High Redshift Starformation in the Current and Next Generation of RadioSurveys. In EPRINTS-BOOK-TITLE University of Groningen.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 09-09-2023arXiv:0911.4599v1  [astro-ph.CO]  24 Nov 2009Tracing High Redshift Starformation in the Currentand Next Generation of Radio SurveysN. Seymour∗University College London, Department of Space & Climate PhysicsMullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UKE-mail: nps@mssl.ucl.ac.ukThe current deepest radio surveys detect hundreds of sources per square degree below 0.1mJy.There is a growing consensus that a large fraction of these sources are dominated by star formationalthough the exact proportion has been debated in the literatur . However, the low luminosity ofthese galaxies at most other wavelengths makes determiningthe nature of individual sourcesdifficult. If future, deeper surveys performed with the nextgeneration of radio instrumentationare to reap high scientific reward we need to develop reliablemethods of distinguishing betweenradio emission powered by active galactic nuclei (AGN) and that powered by star formation.In particular, we believe that such discriminations shouldbe based on purely radio, or relative toradio, diagnostics. These diagnostics include radio morphology, radio spectral index, polarisation,variability, radio luminosity and flux density ratios with no -radio wavelengths e.g. with differentparts of the infrared (IR) regime. We discuss the advantagesnd limitations of these variousdiagnostics methods with current and future surveys. However, weeding AGN out of deep radiosurveys can already provide several insights into the star formation at high redshift. As well asreproducing the well known rise with redshift in the comoving star formation rate density, wealso see evidence for the continued dominance of LIRGs and ULIRGs to the total star formingbudget across redshifts 1−3. Additionally, while we see that the IR-radio relation forstar forminggalaxies does hold to high redshifts (z> 1) there is a mild deviation depending on the IR wavebandused and the range of IR SEDs found. We will discuss the possible reasons behind this change inproperties.Panoramic Radio Astronomy: Wide-field 1-2 GHz research on galaxy evolutionJune 2-5 2009Groningen, the Netherlands∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licen. http://pos.sissa.it/Tracing High Redshift Starformation in the Current and NextGeneration of Radio Surveys N. SeymourFigure 1: Left The 1.4GHz Euclidean normalised source counts from the 13H field divided into SFGsand AGN [2]. Models of the contribution from different populations are overlaid: S04 [3], J08 [4].RightCompilation from the literature of the fraction of SFGs as a function of 1.4GHz flux density [2,5,6,7,8,9].1. IntroductionTraditionally radio surveys have been dominated by powerful radio-loud active galactic nuclei(AGN) and radio galaxies were the first probe of the distant Universe [1], and references therein.However, surveys by the next generation of radio telescopeswe will obtain vast samples of starforming galaxies (SFGs). In fact, as studies of star formation, these surveys will have severaladvantages over current surveys at other wavelengths. Firstly, radio luminosity is a direct tracer ofstar formation rate (SFR) once AGN activity has been accounted for and secondly, the combinationof area and the sensitivity will provide a fast survey speed allowing for an unprecedented census ofstar formation in the distant Universe.The sub-mJy regime of the radio source counts has been studied for several decades now andthe up-turn below∼ 1mJy has been well characterised albeit with some scatter. Cu rently thedebate is over the normalisation of the source counts and type of sources that are responsible forthe up-turn. However, the most important question is how do we distinguish between AGN andSFGs in the deepest surveys?We believe that as some AGN have very low emission in the radio, compared to even lowMilky Way rates of star formation, the simple presence of an AGN does not necessarily infer thatthe radio emission of a galaxy is from an accretion driven source. Hence, methods to discriminatebetween whether AGN or star forming activity contribute to the observed radio flux density shouldbe related to the radio emission, and we list some of these methods here:• Radio morphology• Radio spectral index/radio Spectral Energy Distribution (SED)• Radio variability• Radio polarisation• Flux density ratios/full SED modeling2Tracing High Redshift Starformation in the Current and NextGeneration of Radio Surveys N. SeymourFigure 2: Left Star formation rate plotted against redshift for a sample ofradio selected SFGs from the13H field (coloured symbols indicating host galaxies of different stellar masses). Local radio-selected starforming galaxies are illustrated by black dots. EquivalentIR luminosity is plotted on the right axis. At highredshift we see massive galaxies with star formation rates in xcess of that seen in the local Universe.RightDerived comoving star formation rate density (SFRD) as a functio of redshift compared with the meanSFRD presented in [11]. The results in the radio agree well with those at other wavelengths which bodeswell for future radio surveys making detailed studies of star formation at high redshift.This list is not exhaustive and several of these methods havealready been used in variouscombinations (e.g. [2,5,6,7,8,9]). We have found that a combination of the radio morphology andspectral index is the most effective method if you have data of sufficient quality, but in practice forthe deepest surveys the flux ratio method, based on SEDs of SFGs and AGN, is the most productive.So employing these techniques, and on the assumption that one pr cess dominates we can separatethe faint radio source counts by galaxy type (e.g. Fig. 1 left).We have demonstrated that while AGN dominate the 1.4 GHz source counts around 0.4 mJy,SFGs make up 70−80% below 0.1 mJy [2] with the transition between the two populations occur-ing between 0.1 and 0.2mJy. So while SFGs dominate at the faintest flux densities thre remainsa non-negligible contribution from radio quiet AGN around 50µJy. This result is consistent withdeterminations and constraints in the literature (Fig. 1 right) which are reaching a consensus aboutthe nature of the faint radio source counts. The observed differences in this plot are easily explainedby the different methods used in discriminating between AGNand SFGs as well as sample variancebetween the different deep narrow survey fields used in each study.Our knowledge of the faint radio source population will be gratly improved with surveys fromup-coming radio facilities (see other contributions to these proceedings). At fainter flux densitiesthe modeled evolution of the luminosity function of both populations suggests that the fraction ofSFGs will asymptote towards 100% [4]. The eVLA and eMERLIN will push to such flux densitieswhere star formation completely dominates and provide samples of radio-selecting SFGs out tothe highest redshifts. Other projects, like the Evolutionary Map of the Universe (EMU, see [10]in these proceedings) on the Australian Square Kilometer Array Pathfinder, will probe the starforming regime well below 0.1mJy over a large fraction of the whole sky. However, in prepaationfor such surveys we must obtain a better understanding of radio emission from star formation inthe local Universe and a better understanding of the radio luminosity to star formation rate (SFR)conversion.3Tracing High Redshift Starformation in the Current and NextGeneration of Radio Surveys N. SeymourFigure 3: The comoving IR luminosity density (IRLD) plotted as a function of redshift. The gray pointsare SFRD data converted to IRLD from [15], the shaded regions(from [14]) are the IRLD separated by IRluminosity as indicated in the plot. The coloured points arebased on radio-selected SFGs in the 13H fieldand follow the same division by IR luminosity. We observe that the contribution of LIRGs and ULIRGsremains high across 1≤ z≤ 3.2. Science with Radio Selected Star Forming GalaxiesWith the data we have in hand we can already study the properties of host galaxies of powerfulstarbursts selected at radio wavelengths. We discover above z= 1 galaxies with high SFRs whichare simply not detected in the local Universe (Fig. 2 left), even allowing for volume effects. In thelocal Universe the most extreme galaxies have SFRs of∼ 300M⊙yr−1 whereas at high redshift wesee galaxies with SFRs up to almost an order of magnitude greater. Furthermore, we find that suchgalaxies are always have high stellar masses implying they have already built up a large populationof stars [11].We can also use such samples to study the comoving star formati n r e density (SFRD) asa function of redshift (e.g. [12], [13]). We find that the radio selected sample produce resultsconsistent with those from other wavelengths (Fig. 2 right and [11]). The future radio surveysdiscussed in the previous section will no doubt greatly increase the accuracy with which we cantrace the formation of stars in such a plot.From radio surveys we can not only study when stars formed, but also where. The contri-bution to the comoving IR luminosity density (a proxy for theSFRD) by galaxies of differentIR luminosities changes with redshift [14]. The contribution of luminous IR galaxies (LIRGs,1011 ≤ L/L⊙ ≤ 1012) and ultra-luminous IR galaxies (ULIRGs, 1012 ≤ L/L⊙ ≤ 1013) increasesfrom z= 0 to z= 1. However, this is not surprising as it is a natural result ofintegrating a rapidlyevolving luminosity function.By exploiting the IR/radio correlation for SFGs we find that results from radio surveys agreewell those from the IR and can extend such studies to redshifts greater than unity. In Fig. 3 we seethat LIRGs and ULIRGs continue to make a high contribution tothe IR luminosity density across1 ≤ z≤ 3. The extreme starbursts, hyper-luminous IR galaxies (HyLIRGs,L/L⊙ ≥ 1013) whichare not seen in the local Universe only contribute a few percent to the total star forming budget atz= 2−3.4Tracing High Redshift Starformation in the Current and NextGeneration of Radio Surveys N. SeymourFigure 4: The mean observed value of q70 (= log[S70µm/S1.4GHz]) for radio-selected SFGs plotted as a func-tion of redshift (red squares). The numbers beside each datapoint indicate the mean total IR luminosityderived from the radio SFR. The black lines represent the tracks from local templates from [16] at lumi-nosities of as indicted in the figure. These lines become thinner at the redshift a given template becomesundetectable in our radio survey. The trend of a decrease in q70 toward higher redshifts is generally consistentwith the tracks with local SEDs.3. The IR/Radio Relation for Star Forming GalaxiesAn important question for future surveys is does the radio luminosity/star formation rate re-lation hold at high redshifts and extreme luminosities? This relation primarily depends on theIR/radio correlation. What do we expect at high redshift from local galaxies? In Fig. 4 we showthe observed 70µm to 1.4GHz flux density ratio for average SEDs of local galaxies [16]. Weobserve that the flux density ratio increases with IR luminosity which is due to more luminous IRgalaxies being characterised by hotter dust temperatures and hence peaking at shorter wavelengths(i.e. they contribute relatively more to the 70µm band). However, all the tracks decrease withincreasing redshifts due to k-correction effects.We have stacked 70µm images at positions of radio-selected SFGs and hence we canobt ia flux density ratio as a function of redshift, [17] and Fig. 4.We note, however, that we areprobing different luminosities at each redshift, hence must be careful to compare each point tothe correct SED track. The data broadly agree with the general tr nd of decreasing flux densityratio toward higher redshift, but there are differences in the detail. We see that the bin atz∼ 0.9,which contains SFGs close to ULIRG power (L̄ ∼ 1011.7 L⊙), lies next to the low luminosity tracks(log(LIR) = 10) and 2σ away from the ULIRG track. Why might the 70um/radio correlationchange at high redshift? Locally the IR SED is luminosity dependent so it is possible that thesehigh redshift luminous starburst may have different IR SEDscompared to local luminous starbursts.If high redshift star forming regions in luminous starbursts are more extended, they may be moreoptically thin and hence have less free-free absorption (and therefore have a higher radio flux).Such SEDs would also be characterised as being cooler and have a lower 70µm flux density.There is now strong evidence for cold LIRGs and ULIRGs at highredshift. We have found thatsuch galaxies at 0.5 ≤ z≤ 1.0 span a range of characteristic dust temperatures from the hot localstarbursts to that of higher-z sub-mm selected galaxies [18]. We have also found that luminousgalaxies colder than local hot starbursts dominate the luminosity density at these redshifts [19].5Tracing High Redshift Starformation in the Current and NextGeneration of Radio Surveys N. Seymour4. ConclusionsRadio observations of the distant Universe have traditionally been used to study AGN, butwe are now begining to obtain a census of star formation from deep, wide radio surveys. Thereare three crucial issues in exploiting such data: (i) distingu shing between AGN and SFGs, (ii)calibrating the radio luminosity/SFR relation across all redshifts, radio luminosities and type ofgalaxy, and (iii) obtaining redshifts from ancillary data.The radio/IR relation for SFGs appears todepend on IR SED and hence waveband used in the IR. We must better understand this relationshiplocally before applying it to galaxies at high redshift.References[1] G. Miley & C. De Breuck, 2008, A&AR, 15, 67[2] N. Seymour et al., 2008, MNRAS, 386, 1695[3] N. Seymour et al., 2004, MNRAS, 352, 131[4] R. Wilman et al., 2008, MNRAS, 388, 1335[5] T. Muxlow et al., 2005, MNRAS, 358, 1159[6] C. Simpson et al., 2006, MNRAS, 372, 741[7] V. Smolčić et al., 2008, ApJS, 177, 14[8] E. Ibar et al., 2008, MNRAS, 386, 953[9] P. Padovani et al., 2009, ApJ, 694, 235[10] R. Norris et al., 2009,POS(PRA2009)003[11] A. Hopkins et al., 2007, ASPC, 380, 423[12] S. Lilly et al., 1996, ApJ, 460, L1[13] P. Madau et al., 1996, MNRAS, 283, 1388[14] E. Le Floc’h et al., 2005, ApJ, 632, 169[15] A. Hopkins & J.F. Beacom, 2006, ApJ, 651, 142[16] G. Rieke et al., 2009, ApJ, 692, 556[17] N. Seymour et al., 2009a, MNRAS, 398, 1573[18] M. Symeonidis et al., 2009, MNRAS, 397, 1728[19] N. Seymour et al., 2009b, MNRAS,in press6

   University of GroningenTracing High Redshift Starformation in the Current and Next Generation of Radio SurveysSeymour, NickPublished in:EPRINTS-BOOK-TITLEIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2009Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Seymour, N. (2009). Tracing High Redshift Starformation in the Current and Next Generation of RadioSurveys. In EPRINTS-BOOK-TITLE University of Groningen.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 16-07-2023arXiv:0911.4599v1  [astro-ph.CO]  24 Nov 2009Tracing High Redshift Starformation in the Currentand Next Generation of Radio SurveysN. Seymour∗University College London, Department of Space & Climate PhysicsMullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UKE-mail: nps@mssl.ucl.ac.ukThe current deepest radio surveys detect hundreds of sources per square degree below 0.1mJy.There is a growing consensus that a large fraction of these sources are dominated by star formationalthough the exact proportion has been debated in the literatur . However, the low luminosity ofthese galaxies at most other wavelengths makes determiningthe nature of individual sourcesdifficult. If future, deeper surveys performed with the nextgeneration of radio instrumentationare to reap high scientific reward we need to develop reliablemethods of distinguishing betweenradio emission powered by active galactic nuclei (AGN) and that powered by star formation.In particular, we believe that such discriminations shouldbe based on purely radio, or relative toradio, diagnostics. These diagnostics include radio morphology, radio spectral index, polarisation,variability, radio luminosity and flux density ratios with no -radio wavelengths e.g. with differentparts of the infrared (IR) regime. We discuss the advantagesnd limitations of these variousdiagnostics methods with current and future surveys. However, weeding AGN out of deep radiosurveys can already provide several insights into the star formation at high redshift. As well asreproducing the well known rise with redshift in the comoving star formation rate density, wealso see evidence for the continued dominance of LIRGs and ULIRGs to the total star formingbudget across redshifts 1−3. Additionally, while we see that the IR-radio relation forstar forminggalaxies does hold to high redshifts (z> 1) there is a mild deviation depending on the IR wavebandused and the range of IR SEDs found. We will discuss the possible reasons behind this change inproperties.Panoramic Radio Astronomy: Wide-field 1-2 GHz research on galaxy evolutionJune 2-5 2009Groningen, the Netherlands∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licen. http://pos.sissa.it/Tracing High Redshift Starformation in the Current and NextGeneration of Radio Surveys N. SeymourFigure 1: Left The 1.4GHz Euclidean normalised source counts from the 13H field divided into SFGsand AGN [2]. Models of the contribution from different populations are overlaid: S04 [3], J08 [4].RightCompilation from the literature of the fraction of SFGs as a function of 1.4GHz flux density [2,5,6,7,8,9].1. IntroductionTraditionally radio surveys have been dominated by powerful radio-loud active galactic nuclei(AGN) and radio galaxies were the first probe of the distant Universe [1], and references therein.However, surveys by the next generation of radio telescopeswe will obtain vast samples of starforming galaxies (SFGs). In fact, as studies of star formation, these surveys will have severaladvantages over current surveys at other wavelengths. Firstly, radio luminosity is a direct tracer ofstar formation rate (SFR) once AGN activity has been accounted for and secondly, the combinationof area and the sensitivity will provide a fast survey speed allowing for an unprecedented census ofstar formation in the distant Universe.The sub-mJy regime of the radio source counts has been studied for several decades now andthe up-turn below∼ 1mJy has been well characterised albeit with some scatter. Cu rently thedebate is over the normalisation of the source counts and type of sources that are responsible forthe up-turn. However, the most important question is how do we distinguish between AGN andSFGs in the deepest surveys?We believe that as some AGN have very low emission in the radio, compared to even lowMilky Way rates of star formation, the simple presence of an AGN does not necessarily infer thatthe radio emission of a galaxy is from an accretion driven source. Hence, methods to discriminatebetween whether AGN or star forming activity contribute to the observed radio flux density shouldbe related to the radio emission, and we list some of these methods here:• Radio morphology• Radio spectral index/radio Spectral Energy Distribution (SED)• Radio variability• Radio polarisation• Flux density ratios/full SED modeling2Tracing High Redshift Starformation in the Current and NextGeneration of Radio Surveys N. SeymourFigure 2: Left Star formation rate plotted against redshift for a sample ofradio selected SFGs from the13H field (coloured symbols indicating host galaxies of different stellar masses). Local radio-selected starforming galaxies are illustrated by black dots. EquivalentIR luminosity is plotted on the right axis. At highredshift we see massive galaxies with star formation rates in xcess of that seen in the local Universe.RightDerived comoving star formation rate density (SFRD) as a functio of redshift compared with the meanSFRD presented in [11]. The results in the radio agree well with those at other wavelengths which bodeswell for future radio surveys making detailed studies of star formation at high redshift.This list is not exhaustive and several of these methods havealready been used in variouscombinations (e.g. [2,5,6,7,8,9]). We have found that a combination of the radio morphology andspectral index is the most effective method if you have data of sufficient quality, but in practice forthe deepest surveys the flux ratio method, based on SEDs of SFGs and AGN, is the most productive.So employing these techniques, and on the assumption that one pr cess dominates we can separatethe faint radio source counts by galaxy type (e.g. Fig. 1 left).We have demonstrated that while AGN dominate the 1.4 GHz source counts around 0.4 mJy,SFGs make up 70−80% below 0.1 mJy [2] with the transition between the two populations occur-ing between 0.1 and 0.2mJy. So while SFGs dominate at the faintest flux densities thre remainsa non-negligible contribution from radio quiet AGN around 50µJy. This result is consistent withdeterminations and constraints in the literature (Fig. 1 right) which are reaching a consensus aboutthe nature of the faint radio source counts. The observed differences in this plot are easily explainedby the different methods used in discriminating between AGNand SFGs as well as sample variancebetween the different deep narrow survey fields used in each study.Our knowledge of the faint radio source population will be gratly improved with surveys fromup-coming radio facilities (see other contributions to these proceedings). At fainter flux densitiesthe modeled evolution of the luminosity function of both populations suggests that the fraction ofSFGs will asymptote towards 100% [4]. The eVLA and eMERLIN will push to such flux densitieswhere star formation completely dominates and provide samples of radio-selecting SFGs out tothe highest redshifts. Other projects, like the Evolutionary Map of the Universe (EMU, see [10]in these proceedings) on the Australian Square Kilometer Array Pathfinder, will probe the starforming regime well below 0.1mJy over a large fraction of the whole sky. However, in prepaationfor such surveys we must obtain a better understanding of radio emission from star formation inthe local Universe and a better understanding of the radio luminosity to star formation rate (SFR)conversion.3Tracing High Redshift Starformation in the Current and NextGeneration of Radio Surveys N. SeymourFigure 3: The comoving IR luminosity density (IRLD) plotted as a function of redshift. The gray pointsare SFRD data converted to IRLD from [15], the shaded regions(from [14]) are the IRLD separated by IRluminosity as indicated in the plot. The coloured points arebased on radio-selected SFGs in the 13H fieldand follow the same division by IR luminosity. We observe that the contribution of LIRGs and ULIRGsremains high across 1≤ z≤ 3.2. Science with Radio Selected Star Forming GalaxiesWith the data we have in hand we can already study the properties of host galaxies of powerfulstarbursts selected at radio wavelengths. We discover above z= 1 galaxies with high SFRs whichare simply not detected in the local Universe (Fig. 2 left), even allowing for volume effects. In thelocal Universe the most extreme galaxies have SFRs of∼ 300M⊙yr−1 whereas at high redshift wesee galaxies with SFRs up to almost an order of magnitude greater. Furthermore, we find that suchgalaxies are always have high stellar masses implying they have already built up a large populationof stars [11].We can also use such samples to study the comoving star formati n r e density (SFRD) asa function of redshift (e.g. [12], [13]). We find that the radio selected sample produce resultsconsistent with those from other wavelengths (Fig. 2 right and [11]). The future radio surveysdiscussed in the previous section will no doubt greatly increase the accuracy with which we cantrace the formation of stars in such a plot.From radio surveys we can not only study when stars formed, but also where. The contri-bution to the comoving IR luminosity density (a proxy for theSFRD) by galaxies of differentIR luminosities changes with redshift [14]. The contribution of luminous IR galaxies (LIRGs,1011 ≤ L/L⊙ ≤ 1012) and ultra-luminous IR galaxies (ULIRGs, 1012 ≤ L/L⊙ ≤ 1013) increasesfrom z= 0 to z= 1. However, this is not surprising as it is a natural result ofintegrating a rapidlyevolving luminosity function.By exploiting the IR/radio correlation for SFGs we find that results from radio surveys agreewell those from the IR and can extend such studies to redshifts greater than unity. In Fig. 3 we seethat LIRGs and ULIRGs continue to make a high contribution tothe IR luminosity density across1 ≤ z≤ 3. The extreme starbursts, hyper-luminous IR galaxies (HyLIRGs,L/L⊙ ≥ 1013) whichare not seen in the local Universe only contribute a few percent to the total star forming budget atz= 2−3.4Tracing High Redshift Starformation in the Current and NextGeneration of Radio Surveys N. SeymourFigure 4: The mean observed value of q70 (= log[S70µm/S1.4GHz]) for radio-selected SFGs plotted as a func-tion of redshift (red squares). The numbers beside each datapoint indicate the mean total IR luminosityderived from the radio SFR. The black lines represent the tracks from local templates from [16] at lumi-nosities of as indicted in the figure. These lines become thinner at the redshift a given template becomesundetectable in our radio survey. The trend of a decrease in q70 toward higher redshifts is generally consistentwith the tracks with local SEDs.3. The IR/Radio Relation for Star Forming GalaxiesAn important question for future surveys is does the radio luminosity/star formation rate re-lation hold at high redshifts and extreme luminosities? This relation primarily depends on theIR/radio correlation. What do we expect at high redshift from local galaxies? In Fig. 4 we showthe observed 70µm to 1.4GHz flux density ratio for average SEDs of local galaxies [16]. Weobserve that the flux density ratio increases with IR luminosity which is due to more luminous IRgalaxies being characterised by hotter dust temperatures and hence peaking at shorter wavelengths(i.e. they contribute relatively more to the 70µm band). However, all the tracks decrease withincreasing redshifts due to k-correction effects.We have stacked 70µm images at positions of radio-selected SFGs and hence we canobt ia flux density ratio as a function of redshift, [17] and Fig. 4.We note, however, that we areprobing different luminosities at each redshift, hence must be careful to compare each point tothe correct SED track. The data broadly agree with the general tr nd of decreasing flux densityratio toward higher redshift, but there are differences in the detail. We see that the bin atz∼ 0.9,which contains SFGs close to ULIRG power (L̄ ∼ 1011.7 L⊙), lies next to the low luminosity tracks(log(LIR) = 10) and 2σ away from the ULIRG track. Why might the 70um/radio correlationchange at high redshift? Locally the IR SED is luminosity dependent so it is possible that thesehigh redshift luminous starburst may have different IR SEDscompared to local luminous starbursts.If high redshift star forming regions in luminous starbursts are more extended, they may be moreoptically thin and hence have less free-free absorption (and therefore have a higher radio flux).Such SEDs would also be characterised as being cooler and have a lower 70µm flux density.There is now strong evidence for cold LIRGs and ULIRGs at highredshift. We have found thatsuch galaxies at 0.5 ≤ z≤ 1.0 span a range of characteristic dust temperatures from the hot localstarbursts to that of higher-z sub-mm selected galaxies [18]. We have also found that luminousgalaxies colder than local hot starbursts dominate the luminosity density at these redshifts [19].5Tracing High Redshift Starformation in the Current and NextGeneration of Radio Surveys N. Seymour4. ConclusionsRadio observations of the distant Universe have traditionally been used to study AGN, butwe are now begining to obtain a census of star formation from deep, wide radio surveys. Thereare three crucial issues in exploiting such data: (i) distingu shing between AGN and SFGs, (ii)calibrating the radio luminosity/SFR relation across all redshifts, radio luminosities and type ofgalaxy, and (iii) obtaining redshifts from ancillary data.The radio/IR relation for SFGs appears todepend on IR SED and hence waveband used in the IR. We must better understand this relationshiplocally before applying it to galaxies at high redshift.References[1] G. Miley & C. De Breuck, 2008, A&AR, 15, 67[2] N. Seymour et al., 2008, MNRAS, 386, 1695[3] N. Seymour et al., 2004, MNRAS, 352, 131[4] R. Wilman et al., 2008, MNRAS, 388, 1335[5] T. Muxlow et al., 2005, MNRAS, 358, 1159[6] C. Simpson et al., 2006, MNRAS, 372, 741[7] V. Smolčić et al., 2008, ApJS, 177, 14[8] E. Ibar et al., 2008, MNRAS, 386, 953[9] P. Padovani et al., 2009, ApJ, 694, 235[10] R. Norris et al., 2009,POS(PRA2009)003[11] A. Hopkins et al., 2007, ASPC, 380, 423[12] S. Lilly et al., 1996, ApJ, 460, L1[13] P. Madau et al., 1996, MNRAS, 283, 1388[14] E. Le Floc’h et al., 2005, ApJ, 632, 169[15] A. Hopkins & J.F. Beacom, 2006, ApJ, 651, 142[16] G. Rieke et al., 2009, ApJ, 692, 556[17] N. Seymour et al., 2009a, MNRAS, 398, 1573[18] M. Symeonidis et al., 2009, MNRAS, 397, 1728[19] N. Seymour et al., 2009b, MNRAS,in press6

University of Groningen Research Database

  
 University of Groningen
Tracing High Redshift Starformation in the Current and Next Generation of Radio Surveys
Seymour, Nick
Published in:
EPRINTS-BOOK-TITLE
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
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Publication date:
2009
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Seymour, N. (2009). Tracing High Redshift Starformation in the Current and Next Generation of Radio
Surveys. In EPRINTS-BOOK-TITLE University of Groningen.
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the
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If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
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number of authors shown on this cover page is limited to 10 maximum.
Download date: 11-02-2018
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09
Tracing High Redshift Starformation in the Current
and Next Generation of Radio Surveys
N. Seymour∗
University College London, Department of Space & Climate Physics
Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
E-mail: nps@mssl.ucl.ac.uk
The current deepest radio surveys detect hundreds of sources per square degree below 0.1mJy.
There is a growing consensus that a large fraction of these sources are dominated by star formation
although the exact proportion has been debated in the literature. However, the low luminosity of
these galaxies at most other wavelengths makes determining the nature of individual sources
difficult. If future, deeper surveys performed with the next generation of radio instrumentation
are to reap high scientific reward we need to develop reliable methods of distinguishing between
radio emission powered by active galactic nuclei (AGN) and that powered by star formation.
In particular, we believe that such discriminations should be based on purely radio, or relative to
radio, diagnostics. These diagnostics include radio morphology, radio spectral index, polarisation,
variability, radio luminosity and flux density ratios with non-radio wavelengths e.g. with different
parts of the infrared (IR) regime. We discuss the advantages and limitations of these various
diagnostics methods with current and future surveys. However, weeding AGN out of deep radio
surveys can already provide several insights into the star formation at high redshift. As well as
reproducing the well known rise with redshift in the comoving star formation rate density, we
also see evidence for the continued dominance of LIRGs and ULIRGs to the total star forming
budget across redshifts 1−3. Additionally, while we see that the IR-radio relation for star forming
galaxies does hold to high redshifts (z > 1) there is a mild deviation depending on the IR waveband
used and the range of IR SEDs found. We will discuss the possible reasons behind this change in
properties.
Panoramic Radio Astronomy: Wide-field 1-2 GHz research on galaxy evolution
June 2-5 2009
Groningen, the Netherlands
∗Speaker.
c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/
Tracing High Redshift Starformation in the Current and Next Generation of Radio Surveys N. Seymour
Figure 1: Left The 1.4GHz Euclidean normalised source counts from the 13H field divided into SFGs
and AGN [2]. Models of the contribution from different populations are overlaid: S04 [3], J08 [4]. Right
Compilation from the literature of the fraction of SFGs as a function of 1.4GHz flux density [2,5,6,7,8,9].
1. Introduction
Traditionally radio surveys have been dominated by powerful radio-loud active galactic nuclei
(AGN) and radio galaxies were the first probe of the distant Universe [1], and references therein.
However, surveys by the next generation of radio telescopes we will obtain vast samples of star
forming galaxies (SFGs). In fact, as studies of star formation, these surveys will have several
advantages over current surveys at other wavelengths. Firstly, radio luminosity is a direct tracer of
star formation rate (SFR) once AGN activity has been accounted for and secondly, the combination
of area and the sensitivity will provide a fast survey speed allowing for an unprecedented census of
star formation in the distant Universe.
The sub-mJy regime of the radio source counts has been studied for several decades now and
the up-turn below ∼ 1mJy has been well characterised albeit with some scatter. Currently the
debate is over the normalisation of the source counts and type of sources that are responsible for
the up-turn. However, the most important question is how do we distinguish between AGN and
SFGs in the deepest surveys?
We believe that as some AGN have very low emission in the radio, compared to even low
Milky Way rates of star formation, the simple presence of an AGN does not necessarily infer that
the radio emission of a galaxy is from an accretion driven source. Hence, methods to discriminate
between whether AGN or star forming activity contribute to the observed radio flux density should
be related to the radio emission, and we list some of these methods here:
• Radio morphology
• Radio spectral index/radio Spectral Energy Distribution (SED)
• Radio variability
• Radio polarisation
• Flux density ratios/full SED modeling
2
Tracing High Redshift Starformation in the Current and Next Generation of Radio Surveys N. Seymour
Figure 2: Left Star formation rate plotted against redshift for a sample of radio selected SFGs from the
13H field (coloured symbols indicating host galaxies of different stellar masses). Local radio-selected star
forming galaxies are illustrated by black dots. Equivalent IR luminosity is plotted on the right axis. At high
redshift we see massive galaxies with star formation rates in excess of that seen in the local Universe. Right
Derived comoving star formation rate density (SFRD) as a function of redshift compared with the mean
SFRD presented in [11]. The results in the radio agree well with those at other wavelengths which bodes
well for future radio surveys making detailed studies of star formation at high redshift.
This list is not exhaustive and several of these methods have already been used in various
combinations (e.g. [2,5,6,7,8,9]). We have found that a combination of the radio morphology and
spectral index is the most effective method if you have data of sufficient quality, but in practice for
the deepest surveys the flux ratio method, based on SEDs of SFGs and AGN, is the most productive.
So employing these techniques, and on the assumption that one process dominates we can separate
the faint radio source counts by galaxy type (e.g. Fig. 1 left).
We have demonstrated that while AGN dominate the 1.4 GHz source counts around 0.4 mJy,
SFGs make up 70−80% below 0.1 mJy [2] with the transition between the two populations occur-
ing between 0.1 and 0.2mJy. So while SFGs dominate at the faintest flux densities there remains
a non-negligible contribution from radio quiet AGN around 50 µJy. This result is consistent with
determinations and constraints in the literature (Fig. 1 right) which are reaching a consensus about
the nature of the faint radio source counts. The observed differences in this plot are easily explained
by the different methods used in discriminating between AGN and SFGs as well as sample variance
between the different deep narrow survey fields used in each study.
Our knowledge of the faint radio source population will be greatly improved with surveys from
up-coming radio facilities (see other contributions to these proceedings). At fainter flux densities
the modeled evolution of the luminosity function of both populations suggests that the fraction of
SFGs will asymptote towards 100% [4]. The eVLA and eMERLIN will push to such flux densities
where star formation completely dominates and provide samples of radio-selecting SFGs out to
the highest redshifts. Other projects, like the Evolutionary Map of the Universe (EMU, see [10]
in these proceedings) on the Australian Square Kilometer Array Pathfinder, will probe the star
forming regime well below 0.1mJy over a large fraction of the whole sky. However, in preparation
for such surveys we must obtain a better understanding of radio emission from star formation in
the local Universe and a better understanding of the radio luminosity to star formation rate (SFR)
conversion.
3
Tracing High Redshift Starformation in the Current and Next Generation of Radio Surveys N. Seymour
Figure 3: The comoving IR luminosity density (IRLD) plotted as a function of redshift. The gray points
are SFRD data converted to IRLD from [15], the shaded regions (from [14]) are the IRLD separated by IR
luminosity as indicated in the plot. The coloured points are based on radio-selected SFGs in the 13H field
and follow the same division by IR luminosity. We observe that the contribution of LIRGs and ULIRGs
remains high across 1≤ z≤ 3.
2. Science with Radio Selected Star Forming Galaxies
With the data we have in hand we can already study the properties of host galaxies of powerful
starbursts selected at radio wavelengths. We discover above z = 1 galaxies with high SFRs which
are simply not detected in the local Universe (Fig. 2 left), even allowing for volume effects. In the
local Universe the most extreme galaxies have SFRs of ∼ 300Myr−1 whereas at high redshift we
see galaxies with SFRs up to almost an order of magnitude greater. Furthermore, we find that such
galaxies are always have high stellar masses implying they have already built up a large population
of stars [11].
We can also use such samples to study the comoving star formation rate density (SFRD) as
a function of redshift (e.g. [12], [13]). We find that the radio selected sample produce results
consistent with those from other wavelengths (Fig. 2 right and [11]). The future radio surveys
discussed in the previous section will no doubt greatly increase the accuracy with which we can
trace the formation of stars in such a plot.
From radio surveys we can not only study when stars formed, but also where. The contri-
bution to the comoving IR luminosity density (a proxy for the SFRD) by galaxies of different
IR luminosities changes with redshift [14]. The contribution of luminous IR galaxies (LIRGs,
1011 ≤ L/L ≤ 1012) and ultra-luminous IR galaxies (ULIRGs, 1012 ≤ L/L ≤ 1013) increases
from z = 0 to z = 1. However, this is not surprising as it is a natural result of integrating a rapidly
evolving luminosity function.
By exploiting the IR/radio correlation for SFGs we find that results from radio surveys agree
well those from the IR and can extend such studies to redshifts greater than unity. In Fig. 3 we see
that LIRGs and ULIRGs continue to make a high contribution to the IR luminosity density across
1 ≤ z ≤ 3. The extreme starbursts, hyper-luminous IR galaxies (HyLIRGs, L/L ≥ 1013) which
are not seen in the local Universe only contribute a few percent to the total star forming budget at
z = 2−3.
4
Tracing High Redshift Starformation in the Current and Next Generation of Radio Surveys N. Seymour
Figure 4: The mean observed value of q70 (= log[S70µm/S1.4GHz]) for radio-selected SFGs plotted as a func-
tion of redshift (red squares). The numbers beside each data point indicate the mean total IR luminosity
derived from the radio SFR. The black lines represent the tracks from local templates from [16] at lumi-
nosities of as indicted in the figure. These lines become thinner at the redshift a given template becomes
undetectable in our radio survey. The trend of a decrease in q70 toward higher redshifts is generally consistent
with the tracks with local SEDs.
3. The IR/Radio Relation for Star Forming Galaxies
An important question for future surveys is does the radio luminosity/star formation rate re-
lation hold at high redshifts and extreme luminosities? This relation primarily depends on the
IR/radio correlation. What do we expect at high redshift from local galaxies? In Fig. 4 we show
the observed 70 µm to 1.4GHz flux density ratio for average SEDs of local galaxies [16]. We
observe that the flux density ratio increases with IR luminosity which is due to more luminous IR
galaxies being characterised by hotter dust temperatures and hence peaking at shorter wavelengths
(i.e. they contribute relatively more to the 70 µm band). However, all the tracks decrease with
increasing redshifts due to k-correction effects.
We have stacked 70 µm images at positions of radio-selected SFGs and hence we can obtain
a flux density ratio as a function of redshift, [17] and Fig. 4. We note, however, that we are
probing different luminosities at each redshift, hence must be careful to compare each point to
the correct SED track. The data broadly agree with the general trend of decreasing flux density
ratio toward higher redshift, but there are differences in the detail. We see that the bin at z ∼ 0.9,
which contains SFGs close to ULIRG power (¯L∼ 1011.7 L), lies next to the low luminosity tracks
(log(LIR) = 10) and 2σ away from the ULIRG track. Why might the 70um/radio correlation
change at high redshift? Locally the IR SED is luminosity dependent so it is possible that these
high redshift luminous starburst may have different IR SEDs compared to local luminous starbursts.
If high redshift star forming regions in luminous starbursts are more extended, they may be more
optically thin and hence have less free-free absorption (and therefore have a higher radio flux).
Such SEDs would also be characterised as being cooler and have a lower 70 µm flux density.
There is now strong evidence for cold LIRGs and ULIRGs at high redshift. We have found that
such galaxies at 0.5 ≤ z ≤ 1.0 span a range of characteristic dust temperatures from the hot local
starbursts to that of higher-z sub-mm selected galaxies [18]. We have also found that luminous
galaxies colder than local hot starbursts dominate the luminosity density at these redshifts [19].
5
Tracing High Redshift Starformation in the Current and Next Generation of Radio Surveys N. Seymour
4. Conclusions
Radio observations of the distant Universe have traditionally been used to study AGN, but
we are now begining to obtain a census of star formation from deep, wide radio surveys. There
are three crucial issues in exploiting such data: (i) distinguishing between AGN and SFGs, (ii)
calibrating the radio luminosity/SFR relation across all redshifts, radio luminosities and type of
galaxy, and (iii) obtaining redshifts from ancillary data. The radio/IR relation for SFGs appears to
depend on IR SED and hence waveband used in the IR. We must better understand this relationship
locally before applying it to galaxies at high redshift.
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6


The current deepest radio surveys detect hundreds of sources per square degree below 0.1mJy.
There is a growing consensus that a large fraction of these sources are dominated by star formation
although the exact proportion has been debated in the literature. However, the low luminosity of
these galaxies at most other wavelengths makes determining the nature of individual sources
difficult. If future, deeper surveys performed with the next generation of radio instrumentation
are to reap high scientific reward we need to develop reliable methods of distinguishing between
radio emission powered by active galactic nuclei (AGN) and that powered by star formation.
In particular, we believe that such discriminations should be based on purely radio, or relative to
radio, diagnostics. These diagnostics include radiomorphology, radio spectral index, polarisation,
variability, radio luminosity and flux density ratios with non-radio wavelengths e.g. with different
parts of the infrared (IR) regime. We discuss the advantages and limitations of these various
diagnostics methods with current and future surveys. However, weeding AGN out of deep radio
surveys can already provide several insights into the star formation at high redshift. As well as
reproducing the well known rise with redshift in the comoving star formation rate density, we
also see evidence for the continued dominance of LIRGs and ULIRGs to the total star forming
budget across redshifts 1−3. Additionally, while we see that the IR-radio relation for star forming
galaxies does hold to high redshifts (z>1) there is amild deviation depending on the IR waveband
used and the range of IR SEDs found. We will discuss the possible reasons behind this change in
properties.
Panoramic

Seymour, Nick,

University of Groningen Digital Archive

https://pure.rug.nl/ws/files/14696995/Tracing_high_Redshift_Star.pdf

Tracing High Redshift Starformation in the Current and Next Generation of Radio Surveys

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