406 research outputs found
VALES: IV. Exploring the transition of star formation efficiencies between normal and starburst galaxies using APEX/SEPIA Band-5 and ALMA at low redshift
In this work we present new APEX/SEPIA Band-5 observations targeting the CO
() emission line of 24 Herschel-detected galaxies at .
Combining this sample {with} our recent new Valpara\'iso ALMA Line Emission
Survey (VALES), we investigate the star formation efficiencies (SFEs =
SFR/) of galaxies at low redshift. We find the SFE of our sample
bridges the gap between normal star-forming galaxies and Ultra-Luminous
Infrared Galaxies (ULIRGs), which are thought to be triggered by different star
formation modes. Considering the as the SFR and the
ratio, our data show a continuous and smooth increment as a function of
infrared luminosity (or star formation rate) with a scatter about 0.5 dex,
instead of a steep jump with a bimodal behaviour. This result is due to the use
of a sample with a much larger range of sSFR/sSFR using LIRGs, with
luminosities covering the range between normal and ULIRGs. We conclude that the
main parameters controlling the scatter of the SFE in star-forming galaxies are
the systematic uncertainty of the conversion factor, the gas
fraction and physical size.Comment: 9pages, 7 figures, 1 table, accepted for publication in MNRA
Infrared-Faint Radio Sources: A Cosmological View - AGN Number Counts, the Cosmic X-Ray Background and SMBH Formation
Context. Infrared Faint Radio Sources (IFRS) are extragalactic emitters
clearly detected at radio wavelengths but barely detected or undetected at
optical and infrared wavelengths, with 5 sigma sensitivities as low as 1 uJy.
Aims. Recent SED-modelling and analysis of their radio properties shows that
IFRS are consistent with a population of (potentially extremely obscured)
high-redshift AGN at 3<z<6. We demonstrate some astrophysical implications of
this population and compare them to predictions from models of galaxy evolution
and structure formation.
Methods. We compiled a list of IFRS from four deep extragalactic surveys and
extrapolated the IFRS number density to a survey-independent value of (30.8 +-
15.0) per square degree. We computed the IFRS contribution to the total number
of AGN in the Universe to account for the Cosmic X-ray Background. By
estimating the black hole mass contained in IFRS, we present conclusions for
the SMBH mass density in the early universe and compare it to relevant
simulations of structure formation after the Big Bang.
Results. The number density of AGN derived from the IFRS density was found to
be about 310 deg^-2, which is equivalent to a SMBH mass density of the order of
10^3 M_sun Mpc^-3 in the redshift range 3<z<6. This produces an X-ray flux of 9
10^-16 W m^-2 deg^-2 in the 0.5-2.0 keV band and 3 10^-15 W m^-2 deg^-2 in the
2.0-10 keV band, in agreement with the missing unresolved components of the
Cosmic X-ray Background. Concerning the problem of SMBH formation after the Big
Bang we find evidence for a scenario involving both halo gas accretion and
major mergers.Comment: 8 pages, 4 figures, accepted for publication in A&
Spectroscopy of superluminous supernova host galaxies. A preference of hydrogen-poor events for extreme emission line galaxies
Superluminous supernovae (SLSNe) are very bright explosions that were only
discovered recently and that show a preference for occurring in faint dwarf
galaxies. Understanding why stellar evolution yields different types of stellar
explosions in these environments is fundamental in order to both uncover the
elusive progenitors of SLSNe and to study star formation in dwarf galaxies. In
this paper, we present the first results of our project to study SUperluminous
Supernova Host galaxIES, focusing on the sample for which we have obtained
spectroscopy. We show that SLSNe-I and SLSNe-R (hydrogen-poor) often (~50% in
our sample) occur in a class of galaxies that is known as Extreme Emission Line
Galaxies (EELGs). The probability of this happening by chance is negligible and
we therefore conclude that the extreme environmental conditions and the SLSN
phenomenon are related. In contrast, SLSNe-II (hydrogen-rich) occur in more
massive, more metal-rich galaxies with softer radiation fields. Therefore, if
SLSNe-II constitute a uniform class, their progenitor systems are likely
different from those of H-poor SLSNe. Gamma-ray bursts (GRBs) are, on average,
not found in as extreme environments as H-poor SLSNe. We propose that H-poor
SLSNe result from the very first stars exploding in a starburst, even earlier
than GRBs. This might indicate a bottom-light initial mass function in these
systems. SLSNe present a novel method of selecting candidate EELGs independent
of their luminosity.Comment: Published version, matches proofs. Accepted 2015 February 13. 23
pages, 8 figures, 4 tables. Minor changes with respect to previous versio
Linking the X-ray and infrared properties of star-forming galaxies at z < 1.5
We present the most complete study to date of the X-ray emission from star formation in high-redshift (median z = 0.7; z −3 in both hard and soft X-ray bands. From the sources which are star formation dominated, only a small fraction are individually X-ray detected and for the bulk of the sample we calculate average X-ray luminosities through stacking. We find an average soft X-ray to infrared ratio of log ?L SX /L IR ? = −4.3 and an average hard X-ray to infrared ratio of log?L HX /L IR ?=−3.8.WereportthattheX-ray/IRcorrelationisapproximatelylinearthrough the entire range of L IR and z probed and, although broadly consistent with the local (z < 0.1) one, it does display some discrepancies. We suggest that these discrepancies are unlikely to be physical, i.e. due to an intrinsic change in the X-ray properties of star-forming galaxies with cosmic time, as there is no significant evidence for evolution of the L X /L IR ratio with redshift. Instead, they are possibly due to selection effects and remaining AGN contamination.
We also examine whether dust obscuration in the galaxy plays a role in attenuating X-rays from star formation, by investigating changes in the L X /L IR ratio as a function of the average dust temperature. We conclude that X-rays do not suffer any measurable attenuation in the host galaxy
ALMA reveals the molecular gas properties of five star-forming galaxies across the main sequence at 3
International audienceWe present the detection of CO(5-4) with S/N> 7 - 13 and a lower CO transition with S/N > 3 (CO(4-3) for 4 galaxies, and CO(3-2) for one) with ALMA in band 3 and 4 in five main sequence star-forming galaxies with stellar masses 3-6x10^10 M/M_sun at 3 < z < 3.5. We find a good correlation between the total far-infrared luminosity LFIR and the luminosity of the CO(5-4) transition L'CO(5-4), where L'CO(5-4) increases with SFR, indicating that CO(5-4) is a good tracer of the obscured SFR in these galaxies. The two galaxies that lie closer to the star-forming main sequence have CO SLED slopes that are comparable to other star-forming populations, such as local SMGs and BzK star-forming galaxies; the three objects with higher specific star formation rates (sSFR) have far steeper CO SLEDs, which possibly indicates a more concentrated episode of star formation. By exploiting the CO SLED slopes to extrapolate the luminosity of the CO(1-0) transition, and using a classical conversion factor for main sequence galaxies of alpha_CO = 3.8 M_sun(K km s^-1 pc^-2)^-1, we find that these galaxies are very gas rich, with molecular gas fractions between 60 and 80%, and quite long depletion times, between 0.2 and 1 Gyr. Finally, we obtain dynamical masses that are comparable with the sum of stellar and gas mass (at least for four out of five galaxies), allowing us to put a first constraint on the alpha_CO parameter for main sequence galaxies at an unprecedented redshift
The ALMA Frontier Fields Survey - IV. Lensing-corrected 1.1 mm number counts in Abell 2744, MACSJ0416.1-2403 and MACSJ1149.5+2223
[abridged] Characterizing the number counts of faint, dusty star-forming
galaxies is currently a challenge even for deep, high-resolution observations
in the FIR-to-mm regime. They are predicted to account for approximately half
of the total extragalactic background light at those wavelengths. Searching for
dusty star-forming galaxies behind massive galaxy clusters benefits from strong
lensing, enhancing their measured emission while increasing spatial resolution.
Derived number counts depend, however, on mass reconstruction models that
properly constrain these clusters. We estimate the 1.1 mm number counts along
the line of sight of three galaxy clusters, i.e. Abell 2744, MACSJ0416.1-2403
and MACSJ1149.5+2223, which are part of the ALMA Frontier Fields Survey. We
perform detailed simulations to correct these counts for lensing effects. We
use several publicly available lensing models for the galaxy clusters to derive
the intrinsic flux densities of our sources. We perform Monte Carlo simulations
of the number counts for a detailed treatment of the uncertainties in the
magnifications and adopted source redshifts. We find an overall agreement among
the number counts derived for the different lens models, despite their
systematic variations regarding source magnifications and effective areas. Our
number counts span ~2.5 dex in demagnified flux density, from several mJy down
to tens of uJy. Our number counts are consistent with recent estimates from
deep ALMA observations at a 3 level. Below 0.1 mJy, however,
our cumulative counts are lower by 1 dex, suggesting a flattening in
the number counts. In our deepest ALMA mosaic, we estimate number counts for
intrinsic flux densities 4 times fainter than the rms level. This
highlights the potential of probing the sub-10 uJy population in larger samples
of galaxy cluster fields with deeper ALMA observations.Comment: 19 pages, 14 figures, 3 tables. Accepted for publication in A&
Dust properties of Lyman break galaxies at
We explore from a statistical point of view the far-infrared (far-IR) and
sub-millimeter (sub-mm) properties of a large sample of LBGs (22,000) at z~3 in
the COSMOS field. The large number of galaxies allows us to split it in several
bins as a function of UV luminosity, UV slope, and stellar mass to better
sample their variety. We perform stacking analysis in PACS (100 and 160 um),
SPIRE (250, 350 and 500 um) and AzTEC (1.1 mm) images. Our stacking procedure
corrects the biases induced by galaxy clustering and incompleteness of our
input catalogue in dense regions. We obtain the full IR spectral energy
distributions (SED) of subsamples of LBGs and derive the mean IR luminosity as
a function of UV luminosity, UV slope, and stellar mass. The average IRX is
roughly constant over the UV luminosity range, with a mean of 7.9 (1.8 mag).
However, it is correlated with UV slope, and stellar mass. We investigate using
a statistically-controlled stacking analysis as a function of (stellar mass, UV
slope) the dispersion of the IRX-UVslope and IRX-M* plane. Our results enable
us to study the average relation between star-formation rate (SFR) and stellar
mass, and we show that our LBG sample lies on the main sequence of star
formation at z~3.Comment: Accepted to A&A, 17 Pages, 14 Figures, 2 Table
The second Herschel–ATLAS Data Release – III. Optical and near-infrared counterparts in the North Galactic Plane field
This paper forms part of the second major public data release of the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS). In this work, we describe the identification of optical and near-infrared counterparts to the submillimetre detected sources in the 177 deg2 North Galactic Plane (NGP) field. We used the likelihood ratio method to identify counterparts in the Sloan Digital Sky Survey and in the United Kingdom InfraRed Telescope Imaging Deep Sky Survey within a search radius of 10 arcsec of the H-ATLAS sources with a 4σ detection at 250 μm. We obtained reliable (R ≥ 0.8) optical counterparts with r < 22.4 for 42 429 H-ATLAS sources (37.8 per cent), with an estimated completeness of 71.7 per cent and a false identification rate of 4.7 per cent. We also identified counterparts in the near-infrared using deeper K-band data which covers a smaller ∼25 deg2. We found reliable near-infrared counterparts to 61.8 per cent of the 250-μm-selected sources within that area. We assessed the performance of the likelihood ratio method to identify optical and near-infrared counterparts taking into account the depth and area of both input catalogues. Using catalogues with the same surface density of objects in the overlapping ∼25 deg2 area, we obtained that the reliable fraction in the near-infrared (54.8 per cent) is significantly higher than in the optical (36.4 per cent). Finally, using deep radio data which covers a small region of the NGP field, we found that 80–90 per cent of our reliable identifications are correct
Resolving a dusty, star-forming SHiZELS galaxy at z = 2.2 with HST, ALMA, and SINFONI on kiloparsec scales
We present ∼0.15 arcsec spatial resolution imaging of SHiZELS-14, a massive (M∗∼1011M⊙), dusty, star-forming galaxy at z = 2.24. Our rest-frame ∼1kpc-scale, matched-resolution data comprise four different widely used tracers of star formation: the Hα emission line (from SINFONI/VLT), rest-frame UV continuum (from HST F606W imaging), the rest-frame far-infrared (from ALMA), and the radio continuum (from JVLA). Although originally identified by its modest Hα emission line flux, SHiZELS-14 appears to be a vigorously star-forming (SFR∼1000M⊙yr−1) example of a submillimetre galaxy, probably undergoing a merger. SHiZELS-14 displays a compact, dusty central starburst, as well as extended emission in Hα and the rest-frame optical and FIR. The UV emission is spatially offset from the peak of the dust continuum emission, and appears to trace holes in the dust distribution. We find that the dust attenuation varies across the spatial extent of the galaxy, reaching a peak of at least AH α ∼ 5 in the most dusty regions, although the extinction in the central starburst is likely to be much higher. Global star-formation rates inferred using standard calibrations for the different tracers vary from ∼10−1000M⊙yr−1, and are particularly discrepant in the galaxy’s dusty centre. This galaxy highlights the biased view of the evolution of star-forming galaxies provided by shorter wavelength data
SPT 0538-50: Physical conditions in the ISM of a strongly lensed dusty star-forming galaxy at z=2.8
We present observations of SPT-S J053816-5030.8, a gravitationally-lensed
dusty star forming galaxy (DSFG) at z = 2.7817, first discovered at millimeter
wavelengths by the South Pole Telescope. SPT 0538-50 is typical of the
brightest sources found by wide-field millimeter-wavelength surveys, being
lensed by an intervening galaxy at moderate redshift (in this instance, at z =
0.441). We present a wide array of multi-wavelength spectroscopic and
photometric data on SPT 0538-50, including data from ALMA, Herschel PACS and
SPIRE, Hubble, Spitzer, VLT, ATCA, APEX, and the SMA. We use high resolution
imaging from HST to de-blend SPT 0538-50, separating DSFG emission from that of
the foreground lens. Combined with a source model derived from ALMA imaging
(which suggests a magnification factor of 21 +/- 4), we derive the intrinsic
properties of SPT 0538-50, including the stellar mass, far-IR luminosity, star
formation rate, molecular gas mass, and - using molecular line fluxes - the
excitation conditions within the ISM. The derived physical properties argue
that we are witnessing compact, merger-driven star formation in SPT 0538-50,
similar to local starburst galaxies, and unlike that seen in some other DSFGs
at this epoch.Comment: 16 pages, 11 figures. Accepted for publication in Ap
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