We investigate the physical properties of the progenitors of today living
Milky Way-like galaxies that are visible as Damped Lya Absorption systems and
Lya Emitters at higher redshifts (z ~ 2.3,5.7). To this aim we use a
statistical merger-tree approach that follows the formation of the Galaxy and
its dwarf satellites in a cosmological context, tracing the chemical evolution
and stellar population history of the progenitor halos. The model accounts for
the properties of the most metal-poor stars and local dwarf galaxies, providing
insights on the early cosmic star-formation. Fruitful links between Galactic
Archaeology and more distant galaxies are presented.Comment: 6 pages, 6 figures; to appear in the proceedings of the Subaru
  conference on Galactic Archaeology, Shuzenji, Japan (Nov. 1-4 2011);
  Astronomical Society of the Pacific Conference Series 201

Ferrara, Andrea

Salvadori, Stefania

English

arXiv

University of Groningen

The Infant Milky Way

We investigate the physical properties of the progenitors of today living Milky Way-like galaxies that are visible as Damped Lyα Absorption systems and Lyα Emitters at higher redshifts (z ≍ 2.3,5.7). To this aim we use a statistical merger-tree approach that follows the formation of the Galaxy and its dwarf satellites in a cosmological context, tracing the chemical evolution and stellar population history of the progenitor halos. The model accounts for the properties of the most metal-poor stars and local dwarf galaxies, providing insights on the early cosmic star-formation. Fruitful links between Galactic Archaeology and more distant galaxies are presented

Dissertations of the University of Groningen

Proceedings - University of Groningen

We investigate the physical properties of the progenitors of today living Milky Way-like galaxies that are visible as Damped Ly Absorption systems and Ly Emitters at higher redshifts (z ≈ 2.3, 5.7). To this aim we use a statistical mergertree approach that follows the formation of the Galaxy and its dwarf satellites in a cosmological context, tracing the chemical evolution and stellar population history of the progenitor halos. The model accounts for the properties of the most metal-poor stars and local dwarf galaxies, providing insights on the early cosmic star-formation. Fruitful links between Galactic Archaeology and more distant galaxies are presented

   University of GroningenThe infant MilkyWaySalvadori, Stefania; Ferrara, AndreaPublished in:ASP Conference SeriesIMPORTANT 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:2012Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Salvadori, S., & Ferrara, A. (2012). The infant MilkyWay. ASP Conference Series.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: 04-06-2022arXiv:1204.1943v1  [astro-ph.CO]  9 Apr 2012**Volume Title**ASP Conference Series, Vol. **Volume Number****Author**c© **Copyright Year** Astronomical Society of the PacificThe infant Milky WayStefania Salvadori1 and Andrea Ferrara21Kapteyn Astronomical Institute, Landleven 12, 9747AD Groningen, NL2Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, ITAbstract. We investigate the physical properties of the progenitors of today livingMilky Way-like galaxies that are visible as Damped Lyα Absorption systems and LyαEmitters at higher redshifts (z ≈ 2.3, 5.7). To this aim we use a statistical merger-tree approach that follows the formation of the Galaxy and its dwarf satellites in acosmological context, tracing the chemical evolution and stellar population history ofthe progenitor halos. The model accounts for the propertiesof the most metal-poorstars and local dwarf galaxies, providing insights on the early cosmic star-formation.Fruitful links between Galactic Archaeology and more distant galaxies are presented.1. BackgroundOne of the most popular methods to identify high-redshifts galaxies (z ≈ 2 − 7) isby detecting their strong Lyα line. These Lyman Alpha Emitters (LAEs) are mainlyassociated tostar-forming galaxies, and they have been extensively used to probeboth the ionization state of the Inter Galactic Medium and the early galaxy evolu-tion. At lower redshifts,z < 5, galaxies withhigh neutral hydrogen column den-sities, NHI > 1020.3cm−2, can be identified in the spectra of more distant quasarsby means of their strong Lyα absorption line. The most metal-poor among theseDamped Lyα Absorption systems (DLAs), can provide insights on the initial metal-enrichment phases of galaxy formation. Recently a DLA with [Fe/H]≈ −3 observed atzabs≈ 2.3, has indeed revealed strong carbon-enhancement and evident odd-even effect(Cooke et al. 2011b), consistent with the chemical imprint by Z = 0 faint supernovae(Kobayashi et al. 2011). Furthermore, all others DLAs with [Fe/H]< −2 observed athigh-resolution show chemical abundance ratios consistent with those of very metal-poor Galactic halo stars (Cooke et al. 2011a), thus suggestin possible connections be-tween these absorbers and the early building blocks of MilkyWa (MW) -like galaxies.We determine the physical properties of the progenitors of the MW and its dwarf com-panions by using the merger-tree code GAMETE (GAlaxy MErgerTr e & Evolution),which reconstructs the possible star-formation and chemical evolution histories of theMW system. The observed Lyα luminosity and Lyα line equivalent width are computedusing the LAE model by Dayal and collaborators (Dayal et al. 2008, 2010) that repro-duces a number of important observations for high-z LAEs. Adopting the canonicalobservational criteria we identify the progenitors visible as DLAs and LAEs at red-shifts respectively equal toz ≈ 2.3 andz ≈ 5.7. The observable properties of the MWGalaxy and its neighboring companions are presented below,from present days back tothe time when the Universe was only 1 Gyr old.12 Stefania Salvadori & Andrea Ferrara2. The Milky Way system at z= 0 : Galactic ArchaeologyVery metal-poor stars represent the living fossils of the first stellar generations. Theirchemical abundance patterns and Metallicity DistributionFu ctions (MDFs) observedin both the stellar halo and in nearby dSph galaxies can provide fundamental insightson the properties of the first cosmic sources.2.1. First stars and their Cosmic RelicsCosmological simulations suggest that first stars formed atz ≈ 15− 20 in primordialH2-cooling minihaloes and that were possibly more massive than ypical stars formingtoday (Hosokawa et al. 2011). The transition from massive tonormal stars is expectedto be driven by metalsanddust cooling, becoming important when the metallicity of thestar-forming gas exceeds the critical value,Zcr = 10−4±1Z⊙ (Schneider et al. 2002). Weuse our cosmological model to interpret the observed Galactic halo MDF. We find thatthe low-metallicity tail of the MDF strongly depends on the assumedZcr value (Fig. 1).If the observed cut-off (Schörck et al. 2009) suggestsZcr ≈ 10−4Z⊙, the presence of thefour stars at [Fe/H]< −4.5 can only be accounted ifZcr < 10−5Z⊙. In particular, theexistence of the most metal-deficient star ever, which hastot l metallicityZ ≈ 10−4.5Z⊙(Caffau et al. 2011) clearly requiresZcr < 10−4Z⊙. Such a recent discovery definitelyproves that dust strongly governs the transition from massive to normal stars in the low-Z regimes (Schneider et al. 2002).Figure 1. The Galactic halo MDF: observations (points (Beers & Christlieb2005)) vs simulations averaged over 100 possible MW merger histories (histogramsand shaded area). The starred symbol indicates the most metal-deficient star.The chemical abundance patterns of halo stars with−4.5 <[Fe/H]< −2.7 (Cayrel et al.2004; Caffau et al. 2011), do not show any peculiar imprint from very massive primor-dial stars, and have small chemical abundance scatter unlikely resulting from individualsupernovae (SN) ejecta. According to our cosmological model (Salvadori et al. 2007)the number of stars formed out of gas pollutedonlyby Z < Zcr stars is extremely small,and thus negligible in current data sample. These ”second-ge eration” stars can ei-ther have low or high [Fe/H] (Fig. 2) if they form in halos that accreted metal-enrichedgas from the MW environment or that are self-enriched by the first stars. To have thechance to detect the chemical imprint by first stars an highernumber of [Fe/H]< −2stars is clearly needed. To this aim it is useful to survey thestellar halo between20 kpc <∼ r <∼ 40 kpc, where the contribution of [Fe/H]< −2 stars with respect to theoverall stellar population is expected to be maximal (Salvadori et al. 2010b).S. Salvadori & A. Ferrara 3Figure 2. Number of stars predicted to exist atz= 0 as a function of their [Fe/H]for differentZcr models. The magenta histograms show second-generation stars, thecyan histograms the overall stellar populations.2.2. First Galaxies and their Cosmic RelicsAn alternative way to find very metal-poor stars is by surveying dSph galaxies and inparticular ultra-faint dSphs (L < 105L⊙, Fig. 2), in which [Fe/H]< −3 stars representthe 25% of the total stellar mass. These faint dwarfs are predicted to be among the firststar-forming galaxies in the MW system, left-overs of H2-cooling minihaloes formedat z > 8.5 (Salvadori & Ferrara 2009), i.e. before the end of reionization (zrei = 6). Inthese galaxies the higher fraction of [Fe/H]< −3 stars with respect to the more luminous”classical dSphs” reflects both the lower star-formation rate, caused by ineffective H2cooling, and the lower metal (pre-)enrichment of the MW-environment at their furtherformation epoch. Indeed classical dSphs are find to finally assemble atz < 7 when thepre-enrichment of the MW environment was [Fe/H]≈ −3. The few stars at [Fe/H]< −3observed in classical dSphs (Starkenburg et al. 2010) are predicted to form in progenitorminihaloes atz> 8.5 (Salvadori & Ferrara 2009), some of which might host first stars.The unusual composition of two stars at [Fe/H]≈ −2 observed in Hercules (Koch et al.2008) might be the result of self-enrichment by first stars inearly progenitor halos.Figure 3. Observed (points with error-bars, (Kirby et al. 2008)) vs simulated (con-tours/histograms) properties of dSph galaxies atz = 0. Left: the iron-luminosity re-lation. The colored shaded areas correspond to regions includi g the (99, 95, 68)% ofthe total number of dSph candidates in 50 MW merger histories(Salvadori & Ferrara2012).Right: MDF of ultra-faint dSph galaxies (Salvadori & Ferrara 2009).4 Stefania Salvadori & Andrea FerraraFigure 4. MW progenitors visible as DLAs atz ≈ 2.3 (contours), with color in-tensity corresponding to regions containing the (99, 95, 68)% of DLAs in 50 mergerhistories (Salvadori & Ferrara 2012). Points with error-bas are observations: circles(Prochaska et al. 2007), triangles (Cooke et al. 2011a), square (Cooke et al. 2011b).3. The Milky Way system at z≈ 2.3: DLAsThe predictedNHI vs [Fe/H] values of the MW progenitors visible as DLAs atz ≈ 2.3follows the observed relation (Fig. 4). In our picture very metal-poor DLAs, [Fe/H]<−2, are associated to starlessM ≈ 108M⊙ minihaloes that virialize from metal-enrichedregions of the MW environment before the end of reionizationand passively evolvedown toz ≈ 2.3. These sterile absorbers retain the chemical imprint of the dominantstellar populations that pollute the MW environment at their formation epoch: low-ZSN type II (Salvadori et al. 2007). This finding agrees with the observational results byBecker et al. (2012) that show that the gas chemical abundance r tios in very metal-poorDLAs/sub-DLAs do not significantly evolve between 2< z < 5. The recently discov-ered C-enhanced DLA is instead pertaining to a new class of absorbers hosting firststars along with second-generation of long-living low-mass stars. These peculiar DLAsare descendants ofM ≈ 107M⊙ minihaloes, that virialize atz> 8 in neutral primordialregions of the MW environment and passively evolve after a short initial period of starformation. These conditions are only satisfied by≈ 0.01% of the total amount of DLAs,making these absorbers extremely rare. The peculiar abundance pattern observed in theC-enhanced DLA results from the enrichment by low-metallicity SN typeII and AGBstars, which may start to form as soon asZ > Zcr. While SNII nucleosynthetic prod-ucts are mostly lost in winds, AGB metals are retained in the ISM, causing a dramaticincrease of [C/Fe]. The amount of N produced byZ < 5 × 10−4Z⊙ AGB stars is verylimited (Meynet & Maeder 2002), resulting in a gas abundance[N/H]= −3.8± 0.9 (seeFig. 5). The mass of relic stars in C-enhanced DLAs isM∗ ≈ 102−4M⊙, making themthe gas-rich counterpart of the faintest dwarfs.4. The Milky Way system at z≈ 5.7: LAEsAt z ≈ 5.7 the star-forming progenitors of MW-like galaxies cover a wide range ofobserved Lyα luminosity, Lα = 1039−43.25 erg s−1. The probability to have at leastone progenitor observable as LAE (Lα ≥ 1042, EW ≥ 20 Å) is therefore very highP ≥ 68%. Such visible progenitors are mainly associated with the most massive halosof the hierarchy, i.e. the major branch, with total massM > 109.5M⊙. On averageS. Salvadori & A. Ferrara 5Figure 5. Gas chemical abundances in the C-enhanced DLA: observations (points,Cooke et al. (2011a)) vs simulations (average value±1σ, shaded areas).the identified LAEs have star-formation rateṡM∗ ≈ 2.3M⊙/yr andLα ≈ 1042.2erg/s.They are populated by intermediate age stars,t∗ ≈ 150− 400 Myr, which have averagemetallicitiesZ ≈ (0.3−1)Z⊙ (Fig.6). Interestingly these visible MW progenitors providemore than the 10% of the very metal-poor stars that are observed today in the Galactichalo. Indeed, most of these [Fe/H]< −2 stars formed atz> 6 in newly virializing halos,accreting metal-enriched gas from the MW environment. Byz ≈ 5.7 many of thesepremature building blocks have already merged into the major branch of the hierarchy,i.e. the visible progenitor (Salvadori et al. 2010a).Figure 6. Physical properties of MW progenitors (yellow circles) atz≈ 5.7. Bluetriangles identify the objects visible as LAEs. Points witherror-bars are data byOno et al. (2010) (magenta squares, 4 models for 1 LAE), Pirzkal et al. (2007) (bluetriangles) and Lai et al. (2007) (green circles). As a function of the total stellar massthe panels show: the instantaneous star formation rate (a),the average stellar age (c)and (d) metallicity (d). Panel (b) shows the relation between the halo and the gasmass, with the cosmic value pointed out by the dashed line.6 Stefania Salvadori & Andrea Ferrara5. ConclusionsThe MW system is a powerful laboratory to study early galaxy formation. On the onehand the properties of the first cosmic sources can be studiedby xploiting the obser-vations of today living metal-poor stars and galaxies. Fromthe other hand the earlyevolutionary stages of different MW progenitors can be investigated using complemen-tary observations of high-z galaxies. In particular, by identifying the MW progenitorsamong the faintest LAEs observed atz ≈ 5.7 it will be possible to observe the MW inits infancy, when it was only 1 Gyr old.Acknowledgments. We thank R. Schneider and P. Dayal for their contribution tothe works on Galactic Archaeology and MW-LAE connection.ReferencesBecker, G. D., Sargent, W. L. W., Rauch, M., & Carswell, R. F. 201 , ApJ, 744, 91Beers, T. C., & Christlieb, N. 2005, ARA&A, 43, 531Caffau, E., Bonifacio, P., François, P., Sbordone, L., Monaco,L., Spite, M., Spite, F., Ludwig,H.-G., Cayrel, R., Zaggia, S., Hammer, F., Randich, S., Molaro, P., & Hill, V. 2011, Nat,477, 67Cayrel, R., Depagne, E., Spite, M., Hill, V., Spite, F., François, P., Plez, B., Beers, T., Primas,F., Andersen, J., Barbuy, B., Bonifacio, P., Molaro, P., & Nordström, B. 2004, A&A,416, 1117Cooke, R., Pettini, M., Steidel, C. C., Rudie, G. C., & Jorgenson, R. A. 2011a, MNRAS, 412,1047Cooke, R., Pettini, M., Steidel, C. C., Rudie, G. C., & Nissen, P. E. 2011b, MNRAS, 417, 1534Dayal, P., Ferrara, A., & Gallerani, S. 2008, MNRAS, 389, 1683Dayal, P., Ferrara, A., & Saro, A. 2010, MNRAS, 402, 1449Kirby, E. N., Simon, J. D., Geha, M., Guhathakurta, P., & Frebel, A. 2008, ApJ, 685, L43Kobayashi, C., Tominaga, N., & Nomoto, K. 2011, ApJ, 730, L14Koch, A., McWilliam, A., Grebel, E. K., Zucker, D. B., & Belokurov, V. 2008, ApJ, 688, L13Lai, K., Huang, J.-S., Fazio, G., Cowie, L. L., Hu, E. M., & Kakazu, Y. 2007, ApJ, 655, 704Meynet, G., & Maeder, A. 2002, A&A, 390, 561Ono, Y., Ouchi, M., Shimasaku, K., Dunlop, J., Farrah, D., McLure, R., & Okamura, S. 2010,ApJ, 724, 1524Pirzkal, N., Malhotra, S., Rhoads, J. E., & Xu, C. 2007, ApJ, 667, 49Prochaska, J. X., Wolfe, A. M., Howk, J. C., Gawiser, E., Burles, S. M., & Cooke, J. 2007,ApJS, 171, 29Salvadori, S., Dayal, P., & Ferrara, A. 2010a, MNRAS, 407, L1Salvadori, S., & Ferrara, A. 2009, MNRAS, 395, L6— 2012, MNRAS, 421, L29Salvadori, S., Ferrara, A., Schneider, R., Scannapieco, E., & Kawata, D. 2010b, MNRAS, 401,L5Salvadori, S., Schneider, R., & Ferrara, A. 2007, MNRAS, 381, 647Schneider, R., Ferrara, A., Natarajan, P., & Omukai, K. 2002, ApJ, 571, 30Schörck, T., Christlieb, N., Cohen, J. G., Beers, T. C., Shectman, S., Thompson, I., McWilliam,A., Bessell, M. S., Norris, J. E., Meléndez, J., Ramı́rez, S., Haynes, D., Cass, P., Hartley,M., Russell, K., Watson, F., Zickgraf, F.-J., Behnke, B., Fechner, C., Fuhrmeister, B.,Barklem, P. S., Edvardsson, B., Frebel, A., Wisotzki, L., & Reimers, D. 2009, A&A,507, 817Starkenburg, E., Hill, V., Tolstoy, E., González Hernández, J. I., Irwin, M., Helmi, A., Battaglia,G., Jablonka, P., Tafelmeyer, M., Shetrone, M., Venn, K., & de Boer, T. 2010, A&A,513, A34

The infant MilkyWay

   University of GroningenThe infant MilkyWaySalvadori, Stefania; Ferrara, AndreaPublished in:ASP Conference SeriesIMPORTANT 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:2012Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Salvadori, S., & Ferrara, A. (2012). The infant MilkyWay. ASP Conference Series.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:1204.1943v1  [astro-ph.CO]  9 Apr 2012**Volume Title**ASP Conference Series, Vol. **Volume Number****Author**c© **Copyright Year** Astronomical Society of the PacificThe infant Milky WayStefania Salvadori1 and Andrea Ferrara21Kapteyn Astronomical Institute, Landleven 12, 9747AD Groningen, NL2Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, ITAbstract. We investigate the physical properties of the progenitors of today livingMilky Way-like galaxies that are visible as Damped Lyα Absorption systems and LyαEmitters at higher redshifts (z ≈ 2.3, 5.7). To this aim we use a statistical merger-tree approach that follows the formation of the Galaxy and its dwarf satellites in acosmological context, tracing the chemical evolution and stellar population history ofthe progenitor halos. The model accounts for the propertiesof the most metal-poorstars and local dwarf galaxies, providing insights on the early cosmic star-formation.Fruitful links between Galactic Archaeology and more distant galaxies are presented.1. BackgroundOne of the most popular methods to identify high-redshifts galaxies (z ≈ 2 − 7) isby detecting their strong Lyα line. These Lyman Alpha Emitters (LAEs) are mainlyassociated tostar-forming galaxies, and they have been extensively used to probeboth the ionization state of the Inter Galactic Medium and the early galaxy evolu-tion. At lower redshifts,z < 5, galaxies withhigh neutral hydrogen column den-sities, NHI > 1020.3cm−2, can be identified in the spectra of more distant quasarsby means of their strong Lyα absorption line. The most metal-poor among theseDamped Lyα Absorption systems (DLAs), can provide insights on the initial metal-enrichment phases of galaxy formation. Recently a DLA with [Fe/H]≈ −3 observed atzabs≈ 2.3, has indeed revealed strong carbon-enhancement and evident odd-even effect(Cooke et al. 2011b), consistent with the chemical imprint by Z = 0 faint supernovae(Kobayashi et al. 2011). Furthermore, all others DLAs with [Fe/H]< −2 observed athigh-resolution show chemical abundance ratios consistent with those of very metal-poor Galactic halo stars (Cooke et al. 2011a), thus suggestin possible connections be-tween these absorbers and the early building blocks of MilkyWa (MW) -like galaxies.We determine the physical properties of the progenitors of the MW and its dwarf com-panions by using the merger-tree code GAMETE (GAlaxy MErgerTr e & Evolution),which reconstructs the possible star-formation and chemical evolution histories of theMW system. The observed Lyα luminosity and Lyα line equivalent width are computedusing the LAE model by Dayal and collaborators (Dayal et al. 2008, 2010) that repro-duces a number of important observations for high-z LAEs. Adopting the canonicalobservational criteria we identify the progenitors visible as DLAs and LAEs at red-shifts respectively equal toz ≈ 2.3 andz ≈ 5.7. The observable properties of the MWGalaxy and its neighboring companions are presented below,from present days back tothe time when the Universe was only 1 Gyr old.12 Stefania Salvadori & Andrea Ferrara2. The Milky Way system at z= 0 : Galactic ArchaeologyVery metal-poor stars represent the living fossils of the first stellar generations. Theirchemical abundance patterns and Metallicity DistributionFu ctions (MDFs) observedin both the stellar halo and in nearby dSph galaxies can provide fundamental insightson the properties of the first cosmic sources.2.1. First stars and their Cosmic RelicsCosmological simulations suggest that first stars formed atz ≈ 15− 20 in primordialH2-cooling minihaloes and that were possibly more massive than ypical stars formingtoday (Hosokawa et al. 2011). The transition from massive tonormal stars is expectedto be driven by metalsanddust cooling, becoming important when the metallicity of thestar-forming gas exceeds the critical value,Zcr = 10−4±1Z⊙ (Schneider et al. 2002). Weuse our cosmological model to interpret the observed Galactic halo MDF. We find thatthe low-metallicity tail of the MDF strongly depends on the assumedZcr value (Fig. 1).If the observed cut-off (Schörck et al. 2009) suggestsZcr ≈ 10−4Z⊙, the presence of thefour stars at [Fe/H]< −4.5 can only be accounted ifZcr < 10−5Z⊙. In particular, theexistence of the most metal-deficient star ever, which hastot l metallicityZ ≈ 10−4.5Z⊙(Caffau et al. 2011) clearly requiresZcr < 10−4Z⊙. Such a recent discovery definitelyproves that dust strongly governs the transition from massive to normal stars in the low-Z regimes (Schneider et al. 2002).Figure 1. The Galactic halo MDF: observations (points (Beers & Christlieb2005)) vs simulations averaged over 100 possible MW merger histories (histogramsand shaded area). The starred symbol indicates the most metal-deficient star.The chemical abundance patterns of halo stars with−4.5 <[Fe/H]< −2.7 (Cayrel et al.2004; Caffau et al. 2011), do not show any peculiar imprint from very massive primor-dial stars, and have small chemical abundance scatter unlikely resulting from individualsupernovae (SN) ejecta. According to our cosmological model (Salvadori et al. 2007)the number of stars formed out of gas pollutedonlyby Z < Zcr stars is extremely small,and thus negligible in current data sample. These ”second-ge eration” stars can ei-ther have low or high [Fe/H] (Fig. 2) if they form in halos that accreted metal-enrichedgas from the MW environment or that are self-enriched by the first stars. To have thechance to detect the chemical imprint by first stars an highernumber of [Fe/H]< −2stars is clearly needed. To this aim it is useful to survey thestellar halo between20 kpc <∼ r <∼ 40 kpc, where the contribution of [Fe/H]< −2 stars with respect to theoverall stellar population is expected to be maximal (Salvadori et al. 2010b).S. Salvadori & A. Ferrara 3Figure 2. Number of stars predicted to exist atz= 0 as a function of their [Fe/H]for differentZcr models. The magenta histograms show second-generation stars, thecyan histograms the overall stellar populations.2.2. First Galaxies and their Cosmic RelicsAn alternative way to find very metal-poor stars is by surveying dSph galaxies and inparticular ultra-faint dSphs (L < 105L⊙, Fig. 2), in which [Fe/H]< −3 stars representthe 25% of the total stellar mass. These faint dwarfs are predicted to be among the firststar-forming galaxies in the MW system, left-overs of H2-cooling minihaloes formedat z > 8.5 (Salvadori & Ferrara 2009), i.e. before the end of reionization (zrei = 6). Inthese galaxies the higher fraction of [Fe/H]< −3 stars with respect to the more luminous”classical dSphs” reflects both the lower star-formation rate, caused by ineffective H2cooling, and the lower metal (pre-)enrichment of the MW-environment at their furtherformation epoch. Indeed classical dSphs are find to finally assemble atz < 7 when thepre-enrichment of the MW environment was [Fe/H]≈ −3. The few stars at [Fe/H]< −3observed in classical dSphs (Starkenburg et al. 2010) are predicted to form in progenitorminihaloes atz> 8.5 (Salvadori & Ferrara 2009), some of which might host first stars.The unusual composition of two stars at [Fe/H]≈ −2 observed in Hercules (Koch et al.2008) might be the result of self-enrichment by first stars inearly progenitor halos.Figure 3. Observed (points with error-bars, (Kirby et al. 2008)) vs simulated (con-tours/histograms) properties of dSph galaxies atz = 0. Left: the iron-luminosity re-lation. The colored shaded areas correspond to regions includi g the (99, 95, 68)% ofthe total number of dSph candidates in 50 MW merger histories(Salvadori & Ferrara2012).Right: MDF of ultra-faint dSph galaxies (Salvadori & Ferrara 2009).4 Stefania Salvadori & Andrea FerraraFigure 4. MW progenitors visible as DLAs atz ≈ 2.3 (contours), with color in-tensity corresponding to regions containing the (99, 95, 68)% of DLAs in 50 mergerhistories (Salvadori & Ferrara 2012). Points with error-bas are observations: circles(Prochaska et al. 2007), triangles (Cooke et al. 2011a), square (Cooke et al. 2011b).3. The Milky Way system at z≈ 2.3: DLAsThe predictedNHI vs [Fe/H] values of the MW progenitors visible as DLAs atz ≈ 2.3follows the observed relation (Fig. 4). In our picture very metal-poor DLAs, [Fe/H]<−2, are associated to starlessM ≈ 108M⊙ minihaloes that virialize from metal-enrichedregions of the MW environment before the end of reionizationand passively evolvedown toz ≈ 2.3. These sterile absorbers retain the chemical imprint of the dominantstellar populations that pollute the MW environment at their formation epoch: low-ZSN type II (Salvadori et al. 2007). This finding agrees with the observational results byBecker et al. (2012) that show that the gas chemical abundance r tios in very metal-poorDLAs/sub-DLAs do not significantly evolve between 2< z < 5. The recently discov-ered C-enhanced DLA is instead pertaining to a new class of absorbers hosting firststars along with second-generation of long-living low-mass stars. These peculiar DLAsare descendants ofM ≈ 107M⊙ minihaloes, that virialize atz> 8 in neutral primordialregions of the MW environment and passively evolve after a short initial period of starformation. These conditions are only satisfied by≈ 0.01% of the total amount of DLAs,making these absorbers extremely rare. The peculiar abundance pattern observed in theC-enhanced DLA results from the enrichment by low-metallicity SN typeII and AGBstars, which may start to form as soon asZ > Zcr. While SNII nucleosynthetic prod-ucts are mostly lost in winds, AGB metals are retained in the ISM, causing a dramaticincrease of [C/Fe]. The amount of N produced byZ < 5 × 10−4Z⊙ AGB stars is verylimited (Meynet & Maeder 2002), resulting in a gas abundance[N/H]= −3.8± 0.9 (seeFig. 5). The mass of relic stars in C-enhanced DLAs isM∗ ≈ 102−4M⊙, making themthe gas-rich counterpart of the faintest dwarfs.4. The Milky Way system at z≈ 5.7: LAEsAt z ≈ 5.7 the star-forming progenitors of MW-like galaxies cover a wide range ofobserved Lyα luminosity, Lα = 1039−43.25 erg s−1. The probability to have at leastone progenitor observable as LAE (Lα ≥ 1042, EW ≥ 20 Å) is therefore very highP ≥ 68%. Such visible progenitors are mainly associated with the most massive halosof the hierarchy, i.e. the major branch, with total massM > 109.5M⊙. On averageS. Salvadori & A. Ferrara 5Figure 5. Gas chemical abundances in the C-enhanced DLA: observations (points,Cooke et al. (2011a)) vs simulations (average value±1σ, shaded areas).the identified LAEs have star-formation rateṡM∗ ≈ 2.3M⊙/yr andLα ≈ 1042.2erg/s.They are populated by intermediate age stars,t∗ ≈ 150− 400 Myr, which have averagemetallicitiesZ ≈ (0.3−1)Z⊙ (Fig.6). Interestingly these visible MW progenitors providemore than the 10% of the very metal-poor stars that are observed today in the Galactichalo. Indeed, most of these [Fe/H]< −2 stars formed atz> 6 in newly virializing halos,accreting metal-enriched gas from the MW environment. Byz ≈ 5.7 many of thesepremature building blocks have already merged into the major branch of the hierarchy,i.e. the visible progenitor (Salvadori et al. 2010a).Figure 6. Physical properties of MW progenitors (yellow circles) atz≈ 5.7. Bluetriangles identify the objects visible as LAEs. Points witherror-bars are data byOno et al. (2010) (magenta squares, 4 models for 1 LAE), Pirzkal et al. (2007) (bluetriangles) and Lai et al. (2007) (green circles). As a function of the total stellar massthe panels show: the instantaneous star formation rate (a),the average stellar age (c)and (d) metallicity (d). Panel (b) shows the relation between the halo and the gasmass, with the cosmic value pointed out by the dashed line.6 Stefania Salvadori & Andrea Ferrara5. ConclusionsThe MW system is a powerful laboratory to study early galaxy formation. On the onehand the properties of the first cosmic sources can be studiedby xploiting the obser-vations of today living metal-poor stars and galaxies. Fromthe other hand the earlyevolutionary stages of different MW progenitors can be investigated using complemen-tary observations of high-z galaxies. In particular, by identifying the MW progenitorsamong the faintest LAEs observed atz ≈ 5.7 it will be possible to observe the MW inits infancy, when it was only 1 Gyr old.Acknowledgments. We thank R. Schneider and P. Dayal for their contribution tothe works on Galactic Archaeology and MW-LAE connection.ReferencesBecker, G. D., Sargent, W. L. W., Rauch, M., & Carswell, R. F. 201 , ApJ, 744, 91Beers, T. C., & Christlieb, N. 2005, ARA&A, 43, 531Caffau, E., Bonifacio, P., François, P., Sbordone, L., Monaco,L., Spite, M., Spite, F., Ludwig,H.-G., Cayrel, R., Zaggia, S., Hammer, F., Randich, S., Molaro, P., & Hill, V. 2011, Nat,477, 67Cayrel, R., Depagne, E., Spite, M., Hill, V., Spite, F., François, P., Plez, B., Beers, T., Primas,F., Andersen, J., Barbuy, B., Bonifacio, P., Molaro, P., & Nordström, B. 2004, A&A,416, 1117Cooke, R., Pettini, M., Steidel, C. C., Rudie, G. C., & Jorgenson, R. A. 2011a, MNRAS, 412,1047Cooke, R., Pettini, M., Steidel, C. C., Rudie, G. C., & Nissen, P. E. 2011b, MNRAS, 417, 1534Dayal, P., Ferrara, A., & Gallerani, S. 2008, MNRAS, 389, 1683Dayal, P., Ferrara, A., & Saro, A. 2010, MNRAS, 402, 1449Kirby, E. N., Simon, J. D., Geha, M., Guhathakurta, P., & Frebel, A. 2008, ApJ, 685, L43Kobayashi, C., Tominaga, N., & Nomoto, K. 2011, ApJ, 730, L14Koch, A., McWilliam, A., Grebel, E. K., Zucker, D. B., & Belokurov, V. 2008, ApJ, 688, L13Lai, K., Huang, J.-S., Fazio, G., Cowie, L. L., Hu, E. M., & Kakazu, Y. 2007, ApJ, 655, 704Meynet, G., & Maeder, A. 2002, A&A, 390, 561Ono, Y., Ouchi, M., Shimasaku, K., Dunlop, J., Farrah, D., McLure, R., & Okamura, S. 2010,ApJ, 724, 1524Pirzkal, N., Malhotra, S., Rhoads, J. E., & Xu, C. 2007, ApJ, 667, 49Prochaska, J. X., Wolfe, A. M., Howk, J. C., Gawiser, E., Burles, S. M., & Cooke, J. 2007,ApJS, 171, 29Salvadori, S., Dayal, P., & Ferrara, A. 2010a, MNRAS, 407, L1Salvadori, S., & Ferrara, A. 2009, MNRAS, 395, L6— 2012, MNRAS, 421, L29Salvadori, S., Ferrara, A., Schneider, R., Scannapieco, E., & Kawata, D. 2010b, MNRAS, 401,L5Salvadori, S., Schneider, R., & Ferrara, A. 2007, MNRAS, 381, 647Schneider, R., Ferrara, A., Natarajan, P., & Omukai, K. 2002, ApJ, 571, 30Schörck, T., Christlieb, N., Cohen, J. G., Beers, T. C., Shectman, S., Thompson, I., McWilliam,A., Bessell, M. S., Norris, J. E., Meléndez, J., Ramı́rez, S., Haynes, D., Cass, P., Hartley,M., Russell, K., Watson, F., Zickgraf, F.-J., Behnke, B., Fechner, C., Fuhrmeister, B.,Barklem, P. S., Edvardsson, B., Frebel, A., Wisotzki, L., & Reimers, D. 2009, A&A,507, 817Starkenburg, E., Hill, V., Tolstoy, E., González Hernández, J. I., Irwin, M., Helmi, A., Battaglia,G., Jablonka, P., Tafelmeyer, M., Shetrone, M., Venn, K., & de Boer, T. 2010, A&A,513, A34

   University of GroningenThe infant MilkyWaySalvadori, Stefania; Ferrara, AndreaPublished in:ASP Conference SeriesIMPORTANT 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:2012Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Salvadori, S., & Ferrara, A. (2012). The infant MilkyWay. ASP Conference Series.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: 05-12-2023arXiv:1204.1943v1  [astro-ph.CO]  9 Apr 2012**Volume Title**ASP Conference Series, Vol. **Volume Number****Author**c© **Copyright Year** Astronomical Society of the PacificThe infant Milky WayStefania Salvadori1 and Andrea Ferrara21Kapteyn Astronomical Institute, Landleven 12, 9747AD Groningen, NL2Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, ITAbstract. We investigate the physical properties of the progenitors of today livingMilky Way-like galaxies that are visible as Damped Lyα Absorption systems and LyαEmitters at higher redshifts (z ≈ 2.3, 5.7). To this aim we use a statistical merger-tree approach that follows the formation of the Galaxy and its dwarf satellites in acosmological context, tracing the chemical evolution and stellar population history ofthe progenitor halos. The model accounts for the propertiesof the most metal-poorstars and local dwarf galaxies, providing insights on the early cosmic star-formation.Fruitful links between Galactic Archaeology and more distant galaxies are presented.1. BackgroundOne of the most popular methods to identify high-redshifts galaxies (z ≈ 2 − 7) isby detecting their strong Lyα line. These Lyman Alpha Emitters (LAEs) are mainlyassociated tostar-forming galaxies, and they have been extensively used to probeboth the ionization state of the Inter Galactic Medium and the early galaxy evolu-tion. At lower redshifts,z < 5, galaxies withhigh neutral hydrogen column den-sities, NHI > 1020.3cm−2, can be identified in the spectra of more distant quasarsby means of their strong Lyα absorption line. The most metal-poor among theseDamped Lyα Absorption systems (DLAs), can provide insights on the initial metal-enrichment phases of galaxy formation. Recently a DLA with [Fe/H]≈ −3 observed atzabs≈ 2.3, has indeed revealed strong carbon-enhancement and evident odd-even effect(Cooke et al. 2011b), consistent with the chemical imprint by Z = 0 faint supernovae(Kobayashi et al. 2011). Furthermore, all others DLAs with [Fe/H]< −2 observed athigh-resolution show chemical abundance ratios consistent with those of very metal-poor Galactic halo stars (Cooke et al. 2011a), thus suggestin possible connections be-tween these absorbers and the early building blocks of MilkyWa (MW) -like galaxies.We determine the physical properties of the progenitors of the MW and its dwarf com-panions by using the merger-tree code GAMETE (GAlaxy MErgerTr e & Evolution),which reconstructs the possible star-formation and chemical evolution histories of theMW system. The observed Lyα luminosity and Lyα line equivalent width are computedusing the LAE model by Dayal and collaborators (Dayal et al. 2008, 2010) that repro-duces a number of important observations for high-z LAEs. Adopting the canonicalobservational criteria we identify the progenitors visible as DLAs and LAEs at red-shifts respectively equal toz ≈ 2.3 andz ≈ 5.7. The observable properties of the MWGalaxy and its neighboring companions are presented below,from present days back tothe time when the Universe was only 1 Gyr old.12 Stefania Salvadori & Andrea Ferrara2. The Milky Way system at z= 0 : Galactic ArchaeologyVery metal-poor stars represent the living fossils of the first stellar generations. Theirchemical abundance patterns and Metallicity DistributionFu ctions (MDFs) observedin both the stellar halo and in nearby dSph galaxies can provide fundamental insightson the properties of the first cosmic sources.2.1. First stars and their Cosmic RelicsCosmological simulations suggest that first stars formed atz ≈ 15− 20 in primordialH2-cooling minihaloes and that were possibly more massive than ypical stars formingtoday (Hosokawa et al. 2011). The transition from massive tonormal stars is expectedto be driven by metalsanddust cooling, becoming important when the metallicity of thestar-forming gas exceeds the critical value,Zcr = 10−4±1Z⊙ (Schneider et al. 2002). Weuse our cosmological model to interpret the observed Galactic halo MDF. We find thatthe low-metallicity tail of the MDF strongly depends on the assumedZcr value (Fig. 1).If the observed cut-off (Schörck et al. 2009) suggestsZcr ≈ 10−4Z⊙, the presence of thefour stars at [Fe/H]< −4.5 can only be accounted ifZcr < 10−5Z⊙. In particular, theexistence of the most metal-deficient star ever, which hastot l metallicityZ ≈ 10−4.5Z⊙(Caffau et al. 2011) clearly requiresZcr < 10−4Z⊙. Such a recent discovery definitelyproves that dust strongly governs the transition from massive to normal stars in the low-Z regimes (Schneider et al. 2002).Figure 1. The Galactic halo MDF: observations (points (Beers & Christlieb2005)) vs simulations averaged over 100 possible MW merger histories (histogramsand shaded area). The starred symbol indicates the most metal-deficient star.The chemical abundance patterns of halo stars with−4.5 <[Fe/H]< −2.7 (Cayrel et al.2004; Caffau et al. 2011), do not show any peculiar imprint from very massive primor-dial stars, and have small chemical abundance scatter unlikely resulting from individualsupernovae (SN) ejecta. According to our cosmological model (Salvadori et al. 2007)the number of stars formed out of gas pollutedonlyby Z < Zcr stars is extremely small,and thus negligible in current data sample. These ”second-ge eration” stars can ei-ther have low or high [Fe/H] (Fig. 2) if they form in halos that accreted metal-enrichedgas from the MW environment or that are self-enriched by the first stars. To have thechance to detect the chemical imprint by first stars an highernumber of [Fe/H]< −2stars is clearly needed. To this aim it is useful to survey thestellar halo between20 kpc <∼ r <∼ 40 kpc, where the contribution of [Fe/H]< −2 stars with respect to theoverall stellar population is expected to be maximal (Salvadori et al. 2010b).S. Salvadori & A. Ferrara 3Figure 2. Number of stars predicted to exist atz= 0 as a function of their [Fe/H]for differentZcr models. The magenta histograms show second-generation stars, thecyan histograms the overall stellar populations.2.2. First Galaxies and their Cosmic RelicsAn alternative way to find very metal-poor stars is by surveying dSph galaxies and inparticular ultra-faint dSphs (L < 105L⊙, Fig. 2), in which [Fe/H]< −3 stars representthe 25% of the total stellar mass. These faint dwarfs are predicted to be among the firststar-forming galaxies in the MW system, left-overs of H2-cooling minihaloes formedat z > 8.5 (Salvadori & Ferrara 2009), i.e. before the end of reionization (zrei = 6). Inthese galaxies the higher fraction of [Fe/H]< −3 stars with respect to the more luminous”classical dSphs” reflects both the lower star-formation rate, caused by ineffective H2cooling, and the lower metal (pre-)enrichment of the MW-environment at their furtherformation epoch. Indeed classical dSphs are find to finally assemble atz < 7 when thepre-enrichment of the MW environment was [Fe/H]≈ −3. The few stars at [Fe/H]< −3observed in classical dSphs (Starkenburg et al. 2010) are predicted to form in progenitorminihaloes atz> 8.5 (Salvadori & Ferrara 2009), some of which might host first stars.The unusual composition of two stars at [Fe/H]≈ −2 observed in Hercules (Koch et al.2008) might be the result of self-enrichment by first stars inearly progenitor halos.Figure 3. Observed (points with error-bars, (Kirby et al. 2008)) vs simulated (con-tours/histograms) properties of dSph galaxies atz = 0. Left: the iron-luminosity re-lation. The colored shaded areas correspond to regions includi g the (99, 95, 68)% ofthe total number of dSph candidates in 50 MW merger histories(Salvadori & Ferrara2012).Right: MDF of ultra-faint dSph galaxies (Salvadori & Ferrara 2009).4 Stefania Salvadori & Andrea FerraraFigure 4. MW progenitors visible as DLAs atz ≈ 2.3 (contours), with color in-tensity corresponding to regions containing the (99, 95, 68)% of DLAs in 50 mergerhistories (Salvadori & Ferrara 2012). Points with error-bas are observations: circles(Prochaska et al. 2007), triangles (Cooke et al. 2011a), square (Cooke et al. 2011b).3. The Milky Way system at z≈ 2.3: DLAsThe predictedNHI vs [Fe/H] values of the MW progenitors visible as DLAs atz ≈ 2.3follows the observed relation (Fig. 4). In our picture very metal-poor DLAs, [Fe/H]<−2, are associated to starlessM ≈ 108M⊙ minihaloes that virialize from metal-enrichedregions of the MW environment before the end of reionizationand passively evolvedown toz ≈ 2.3. These sterile absorbers retain the chemical imprint of the dominantstellar populations that pollute the MW environment at their formation epoch: low-ZSN type II (Salvadori et al. 2007). This finding agrees with the observational results byBecker et al. (2012) that show that the gas chemical abundance r tios in very metal-poorDLAs/sub-DLAs do not significantly evolve between 2< z < 5. The recently discov-ered C-enhanced DLA is instead pertaining to a new class of absorbers hosting firststars along with second-generation of long-living low-mass stars. These peculiar DLAsare descendants ofM ≈ 107M⊙ minihaloes, that virialize atz> 8 in neutral primordialregions of the MW environment and passively evolve after a short initial period of starformation. These conditions are only satisfied by≈ 0.01% of the total amount of DLAs,making these absorbers extremely rare. The peculiar abundance pattern observed in theC-enhanced DLA results from the enrichment by low-metallicity SN typeII and AGBstars, which may start to form as soon asZ > Zcr. While SNII nucleosynthetic prod-ucts are mostly lost in winds, AGB metals are retained in the ISM, causing a dramaticincrease of [C/Fe]. The amount of N produced byZ < 5 × 10−4Z⊙ AGB stars is verylimited (Meynet & Maeder 2002), resulting in a gas abundance[N/H]= −3.8± 0.9 (seeFig. 5). The mass of relic stars in C-enhanced DLAs isM∗ ≈ 102−4M⊙, making themthe gas-rich counterpart of the faintest dwarfs.4. The Milky Way system at z≈ 5.7: LAEsAt z ≈ 5.7 the star-forming progenitors of MW-like galaxies cover a wide range ofobserved Lyα luminosity, Lα = 1039−43.25 erg s−1. The probability to have at leastone progenitor observable as LAE (Lα ≥ 1042, EW ≥ 20 Å) is therefore very highP ≥ 68%. Such visible progenitors are mainly associated with the most massive halosof the hierarchy, i.e. the major branch, with total massM > 109.5M⊙. On averageS. Salvadori & A. Ferrara 5Figure 5. Gas chemical abundances in the C-enhanced DLA: observations (points,Cooke et al. (2011a)) vs simulations (average value±1σ, shaded areas).the identified LAEs have star-formation rateṡM∗ ≈ 2.3M⊙/yr andLα ≈ 1042.2erg/s.They are populated by intermediate age stars,t∗ ≈ 150− 400 Myr, which have averagemetallicitiesZ ≈ (0.3−1)Z⊙ (Fig.6). Interestingly these visible MW progenitors providemore than the 10% of the very metal-poor stars that are observed today in the Galactichalo. Indeed, most of these [Fe/H]< −2 stars formed atz> 6 in newly virializing halos,accreting metal-enriched gas from the MW environment. Byz ≈ 5.7 many of thesepremature building blocks have already merged into the major branch of the hierarchy,i.e. the visible progenitor (Salvadori et al. 2010a).Figure 6. Physical properties of MW progenitors (yellow circles) atz≈ 5.7. Bluetriangles identify the objects visible as LAEs. Points witherror-bars are data byOno et al. (2010) (magenta squares, 4 models for 1 LAE), Pirzkal et al. (2007) (bluetriangles) and Lai et al. (2007) (green circles). As a function of the total stellar massthe panels show: the instantaneous star formation rate (a),the average stellar age (c)and (d) metallicity (d). Panel (b) shows the relation between the halo and the gasmass, with the cosmic value pointed out by the dashed line.6 Stefania Salvadori & Andrea Ferrara5. ConclusionsThe MW system is a powerful laboratory to study early galaxy formation. On the onehand the properties of the first cosmic sources can be studiedby xploiting the obser-vations of today living metal-poor stars and galaxies. Fromthe other hand the earlyevolutionary stages of different MW progenitors can be investigated using complemen-tary observations of high-z galaxies. In particular, by identifying the MW progenitorsamong the faintest LAEs observed atz ≈ 5.7 it will be possible to observe the MW inits infancy, when it was only 1 Gyr old.Acknowledgments. We thank R. Schneider and P. Dayal for their contribution tothe works on Galactic Archaeology and MW-LAE connection.ReferencesBecker, G. D., Sargent, W. L. W., Rauch, M., & Carswell, R. F. 201 , ApJ, 744, 91Beers, T. C., & Christlieb, N. 2005, ARA&A, 43, 531Caffau, E., Bonifacio, P., François, P., Sbordone, L., Monaco,L., Spite, M., Spite, F., Ludwig,H.-G., Cayrel, R., Zaggia, S., Hammer, F., Randich, S., Molaro, P., & Hill, V. 2011, Nat,477, 67Cayrel, R., Depagne, E., Spite, M., Hill, V., Spite, F., François, P., Plez, B., Beers, T., Primas,F., Andersen, J., Barbuy, B., Bonifacio, P., Molaro, P., & Nordström, B. 2004, A&A,416, 1117Cooke, R., Pettini, M., Steidel, C. C., Rudie, G. C., & Jorgenson, R. A. 2011a, MNRAS, 412,1047Cooke, R., Pettini, M., Steidel, C. C., Rudie, G. C., & Nissen, P. E. 2011b, MNRAS, 417, 1534Dayal, P., Ferrara, A., & Gallerani, S. 2008, MNRAS, 389, 1683Dayal, P., Ferrara, A., & Saro, A. 2010, MNRAS, 402, 1449Kirby, E. N., Simon, J. D., Geha, M., Guhathakurta, P., & Frebel, A. 2008, ApJ, 685, L43Kobayashi, C., Tominaga, N., & Nomoto, K. 2011, ApJ, 730, L14Koch, A., McWilliam, A., Grebel, E. K., Zucker, D. B., & Belokurov, V. 2008, ApJ, 688, L13Lai, K., Huang, J.-S., Fazio, G., Cowie, L. L., Hu, E. M., & Kakazu, Y. 2007, ApJ, 655, 704Meynet, G., & Maeder, A. 2002, A&A, 390, 561Ono, Y., Ouchi, M., Shimasaku, K., Dunlop, J., Farrah, D., McLure, R., & Okamura, S. 2010,ApJ, 724, 1524Pirzkal, N., Malhotra, S., Rhoads, J. E., & Xu, C. 2007, ApJ, 667, 49Prochaska, J. X., Wolfe, A. M., Howk, J. C., Gawiser, E., Burles, S. M., & Cooke, J. 2007,ApJS, 171, 29Salvadori, S., Dayal, P., & Ferrara, A. 2010a, MNRAS, 407, L1Salvadori, S., & Ferrara, A. 2009, MNRAS, 395, L6— 2012, MNRAS, 421, L29Salvadori, S., Ferrara, A., Schneider, R., Scannapieco, E., & Kawata, D. 2010b, MNRAS, 401,L5Salvadori, S., Schneider, R., & Ferrara, A. 2007, MNRAS, 381, 647Schneider, R., Ferrara, A., Natarajan, P., & Omukai, K. 2002, ApJ, 571, 30Schörck, T., Christlieb, N., Cohen, J. G., Beers, T. C., Shectman, S., Thompson, I., McWilliam,A., Bessell, M. S., Norris, J. E., Meléndez, J., Ramı́rez, S., Haynes, D., Cass, P., Hartley,M., Russell, K., Watson, F., Zickgraf, F.-J., Behnke, B., Fechner, C., Fuhrmeister, B.,Barklem, P. S., Edvardsson, B., Frebel, A., Wisotzki, L., & Reimers, D. 2009, A&A,507, 817Starkenburg, E., Hill, V., Tolstoy, E., González Hernández, J. I., Irwin, M., Helmi, A., Battaglia,G., Jablonka, P., Tafelmeyer, M., Shetrone, M., Venn, K., & de Boer, T. 2010, A&A,513, A34

We investigate the physical properties of the progenitors of today living
Milky Way-like galaxies that are visible as Damped Ly Absorption systems and Ly
Emitters at higher redshifts (z ≈ 2.3, 5.7). To this aim we use a statistical mergertree
approach that follows the formation of the Galaxy and its dwarf satellites in a
cosmological context, tracing the chemical evolution and stellar population history of
the progenitor halos. The model accounts for the properties of the most metal-poor
stars and local dwarf galaxies, providing insights on the early cosmic star-formation.
Fruitful links between Galactic Archaeology and more distant galaxies are presented.

Ferrara, Andrea,

Salvadori, Stefania,

University of Groningen Digital Archive

ARTS repository - University of Groningen

   University of GroningenThe infant MilkyWaySalvadori, Stefania; Ferrara, AndreaPublished in:ASP Conference SeriesIMPORTANT 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:2012Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Salvadori, S., & Ferrara, A. (2012). The infant MilkyWay. ASP Conference Series.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: 08-06-2022arXiv:1204.1943v1  [astro-ph.CO]  9 Apr 2012**Volume Title**ASP Conference Series, Vol. **Volume Number****Author**c© **Copyright Year** Astronomical Society of the PacificThe infant Milky WayStefania Salvadori1 and Andrea Ferrara21Kapteyn Astronomical Institute, Landleven 12, 9747AD Groningen, NL2Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, ITAbstract. We investigate the physical properties of the progenitors of today livingMilky Way-like galaxies that are visible as Damped Lyα Absorption systems and LyαEmitters at higher redshifts (z ≈ 2.3, 5.7). To this aim we use a statistical merger-tree approach that follows the formation of the Galaxy and its dwarf satellites in acosmological context, tracing the chemical evolution and stellar population history ofthe progenitor halos. The model accounts for the propertiesof the most metal-poorstars and local dwarf galaxies, providing insights on the early cosmic star-formation.Fruitful links between Galactic Archaeology and more distant galaxies are presented.1. BackgroundOne of the most popular methods to identify high-redshifts galaxies (z ≈ 2 − 7) isby detecting their strong Lyα line. These Lyman Alpha Emitters (LAEs) are mainlyassociated tostar-forming galaxies, and they have been extensively used to probeboth the ionization state of the Inter Galactic Medium and the early galaxy evolu-tion. At lower redshifts,z < 5, galaxies withhigh neutral hydrogen column den-sities, NHI > 1020.3cm−2, can be identified in the spectra of more distant quasarsby means of their strong Lyα absorption line. The most metal-poor among theseDamped Lyα Absorption systems (DLAs), can provide insights on the initial metal-enrichment phases of galaxy formation. Recently a DLA with [Fe/H]≈ −3 observed atzabs≈ 2.3, has indeed revealed strong carbon-enhancement and evident odd-even effect(Cooke et al. 2011b), consistent with the chemical imprint by Z = 0 faint supernovae(Kobayashi et al. 2011). Furthermore, all others DLAs with [Fe/H]< −2 observed athigh-resolution show chemical abundance ratios consistent with those of very metal-poor Galactic halo stars (Cooke et al. 2011a), thus suggestin possible connections be-tween these absorbers and the early building blocks of MilkyWa (MW) -like galaxies.We determine the physical properties of the progenitors of the MW and its dwarf com-panions by using the merger-tree code GAMETE (GAlaxy MErgerTr e & Evolution),which reconstructs the possible star-formation and chemical evolution histories of theMW system. The observed Lyα luminosity and Lyα line equivalent width are computedusing the LAE model by Dayal and collaborators (Dayal et al. 2008, 2010) that repro-duces a number of important observations for high-z LAEs. Adopting the canonicalobservational criteria we identify the progenitors visible as DLAs and LAEs at red-shifts respectively equal toz ≈ 2.3 andz ≈ 5.7. The observable properties of the MWGalaxy and its neighboring companions are presented below,from present days back tothe time when the Universe was only 1 Gyr old.12 Stefania Salvadori & Andrea Ferrara2. The Milky Way system at z= 0 : Galactic ArchaeologyVery metal-poor stars represent the living fossils of the first stellar generations. Theirchemical abundance patterns and Metallicity DistributionFu ctions (MDFs) observedin both the stellar halo and in nearby dSph galaxies can provide fundamental insightson the properties of the first cosmic sources.2.1. First stars and their Cosmic RelicsCosmological simulations suggest that first stars formed atz ≈ 15− 20 in primordialH2-cooling minihaloes and that were possibly more massive than ypical stars formingtoday (Hosokawa et al. 2011). The transition from massive tonormal stars is expectedto be driven by metalsanddust cooling, becoming important when the metallicity of thestar-forming gas exceeds the critical value,Zcr = 10−4±1Z⊙ (Schneider et al. 2002). Weuse our cosmological model to interpret the observed Galactic halo MDF. We find thatthe low-metallicity tail of the MDF strongly depends on the assumedZcr value (Fig. 1).If the observed cut-off (Schörck et al. 2009) suggestsZcr ≈ 10−4Z⊙, the presence of thefour stars at [Fe/H]< −4.5 can only be accounted ifZcr < 10−5Z⊙. In particular, theexistence of the most metal-deficient star ever, which hastot l metallicityZ ≈ 10−4.5Z⊙(Caffau et al. 2011) clearly requiresZcr < 10−4Z⊙. Such a recent discovery definitelyproves that dust strongly governs the transition from massive to normal stars in the low-Z regimes (Schneider et al. 2002).Figure 1. The Galactic halo MDF: observations (points (Beers & Christlieb2005)) vs simulations averaged over 100 possible MW merger histories (histogramsand shaded area). The starred symbol indicates the most metal-deficient star.The chemical abundance patterns of halo stars with−4.5 <[Fe/H]< −2.7 (Cayrel et al.2004; Caffau et al. 2011), do not show any peculiar imprint from very massive primor-dial stars, and have small chemical abundance scatter unlikely resulting from individualsupernovae (SN) ejecta. According to our cosmological model (Salvadori et al. 2007)the number of stars formed out of gas pollutedonlyby Z < Zcr stars is extremely small,and thus negligible in current data sample. These ”second-ge eration” stars can ei-ther have low or high [Fe/H] (Fig. 2) if they form in halos that accreted metal-enrichedgas from the MW environment or that are self-enriched by the first stars. To have thechance to detect the chemical imprint by first stars an highernumber of [Fe/H]< −2stars is clearly needed. To this aim it is useful to survey thestellar halo between20 kpc <∼ r <∼ 40 kpc, where the contribution of [Fe/H]< −2 stars with respect to theoverall stellar population is expected to be maximal (Salvadori et al. 2010b).S. Salvadori & A. Ferrara 3Figure 2. Number of stars predicted to exist atz= 0 as a function of their [Fe/H]for differentZcr models. The magenta histograms show second-generation stars, thecyan histograms the overall stellar populations.2.2. First Galaxies and their Cosmic RelicsAn alternative way to find very metal-poor stars is by surveying dSph galaxies and inparticular ultra-faint dSphs (L < 105L⊙, Fig. 2), in which [Fe/H]< −3 stars representthe 25% of the total stellar mass. These faint dwarfs are predicted to be among the firststar-forming galaxies in the MW system, left-overs of H2-cooling minihaloes formedat z > 8.5 (Salvadori & Ferrara 2009), i.e. before the end of reionization (zrei = 6). Inthese galaxies the higher fraction of [Fe/H]< −3 stars with respect to the more luminous”classical dSphs” reflects both the lower star-formation rate, caused by ineffective H2cooling, and the lower metal (pre-)enrichment of the MW-environment at their furtherformation epoch. Indeed classical dSphs are find to finally assemble atz < 7 when thepre-enrichment of the MW environment was [Fe/H]≈ −3. The few stars at [Fe/H]< −3observed in classical dSphs (Starkenburg et al. 2010) are predicted to form in progenitorminihaloes atz> 8.5 (Salvadori & Ferrara 2009), some of which might host first stars.The unusual composition of two stars at [Fe/H]≈ −2 observed in Hercules (Koch et al.2008) might be the result of self-enrichment by first stars inearly progenitor halos.Figure 3. Observed (points with error-bars, (Kirby et al. 2008)) vs simulated (con-tours/histograms) properties of dSph galaxies atz = 0. Left: the iron-luminosity re-lation. The colored shaded areas correspond to regions includi g the (99, 95, 68)% ofthe total number of dSph candidates in 50 MW merger histories(Salvadori & Ferrara2012).Right: MDF of ultra-faint dSph galaxies (Salvadori & Ferrara 2009).4 Stefania Salvadori & Andrea FerraraFigure 4. MW progenitors visible as DLAs atz ≈ 2.3 (contours), with color in-tensity corresponding to regions containing the (99, 95, 68)% of DLAs in 50 mergerhistories (Salvadori & Ferrara 2012). Points with error-bas are observations: circles(Prochaska et al. 2007), triangles (Cooke et al. 2011a), square (Cooke et al. 2011b).3. The Milky Way system at z≈ 2.3: DLAsThe predictedNHI vs [Fe/H] values of the MW progenitors visible as DLAs atz ≈ 2.3follows the observed relation (Fig. 4). In our picture very metal-poor DLAs, [Fe/H]<−2, are associated to starlessM ≈ 108M⊙ minihaloes that virialize from metal-enrichedregions of the MW environment before the end of reionizationand passively evolvedown toz ≈ 2.3. These sterile absorbers retain the chemical imprint of the dominantstellar populations that pollute the MW environment at their formation epoch: low-ZSN type II (Salvadori et al. 2007). This finding agrees with the observational results byBecker et al. (2012) that show that the gas chemical abundance r tios in very metal-poorDLAs/sub-DLAs do not significantly evolve between 2< z < 5. The recently discov-ered C-enhanced DLA is instead pertaining to a new class of absorbers hosting firststars along with second-generation of long-living low-mass stars. These peculiar DLAsare descendants ofM ≈ 107M⊙ minihaloes, that virialize atz> 8 in neutral primordialregions of the MW environment and passively evolve after a short initial period of starformation. These conditions are only satisfied by≈ 0.01% of the total amount of DLAs,making these absorbers extremely rare. The peculiar abundance pattern observed in theC-enhanced DLA results from the enrichment by low-metallicity SN typeII and AGBstars, which may start to form as soon asZ > Zcr. While SNII nucleosynthetic prod-ucts are mostly lost in winds, AGB metals are retained in the ISM, causing a dramaticincrease of [C/Fe]. The amount of N produced byZ < 5 × 10−4Z⊙ AGB stars is verylimited (Meynet & Maeder 2002), resulting in a gas abundance[N/H]= −3.8± 0.9 (seeFig. 5). The mass of relic stars in C-enhanced DLAs isM∗ ≈ 102−4M⊙, making themthe gas-rich counterpart of the faintest dwarfs.4. The Milky Way system at z≈ 5.7: LAEsAt z ≈ 5.7 the star-forming progenitors of MW-like galaxies cover a wide range ofobserved Lyα luminosity, Lα = 1039−43.25 erg s−1. The probability to have at leastone progenitor observable as LAE (Lα ≥ 1042, EW ≥ 20 Å) is therefore very highP ≥ 68%. Such visible progenitors are mainly associated with the most massive halosof the hierarchy, i.e. the major branch, with total massM > 109.5M⊙. On averageS. Salvadori & A. Ferrara 5Figure 5. Gas chemical abundances in the C-enhanced DLA: observations (points,Cooke et al. (2011a)) vs simulations (average value±1σ, shaded areas).the identified LAEs have star-formation rateṡM∗ ≈ 2.3M⊙/yr andLα ≈ 1042.2erg/s.They are populated by intermediate age stars,t∗ ≈ 150− 400 Myr, which have averagemetallicitiesZ ≈ (0.3−1)Z⊙ (Fig.6). Interestingly these visible MW progenitors providemore than the 10% of the very metal-poor stars that are observed today in the Galactichalo. Indeed, most of these [Fe/H]< −2 stars formed atz> 6 in newly virializing halos,accreting metal-enriched gas from the MW environment. Byz ≈ 5.7 many of thesepremature building blocks have already merged into the major branch of the hierarchy,i.e. the visible progenitor (Salvadori et al. 2010a).Figure 6. Physical properties of MW progenitors (yellow circles) atz≈ 5.7. Bluetriangles identify the objects visible as LAEs. Points witherror-bars are data byOno et al. (2010) (magenta squares, 4 models for 1 LAE), Pirzkal et al. (2007) (bluetriangles) and Lai et al. (2007) (green circles). As a function of the total stellar massthe panels show: the instantaneous star formation rate (a),the average stellar age (c)and (d) metallicity (d). Panel (b) shows the relation between the halo and the gasmass, with the cosmic value pointed out by the dashed line.6 Stefania Salvadori & Andrea Ferrara5. ConclusionsThe MW system is a powerful laboratory to study early galaxy formation. On the onehand the properties of the first cosmic sources can be studiedby xploiting the obser-vations of today living metal-poor stars and galaxies. Fromthe other hand the earlyevolutionary stages of different MW progenitors can be investigated using complemen-tary observations of high-z galaxies. In particular, by identifying the MW progenitorsamong the faintest LAEs observed atz ≈ 5.7 it will be possible to observe the MW inits infancy, when it was only 1 Gyr old.Acknowledgments. We thank R. Schneider and P. 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