Histories of Dwarf Galaxies

Dan Weisz University of Washington University of California at Santa Cruz

Sexten Center for Astrophysics Date 7.29.13 Optical CMDs

MS RHeBs BHeBs AGB Brighter

RGB Oldest SGB MSTO Lower MS

Hotter “A great variety of evolutionary tracks therefore is involved in [the Hertzsprung-Russell diagram] and their disentanglement seems somewhat hopeless.”

~ Martin Schwartzchild, 1958 “The Evolution of Stars”

CMD Modeling Algorithms e.g., Tosi+ 1989; Tolstoy & Saha 1996; Gallart+ 1996; Mighell 1997; Holtzman+ 1999; Harris & Zaritsky 2001; Dolphin 2002; Ikuta & Arimoto 2002; Cole+ 2007; Yuk & Lee 2007; Aparicio & Hidalgo 2009; Cignoni & Tosi 2010; de Boer+ 2012, ... Measuring SFHs

Ingredients IMF Binary Fraction Stellar Models Extinction Differential Extinction Filter Convolution Observational Errors Bin the CMD

Model CMD = Linear combination of SSPs Measuring SFHs

“Observed” CMD Model CMD

observed density model density in bin i: ni in bin i: mi

n = ln P(data model) = 2 m n + n ln i L | i i i m i i X Maximize to find most likely SFH L Dolphin 2002 What can we learn from the SFHs of many LG dwarf galaxies? Mon. Not. R. Astron. Soc. 000,1–6(2013) Printed24July2013 (MNLATEXstylefilev2.2)

The Star Formation Histories of Local Group Dwarf Galaxies I. Hubble Space Telescope / Wide Field Planetary Camera 2 Observations?

Daniel R. Weisz1,2,3 , Andrew E. Dolphin4, Evan D. Skillman5, Jon Holtzman6, † Karoline Gilbert1,3, Julianne J. Dalcanton1, Benjamin F. Williams1 1Department of Astronomy, Box 351580, University of Washington, Seattle, WA 98195 2Department of Astronomy, University of California at Santa Cruz, 1156 High Street, Santa Cruz, CA, 95064 3Hubble Fellow 4Raytheon, 1151 E. Hermans Road, Tucson, AZ 85756 5Minnesota Institute for Astrophysics, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455, USA 6Department of Astronomy, New Mexico State University, Box 30001, 1320 Frenger St., Las Cruces, NM 88003

24 July 2013 55 WFPC2 pointings in 41 LG dwarfs

Photometry fromABSTRACT the HST LG Stellar Photometry Archive XXX We present uniformly measured star formation and chemical evolution his- tories (SFHs)(Holtzman+ of 50 Local Group 2006) dwarf galaxies based on a rich set of archival Hubble http://astronomy.nmsu.edu/holtz/archival/html/lg.htmlSpace Telescope / Wide Field Planetary Camera 2 imaging. Our sample spans a wide range of types including faint, gas-poor satellites of the and Andromeda such as Canes Venetici I and Andromeda XII to dense, luminous dwarf ellipticals such as NGC 185, and star-forming, gas-rich, and more isolated galaxies such as WLM. From the SFHs, we demonstrate the existence of an age-density re- lationship in the LG, such that galaxies within the virial radii of either M31 or the MW generally ceased star formation prior to those located at larger distances. We compare mean metallicities for stars older than 10 Gyr for all galaxies, and find that dwarf spheroidals (dSphs) and dwarf irregulars (dIs) have similar mean metallicity distributions, in contrast to some previous suggestions, which we interpret as further evidence that dSphs and dIs shared a common progenitor galaxy. We compare the derived stellar and chemical properties of M31 and Milky Way satellites and find that on average: (1) the M31 satellites appear to have stopped forming stars 2 Gyr after the Milky Way satellites; (2) the mean metallicies of stars older than 10⇠ Gyr appears to be slightly lower in the M31 satellites than in the MW satellites. We discuss how both selection e↵ects (e.g., sample construction, field placement) and physical e↵ects (e.g., tidal and radiative influences of host galaxies) may contribute to these finding. XXX Too much/ too controversial XXX Combined with known di↵erences in the structural properties of M31 and MW satellites, we suggest that properties of the host galaxy can strongly influence the evolution of its companions XXX Lastly, we consider the precision to which a SFH can be derived given the depth of photometry below the oldest main sequence turno↵, and conclude XXX. We provide tabulated data of all star formation and chemical evolutionary histories presented in this paper, including both random uncertainties, due to num- ber of stars on the CMD, and systematic uncertainties, due to uncertainties with the underlying stellar models. Key words: galaxies: stellar content, galaxies: dwarf, Local Group, color-magnitude diagrams (HR diagram)

? Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space c 2013 RAS

MW M31 MV = -4.9 Absolute Magnitude

MV = -16.5 Color Absolute Magnitude

Color New! 0.5 orbits 24 orbits

PI A. Cole MV = -4.9 Cumulative SFH

MV = -16.5 Time (Gyr ago) Number Weighted Median SFHs Number Weighted Median SFHs Number Weighted Median SFHs

Interesting. But does it actually mean anything? Truncation in dwarf galaxy SFHs

or

When/Why does SF stop in dwarfs? Definition of “Truncation” Time IV

11 Gyr 90%

Blue Stragglers or Young Stars?

Truncation Age Differences between M31 and MW dwarfs? Some Mechanisms for Truncating SF in Dwarfs The Astrophysical Journal,741:18(19pp),2011November1 Bovill & Ricotti

Table 1 Table 2 Parameters of “Milky Ways” All the Local Group Dwarfs Divided by Their Host (or Lack thereof) and Their Non-fossil or Fossil Status Name Run Mass Rvir vmax 12 1 Non-fossils Fossils (10 M )(kpc)(kms− ) ! MW.1 C 1.82 248 203 RG05 only RG05 & BR11a BR11a only MW.2 D 0.87 222 196 Milky Way LMC Sculptor Draco1 Bootes I & II MW.3 D 1.32 194 177 NGC 55 Phoenix CVn I & II Sextans A & B Sextans SMC Ursa Minor Leo IV & T2 not need to resolve these pre-reionization halos, just trace their WLM Pisces II merger history, tidal disruption, and positions from reionization Carina to present day. Fornax Paper I details the two realizations of our initial conditions. GR8 Our first-order realization (see the Appendices in Paper I for , II & A details) uses the same pre-reionization output across the entire Sagittarius high-resolution region, and consequently does not account for M31 IC 10 And I & II And V And XI XII the slower rate of structure evolution in the voids. In contrast, IC 1613 And III And XIII & IV our second-order initial conditions use four outputs from the IC 5152 And VI And XV & XVI pre-reionization simulations and do account for a slower rate M32 Antila AndXVII & XVIII of structure formation in the voids (Barkana & Loeb 2004). NGC 185 KKR 25 And XX & And XXI NGC 205 And XXII & XXIII Near the Milky Way, where this paper is concerned, we NGC 3109 And XXIV & XXV find no substantive difference between the two realizations. NGC 6822 And XXVI & XXVII We therefore focus our discussion on the second-order initial DDO 210 conditions. LGC3 All the simulations discussed in this paper were run using Pegasus Gadget 2 (Springel 2005)andanalyzedusingtheAmigahalo Isolated Cetus3 ··· ··· ··· finder AHF (Knollmann & Knebe 2009;Gilletal.2004). Tucana Simulation and data analysis were run on the beowulf cluster Deepthought at the HPCC at the University of Maryland. At Notes. We have further divided the fossils into three groups, those considered 6 z 0wedefineaboundhaloasanythingidentifiedbyAHF fossils in RG05, but with LV > 10 L ,the“classical”fossildwarfswith that,= for the parameters we used, produces mass functions luminosities below the 106 L threshold! from Bovill & Ricotti (2011,BR11ain ! complete for halos with a number of particles N>50. Paper I this table), and, finally, those satellites that were only included in BR11a and in shows that our hybrid initial conditions and non-uniform particle this paper after their post-2004 discovery. Three of the dwarfs initially identified masses produce results consistent with traditional CDM N-body as fossils show interesting properties: (1) a small fraction of the stars in Draco are of intermediate age (Cioni & Habing 2005), (2) has 105 M of simulations on both Local Volume and galactic subhalo scales. ∼ ! gas and a young stellar population (de Jong et al. 2008), and (3) Cetus shows The parameters of the three “Milky Ways” contained within our evidence for star formation through z 1(Monellietal.2010). two highest resolution runs are listed in Table 1. ∼ 2.1. Fossil Definition term fossil will only refer to “true fossils.” Unless otherwise The Astrophysical Journal,741:18(19pp),2011November1specified, the term non-fossil will apply doi:10.1088/0004-637X/741/1/18 to any halo that could C 2011. The AmericanDark matter Astronomical halos Society. identified All rights reserved. in our Printed simulations in the U.S.A. at z 0are ! Re-ionization= have accreted gas after reionization regardless of its maximum divided into three populations based on their ability to accrete circular velocity at z 0. gas from the IGM and form stars after reionization. A just- The division of the= Milky Way and M31 satellites into fossils WHEREvirialized ARE halo THEis able FOSSILS to accrete gas OF after THE reionization FIRST GALAXIES? only if and II. non-fossilsTRUE FOSSILS, is shown GHOST in Table 2 HALOS,.Wedistinguishbetween AND THE its maximum circular velocity, vmax,islargerthanafiltering 6 MISSING BRIGHT1 SATELLITESthe seven classical RG05 fossils above the 10 L threshold, velocity, vfilt. In this work, we use vfilt 20–30 km s− , 6 ! = the RG05 fossils with LV < 10 L , and the ultra-faint dwarfs corresponding to the threshold for coolingMia via S. Ly Bovillα.PaperI and Massimodiscovered Ricotti since 2005. ! and GK06 showDepartment that the exact of Astronomy, choice of University the filtering of Maryland, velocity College Park, MD 20740, USA; [email protected] does not significantlyThe Astrophysical change JournalReceived the,741:18(19pp),2011November1 results. 2010 IfOctober a z 12;0halohas accepted 2011 July 21; published 20113. A October NOTE 11 ON OBSERVATIONSBovill & Ricotti v >v , we classify it as a non-fossil. In the modern= epoch, max filt Table 1 Table 2 non-fossils can be identifiedParameters as dIrrs of which “Milky Ways” have been accretingABSTRACTAllWe the Local approach Group Dwarfs the Divided observations by Their Host as (or follows. Lack thereof) The and majorityTheir of gas and forming stars continuously since reionization. the information onNon-fossil the classical or Fossil dwarfs Status comes from the Mateo Name Run Mass Rvir vmax AnyWe use halos a new whose set present of cold day dark12v matteris below simulations the filtering of the1 local(1998 ) review.Non-fossils to For investigate the ultra-faint the distribution dwarfs Fossils we generallyof fossils defer to (10 Mmax)(kpc)(kms− ) ! velocityof primordial is a candidate dwarf fossil galaxies.Acandidatefossilforwhich within and around the Milky Way.measurements Throughout, withRG05 the we only smallest build RG05 upon & error BR11a previous bars BR11a with results only some weight showingMW.1 agreement C between the 1.82 observed 248 stellar properties 203 of a subset of the ultra-faint dwarfs and our simulated v‘Truemax >vfilt MW.2Fossils’at any point D during= Primordial its 0.87evolution may 222Galaxies have accreted 196 givenMilky Way to more LMC recent Sculptor work (Walker Draco1 et al. 2009Bootes). I We& II direct the gasfossils. and/orMW.3 Here, formed we stars show D after that reionization. fossils 1.32 of the We first include 194 galaxies them 177 have in galactocentricreader to PaperNGC distributions 55 I of this series and and Phoenix cumulative to BR09 for luminosity CVn a I more & II complete thefunctions “non-fossil” consistent group, withas post-reionization observations. In stars our may model, be we the predictdiscussion300Sextans luminous of these A & B criteria. satellites orbiting Sextans the Milky Hercules Way, 2 dominant stellar population. When we need to distinguish these When∼ calculatingSMC the observed Ursa distributionsMinor Leo of IV &dwarfs T around 50%–70%not need of whichto resolve are these well-preserved pre-reionization fossils. halos, just Within trace their the Milky Way virialWLM radius, the majority of these Pisces fossils II 6 “candidatehave luminositiesmerger fossils” history, fromL tidalV the< disruption, broader10 L “non-fossil” and.Despiteourmultidimensionalagreementwithobservationsatlowmassesand positions group, from reionization we will the Milky Way,Carina we account for two effects, the sky coverage # Fornax 4 referluminosities, to themto present as polluted the day. primordial fossils.Finally,anycandidatefossilfor model produces an overabundanceof the of bright Sloan dwarf Digital satellites Sky Survey (LV (SDSS)> 10 L and)with its detection GR8 whichrespectvmax toPaper 70%) in Paper of their I for the classicalLeo I, dwarfs, II & A we assume that the entire sky has been truethe fossil outer parts of the Milky Way. We estimate that, although relativelySagittarius bright, the primordial stellar populations stars beforedetails) reionization uses the same and todaypre-reionization would be output relatively across diffuse the entire covered and only apply sky coverage corrections to the ultra- are very diffuse, producing a population with surface brightnessesM31 below IC surveys’ 10 And detection I & II And limits, V and And are XI easily XII systems ofhigh-resolution old stars devoid region, of gas and (Ricotti consequently & Gnedin does not2005 account). The for faint population. To correct for the SDSS sky coverage, we strippedthe by slower tidal rate forces. of structure Although evolution we in cannot the voids. yet In present contrast, unmistakable evidenceIC 1613 for And the III existence of the And fossils XIII & IV of first galaxiesour second-order in the Local initial Group, conditions the use results four outputs of our from studies the suggest observationalIC 5152 And strategies VI that may demonstrateAnd XV & XVI pre-reionization simulations and do account for a slower rate 3 M32 Antila AndXVII & XVIII their existence:of structure (1) formation the detection in the voids of (Barkana “ghost halos” & Loeb of2004 primordial). starsNGC around 185 isolated KKR 25 dwarfs would And XX prove & And that XXI 8 NGC 205 And XXII & XXIII stars formedNear the in Milky minihalos Way, (whereM< this10 paperM )beforereionizationandstronglysuggestthatatleastafractionofthe is concerned, we # NGC 3109 And XXIV & XXV ultra-faintfind dwarfs no substantive are fossils difference of the between first galaxies; the two realizations. and (2) the existence ofNGC a yet 6822 unknown population of And XXVI150 &Milky XXVII LCID 4 ∼ Way ultra-faintsWe therefore with focus half-light our discussion radii onrhl the second-order100–1000 initial pc and luminositiesDDOLV 210< 10 L ,detectablebyfuturedeep surveys.conditions. These undetected dwarfs would≈ have the mass-to-light ratios, stellarLGC3 velocity dispersions,# and metallicities All the simulations discussed in this paper were run using Pegasus predictedMonelli+ in this work. 2010a,b Gadget 2 (Springel 2005)andanalyzedusingtheAmigahalo Isolated Cetus3 ··· ··· ··· Key words:finderdark AHF matter(Knollmann – galaxies: & Knebe dwarf2009;Gilletal. – galaxies:2004 evolution). – galaxies: formation – LocalTucana Group Simulation and data analysis were run on the beowulf cluster Online-onlyDeepthought material: at thecolor HPCC figures at the University of Maryland. At Notes. We have further divided the fossils into three groups, those considered 6 z 0wedefineaboundhaloasanythingidentifiedbyAHF fossils in RG05, but with LV > 10 L ,the“classical”fossildwarfswith that,= for the parameters we used, produces mass functions luminosities below the 106 L threshold! from Bovill & Ricotti (2011,BR11ain ! complete for halos with a number of particles N>50. Paper I this table), and, finally, those satellites that were only included in BR11a and in shows that our hybrid initial conditions and non-uniform particle this paper after their post-2004 discovery. Three of the dwarfs initially identified masses produce results consistent with traditional CDM N-body loopsas fossils operating show interesting before properties: reionization (1) a small fraction which of the determine stars in Draco whether 1. INTRODUCTION are of intermediate age (Cioni & Habing 2005),8 (2) Leo T has 105 M of simulations on both Local Volume and galactic subhalo scales. low-mass minihalos with M<10 M are able∼ to! accrete gas gas and a young stellar population (de Jong et al. 2008#), and (3) Cetus shows Over the lastThe decade, parameters since of theKlypin three “Milky et al. (Ways”1999)andMoore contained within our fromevidence the for IGM star formation and form through stars.z 1(Monellietal. Simulations2010). show that for halos et al. (1999)showedthatthenumberofdarkmattersubhalostwo highest resolution runs are listed in Table 1. with masses M<108 M the∼ local and stochastic components # expected around a Milky Way mass2.1. halo Fossil is Definition two orders of magni- of galaxyterm fossil feedbackwill only produce refer to minihalos “true fossils.” with Unless the sameotherwise tude above the number of known satellites, significant effort has mass,specified, but with the term stellar non-fossil masses will which apply to vary any halo by thatseveral could orders of Dark matter halos identified in our simulations at z 0are have accreted gas after reionization regardless of its maximum been made in observationdivided into three and populations theory to basedsolve on the their substructure ability to= accrete magnitude (Ricotti et al. 2008). It is likely unjustified to assume a sharpcircular mass velocity threshold at z separating0. dark and luminous halos and/or problem in coldgas dark from matter the IGM (CDM) and form cosmology. stars after Observational reionization. A just- The division of the= Milky Way and M31 satellites into fossils discoveries havevirialized redefined halo the is able “missing to accrete galactic gas after satellite reionization prob- only if atightrelationshipbetweendwarfs’luminositiesandtheirtotaland non-fossils is shown in Table 2.Wedistinguishbetween its maximum circular velocity, vmax,islargerthanafiltering 6 lem” with the number of observed Milky Way satellites now 1 mass,the seven at the classical faint end RG05 of thefossils luminosity above the 10 function.L threshold, This is one of velocity, vfilt. In this work, we use vfilt 20–30 km s− , 6 ! somewhat closer to theoretical expectations (Simon= & Geha thethe main RG05 motivations fossils with L forV < the10 presentL , and study. the ultra-faint dwarfs corresponding to the threshold for cooling via Lyα.PaperI discovered since 2005. ! 8 2007; Tollerud etand al. GK062008 show;Bovill&Ricotti that the exact choice2009 of;Macci the filteringoetal.` velocity Cooling in halos with masses at formation, M>10 M 1 # 2010). The discoverydoes not of significantly the ultra-faint change dwarfs the results. (Belokurov If a z et0halohas al. (vmax > 20 km3. s A− NOTE) is initiated ON OBSERVATIONS via readily available hydrogen v >v , we classify it as a non-fossil. In the modern= epoch, 4 2006, 2007; Irwinmax et al.filt2007; Walsh et al. 2007; Willman et al. Lyα emission. However, cooling in minihalos with Tvir < 10 K non-fossils can be identified as dIrrs which have been accreting requiresWe approach the formation the observations of either as follows. molecular The majority hydrogen of or pre- 2005a, 2005b;Zuckeretal.gas and forming2006a stars, continuously2006b;Gehaetal. since reionization.2009)has the information on the classical dwarfs comes from the Mateo roughly doubled theAny known halos whose Milky present Way and day v M31max is satellite below the pop- filtering enrichment(1998) review. with For metals the ultra-faint from dwarfs nearby we generally galaxies. defer The to balance ulations since 2004.velocity However, is a candidate observations fossil.Acandidatefossilforwhich alone cannot fully betweenmeasurements the destruction with the smallest and formation error bars of with H2 someand metal weight transport explain the discrepancyvmax >vfilt betweenat any point luminous during its satellites evolution may and have CDM accreted ingiven the IGM to more governs recent work whether (Walker or et not al. 2009 low-mass). We direct minihalos the can gas and/or formed stars after reionization. We include them in initiatereader cooling to Paper I andof this form series stars. and to BR09 If ultraviolet for a more complete H dissociating substructure. the “non-fossil” group, as post-reionization stars may be the discussion of these criteria. 2 One of the coredominant theoretical stellar population. issues of When the “missing we need to satellites” distinguish these radiationWhen dominates, calculating the star observed formation distributions in the of dwarfs first minihalos around may problem remains“candidate the relationship fossils” from betweenthe broader the “non-fossil” luminosity group, of we will bethe suppressed Milky Way, or we delayed account for (Haiman two effects, et theal. sky2000 coverage; Ciardi et al. satellites and therefer virial to them mass as polluted at formation fossils.Finally,anycandidatefossilfor of their dark matter 2000of;Machaceketal. the Sloan Digital Sky2000 Survey;Wise&Abel (SDSS) and its2007 detection;O’Shea& halos. From a theoreticalwhich vmax perspective, 70%) of their the classical dwarfs, we assume that the entire sky has been to be answeredstars is: “What before reionization is the minimum and today halo would mass be relatively that can diffuse radiationcovered ( andhν only> 13 apply.6 eV) sky is coverage the dominant corrections feedback to the ultra- mechanism, host a luminoussystems galaxy?” of old Previous stars devoid studies of gas (Ricotti (Efstathiou & Gnedin19922005).; The H2faintformation population. can To be correct catalyzed for the insideSDSS sky relic coverage, H ii regions we and Thoul & Weinberg 1996;Bullocketal.2001;Venkatesanetal. on the edges of Stromegen spheres (Ricotti et al. 2001, 2002a, 2001;Ricotti&Ostriker2004;Ricottietal.2005)haveshown 32002b;Ahnetal.2006;Whalenetal.2008;Wise&Abel2008), that this critical mass is set by the reheating history of the allowing star formation to be more widespread in minihalos 8 intergalactic medium (IGM; Ricotti et al. 2000)andbyfeedback with mass M ! 10 M ,beforereionization.Itisimportantto # 1 ‘Classical’ Dwarfs

Ursa Draco And V Phoenix Minor Cumulative SFH Time (Gyr ago) ‘Ultra-Faint’ Dwarfs

The Astrophysical Journal,629:259–267,2005August10 # 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A. CvN II Leo IV And XII Hercules FORMATION HISTORIES OF DWARF GALAXIES IN THE LOCAL GROUP Massimo Ricotti1 and Nickolay Y. Gnedin2 Received 2004 August 5; accepted 2005 April 28 No. 1, 2005 DWARF GALAXIES IN LOCAL GROUP 265 ABSTRACT Cumulative SFH We compare the properties of dwarf galaxies in the Local Group with the simulated galaxies formed before reionization in a cosmological1. True fossils simulation formed of unprecedented most of their spatial stars and mass in the resolution, pre-reionization including radiative feedback effects. We findera that a and subset have of the had Local little Group (say, dwarfs< are30%) already star remarkably formation similar since to the then. simulated dwarf galaxies in all their properties2. before Polluted reionization. fossils On started the basis of as this true similarity, fossils we but propose had the substantial hypothesis that Local Group dwarfs form inAnd a variety XIII of ways: some of themAnd are ‘‘true XI fossils’’ of the pre-reionizationCvN I era, some of them form most of their stars later,episode(s) after reionization of subsequent (we call them star ‘‘survivors’’ formation of the reionization as they continued era), and the rest ac- of them form an intermediate groupcreting of ‘‘polluted mass fossils.’’ and were We also tidally identify shocked a simple observational during the test formation that is able to testof our hypothesis. Subject headingthegs: cosmology: parent galaxy theory —halo. galaxies: dwarf — galaxies: formation — Local Group — 3.stars: Survivors formation startedTime forming (Gyr stars ago) mostly after reionization. 1. INTRODUCTIONThis conclusion is motivated by( 10 considering9 1010 M ) than star in formation the reionization scenario. While this idea resolves the ‘‘crisis,’’ it does not provide an explanation for Nothing emphasizes ourhistories lack of understanding of Local of Group the origin dwarfs of and by comparing their prop- the origin of dwarf galaxies. the dwarf galaxies of the Localerties Group with more properties than the well-known of simulated galaxies that formed all their Amorecomprehensiveandcomplexpicturehasbeenrecently ‘‘substructure crisis’’ of the cold dark matter (CDM) paradigm stars before cosmological reionization.proposed The by Kravtsovfact that et our al. (2004). simu- Under their hypothesis— (Klypinetal.1999;Mooreetal.1999)—thefactthatthenumber hereafter the ‘‘tidal scenario’’—most dwarf galaxies used to be of dark matter halos inlated the Local galaxies Group are predicted remarkably by CDM similar to the subset of the Local much (up to a factor of 100) more massive during the forma- simulations is much largerGroups than the number dwarfs of observed that we Galactic identified as true fossils in Figure 5 satellites. Several explanations have been suggested so far for tion episodes of the Milky Way and Andromeda and thus were renders support to our conclusions.not This affected subset by photoionization includes almost feedback. They formed most of resolving the ‘‘crisis.’’ all known dSphs. their stars at z 3, but later most of the dark matter was tid- The hypothesis of Bullock et al. (2001)—hereafter the ‘‘re-  ionization scenario’’—proposedTo avoidthat dwarf confusion, spheroidal galaxies it is worthally noting stripped during that the their structuralevolution within the Milky Way and Andromeda halos. Kravtsov et al. (2004) emphasize that in their Fig. 11.—Stellar metallicity distribution of one simulated galaxy.(dSphs) The in the solid Local Group formed during the pre-reionization properties of the galaxies that we callmodel true a large fossils variety are of the star result formation of histories is observed, and and dashed lines show the number- and mass-weighted distributions,era, respectively. and processes of photoionization feedback (Babul & Rees some of the dwarf galaxies do indeed follow the ‘‘reionization 1992; Efstathiou 1992; Shapirofeedback et al. processes1994, 2004; Haiman in action et al. prior to reionization. The effect of track,’’ but the bulk of stars in the Local Group dwarfs are formed 1996; Thoul & Weinbergreionization 1996; Quinn et is al. simply 1996; Weinberg to preserve those properties by suppress- after reionization. We have already anticipated that our simulation reproduceet al. 1997; Navarro the & Steinmetz 1997; Gnedin 2000b; Dijkstra ing the star formation rate in halosThus, with the masses key difference that between remain the reionization and tidal et al. 2004; Susa & Umemura 2004) resulted in the suppression large metallicity spreads observed in dSphs. In Figure 10 we show smaller than the filtering mass inhypotheses the IGM. is whether In this most sense of the our stars in a given dwarf galaxy of star formation below the observable level in 90% of the low- formed in halos of mass 108 M before reionization (z 6, a comparison between the simulated and the observationalmass dark matter data halos populatingmodel differs the Local from Group. the The standard reion- ‘‘reionization’’ scenarioP for the > 12.5–13 Gyr ago) or in more massi vehalosafter reionization, for the metallicity spreads,  Fe/ H , both as a functionization of the scenario mean is promptedformation by the fact of dwarf that almost galaxies. all Local ½ Š Group dSphs exhibit a prominent old population and a decline of during the epoch of formation of the Milky Way and Andromeda metallicity [Fe/H] (top panel) and as a function of luminosity LV How would one test this hypothesis?(z 2 3, For 10–11 example, Gyr ago). the main their star formation rates about 10 Gyr ago. While in the standard (bottom panel). We find that the typical metallicity variance in each In this paper we propose that dSphs are fossils of galaxies ÃCDM theory (Spergel etargument al. 2003; Tegmark of Kravtsov et al. 2004) et al. stellar (2004) for the tidal origin of the ma- with mass M 108 M that formed before reionization, while dwarf is about 0.5 dex, in good agreement with observations.reionization The took place 12.5 Gyr ago, current observational dm < jority of Local Group dwarfs is theirdwarf preferential Irregular galaxies location (dIrrs) close formed to later in more massive techniques for measuring absolute stellar ages are not able to star formation history in simulated dwarfs is characterized by the parent galaxy. However, true fossils,halos. The living original in the contribution oldest darkof our study is that for the first differentiate between 10 and 12.5 Gyr (e.g., Krauss & Chaboyer multiple instantaneous starbursts extended over a period of 0.5– time we have been able to simulate the first population of gal- 2003). Although the observedmatter drop halos of star of their formation mass, rate are is highly biased and are also pref- axies including the relevant feedback processes that are known to 1Gyr.Thestarburstsoftendooccuratalmostthesametimebutinconsistent with reionization at redshift z 6, as noted by Grebel erentially located closer to the centerself-limit of the their parent formation. halo. Before More- our work (Ricotti et al. 2002a, &Gallagher(2004),themetallicityspreadsobservedintheold distinct dark matter subhalos that are in the process of merging to over, there are at least two examples2002b), of dSphs the mainstream that are wisdom quite was far that negative feedback ef- form a more massive dwarf. The accretion of starspopulations from the may hi- imply a star formation history protracted over about 2 Gyr, to redshift zfrom3. In massive this paper galaxies: we critically Cetus discuss and Tucana,fects (namely, and perhapsthe rapid photodissociation the newly of H2)suppressthe formation of all galaxies with mass smaller than 108 M .Wefind erarchical distribution of subhalos partially explainsthis important the large issue. We showdiscovered that our simulations Apples 1 reproduce (Pasquali the et al. 2005). instead that these galaxies do form, and, at the epoch of their metallicity spread that we find in our simulation. observed metallicity spreadsAmorepowerfultestisbasedonourconclusionthatsurvivors without a protracted period of star formation. The metallicity spreads observed in our simulated formation, they closely resemble dSphs. This second result is the Thus, we conclude that the large metallicity spread observed one we emphasize in this paper. We also show that reionization dwarfs can be understoodform in terms later of than hierarchical the true accretion fossils. of While measuring absolute ages of feedback is not the dominant mechanism that suppresses star in the old stellar population of dSphs is not a definitesubhalos signature containing of starsdwarf with different galaxies metallicities. to 1 Gyr precision is not possible at the moment, formation in most small mass halos. Negative feedback such as An alternative explanation to the substructure crisis was star formation extended over 2–4 Gyr (Ikuta & Arimoto 2002; determining the relative difference ofphotoheating about 1 Gyrby the in stars age inside between each galaxy and inefficient cool- proposed by Stoehr et al. (2002). According to their hypothesis, Grebel & Gallagher 2004). Neither the argument based on the ing produce the observed properties of dSphs well before reion- dwarf galaxies are hostedtwo in dark galaxies matter observed halos more by massive a uniform technique may, in fact, be long timescale (e.g., 1–4 Gyr) needed to have a substantial iron possible for a specific range of metallicitiesization of the and intergalactic ages using medium the (IGM) is complete. It is quite surprising that we find such an astonishing agree- production by Type Ia SNe is a strong argument in favor1 Institute of ofpro- Astronomy, Madingley Road, Cambridge CM3 0HA, England; so-called indexment (G. between Gilmore all properties 2004, of private simulated dwarf galaxies at z 8 [email protected].  longed star formation. The typical timescale for iron enrichment2 CASA, University of Colorado,communication; Boulder, CO 80309; [email protected] also see Zinn 1993;and dSphs. Harbeck Our simulations et al. did 2001; improve on previous ones but by Type Ia SNe is uncertain and is very likely dependent.edu. on the Mackey & Gilmore 2004); youngerare stillgalaxies far from have a complete redder treatment hori- of the problem that, as we mode of star formation. Matteucci & Recchi (2001) estimated zontal branch for the same metallicity,259 although how precise the that this timescale varies from 40 to 50 Myr for an instantaneous horizontal branch index method is and the role of the so-called starburst to 0.3 Gyr for a typical elliptical galaxy to 4–5 Gyr for second parameter effect is still a matter of debate. a disk of a like the Milky Way. The often-quoted Luckily, some of the Local Group dwarfs fall into that range of 1 Gyr timescale is based on observations of [ /Fe] versus [Fe/H] parameters. For example, two galaxies of our true fossil group— in the solar neighborhood stars and is not a universal value. A And I and And II—have Fe/H 1:5, similar to the iron more detailed study would be needed to derive accurate metal abundance in two of the½ survivors—WLMŠÀ and NGC 3109 abundance ratios based on the star formation and merger histo- (havingpairsofgalaxieswiththesamemetallicityisabsolutely ries found in the simulations. This is beyond the aim of this pa- crucial, as even small differences in metallicities can obscure per, but it is worth pursuing in a separate study. large differences in age). Thus, we can make a prediction that We also study the stellar metallicity distribution in each one most of the stars that formed in the first burst of star formation in of our simulated dwarfs. Figure 11 shows such an example. The WLM and NGC 3109 should be about 1.5 Gyr younger than solid and dashed lines show the number- and mass-weighted most of the stars in And I and And II. The color-magnitude distributions, respectively. The metallicity distribution is skewed diagrams of And I, And II, and WLM are measured down to toward low metallicities, but only a small fraction of the stars, V > 26, well below the horizontal branch (Da Costa et al. 2002; 4 having metallicities <10À Z , could be defined as Population III Rejkuba et al. 2000), although to the best of our knowledge, no stars. photometry down to that levels exists for NGC 3109. Visual comparison of the horizontal branch morphologies for the first 4. DISCUSSION three galaxies indicate that And I and And II are indeed sub- In this paper we have proposed that dwarf galaxies of the stantially older than WLM; also a more rigorous, quantitative Local Group (and, by implication, all other dwarf galaxies in the test is required to confirm the visual impression. Such a com- universe) formed in three different evolutionary paths: parison would be a crucial test of our hypothesis—if it is found Mon. Not. R. Astron. Soc. 425, 231–244 (2012) doi:10.1111/j.1365-2966.2012.21432.x Infall Infall times for MilkyMon. Not. Way R. Astron. Soc.satellites425, 231–244 (2012) from their present-day kinematicsdoi:10.1111/j.1365-2966.2012.21432.x

Miguel Rocha,! AnnikaInfall H. G. times Peter for! Milkyand James Way satellites Bullock from! their present-day kinematics Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, CA 92697-4575, USA Miguel Rocha,! Annika H. G. Peter! and James Bullock! Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, CA 92697-4575, USA Accepted 2012 May 31. Received 2012 May 30; in original form 2011 October 2 Accepted 2012 May 31. Received 2012 May 30; in original form 2011 October 2

ABSTRACT ABSTRACT We analyse subhaloes inWe the analyse ViaLacteaII subhaloes in (VL2) the ViaLacteaII cosmological (VL2) cosmological simulation simulation to look to for look correla- for correla- tions among their infall timestions among and theirz infall0dynamicalproperties.Wefindthatthepresent-day times and z 0dynamicalproperties.Wefindthatthepresent-day

= Downloaded from orbital energy is tightly correlated with the time at which subhaloes last entered within the

= Downloaded from orbital energy is tightly correlatedvirial radius. This with energy–infall the time correlation at which provides subhaloes a means last to infer entered infall times within for Milkythe virial radius. This energy–infallWay satellite correlation galaxies. Assuming provides that a the means Milky Way’s to infer assembly infall can times be modelled for Milky by VL2, Way satellite galaxies. Assumingwe show that that the infall the times Milky of some Way’s satellites assembly are well constrained can be modelled given only their by VL2, Galacto- centric positions and line-of-sight velocities. The constraints sharpen for satellites with proper we show that the infall timesmotion of measurements. some satellites We find are that well Carina, constrained Ursa Minor and given Sculptor only were their all accreted Galacto- early, http://mnras.oxfordjournals.org/ centric positions and line-of-sightmore than 8 velocities. Gyr ago. Five The other constraints dwarfs, including sharpen Sextans for and satellites , are with also proper probable motion measurements. Weearly find accreters, that Carina, though with Ursa larger Minor uncertainties. and Sculptor On the other were extreme, all accreted Leo T is just early, falling http://mnras.oxfordjournals.org/ into the Milky Way for the first time while Leo I fell in 2Gyragoandisnowclimbingoutof ∼ more than 8 Gyr ago. Fivethe Milky other Way’s dwarfs, potential including after its first Sextans perigalacticon. and Segue The energies 1, are of several also probable other dwarfs, early accreters, though withincluding larger Fornax uncertainties. and Hercules, point On to the intermediate other extreme, infall times, Leo 2–8 T Gyr is ago. just We falling compare into the Milky Way for theour first infall time time whileestimates Leo to published I fell in star2Gyragoandisnowclimbingoutof formation histories and find hints of a dichotomy between ultrafaint and classical dwarfs.∼ The classical dwarfs appear to have quenched star the Milky Way’s potentialformation after its after first infall perigalacticon. but the ultrafaint dwarfs The tend energies to be quenched of several long before other infall, dwarfs, at least at University of Washington on July 21, 2013 including Fornax and Hercules,for the cases point in which to intermediate our uncertainties infall allow us times, to discern 2–8 differences. Gyr ago. Our We analysis compare suggests our infall time estimatesthat to publishedthe Large Magellanic star formation Cloud crossed histories inside the and Milky find Way hints virial ofradius a dichotomy recently, within the last 4 billion years. between ultrafaint and classical∼ dwarfs. The classical dwarfs appear to have quenched star Key words: methods: numerical – galaxies: evolution – galaxies: formation – galaxies: formation after infall buthaloes the ultrafaint – dark matter. dwarfs tend to be quenched long before infall, at least at University of Washington on July 21, 2013 for the cases in which our uncertainties allow us to discern differences. Our analysis suggests that the crossed inside the Milky Way virial radius recently, within the last 4 billion years. (see e.g. Berrier et al. 2009). Once inside the virial radius of a larger 1INTRODUCTION ∼ host, star formation in the satellites may be quenched either because KeyThe Milky words: Way (MW)methods: is a unique laboratory numerical for understanding – galaxies: the evolutionthey stop accreting – galaxies: fresh gas (‘strangulation’; formation Larson,– galaxies: Tinsley & 8 lives of dwarf galaxies (L ! 10 M ). Dwarf spheroidal galaxies, Caldwell 1980; Bekki, Couch & Shioya 2002) or because their cool haloesin particular, – dark stand out matter. among galaxies" because of their high dark gas is stripped away (‘ram-pressure stripping’; Gunn & Gott 1972) matter content, lack of gas and lack of recent star formation. Like due to interactions with the host’s gas halo. High-speed encounters larger galaxies (Dressler 1980; Butcher & Oemler 1984; Goto et al. with other satellite galaxies or the host itself may similarly affect 2003), dwarf galaxies appear to have a ‘morphology–density’ rela- morphologies and star formation (‘harassment’; Moore et al. 1996). tion, with dwarf spheroidal galaxies preferentially crowding around These processes are also relevant for dwarf galaxies around the normal galaxies (or within groups) instead of the field(see (Mateo e.g. 1998a; Berrier etMW. al. 2009). The MW Once is likely inside surrounded the virial by a hotradius gas halo of a of larger its own, 1INTRODUCTION Weisz et al. 2011). All of the galaxies within the MW’s dark matter which can aid in quenching star formation once galaxies fall within halo except the two are dwarfhost, spheroidals. star formationits in reach the (Mallersatellites & Bullock may be 2004; quenched Fang et al. either 2006; because Peek et al. The Milky Way (MW) is a unique laboratoryIn galaxy for groups understanding and clusters, the the morphology–densitythey stop relation accreting2007; fresh Kaufmann gas (‘strangulation’; et al. 2008; Grcevich Larson, & Putman Tinsley2009, 2010). & In 8 is associated with a colour–density relation or a star-formation– addition, dwarfs can experience dynamical transformation due to lives of dwarf galaxies (L ! 10 M ). Dwarf spheroidal galaxies, Caldwell 1980; Bekki, Couch & Shioya 2002) or because their cool "density relation (Kauffmann et al. 2004; Blanton et al. 2005). The tidal stirring (Łokas et al. 2010; Kazantzidis et al. 2011), though the in particular, stand out among galaxiesorigin because of these ofrelations their is high thought dark to result fromgas the is quenching stripped awaysmallest (‘ram-pressure galaxies may very stripping’; well be born Gunn puffy well& Gott before 1972) entering matter content, lack of gas and lackof star recent formation star once formation. galaxies become Like satellitesdue in larger to interactions systems the with halo the (Kaufmann, host’s gasWheeler halo. & Bullock High-speed 2007). encounters larger galaxies (Dressler 1980; Butcher & Oemler 1984; Goto et al. with other satellite galaxiesOne way in or which the dwarf host satellites itself aremay different similarly than larger affect satel- lite galaxies in groups and clusters is that their low mass makes 2003), dwarf galaxies appear to have! aE-mail: ‘morphology–density’ [email protected] (MR); rela- [email protected] (AHGP); andthem star formation inherently fragile, (‘harassment’; thus more susceptible Moore to et quenching al. 1996). pro- tion, with dwarf spheroidal galaxies [email protected] (JB) crowding around These processes arecesses also that wouldrelevant not otherwise for dwarf affect galaxiesL galaxies. around Specifically, the it ∼ ∗ normal galaxies (or within groups) insteadC of the field (Mateo 1998a; MW. The MW is likely surrounded by a hot gas halo of its own, $ 2012 The Authors C Weisz et al. 2011). All of the galaxiesMonthly within Notices the ofMW’s the Royal dark Astronomical matter Society $which2012 RAS can aid in quenching star formation once galaxies fall within halo except the two Magellanic Clouds are dwarf spheroidals. its reach (Maller & Bullock 2004; Fang et al. 2006; Peek et al. In galaxy groups and clusters, the morphology–density relation 2007; Kaufmann et al. 2008; Grcevich & Putman 2009, 2010). In is associated with a colour–density relation or a star-formation– addition, dwarfs can experience dynamical transformation due to density relation (Kauffmann et al. 2004; Blanton et al. 2005). The tidal stirring (Łokas et al. 2010; Kazantzidis et al. 2011), though the origin of these relations is thought to result from the quenching smallest galaxies may very well be born puffy well before entering of star formation once galaxies become satellites in larger systems the halo (Kaufmann, Wheeler & Bullock 2007). One way in which dwarf satellites are different than larger satel- lite galaxies in groups and clusters is that their low mass makes !E-mail: [email protected] (MR); [email protected] (AHGP); them inherently fragile, thus more susceptible to quenching pro- [email protected] (JB) cesses that would not otherwise affect L galaxies. Specifically, it ∼ ∗

C $ 2012 The Authors C Monthly Notices of the Royal Astronomical Society $ 2012 RAS CvN II Leo IV Hercules LeoT

Ursa CvN I Min. Draco Carina

Leo II Sculptor Leo I Fornax Mon. Not. R. Astron. Soc. 425, 231–244 (2012) doi:10.1111/j.1365-2966.2012.21432.x

Infall times for Milky Way satellites from their present-day kinematics

Miguel Rocha,! Annika H. G. Peter! and James Bullock! Center for Cosmology, Department of Physics and Astronomy,Quench University Before of California, Infall Irvine, CA 92697-4575, USA

Accepted 2012 May 31. Received 2012 May 30; in original form 2011 October 2

ABSTRACT We analyse subhaloes in the ViaLacteaII (VL2) cosmological simulation to look for correla- tions among their infall times and z 0dynamicalproperties.Wefindthatthepresent-day

= Downloaded from orbital energy is tightly correlated with the time at which subhaloes last entered within the virial radius. This energy–infall correlation provides a means to infer infall times for Milky Way satellite galaxies. Assuming that the Milky Way’s assembly can be modelled by VL2, we show that the infall times of some satellites are well constrained given only their Galacto- centric positions and line-of-sight velocities. The constraints sharpen for satellites with proper motion measurements. We find that Carina, Ursa Minor and Sculptor were all accreted early, http://mnras.oxfordjournals.org/ more than 8 Gyr ago. Five other dwarfs, including Sextans and Segue 1, are also probable early accreters, though with larger uncertainties. On the other extreme, Leo T is just falling into the Milky Way for the first time while Leo I fell in 2Gyragoandisnowclimbingoutof ∼ the Milky Way’s potential after its first perigalacticon. The energies of several other dwarfs, including Fornax and Hercules, point to intermediateQuench infall times,After 2–8 Infall Gyr ago. We compare our infall time estimates to published star formation histories and find hints of a dichotomy between ultrafaint and classical dwarfs. The classical dwarfs appear to have quenched star

formation after infall but the ultrafaint dwarfs tend to be quenched long before infall, at least at University of Washington on July 21, 2013 for the cases in which our uncertainties allow us to discern differences. Our analysis suggests that the Large Magellanic Cloud crossed inside the Milky Way virial radius recently, within the last 4 billion years. ∼ Key words: methods: numerical – galaxies: evolution – galaxies: formation – galaxies: haloes – dark matter.

(see e.g. Berrier et al. 2009). Once inside the virial radius of a larger 1INTRODUCTION host, star formation in the satellites may be quenched either because The Milky Way (MW) is a unique laboratory for understanding the they stop accreting fresh gas (‘strangulation’; Larson, Tinsley & 8 lives of dwarf galaxies (L ! 10 M ). Dwarf spheroidal galaxies, Caldwell 1980; Bekki, Couch & Shioya 2002) or because their cool in particular, stand out among galaxies" because of their high dark gas is stripped away (‘ram-pressure stripping’; Gunn & Gott 1972) matter content, lack of gas and lack of recent star formation. Like due to interactions with the host’s gas halo. High-speed encounters larger galaxies (Dressler 1980; Butcher & Oemler 1984; Goto et al. with other satellite galaxies or the host itself may similarly affect 2003), dwarf galaxies appear to have a ‘morphology–density’ rela- morphologies and star formation (‘harassment’; Moore et al. 1996). tion, with dwarf spheroidal galaxies preferentially crowding around These processes are also relevant for dwarf galaxies around the normal galaxies (or within groups) instead of the field (Mateo 1998a; MW. The MW is likely surrounded by a hot gas halo of its own, Weisz et al. 2011). All of the galaxies within the MW’s dark matter which can aid in quenching star formation once galaxies fall within halo except the two Magellanic Clouds are dwarf spheroidals. its reach (Maller & Bullock 2004; Fang et al. 2006; Peek et al. In galaxy groups and clusters, the morphology–density relation 2007; Kaufmann et al. 2008; Grcevich & Putman 2009, 2010). In is associated with a colour–density relation or a star-formation– addition, dwarfs can experience dynamical transformation due to density relation (Kauffmann et al. 2004; Blanton et al. 2005). The tidal stirring (Łokas et al. 2010; Kazantzidis et al. 2011), though the origin of these relations is thought to result from the quenching smallest galaxies may very well be born puffy well before entering of star formation once galaxies become satellites in larger systems the halo (Kaufmann, Wheeler & Bullock 2007). One way in which dwarf satellites are different than larger satel- lite galaxies in groups and clusters is that their low mass makes !E-mail: [email protected] (MR); [email protected] (AHGP); them inherently fragile, thus more susceptible to quenching pro- [email protected] (JB) cesses that would not otherwise affect L galaxies. Specifically, it ∼ ∗

C $ 2012 The Authors C Monthly Notices of the Royal Astronomical Society $ 2012 RAS Lowest Luminosity Dwarfs

UFDs Cumulative SFH Classical And XI, XII, XIII

Time (Gyr ago) Lowest Luminosity Dwarfs

From a SFH perspective, there isn’t a clear ‘Classical’ vs. ‘Ultra Faint’ dichotomy. Cumulative SFH

Time (Gyr ago) Mon. Not. R. Astron. Soc. 425, 231–244 (2012) doi:10.1111/j.1365-2966.2012.21432.x Mon. Not. R. Astron. Soc. 425, 231–244 (2012) doi:10.1111/j.1365-2966.2012.21432.x Infall times for Milky Way satellites from their present-day kinematics Infall times for Milky Way satellites from their present-day kinematics ! ! ! Miguel Rocha, AnnikaMiguel H. G. Rocha, Peter! Annikaand James H. G. Bullock Peter! and James Bullock! Center for Cosmology, Department of PhysicsCenter and for Cosmology,Astronomy, Department University of of Physics California, and Astronomy, Irvine, CA University 92697-4575, of California, USA Irvine, CA 92697-4575, USA

Accepted 2012 May 31. Received 2012 MayAccepted 30; in 2012 original May 31. form Received 2011 October 2012 May 2 30; in original form 2011 October 2

ABSTRACT ABSTRACT We analyse subhaloes in the ViaLacteaII (VL2) cosmological simulation to look for correla- We analyse subhaloes in the ViaLacteaIItions among their (VL2) infall cosmological times and z simulation0dynamicalproperties.Wefindthatthepresent-day to look for correla-

= Downloaded from tions among their infall timesorbital and z energy0dynamicalproperties.Wefindthatthepresent-day is tightly correlated with the time at which subhaloes last entered within the

= Downloaded from orbital energy is tightly correlatedvirial radius. with the This time energy–infall at which correlation subhaloes provides last entered a means within to infer the infall times for Milky virial radius. This energy–infallWay correlation satellite galaxies. provides Assuming a means that to the infer Milky infall Way’s times assembly for Milky can be modelled by VL2, Way satellite galaxies. Assumingwe show that that the the Milky infall times Way’s of assembly some satellites can are be well modelled constrained by VL2, given only their Galacto- we show that the infall timescentric of some positions satellites and are line-of-sight well constrained velocities. given The constraints only their sharpen Galacto- for satellites with proper http://mnras.oxfordjournals.org/ centric positions and line-of-sightmotion velocities. measurements. The constraints We find that sharpen Carina, Ursa for satellites Minor and with Sculptor proper were all accreted early,

more than 8 Gyr ago. Five other dwarfs, including Sextans and Segue 1, are alsohttp://mnras.oxfordjournals.org/ probable motion measurements. We findearly that accreters, Carina, though Ursa Minor with larger and uncertainties. Sculptor were On all the accreted other extreme, early, Leo T is just falling more than 8 Gyr ago. Five otherinto the dwarfs, Milky Way including for the first Sextans time while and Segue Leo I fell 1, in are2Gyragoandisnowclimbingoutof also probable ∼ early accreters, though with largerthe Milky uncertainties. Way’s potential On after the itsother first extreme, perigalacticon. Leo T The is energiesjust falling of several other dwarfs, into the Milky Way for the firstincluding time while Fornax Leo and I fell Hercules, in 2Gyragoandisnowclimbingoutof point to intermediate infall times, 2–8 Gyr ago. We compare ∼ the Milky Way’s potential afterour its infall first time perigalacticon. estimates to published The energies star formation of several histories other and dwarfs, find hints of a dichotomy including Fornax and Hercules,between point ultrafaint to intermediate and classical infall dwarfs. times, The 2–8 classical Gyr ago. dwarfs We compareappear to have quenched star our infall time estimates to publishedformation after star infallformation but the histories ultrafaint and dwarfs find tend hints to be of quenched a dichotomy long before infall, at least at University of Washington on July 21, 2013 for the cases in which our uncertainties allow us to discern differences. Our analysis suggests between ultrafaint and classical dwarfs. The classical dwarfs appear to have quenched star that the Large Magellanic Cloud crossed inside the Milky Way virial radius recently, within formation after infall but thethe ultrafaint last 4 dwarfs billion years. tend to be quenched long before infall, at least at University of Washington on July 21, 2013 for the cases in which our uncertainties∼ allow us to discern differences. Our analysis suggests that the Large Magellanic CloudKey crossedwords: insidemethods: the numerical Milky Way – galaxies: virial radius evolution recently, – galaxies: within formation – galaxies: haloes – dark matter. the last 4 billion years. ∼ Key words: methods: numerical – galaxies: evolution – galaxies: formation – galaxies: haloes – dark matter. (see e.g. Berrier et al. 2009). Once inside the virial radius of a larger 1INTRODUCTION host, star formation in the satellites may be quenched either because The Milky Way (MW) is a unique laboratory for understanding the they stop accreting fresh gas (‘strangulation’; Larson, Tinsley & 8 lives of dwarf galaxies (L ! 10 M ). Dwarf spheroidal galaxies, Caldwell 1980; Bekki, Couch & Shioya 2002) or because their cool in particular, stand out among galaxies" because of their high dark gas is stripped away (‘ram-pressure stripping’; Gunn & Gott 1972) (see e.g. Berrier et al. 2009). Once inside the virial radius of a larger 1INTRODUCTION matter content, lack of gas and lack of recent star formation. Like due to interactions with the host’s gas halo. High-speed encounters larger galaxies (Dressler 1980; Butcher &host, Oemler star 1984; formation Goto et in al. the satelliteswith other may satellite be quenched galaxies either or the because host itself may similarly affect The Milky Way (MW) is a unique laboratory2003), dwarf for galaxies understanding appear to the have a ‘morphology–density’they stop accreting rela- fresh gasmorphologies (‘strangulation’; and star formation Larson, Tinsley (‘harassment’; & Moore et al. 1996). 8 lives of dwarf galaxies (L ! 10 M tion,). Dwarf with dwarf spheroidal spheroidal galaxies, galaxies preferentiallyCaldwell crowding 1980; Bekki, around CouchThese & Shioya processes 2002) are or also because relevant their for cool dwarf galaxies around the in particular, stand out among galaxies"normal because galaxies of (or their within high groups) dark instead ofgas the is field stripped (Mateo away 1998a; (‘ram-pressureMW. The stripping’; MW is likely Gunn surrounded & Gott by 1972) a hot gas halo of its own, matter content, lack of gas and lackWeisz of recent et al. star 2011). formation. All of the Like galaxies withindue the to MW’s interactions dark matter with the host’swhich can gas aid halo. in quenching High-speed star encountersformation once galaxies fall within larger galaxies (Dressler 1980; Butcherhalo & except Oemler the 1984; two Magellanic Goto et al. Clouds arewith dwarf other spheroidals. satellite galaxiesits or reach the host (Maller itself & may Bullock similarly 2004; Fang affect et al. 2006; Peek et al. 2003), dwarf galaxies appear to have aIn ‘morphology–density’ galaxy groups and clusters, rela- the morphology–densitymorphologies and relation star formation2007; (‘harassment’; Kaufmann et al. Moore 2008; Grcevich et al. 1996). & Putman 2009, 2010). In is associated with a colour–density relation or a star-formation– addition, dwarfs can experience dynamical transformation due to tion, with dwarf spheroidal galaxies preferentiallydensity relation crowding (Kauffmann around et al. 2004; BlantonThese processes et al. 2005). are The also relevanttidal stirring for ( dwarfŁokas et galaxies al. 2010; Kazantzidisaround the et al. 2011), though the normal galaxies (or within groups) insteadorigin of of the these field relations (Mateo is 1998a; thought to resultMW. from The the MW quenching is likely surroundedsmallest galaxies by a hot may gas very halo well of be its born own, puffy well before entering Weisz et al. 2011). All of the galaxiesof within star formation the MW’s once dark galaxies matter become satelliteswhich can in larger aid in systems quenching starthe halo formation (Kaufmann, once Wheeler galaxies & fall Bullock within 2007). halo except the two Magellanic Clouds are dwarf spheroidals. its reach (Maller & Bullock 2004;One way Fang in which et al. dwarf 2006; satellites Peek are et different al. than larger satel- In galaxy groups and clusters, the morphology–density relation 2007; Kaufmann et al. 2008;lite Grcevich galaxies in & groups Putman and 2009, clusters 2010). is that In their low mass makes is associated with a colour–density! relationE-mail: [email protected] a star-formation– (MR); [email protected], dwarfs (AHGP); can experiencethem inherently dynamical fragile, transformation thus more susceptible due to to quenching pro- [email protected] (JB) cesses that would not otherwise affect L galaxies. Specifically, it density relation (Kauffmann et al. 2004; Blanton et al. 2005). The tidal stirring (Łokas et al. 2010; Kazantzidis et al. 2011), though∼ the∗ origin of these relations is thought toC result from the quenching smallest galaxies may very well be born puffy well before entering $ 2012 The Authors C of star formation once galaxies becomeMonthly satellites Notices in of larger the Royal systems Astronomical Societythe halo$ 2012 (Kaufmann, RAS Wheeler & Bullock 2007). One way in which dwarf satellites are different than larger satel- lite galaxies in groups and clusters is that their low mass makes !E-mail: [email protected] (MR); [email protected] (AHGP); them inherently fragile, thus more susceptible to quenching pro- [email protected] (JB) cesses that would not otherwise affect L galaxies. Specifically, it ∼ ∗

C $ 2012 The Authors C Monthly Notices of the Royal Astronomical Society $ 2012 RAS The Astrophysical Journal Letters,753:L21(5pp),2012July1 doi:10.1088/2041-8205/753/1/L21 C 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A. !

THE PRIMEVAL POPULATIONS OF THE ULTRA-FAINT DWARF GALAXIES∗ Thomas M. Brown1,JasonTumlinson1,MarlaGeha2,EvanN.Kirby3,9,DonA.VandenBerg4, Ricardo R. Munoz˜ 5,JasonS.Kalirai1,JoshuaD.Simon6,RobertoJ.Avila1, Puragra Guhathakurta7,AlvioRenzini8,andHenryC.Ferguson1 1 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA; [email protected], [email protected], [email protected], [email protected], [email protected] 2 Astronomy Department, Yale University, New Haven, CT 06520, USA; [email protected] 3 Department of Astronomy, California Institute of Technology, 1200 East California Boulevard, MC 249-17, Pasadena, CA 91125, USA; [email protected] 4 Department of Physics and Astronomy, University of Victoria, P. O. Box 3055, Victoria, BC, V8W 3P6, Canada; [email protected] 5 Departamento de Astronom´ıa, Universidad de Chile, Casilla 36-D, Santiago, Chile; [email protected] 6 Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA; [email protected] 7 UCO/Lick Observatory and Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA; [email protected] 8 Osservatorio Astronomico, Vicolo Dell’Osservatorio 5, I-35122 Padova, Italy; [email protected] Received 2012 April 20; accepted 2012 June 4; published 2012 June 15

ABSTRACT We present new constraints on the star formation histories of the ultra-faint dwarf (UFD) galaxies, using deep photometry obtained with the Hubble Space Telescope (HST).AgalaxyclassrecentlydiscoveredintheSloan Digital Sky Survey, the UFDs appear to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in efforts to understand the missing satellite problem. They are the least luminous, most dark- matter-dominated, and least chemically evolved galaxies known. Our HST survey of six UFDs seeks to determine if these galaxies are true fossils from the early universe. We present here the preliminary analysis of three UFD galaxies: Hercules, Leo IV, and Ursa Major I. Classical dwarf spheroidals of the Local Group exhibit extended star formation histories, but these three Milky Way satellites are at least as old as the ancient M92, with no evidence for intermediate-age populations. Their ages also appear to be synchronized to within 1Gyrofeach other, as might be expected if their star formation was truncated by a global event, such as reionization.∼ Key words: galaxies: dwarf – galaxies: evolution – galaxies: formation – galaxies: photometry – galaxies: stellar content – Local Group

1. INTRODUCTION collapse for both classical dSphs and UFDs (z 12; Strigari et al. 2008), but the dSphs apparently continued to∼ evolve (Orban Although the lambda cold dark matter paradigm is consistent et al. 2008;Weiszetal.2011). In contrast, the UFDs are the least with many observable phenomena, discrepancies arise at small chemically evolved galaxies known, with abundance patterns scales. One of the most prominent issues is that it predicts that imply their star formation was brief (Frebel et al. 2010)and many more dark-matter halos than are actually seen as dwarf individual stellar metallicities as low as [Fe/H] 3.7(Norris galaxies (e.g., Moore et al. 1999). A possible solution has arisen et al. 2010). The strict conformance to a metallicity–luminosity= − with the recent discovery of additional satellites around the relation for all Milky Way satellites limits the amount of tidal Milky Way (e.g., Willman et al. 2005;Zuckeretal.2006; stripping to a factor of 3 in stellar mass (Kirby et al. 2011). Belokurov et al. 2007)andAndromeda(e.g.,Zuckeretal.2007) Therefore, UFDs are not∼ tidally stripped versions of classical in the (York et al. 2000)andother dSphs (see also Penarrubia˜ et al. 2008;Norrisetal.2010). wide-field surveys (e.g., McConnachie et al. 2009). As one way of solving the missing satellite problem, The newly discovered ultra-faint dwarf (UFD) galaxies ap- galaxy formation simulations assume that UFDs formed the pear to be an extension of the classical dwarf spheroidals (dSphs) bulk of their stars prior to the epoch of reionization (e.g., to lower luminosities (MV ! 8 mag). UFD luminosities are Tumlinson 2010;Munoz˜ et al. 2009;Bovill&Ricotti2009; comparable to those of globular− clusters, but one distinction Koposov et al. 2009). Mechanisms that could drive an early in the former is the presence of dark matter. Even massive termination of star formation include reionization, gas de- globular clusters have mass-to-light ratios (M/LV )of 2(e.g., pletion, and supernova feedback. Using the Hubble Space Baumgardt et al. 2009; van de Ven et al. 2006), precluding∼ Telescope (HST),weareundertakingadeepimagingsurveyof significant dark matter. In contrast, all known dwarf galax- UFDs that reaches the old main sequence (MS) in each galaxy, ies have higher M/LV (Kalirai et al. 2010 and references yielding high-precision color–magnitude diagrams (CMDs) that therein). UFD kinematics are clearly dark-matter-dominated, provide sensitive probes of their star formation histories. The with M/LV > 100 (e.g., Kleyna et al. 2005; Simon & Geha program includes Hercules, Leo IV, Ursa Major I, Bootes I, 2007;Munoz˜ et al. 2006), even where velocity dispersions have Coma Berenices, and Canes Venatici II. Here, we give prelimi- been revised downward (e.g., Koposov et al. 2011). The inferred nary results for the first three galaxies. dark-matter densities of dwarf galaxies suggest a high-redshift 2. OBSERVATIONS AND DATA REDUCTION

∗ Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at STScI, which is operated by AURA, Inc., under NASA contract We obtained deep optical images of each galaxy (Table 1) NAS 5-26555. using the F606W and F814W filters on the Advanced Cam- 9 Hubble Fellow. era for Surveys (ACS). These filters efficiently enable a high

1 My SFH wishlist

Improved evolved star models (RGB, HB, AGB, etc.)

oMSTO deep HST imaging of M31 Satellites and more distant LG dwarfs (WLM, NGC 6822, etc.) oMSTO deep, wide field imaging of dwarfs in and near the LG

Modelers: Prediction for SFHs; we can test them!