Mem. S.A.It. Vol. 85, 487c© SAIt 2014 Memorie della

The living fossils of the �rst galaxies

M. Ricotti1,2, O. Parry1, E. Polisensky1,3, and M. Bovill4

1 University of Maryland, Department of Astronomy, College Park, 20742 MD, USAe-mail: ricotti@astro.umd.edu

2 Sorbonne Universite, Institut Lagrange de Paris (ILP), 98 bis Bouldevard Arago 75014Paris, France

3 Naval Research Laboratory, Washington, D.C. 20375, USA4 Pontificia Universidad Catolica de Chile, Avda. Libertador Bernardo O’Higgins 340,

Santiago,Chile

Abstract. I will review ideas on the cosmological origin of dwarf galaxies in the LocalGroup focusing on the exciting possibility that some ultra-faint dwarfs are well preservedfossils of the first population of dwarf galaxies formed before reionization.

Key words. Dark ages, reionization, first stars – Stars: Population III – Cosmology: theory– Local Group – Galaxies:dwarf

1. Introduction

In cold dark matter (CDM) cosmologies thenumber density of small mass halos per loga-rithmic mass bin is roughly inversely propor-tional to the halo mass. Is therefore impor-tant to determine what is mass of the small-est dark matter halos hosting luminous galaxiesto answer questions on the number density ofgalaxies in the Local Group, the nature of thesources of ionizing radiation and metals in theearly universe. Currently, the answer to thesequestions is uncertain and is the subject of on-going research. Contrary to the formation ofthe first star in the universe, that is a relativelysimple and well posed initial conditions prob-lem, the formation of the first galaxies is com-plex because sensitive to feedback effects andthe uncertain initial mass function (IMF) of thefirst stars.

As always, progresses can be made onlyif we can constrain our models with observa-

tions. HST has detected a few redshift ten can-didates, but the bulk of the first population ofdwarf galaxies is still undetected. Even withJWST we may not be able to probe the bulk ofthis population if, as simulations seem to sug-gest, is intrinsically faint. However, a promis-ing avenue to constrain models of the firstgalaxies is emerging. It consists in using nearfield observations of the faintest dwarfs in theLocal Group to identify the surviving fossilsof the first galaxies (Ricotti & Gnedin 2005).The discovery of a population of ultra-faintdwarfs satellites of the Milky Way (Zuckeret al. 2006; Belokurov et al. 2007; Majewskiet al. 2007) with properties consistent with pre-dictions of simulations of the fossils of the firstgalaxies(Bovill & Ricotti 2009; Salvadori &Ferrara 2009; Ricotti 2009, 2010), is an excit-ing observational development warranting fur-ther studies. Destruction and tidal transforma-tion of the Milky Way satellites and the identi-fication of the masses at formation of the ultra-

488 Ricotti: The living fossils of the first galaxies

Fig. 1. Portrait of one of the most massive galaxies(∼ 108 M�) at z = 10 in 1 Mpc3 volume in the sim-ulations by Parry et al. (2014), in preparation. Thepanels show the gas density and temperature as in-dicated by the labels.

faint dwarfs remain open questions, althoughthe age and metallicities of the stellar popula-tions of the ultra-faints is consistent with theiridentification as fossils (Brown et al. 2012).

In this presentation I will cover three re-lated topics. First, I summarize the main quali-tative results from simulations of the formationof the first galaxies at z > 10 (§ 2). I will thendiscuss issues related to their subsequent evo-lution to z = 0 and the consistency of the mod-els with observations of the Milky Way satel-lites (§ 3). Finally I will focus on the most mas-sive satellites of the Milky Way (§ 4) and con-clude with a summary (§ 5).

2. New simulations of the firstgalaxies

The first simulations of the the formation ofthe first galaxies in a cosmological volume in-cluding 3D radiation transfer were publishedmore than a decade ago (Ricotti et al. 2001,2002a,b, 2008). Since then, many progresseshave been made on understanding the forma-

tion of the first stars and Population II star for-mation in molecular clouds (e.g., Wise et al.2012; Muratov et al. 2013, Parry et al. 2014, inpreparation). Today’s AMR simulations havesufficient resolution to resolve the multi-phaseISM and molecular clouds in dwarf galaxies(see Figure 1). However, the main qualitativeresults found in early works have been con-firmed by modern adaptive mesh refinement(AMR) simulations. Here is a concise sum-mary of the main qualitative results:

1. Star formation on cosmological volumescales is self-regulated by feedback loopsand is nearly independent of the star for-mation efficiency assumed in molecularclouds. This is illustrated in Figure 2.

2. Near a mass threshold of 107 M� a signif-icant fraction of dark matter halos remaindark and the scatter of the M/L ratio atthese masses is very large.

3. Primordial dwarf galaxies have stars dis-tributed in spheroids with half-light radii100 − 500 pc nearly independently of theirluminosity (see Figure 3).

4. There are about 10-100 luminous dwarfgalaxies per Mpc3 at z = 10, but is notyet clear whether simulations have con-verged. Population III star formation is stillimplemented using a sub-grid recipe, but

Fig. 2. Global star formation rate (SFR) (solidlines) and mass in stars (dashed lines) as a func-tion of redshift in three simulations in which thesub-grid parameter that determines the star forma-tion efficiency in molecular clouds was varied by afactor of 10 above and below a fiducial value. TheSFR is self-regulated on a global scale.

Ricotti: The living fossils of the first galaxies 489

the gravitational potentials at the center ofminihalos of mass 105 − 106 M� are notresolved with a sufficient number of darkmatter particles to allow the collapse ofPopulation III stars that seed the subse-quent Population II star formation.

3. Connection to near fieldcosmology

The population of first galaxies at z = 10has different properties from the subset of sur-viving fossils at z = 0. In Bovill & Ricotti(2011a,b) we presented a method for generat-ing initial conditions for LCDM N-body sim-ulations which provide the dynamical rangenecessary to follow the evolution and distri-bution of the fossils of the first galaxies onLocal Volume scales (5-10 Mpc) We show thathe stellar properties of most of the ultra-faintdwarfs and classical dSph are consistent withthose expected for the fossils and predict theexistence of a yet undetected population of ex-tremely low surface brightness dwarfs whichfall below the SDSS detection limit. Figure 4,taken from Bovill & Ricotti (2011a), showsa comparison of the properties of simulatedfossils (shaded areas) and dwarfs galaxies inthe Local Group. The asterisks are non-fossils(dIrr), crosses are polluted fossils (dE andsome dSph), the filled circles and triangles arethe classical dSphs in the Milky Way and M31respectively, and the opened circles and trian-gles are the ultra-faint populations. We colorthe observed dwarfs whose half-light radii areare inconsistent with our simulations in green.The magenta contours show the undetectablefossils with ΣV below the detection limit ofthe SDSS (Koposov et al. 2008). In both pan-els, the solid black lines show the surfacebrightness limit of the Sloan and the dashedblack lines show the trends from Kormendy &Freeman (2004) for luminous Sc-Im galaxies.A summary of the main properties of the fos-sils of the first galaxies at z = 0 is as follows:

1. Surviving fossils are anti-biased at z = 10and tend to be underluminous for a givenhalo mass: most classical dwarfs leave inhalos with vmax > 20 km/s, thus reioniza-

Fig. 3. Half-light radii as a function of luminosityof the stellar spheroids of the first galaxies at z = 10in the simulations by Parry et al. (2014), in prepara-tion. The triangles refer to the radial extent of Pop IIIstars only and the squares to Pop II stars only. Theresult agrees with previous lower-resolution simula-tions (Ricotti et al. 2002a; Ricotti & Gnedin 2005).

tion does not have a strong effect on theirstar formation history.

2. The observed population of dwarfs withrh < 100 has properties incompatible withsimulated fossils. Their properties are mostlikely shaped by tides.

3. Models in which some of the ultra-faintdwarfs are fossils of the first galaxies agreewell with observations of the ultra-faintdwarfs but show some tension at the brightend of the satellite luminosity function(classical dwarfs and dIrr). A numerouspopulation of ultra-faint dwarfs also pro-duces an overabundance of bright dwarfsatellites especially in the outer parts of theMilky Way. However, this tension is easedby the expected large spatial extent of theold stellar population that forms a “ghosthalo” difficult to detect and easily strippedby tides around bright satellites.

4. Indeed, the existence of diffuse stellar ha-los around isolated dwarfs (Ghost halos) isanother observational test for the existenceof the fossils of the first galaxies.

490 Ricotti: The living fossils of the first galaxies

Fig. 4. Surface brightness and half light radius areplotted against V-band luminosity. The cyan con-tours show the distribution for the fossils in a simu-lation by Ricotti & Gnedin (2005) and the overlaidblack symbols show the observed dwarfs. See thetext for the explanation of symbols and lines.

4. Massive satellites of the Milky Wayin cold and warm dark matter

Recent work focusing on the brightest MilkyWay satellites has highlighted dynamical dis-crepancies with high-resolution CDM simula-tions (Boylan-Kolchin et al. 2011). The mostmassive satellites, either at the present epoch orover the complete infall history, are too denseto be dynamically consistent with the satel-lites. Observations of the stellar velocity dis-persions in the bright satellites are consistentwith dark matter halos with maximum circu-lar velocities < 25 km/s while the AquariusMilky Ways have about 10 subhalos each withvmax > 25 km/s that are also not MagellanicCloud analogues.

We have re-examined this problem, knownas the “too big to fail” (TBTF) problem inCDM and WDM cosmologies (Polisensky &Ricotti 2013). We find that the inconsistencyis largely attributable to the large values ofσ8 and ns adopted in the discrepant simula-tions (the Aquarius simulations has WMAP1cosmology), producing satellites that form tooearly and therefore are too dense. Fig. 5

shows that the tension between observationsand simulations adopting parameters consis-tent with WMAP9 is significantly diminished,making the satellites a sensitive test of CDM.Assuming NFW profiles for the satellites onlya couple of dwarfs are still inconsistent withobservations, suggesting that cored profiles arenot required by current data.

Finally, adopting a power spectrum of per-turbation with truncation at small mass scales,for instance produced by WDM, does not seemto solve the TBTF problem because the effecton the density of the satellites is significantonly when the dark matter is sufficiently warmto suppress the number of satellites below thevalue allowed by observations (Polisensky &Ricotti 2011).

5. Conclusions

Near field cosmology appears the most promis-ing avenue to study the epoch of formationof the first stars and galaxies, also in light ofthe future surveys that will allow us to probefainter and more distant ultra-faint dwarfs.From the theoretical point of view, the problemseems tractable but much progress needs to bemade in our treatment of feedback processes.We need to include the effects of the first blackholes and X-ray heating, study in greater de-tailed chemical enrichment by tracing the pro-duction of different elements, and the transportand mixing of metals in the interstellar and in-tergalactic medium.

From our studies to connect the fossils ofthe first galaxies to the satellites of the MilkyWay, a good agreement is found between thefossils and the ultra-faints, however a problempersists at the bright end of the satellite lumi-nosity function. The fossil light in the mostmassive satellites appears to exceed the obser-vations with an excess of bright satellites es-pecially in the outer parts of the Milky Way.This problem is alleviated if we hypothesizethat the fossil stars are dynamically hot pro-ducing an extended “ghost stellar halo” withsurface brightness below the detection lim-its or stripped by tides for satellites close tothe Milky Way. It is thus conceivable that themass-to-light ratio in small mass halos in not

Ricotti: The living fossils of the first galaxies 491

Fig. 5. NFW circular velocity profiles for the 10 subhalos with largest vmax at z = 0 in each CDM simulationadopting WMAP1 cosmology (top row); and Bolshoi cosmology (bottom row) after filtering MagellanicCloud analogues. Subhalos denser than any observed dwarf (points with error bars) are plotted in bold.Subhalos that are neither among the 10 with largest vin f all or 10 largest vmax at z = 9 are not expected to hosta bright dwarf and are plotted with dotted lines. Note that NFW profiles for the 10 subhalos with largest vmaxover their infall history select a few subhalos with lower values of vmax and Rmax than shown here, furtheralleviating the discrepancy with observations.

a monotonic function of the halo mass, andthus some of the classical dwarf satellites of theMilky Way may not reside in the most massivesubhalos. This conjecture would also solve theTBTF problem noted by Boylan-Kolchin et al.(2011). However, Polisensky & Ricotti (2013)showed that the TBTF problem is very sensi-tive to cosmological parameters, in particularto ns and σ8 that determine the power at smallmass scales and thus the redshift of virializa-tion and density of the Milky Way satellites.Adopting the most recent cosmological param-eters the TBTF problem is not severe, even as-suming NFW profiles for the satellites.

Acknowledgements. MR’s research is sup-ported by NASA grant NNX10AH10G and NSF

CMMI1125285. This work made in the ILPLABEX (under reference ANR-10-LABX-63) wassupported by French state funds managed by theANR within the Investissements d’Avenir pro-gramme under reference ANR-11-IDEX-0004-02.

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