Where are the Fossils of the First Galaxies? II. True Fossils, Ghost Halos, and the Missing Bright Satellites

We use a new set of cold dark matter simulations of the local universe to investigate the distribution of fossils of primordial dwarf galaxies within, and around the Milky Way. Throughout, we build upon previous results showing agreement between the observed stellar properties of a subset of the ultra-faint dwarfs and our simulated fossils. Here, we show that fossils of the first galaxies have galactocentric distributions and cumulative luminosity functions consistent with observations. In our model there are ~ 300 luminous satellites orbiting the Milky Way, ~50-70% of which are well preserved fossils, with this fraction decreasing with galactocentric distance. Within the Milky Way virial radius, the majority of these fossils have luminosities L_V<10^5 L_solar. This work produces an overabundance of bright dwarf satellites (L_V > 10^4 L_solar) with respect to observations where observations are nearly complete. The "bright satellite problem" is most evident in the outer parts of the Milky Way. We estimate that, although relatively bright, the primordial stellar populations are very diffuse, producing a population with surface brightnesses below surveys` detection limits and are easily stripped by tidal forces. Although we cannot yet present unmistakable evidence for the existence of the fossils of first galaxies in the Local Group, the results of our studies suggest observational strategies that may demonstrate their existence. Primarily, the detection of "ghost halos" of primordial stars around isolated dwarfs would prove that stars formed in minihalos (M<10^8 M_solar) before reionization, and strongly suggest that at least a fraction of the ultra-faint dwarfs are fossils of the first galaxies.Comment: publishing in ApJ with minor revisions in October 2011 V. 741 article ID. 1

Where are the Fossils of the First Galaxies? I. Local Volume Maps and Properties of the Undetected Dwarfs

We present a new method for generating initial conditions for LCDM N-body simulations which provides the dynamical range necessary to follow the evolution and distribution of the fossils of the first galaxies on Local Volume, 5-10 Mpc, scales. The initial distribution of particles represents the position, velocity and mass distribution of the dark and luminous halos extracted from pre-reionization simulations. We confirm previous results that ultra-faint dwarfs have properties compatible with being well preserved fossils of the first galaxies. However, because the brightest pre-reionization dwarfs form preferentially in biased regions, they most likely merge into non-fossil halos with circular velocities >20-30 km/s. Hence, we find that the maximum luminosity of true-fossils in the Milky Way is L_V<10^5 L_solar, casting doubts on the interpretation that some classical dSphs are true-fossils. In addition, we argue that most ultra-faints at small galactocentric distance, R<50 kpc, had their stellar properties modified by tides, while a large population of fossils is still undetected due to their extremely low surface brightness log(Sigma_V) < -1.4. We estimate that the region outside R_50 (~ 400 kpc) up to 1 Mpc from the Milky Way contains about a hundred true fossils of the first galaxies with V-band luminosities 10^3 - 10^5 L_solar and half-light radii, r_hl ~ 100-1000 pc.Comment: published in ApJ October 2011 with minor revisions V. 741 article ID. 1

Pre-reionization Fossils, Ultra-faint Dwarfs and the Missing Galactic Satellite Problem

We argue that, at least a fraction of the newly discovered population of ultra-faint dwarf spheroidal galaxies in the Local Group constitute the fossil relic of a once ubiquitous population of dwarf galaxies formed before reionization with circular velocities smaller than $v_{c}^{cr} \sim 20$ km/s. We present several arguments in support of this model. The number of luminous Milky Way satellites inferred from observations is larger than the estimated number of dark halos in the Galaxy that have, or had in the past, circular velocity $>v_{c}^{cr}$, as predicted by the "Via Lactea" simulation. This implies that some ultra-faint dwarfs are fossils. However, this argument is weakened by recent results from the "Aquarius" simulations showing that the number of Galactic dark matter satellites is 2.5 larger than previously believed. Secondly, the existence of a population of ultra-faint dwarfs was predicted by cosmological simulations in which star formation in the first minihalos is reduced -- but not suppressed -- by radiative feedback. Here, we show the statistical properties of the fossil galaxies in those simulations are consistent with observations of the new dwarf population and with the number and radial distribution of Milky Way satellites as a function of their luminosity. Finally, the observed Galactocentric distribution of dwarfs is consistent with a fraction of dSphs being fossils. To make our case more compelling, future work should determine whether stellar chemical abundances of simulated "fossils" can reproduce observations and whether the tidal scenarios for the formation of Local Group dwarf spheroidals are equally consistent with all available observations.Comment: ApJ accepted version, in press, 8 figure, 3 table

A Pathfinder for JWST Spectroscopy: Deep High Spectral Resolution Maps of Galaxies over 1

International audienceWe propose deep, spatially resolved NIRSpec MSA near-IR spectroscopy of ~40 galaxies at redshifts

A Pathfinder for JWST Spectroscopy: Deep High Spectral Resolution Maps of Galaxies over 1

International audienceWe propose deep, spatially resolved NIRSpec MSA near-IR spectroscopy of ~40 galaxies at redshifts