The Fossils of the First Galaxies in the Local Universe

We argue that, at least a fraction of the newly discovered population of ultra-faint dwarfs in the Local Group constitute the fossil relics of a once ubiquitous population of dwarf galaxies formed before reionization with maximum circular velocities, vmax &#8722;1, where vmax = M1/3. To follow the evolution and distribution of the fossils of the first galaxies on Local Volume, 5 &#8722; 10 Mpc, scales, we have developed a new method for generating initial conditions for &#923;CDM N-body simulations which provides the necessary dynamic range. The initial distribution of particles represents the position, velocity and mass distribution of the dark and luminous ha- los extracted from pre-reionization simulations. We find that ultra-faint dwarfs have properties compatible with well preserved fossils of the first galaxies and are able to reproduce the observed luminosity-metallicity relation. However, because the brightest pre-reionization dwarfs form preferentially in overdense regions, they have merged into non-fossil halos with vmax > 20&#8722;30 km s&#8722;1. Hence, we find a luminosity threshold of true-fossils of 6 Lsolar, casting doubts on the classification of some clas- sical dSphs as fossils. We also argue that the ultra-faints at R V ) < &#8722;1.4. Next, we show that fossils of the first galaxies have galactocentric distributions and cumulative luminosity func- tions consistent with observations. We predict there are &#8764; 300 luminous satellites orbiting within Rvir of the Milky Way, &#8764; 50 &#8722; 70% of which are fossils. Despite our multidimensional agreement at low luminosities, our primordial model produces an overabundance of bright dwarf satellites (LV > 105 Lsolar), with this "bright satellite problem" most evident in the outer parts of the Milky Way. We estimate that, although relatively bright (LV > 105 Lsolar), these ghostly primordial populations are very diffuse, producing primordial populations with surface brightnesses below survey detection limits. Although we cannot yet present unmistakable evidence for the existence of the fossils of first galaxies in the Local Group, we suggest observational strategies to prove their existence. (i) The detection of "ghost halos" of primordial stars around isolated dwarfs. (ii) The existence of a yet unknown popu- lation of &#8764; 150 Milky Way ultra-faints with half-light radii rhl &#8776; 100 &#8722; 1000 pc and luminosities LV 4 Lsolar, detectable by future deep surveys

Devouring the Milky Way Satellites: Modeling Dwarf Galaxies with Galacticus

Dwarf galaxies are ubiquitous throughout the universe and are extremely sensitive to various forms of internal and external feedback. Over the last two decades, the census of dwarf galaxies in the Local Group and beyond has increased markedly. While hydrodynamic simulations (e.g., FIRE II, Mint Justice League) have reproduced the observed dwarf properties down to the ultrafaints, such simulations require extensive computational resources to run. In this work, we constrain the standard physical implementations in the semianalytic model Galacticus to reproduce the observed properties of the Milky Way satellites down to the ultrafaint dwarfs found in the Sloan Digital Sky Survey. We run Galacticus on merger trees from our high-resolution N-body simulation of a Milky Way analog. We determine the best-fit parameters by matching the cumulative luminosity function and luminosity-metallicity relation from both observations and hydrodynamic simulations. With the correct parameters, the standard physics in Galacticus can reproduce the observed luminosity function and luminosity-metallicity relation of the Milky Way dwarfs. In addition, we find a multidimensional match with half-light radii, velocity dispersions, and mass to light ratios at z = 0 down to M V ≤ −6 (L ≥ 104 L ⊙). In addition to successfully reproducing the properties of the z = 0 Milky Way satellite population, our modeled dwarfs have star formation histories that are consistent with those of the Local Group dwarfs

Kindling the First Stars II: Dependence of the Predicted PISN Rate on the Pop III Initial Mass Function

Population III (Pop III) stars formed out of metal free gas in minihalos at $z>20$. While their ignition ended the Dark Ages and begin enrichment of the IGM, their mass distribution remains unconstrained. To date, no confirmed Pop III star has been observed and their direct detection is beyond the reach of the James Webb Space Telescope (JWST) without gravitational lensing. However, a subset of massive Pop III stars end their lives in pair instability supernova (PISN). With typical energies of $\sim10^{53}$~erg, PISN light curve peaks are bright enough to be detectable by JWST and the Roman Space Telescope. The fundamental question of this work is whether or not observed PISN can be used as a diagnostic of the Pop III IMF. In this work, we use a model of the formation of the first stars to determine the dependence of PISN rates at $z~>~5$ for a range of Pop III power law IMFs ($\alpha~=~0.2~-~2.35$) and, critically, the method by which the IMF is populated. At $z~>~15$, we predict typical rates of $10^{-2}~-~10^2$ per deg$^{2}$ per year which will produce $10^{-3}~-~0.1$/year in a single NIRCam pointing and $0.003~-~30$/year in a single Roman pointing with $0.1~-~1000$ per year detected in the HLTDS. Our work highlights that theoretical modeling of PISN rates is required if upcoming PISN studies with JWST and Roman are going to constrain the Pop III IMF.Comment: 8 pages, 5 figures, submitted to the Open Journal of Astrophysic

A comprehensive model for the formation and evolution of the faintest Milky Way dwarf satellites

In this study, we modify the semi-analytic model Galacticus in order to accurately reproduce the observed properties of dwarf galaxies in the Milky Way. We find that reproducing observational determinations of the halo occupation fraction and mass-metallicity relation for dwarf galaxies requires us to include H$_2$ cooling, an updated UV background radiation model, and to introduce a model for the metal content of the intergalactic medium. By fine-tuning various model parameters and incorporating empirical constraints, we have tailored the model to match the statistical properties of Milky Way dwarf galaxies, such as their luminosity function and size$-$mass relation. We have validated our modified semi-analytic framework by undertaking a comparative analysis of the resulting galaxy-halo connection. We predict a total of $300 ^{+75} _{-99}$ satellites with an absolute $V$-band magnitude (M$_{V}$) less than $0$ within $300$ kpc from our Milky Way-analogs. The fraction of subhalos that host a galaxy at least this bright drops to $50\%$ by a halo peak mass of $\sim 8.9 \times 10^{7}$ M$_{\odot}$, consistent with the occupation fraction inferred from the latest observations of Milky Way satellite population.Comment: 20 pages, 11 figures. Accepted for publication in MNRA

The Survey of Centaurus A's Baryonic Structures (SCABS) – II. The extended globular cluster system of NGC 5128 and its nearby environment

International audienceNew wide-field u^΄g^΄r^΄i^΄z^΄Dark Energy Camera observations centred on the nearby giant elliptical galaxy NGC 5128 covering ∼21 deg^2 are used to compile a new catalogue of ∼3200 globular clusters (GCs). We report 2404 newly identified candidates, including the vast majority within ∼140 kpc of NGC 5128. We find evidence for a transition at a galactocentric radius of R_gc ≈ 55 kpc from GCs ‘intrinsic’ to NGC 5128 to those likely to have been accreted from dwarf galaxies or that may transition to the intragroup medium of the Centaurus A galaxy group. We fit power-law surface number density profiles of the form $$\Sigma _{N, R_{\rm gc}}\propto R_{\rm gc}^\Gamma$$ and find that inside the transition radius, the red GCs are more centrally concentrated than the blue, with Γ_inner, red ≈ −1.78 and Γ_inner, blue ≈ −1.40, respectively. Outside this region both profiles flatten, more dramatically for the red GCs (Γ_outer, red ≈ −0.33) compared to the blue (Γ_outer, blue ≈ −0.61), although the former is more likely to suffer contamination by background sources. The median (g^΄ − z^΄)_0 = 1.27 mag colour of the inner red population is consistent with arising from the amalgamation of two giant galaxies each less luminous than present-day NGC 5128. Both inwards and outwards of the transition radius, we find the fraction of blue GCs to dominate over the red GCs, indicating a lively history of minor mergers. Assuming the blue GCs to originate primarily in dwarf galaxies, we model the population required to explain them, while remaining consistent with NGC 5128’s present-day spheroid luminosity. We find that several dozen dwarfs of luminosities Ldw, V ≃ 10^6–9.3 LV, ⊙, following a Schechter luminosity function with a faint-end slope of −1.50 ≲ α ≲ −1.25 is favoured, many of which may have already been disrupted in NGC 5128’s tidal field