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 cr c ∼ 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, a circular velocity > v cr c , 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.

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, we predict ∼ 300 luminous satellites orbiting the Milky Way, 50 − 70% of which are well preserved fossils. Within the Milky Way virial radius, the majority of these fossils have luminosities L V < 10 5 L ⊙ . Despite our multidimensional agreement with observations at low masses and luminosities the primordial model produces an overabundance of bright dwarf satellites (L V > 10 4 L ⊙ ) 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. i) The detection of "ghost halos" of primordial stars around isolated dwarfs would prove that stars formed in minihalos (M < 10 8 M ⊙ ) before reionization, and strongly suggest that at least a fraction of the ultrafaint dwarfs are fossils of the first galaxies. ii) The existence of a yet unknown population of ∼ 150 Milky Way ultra-faints with half-light radii r hl ≈ 100 − 1000 pc and luminosities L V < 10 4 L ⊙ , detectable by future deep surveys. These undetected dwarfs would have the mass-to-light ratios, stellar velocity dispersions and metallicities predicted in this work.

We present a new method for generating initial conditions for ΛCDM 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 −1 . Hence, we find that the maximum luminosity of true fossils in the Milky Way is L V < 10 6 L , 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(Σ 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 luminosity 10 3 -10 5 L and half-light radii, r hl ∼ 100-1000 pc.

New 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 ∼ 3 200 globular clusters (GCs). We report 2 404 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 Σ N,Rgc ∝ R Γ gc 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 in-and out-ward of the transition radius, we find the fraction of blue GCs to dominate over the red GCs, indicating a lively history of minormergers. 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 that several dozen dwarfs of luminosities L dw,V 10 6−9.3 L V, , 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.

We report the photometric properties of 16 dwarf galaxies, 15 of which are newly identified, in the Western halo of the nearby giant elliptical galaxy NGC 5128. All candidates are found at projected distances ∼ 100 − 225 kpc from their giant host, with luminosities −10.82 ≤ M V mag ≤ −7.42 and effective radii 4 ′′ r eff 17 ′′ (or 75 r eff pc 300 at the distance of NGC 5128). We compare to other low-mass dwarf galaxies in the local universe and find them to populate the faint/compact extension of the size-luminosity relation that was previously not well-sampled by dwarf galaxies in the Centaurus A system, with optical colors similar to compact stellar systems like globular clusters and ultra-compact dwarf galaxies despite having much more diffuse morphologies. From optical u ′ g ′ r ′ i ′ z ′ photometry, stellar masses are estimated to be 5.17≤logM ⋆ M ⊙ ≤ 6.48, with colors that show them to fall redward of the dwarf galaxy mass-metallicity relation. These colors suggest star formation histories that require some mechanism that would give rise to extra metal enrichment such as primordial formation within the halos of their giant galaxy hosts, non-primordial star formation from previously enriched gas, or extended periods of star formation leading to self-enrichment. We also report the existence of at least two sub-groups of dwarf candidates, each subtending 15 ′ on the sky, corresponding to projected physical separations of 10−20 kpc. True physical associations of these groups, combined with their potentially extended star formation histories, would imply that they may represent dwarf galaxy groups in the early stage of interaction upon infall into a giant elliptical galaxy halo in the very nearby universe.