The Fossils of the First Galaxies in the Local Universe
Abstract
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, v<sub>max<\sub> < 20 km s<super>−1<\super>, where v<sub>max<\sub> = M<super>1/3</super>. To follow the evolution and distribution of the fossils of the first galaxies on Local Volume, 5 − 10 Mpc, scales, we have developed a new method for generating initial conditions for ΛCDM 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 v<sub>max<\sub> > 20−30 km s<super>−1<\super>. Hence, we find a luminosity threshold of true-fossils of < 10<super>6</super> L<sub>solar<\sub>, casting doubts on the classification of some clas- sical dSphs as fossils. We also argue that the ultra-faints at R < 50 kpc, have had their stellar properties significantly modified by tides, and that a large population of fossils remains undetected due to log(Sigma<sub>V<\sub> ) < −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 ∼ 300 luminous satellites
orbiting within R<sub>vir<\sub> of the Milky Way, ∼ 50 − 70% of which are fossils. Despite our multidimensional agreement at low luminosities, our primordial model produces an overabundance of bright dwarf satellites (L<sub>V<\sub> > 10<super>5<\super> L<sub>solar<\sub>), with this "bright satellite problem" most evident in the outer parts of the Milky Way. We estimate that, although relatively bright (L<sub>V<\sub> > 10<super>5</super> L<sub>solar<\sub>), 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 ∼ 150 Milky Way ultra-faints with half-light radii r<sub>hl<\sub> ≈ 100 − 1000 pc and luminosities L<sub>V<\sub> < 10<super>4<\super> L<sub>solar<\sub>, detectable by future deep surveys.