PRE-REIONIZATION FOSSILS, ULTRA-FAINT DWARFS, AND THE MISSING GALACTIC SATELLITE PROBLEM

By M. Bovill, M. Ricotti. 2008

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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 vcrc ∼ 20 km s−1. 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 greater than vcrc, 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 dwarf spheroidals 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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/146531caebaaba3e2630d222bbdd4edcd4cf07cc

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Citations in the corpus

A COMPLETE SPECTROSCOPIC SURVEY OF THE MILKY WAY SATELLITE SEGUE 1: THE DARKEST GALAXY (2010, background): “These objects promise clues to the extreme limits of galaxy formation (Gilmore et al. 2007; Strigari et al. 2008a) and perhaps to the formation of the first galaxies in the early universe (e.g., Bovill & Ricotti 2009), as well as offering insights into dark matter physics.”
Accurate masses for dispersion-supported galaxies (2009, background): “…that we are seeing evidence for a new scale in galaxy formation (Strigari et al. 2008) or that there is a systematic bias that makes less luminous galaxies that sit within lowmass halos more difficult to detect than their more massive counterparts (Bovill & Ricotti 2009; Bullock et al. 2009).”
Small scale problems of the $\Lambda$CDM model: a short review (2016, background): “ause of the photo-ionisation threshold, UFDs’ baryons to stars conversion efﬁciency lies in the range 0.1%–1%, thus making it is not clear whether they are "fossils" from reionisation epoch [390]. Galaxies 2017, 5, 17 23 of 48 0.2 0.4 0.6 0.8 1.0 r[kpc] 5 10 15 20 25 30 V circ ( r ) 8 9 10”
The Evolution of the Intergalactic Medium (2015, background): “IGM pressure can raise the mass threshold above which subhalos can accrete gas and form stars (Quinn, Katz & Efstathiou 1996; Bullock, Kravtsov & Weinberg 2000; Bovill & Ricotti 2009).”
Sweating the small stuff: simulating dwarf galaxies, ultra-faint dwarf galaxies, and their own tiny satellites (2015, background): “If star formation ceased after this time, it would cause them to have uniformly ancient stellar populations (Ricotti & Gnedin 2005) with extremely low metallicity (Bovill & Ricotti 2009; Ricotti 2010).”
A BARYONIC SOLUTION TO THE MISSING SATELLITES PROBLEM (2012, background): “This process is expected to either leave them entirely devoid of stars (“dark”), or very faint if they were able to form a few stars prior to reionization (e.g., Bovill & Ricotti 2009; Salvadori & Ferrara 2009; Li et al. 2010; Wolf et al. 2010), and prone to be missed by current surveys.”
The Milky Way’s bright satellites as an apparent failure of ΛCDM (2011, background): “…therefore may have an important effect on galaxy formation in the host halos of the Milky Way’s dwarf galaxies (Bullock et al. 2000; Benson et al. 2002; Somerville 2002; Kravtsov et al. 2004b; Ricotti & Gnedin 2005; Madau et al. 2008b; Bovill & Ricotti 2009; Busha et al. 2010; Lunnan et al. 2012).”
THE SPLASH SURVEY: SPECTROSCOPY OF 15 M31 DWARF SPHEROIDAL SATELLITE GALAXIES (2011, background): “The missing satellite problem noted above is the classic example, for which a variety of solutions have surfaced (e.g, Bullock et al. 2000; Strigari et al. 2007; Tollerud et al. 2008; Bovill & Ricotti 2009; Koposov et al. 2009; Kravtsov 2010).”
THE QUENCHING OF THE ULTRA-FAINT DWARF GALAXIES IN THE REIONIZATION ERA (2014, background): “…formation in the smallest DM sub-halos QUENCHING OF THEULTRA-FAINT DWARF GALAXIES 11 (e.g., Bullock et al. 2001; Ricotti & Gnedin 2005; Muñoz et al. 2009; Bovill & Ricotti 2009, 2011a, 2011b; Tumlinson 2010; Koposov et al. 2009; Li et al. 2010; Salvadori & Ferrara 2009; Salvadori et al. 2014).”
Small-Scale Challenges to the ΛCDM Paradigm (2017, background): “…feedback likely suppresses gas accretion Mvir ≈ 109 M (Vmax & 30 km s−1; e.g., Efstathiou 1992; Bullock, Kravtsov & Weinberg 2000; Benson et al. 2002; Bovill & Ricotti 2009; Sawala et al. 2016) and the minimum mass for atomic cooling in the early Universe, Mvir ≈ 108 M (Vmax & 15 km…”
Near-Field Cosmology with Extremely Metal-Poor Stars (2015, background): “…galaxy archaeology” because entire metal-poor dwarf galaxies are now being used to study star and galaxy formation processes in the early Universe within the actual dwarf galaxy environment, just as individual stars are used for stellar archaeology (Bovill & Ricotti 2009, Frebel & Bromm 2012).”
CLUMPY STREAMS FROM CLUMPY HALOS: DETECTING MISSING SATELLITES WITH COLD STELLAR STRUCTURES (2010, background): “!) satellites with masses > 108M⊙ (Bovill & Ricotti 2009; Bullock 2010, and references therein) from those that are “missing” (the pure dark-matter subhalos).”
Formation of supermassive black holes (2010, background): “When we calculate the event rate of BH-MBH mergers in dwarf galaxies, we have to further correct for the fact that only a small fraction of these tiny satellites do indeed form stars (Bovill and Ricotti, 2009, and references therein).”