The Faintest Dwarf Galaxies

Abstract

The lowest luminosity ([Formula: see text] L[Formula: see text]) Milky Way satellite galaxies represent the extreme lower limit of the galaxy luminosity function. These ultra-faint dwarfs are the oldest, most dark matter–dominated, most metal-poor, and least chemically evolved stellar systems known. They therefore provide unique windows into the formation of the first galaxies and the behavior of dark matter on small scales. In this review, we summarize the discovery of ultra-faint dwarfs in the Sloan Digital Sky Survey in 2005 and the subsequent observational and theoretical progress in understanding their nature and origin. We describe their stellar kinematics, chemical abundance patterns, structural properties, stellar populations, orbits, and luminosity function, as well as what can be learned from each type of measurement. We conclude the following: ▪ In most cases, the stellar velocity dispersions of ultra-faint dwarfs are robust against systematic uncertainties such as binary stars and foreground contamination. ▪ The chemical abundance patterns of stars in ultra-faint dwarfs require two sources of r-process elements, one of which can likely be attributed to neutron star mergers. ▪ Even under conservative assumptions, only a small fraction of ultra-faint dwarfs may have suffered significant tidal stripping of their stellar components. ▪ Determining the properties of the faintest dwarfs out to the virial radius of the Milky Way will require very large investments of observing time with future telescopes. Finally, we offer a look forward at the observations that will be possible with future facilities as the push toward a complete census of the Local Group dwarf galaxy population continues.

Orbital Evidences for Dark-matter-free Milky Way Dwarf Spheroidal Galaxies

“…However, the most intriguing property of dSphs is the fact that most of them are found near their pericenter (Fritz et al 2018;Simon 2018Simon , 2019, for an MW mass model consistent with its accurate rotation curve (Eilers et al 2019;Mróz et al 2019), with both results coming from Gaia DR2 studies. Cautun et al (2019) have attempted to supersede the Bovy (2015) MW mass model by using a contracted DM halo, but it does not fit the most external part of the MW rotation curve and increase by only 20% the MW total mass.…”

“…Left panels: tidal shocks (or DM) acceleration (in km s −2 ) based on dSph kinematics ((σ 2 los − σ 2 stars ) × r −1 half ) versus MW distance (in kpc). Data (σ los , LV, r half ) are coming from the Table 1 of H19 (see references therein), with an update for the velocity dispersion of UMa II from Simon (2019), and for the Draco II luminosity from Longeard et al (2018). The figure compares the results from dSph data with those deduced through Eq.…”

“…• Why does it becomes so difficult to distinguish heavily DM-dominated UFDs from DM-free star clusters? As both stellar systems substantially overlap in the L V -r half plane, a significant fraction of them are not classified up to date, and this is not only due to the quality of the measurements (Simon 2019). This applies also to Crater, which has been included in the dSph sample of Fritz et al (2018, hereafter F18).…”

The Milky Way’s stellar streams and globular clusters do not align in a Vast Polar Structure

“…Particular attention has been paid to the Milky Way's plane of satellites, as it is the system with the most information and highest quality data by nature of our place as observers. As more (and fainter) satellite galaxies have been discovered in the Sloan Digital Sky Survey, the Dark Energy Survey, and smaller community efforts (Simon 2019), these new satellites continue to preferentially reside in the same spatial plane (Metz et al 2009a;Pawlowski & Kroupa 2014;Pawlowski et al 2015a). Proper motion measurements using data from Gaia Data Release 2 (DR2; Gaia Collaboration et al 2018a) show that the orbital poles for non-classical satellites also preferentially cluster in this plane (Fritz et al 2018).…”

Improved constraints from ultra-faint dwarf galaxies on primordial black holes as dark matter

“…In this paper, we investigate whether observations of ultrafaint dwarf galaxies (UFDGs) are compatible with the O(1-100) M PBH scenario. Owing to their extraordinarily high ratio between dynamical mass and stellar mass, UFDGs represent an excellent candidate to test DM models (Simon 2019). Previous studies of PBHs in UFDGs typically investigated the dynamical imprint of PBHs on the stellar population.…”