The Kinematics of the Ultra-Faint Milky Way Satellites: Solving the Missing Satellite Problem

TL;DR

This study uses Keck/DEIMOS spectroscopy to measure stellar velocities and metallicities for eight ultra-faint Milky Way satellites, deriving velocity dispersions, dynamical masses, and mean metallicities. The ultra-faint dwarfs largely appear to be dark-matter-dominated, with mass-to-light ratios up to ~$10^3$ and metallicities as low as [Fe/H] ≈ -2.3, while UMa II shows compelling evidence for tidal disruption. Including these galaxies alongside previously known dwarfs substantially mitigates the missing satellite problem, though a residual shortfall remains, and models invoking early reionization suppression can bring simulated subhalo populations into closer agreement with observations. The results impose strong constraints on dark matter properties through central densities and phase-space densities and reveal a bifurcation in the halo-mass–luminosity relation at the faint end, suggesting a complex growth and disruption history for the Milky Way’s satellite system.

Abstract

We present Keck/DEIMOS spectroscopy of stars in 8 of the newly discovered ultra-faint dwarf galaxies around the Milky Way. We measure the velocity dispersions of Canes Venatici I and II, Ursa Major I and II, Coma Berenices, Hercules, Leo IV and Leo T from the velocities of 18 - 214 stars in each galaxy and find dispersions ranging from 3.3 to 7.6 km/s. The 6 galaxies with absolute magnitudes M_V < -4 are highly dark matter-dominated, with mass-to-light ratios approaching 1000. The measured velocity dispersions are inversely correlated with their luminosities, indicating that a minimum mass for luminous galactic systems may not yet have been reached. We also measure the metallicities of the observed stars and find that the 6 brightest of the ultra-faint dwarfs extend the luminosity-metallicity relationship followed by brighter dwarfs by 2 orders of magnitude in luminosity; several of these objects have mean metallicities as low as [Fe/H] = -2.3 and therefore represent some of the most metal-poor known stellar systems. We detect metallicity spreads of up to 0.5 dex in several objects, suggesting multiple star formation epochs. Having established the masses of the ultra-faint dwarfs, we re-examine the missing satellite problem. After correcting for the sky coverage of the SDSS, we find that the ultra-faint dwarfs substantially alleviate the discrepancy between the predicted and observed numbers of satellites around the Milky Way, but there are still a factor of ~4 too few dwarf galaxies over a significant range of masses. We show that if galaxy formation in low-mass dark matter halos is strongly suppressed after reionization, the simulated circular velocity function of CDM subhalos can be brought into approximate agreement with the observed circular velocity function of Milky Way satellite galaxies. [slightly abridged]

Figures (15)

• Figure 1: ( a) Distribution of the normalized velocity error, $\sigma_N$ (as defined in Eq. \ref{['sigma_n']}), for 49 pairs of repeated independent velocity measurements. The best fitting systematic error, $\epsilon = 2.2$ km s$^{-1}$, is used to produce the unit Gaussian distribution shown in red. ( b) Combined random ($\sigma_{\rm MC}$) and systematic velocity error for individual measurements plotted as a function of the mean per pixel signal-to-noise ratio. These data include all of our science targets, but not the globular cluster and standard stars observed as spectroscopic templates. Points that fall far from the main locus are typically hot horizontal branch stars that lack the sharp spectral features present in the majority of giant and dwarf stars that comprise our sample. Note that five stars in the sample have velocity uncertainties larger than 18 km s$^{-1}$, and 62 stars have S/N greater than 100, and are therefore not displayed in this plot; we chose the axis ranges so as to make the detailed distribution of uncertainties more visible.
• Figure 2: (a) Color-magnitude diagram of observed stars in Ursa Major II. The large black circles represent stars identified as radial velocity members of the galaxy, the small black dots represent stars identified as non-members, and the blue crosses are spectroscopically confirmed background galaxies and quasars. The red curve shows the location of the red giant branch, subgiant branch, and main sequence turnoff populations in the globular cluster M92 and the blue curve shows the location of the horizontal branch of M13, both corrected for Galactic extinction and shifted to a distance of 32 kpc clem05. (b) Spatial distribution of observed stars in Ursa Major II. Symbols are the same as in (a) (the figure legend applies to both panels), and the ellipse represents the half-light radius of UMa II from zucker06b. (c) Velocity histogram of observed stars in Ursa Major II. Velocities are corrected to the heliocentric rest frame. The filled red histogram represents stars classified as members, and the hatched black-and-white histogram represents non-members. The velocity bins are 2 km s$^{-1}$ wide.
• Figure 3: Same as Figure \ref{['uma2_obsplot']}, but for Leo T.
• Figure 4: Same as Figure \ref{['uma2_obsplot']}, but for Ursa Major I.
• Figure 5: Same as Figure \ref{['uma2_obsplot']}, but for Leo IV.