Observational bias against finding Milky Way (MW) dwarf galaxies at low Galactic latitudes (b < 20 deg) and at low surface brightnesses (fainter than 29 mag arcsec^-2, in the V-band) currently limits our understanding of the faintest limits of the ga
laxy luminosity function. This paper is a proof-of-concept that groups of two or more RR Lyrae stars reveal MW dwarf galaxies at d > 50 kpc in these unmined regions of parameter space, with only modest contamination from interloper groups when large halo structures are excluded. For example, a friends-of-friends (FOF) search with a linking length of 500 pc could reveal dwarf galaxies more luminous than M_V = -3.2 mag and with surface brightnesses as faint as 31 mag arcsec^-2 (or even fainter, depending on RR Lyrae specific frequency). Although existing public RR Lyrae catalogs are highly incomplete at d > 50 kpc and/or include <1% of the MW halos volume, a FOF search reveals two known dwarfs (Bootes I and Sextans) and two dwarf candidate groups possibly worthy of follow-up. PanSTARRS 1 (PS1) may catalog RR Lyrae to 100 kpc which would include ~15% of predicted MW dwarf galaxies. Groups of PS1 RR Lyrae should therefore reveal very low surface brightness and low Galactic latitude dwarfs within its footprint, if they exist. With sensitivity to RR Lyrae to d >600 kpc, LSST is the only planned survey that will be both wide-field and deep enough to use RR Lyrae to definitively measure the Milky Ways dwarf galaxy census to extremely low surface brightnesses, and through the Galactic plane.
We investigate the kinematic properties and stellar population of the Galactic satellite Willman 1 (Wil 1) by combining Keck/DEIMOS spectroscopy with KPNO mosaic camera imaging. Wil 1 is an ultra-low luminosity Milky Way companion. This object lies i
n a region of size-luminosity space (M_V ~ -2 mag, d ~ 38 kpc, r_half ~ 20 pc) also occupied by the Galactic satellites Bootes II and Segue 1 and 2, but no other known old stellar system. We use kinematic and color-magnitude criteria to identify 45 stars as possible members of Wil 1. With a systemic velocity of v_helio = -12.8 +/- 1.0 km/s, Wil 1 stars have velocities similar to those of foreground Milky Way stars. Informed by Monte-Carlo simulations, we identify 5 of the 45 candidate member stars as likely foreground contaminants. We confirm a significant spread in the abundances of the likely Wil 1 red giant branch members ([Fe/H] = -1.73 +/- 0.12 and -2.65 +/- 0.12, [Ca/Fe] = -0.4 +/- 0.18 and +0.13 +/- 0.28). This spread supports the scenario that Wil 1 is an ultra-low luminosity dwarf galaxy rather than a star cluster. Wil 1s innermost stars move with radial velocities offset by 8 km/s from its outer stars and have a velocity dispersion consistent with 0 km/s, suggesting that Wil 1 may not be in dynamical equilibrium. The combination of the foreground contamination and unusual kinematic distribution make it difficult to robustly determine the dark matter mass of Wil 1. As a result, X-ray or gamma-ray observations of Wil 1 that attempt to constrain models of particle dark matter using an equilibrium mass model are strongly affected by the systematics in the observations presented here. We conclude that, despite the unusual features in the Wil 1 kinematic distribution, evidence indicates that this object is, or at least once was, a dwarf galaxy.
The dwarf galaxy companions to the Milky Way are unique cosmological laboratories. With luminosities as low as 10^-7 L_MW, they inhabit the lowest mass dark matter halos known to host stars and are presently the most direct tracers of the distributio
n, mass spectrum, and clustering scale of dark matter. Their resolved stellar populations also facilitate detailed studies of their history and mass content. To fully exploit this potential requires a well-defined census of virtually invisible galaxies to the faintest possible limits and to the largest possible distances. I review the past and present impacts of survey astronomy on the census of Milky Way dwarf galaxy companions, and discuss the future of finding ultra-faint dwarf galaxies around the Milky Way and beyond in wide-field survey data.
[abridged] We present MMT/Megacam imaging in Sloan $g$ and $r$ of the extremely low luminosity Bootes II Milky Way companion. We use a bootstrap approach to perform robust measurements of, and uncertainties on, Bootes IIs distance, luminosity, size,
and morphology. We show that Bootes IIs stellar population is old and metal-poor ([Fe/H] $lta$ -2). Assuming a stellar population like that of M92, Bootes II is at a distance of 42 $pm$ 2 kpc, closer than the initial published estimate of 60 $pm$ 10 kpc. This distance revision, combined with a more robust measurement of Bootes IIs structure with a Plummer model (exponential model) results in a more compact half-light size of $r_hsimeq 36 (33) pm 9 (10)$ pc and lower luminosity of $M_Vsimeq-2.4 (-2.2) pm 0.7 (0.7)$ mag. This revised size and luminosity move Bootes II into a region of size-luminosity space not previously known to be occupied by old stellar populations, but also occupied by the recently discovered Milky Way satellites Willman 1 and SEGUE 1. We show that the apparently distorted morphology of Bootes II is not statistically significant given the present data. We use a tidal argument to support a scenario where Bootes II is a dwarf galaxy (dark matter dominated) rather than a globular cluster (not dark matter dominated). However, we can not rule out that Bootes II is a star cluster on the verge of disruption, such as Palomar 5.
