In Pursuit of the Least Luminous Galaxies

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 distribution, 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.Comment: Review article, 20 pages. Accepted to Advances in Astronomy, Dwarf Galaxy Cosmology issu

A New Milky Way Companion: Unusual Globular Cluster or Extreme Dwarf Satellite?

We report the discovery of SDSS J1049+5103, an overdensity of resolved blue stars at (α2000, δ2000) = (162343, 51051). This object appears to be an old, metal-poor stellar system at a distance of 45 ± 10 kpc, with a half-light radius of 23 ± 10 pc and an absolute magnitude of MV = -3.0. One star that is likely associated with this Milky Way companion has an SDSS spectrum confirming it as a blue horizontal-branch star at 48 kpc. The color-magnitude diagram of SDSS J1049+5103 contains few, if any, horizontal or red giant branch stars, similar to the anomalously faint globular cluster AM 4. The size and luminosity of SDSS J1049+5103 places it at the intersection of the size-luminosity relationships followed by known globular clusters and by Milky Way dwarf spheroidal galaxies. If SDSS J1049+5103 is a globular cluster, then its properties are consistent with the established trend that the largest radius Galactic globular clusters are all in the outer halo. However, the five known globular clusters with similarly faint absolute magnitudes all have half-mass radii that are smaller than SDSS J1049+5103 by a factor of 5. If it is a dwarf spheroidal galaxy, then it is the faintest yet known by 2 orders of magnitude and is the first example of the ultrafaint dwarfs predicted by some theories. The uncertain nature of this new system underscores the sometimes ambiguous distinction between globular clusters and dwarf spheroidal galaxies. A simple friends-of-friends search for similar, blue, small scale length star clusters detected all known globular clusters and dwarfs closer than 50 kpc in the SDSS area but yielded no other candidates as robust as SDSS J1049+5103

The Origin and Properties of Intracluster Stars in a Rich Cluster

We use a multi million particle N-body + SPH simulation to follow the formation of a rich galaxy cluster in a Lambda+CDM cosmology, with the goal of understanding the origin and properties of intracluster stars. The simulation includes gas cooling, star formation, the effects of a uniform UVB and feedback from supernovae. Halos that host galaxies as faint as M_R = -19.0 are resolved by this simulation, which includes 85% of the total galaxy luminosity in a rich cluster. We find that the accumulation of intracluster light (ICL) is an ongoing process, linked to infall and stripping events. The unbound star fraction increases with time and is 20% at z = 0, consistent with observations of galaxy clusters. The surface brightness profile of the cD shows an excess compared to a de Vaucouleur profile near 200 kpc, which is also consistent with observations. Both massive and small galaxies contribute substantially to the formation of the ICL, with stars stripped preferentially from the outer parts of their stellar distributions. Simulated observations of planetary nebulae (PNe) show significant substructure in velocity space. Despite this, individual intracluster PNe might be useful mass tracers if more than 5 fields at a range of radii have measured line-of-sight velocities, where an accurate mass calculation depends more on the number of fields than the number of PNe measured per field. However, the orbits of IC stars are more anisotropic than those of galaxies or dark matter, which leads to a systematic underestimate of cluster mass relative to that calculated with galaxies, if not accounted for in dynamical models. Overall, the properties of ICL formed in a hierarchical scenario are in good agreement with current observations. (Abridged)Comment: Replaced with MNRAS published version. One corrected figure, minor text changes. MNRAS, 355, 15

Too Many, Too Few, or Just Right? The Predicted Number and Distribution of Milky Way Dwarf Galaxies

We predict the spatial distribution and number of Milky Way dwarf galaxies to be discovered in the Dark Energy Survey (DES) and Large Synoptic Survey Telescope (LSST) surveys, by completeness correcting the observed Sloan Digital Sky Survey dwarf population. We apply most massive in the past, earliest forming, and earliest infall toy models to a set of dark matter-only simulated Milky Way/M31 halo pairs from the Exploring the Local Volume In Simulations project. Inclusive of all toy models and simulations, at 90% confidence we predict a total of 37–114 L 103 L☉ dwarfs and 131–782 L 103 L☉ dwarfs within 300 kpc. These numbers of L 103 L☉ dwarfs are dramatically lower than previous predictions, owing primarily to our use of updated detection limits and the decreasing number of SDSS dwarfs discovered per sky area. For an effective rlimit of 25.8 mag, we predict 3–13 L 103 L☉ and 9–99 L 103 L☉ dwarfs for DES, and 18–53 L 103 L☉ and 53–307 L 103 L☉ dwarfs for LSST. We also show that the observed spatial distribution of Milky Way dwarfs in the LSST-era will discriminate between the earliest infall and other simplified models for how dwarf galaxies populate dark matter subhalos

Hunting The Most Distant Stars in the Milky Way: Methods and Initial Results

We present a new catalog of 404 M giant candidates found in the UKIRT Infrared Deep Sky Survey (UKIDSS). The 2,400 deg$^2$ available in the UKIDSS Large Area Survey Data Release 8 resolve M giants through a volume four times larger than that of the entire Two Micron All Sky Survey. Combining near-infrared photometry with optical photometry and proper motions from the Sloan Digital Sky Survey yields an M giant candidate catalog with less M dwarf and quasar contamination than previous searches for similarly distant M giants. Extensive follow-up spectroscopy of this sample will yield the first map of our Galaxy's outermost reaches over a large area of sky. Our initial spectroscopic follow-up of $\sim$ 30 bright candidates yielded the positive identification of five M giants at distances $\sim 20-90$ kpc. Each of these confirmed M giants have positions and velocities consistent with the Sagittarius stream. The fainter M giant candidates in our sample have estimated photometric distances $\sim 200$ kpc (assuming $[Fe/H]$ = 0.0), but require further spectroscopic verification. The photometric distance estimates extend beyond the Milky Way's virial radius, and increase by $\sim 50\%$ for each 0.5 dex decrease in assumed $[Fe/H]$. Given the number of M giant candidates, initial selection efficiency, and volume surveyed, we loosely estimate that at least one additional Sagittarius-like accretion event could have contributed to the hierarchical build-up of the Milky Way's outer halo.Comment: 16 pages, 11 figures, emulateapj format. Accepted by A

Turning the Tides on the Ultra-Faint Dwarf Spheroidal Galaxies: Coma Berenices and Ursa Major

We present deep CFHT/MegaCam photometry of the ultra-faint Milky Way satellite galaxies: Coma Berenices (ComBer) and Ursa Major II (UMa II). These data extend to r ~ 25, corresponding to 3 mag below the main-sequence turn-offs in these galaxies. We robustly calculate a total luminosity of MV = –3.8 ± 0.6 for ComBer and MV = –3.9 ± 0.5 for UMa II, in agreement with previous results and confirming that these galaxies are among the faintest of the known dwarf satellites of the Milky Way. ComBer shows a fairly regular morphology with no signs of active tidal stripping down to a surface brightness limit of 32.4 mag arcsec–2. Using a maximum likelihood analysis, we calculate the half-light radius of ComBer to be r half = 74 ± 4 pc (5.8 ± 03) and its ellipticity = 0.36 ± 0.04. In contrast, UMa II shows signs of ongoing disruption. We map its morphology down to μ V = 32.6 mag arcsec–2 and found that UMa II is larger than previously determined, extending at least ~600 pc (11 on the sky) and it is also quite elongated with an overall ellipticity of = 0.50 ± 0.2. However, our estimate for the half-light radius, 123 ± 3 pc (14.1 ± 03) is similar to previous results. We discuss the implications of these findings in the context of potential indirect dark matter detections and galaxy formation. We conclude that while ComBer appears to be a stable dwarf galaxy, UMa II shows signs of ongoing tidal interaction