3 research outputs found
On the stark difference in satellite distributions around the Milky Way and Andromeda
We compare spherically-averaged radial number counts of bright (> 10^5 Lsun)
dwarf satellite galaxies within 400 kpc of the Milky Way (MW) and M31 and find
that the MW satellites are much more centrally concentrated. Remarkably, the
two satellite systems are almost identical within the central 100 kpc, while
M31 satellites outnumber MW satellites by about a factor of four at deprojected
distances spanning 100 - 400 kpc. We compare the observed distributions to
those predicted for LCDM suhbalos using a suite of 44 high-resolution ~10^12
halo zoom simulations, 22 of which are in pairs like the MW and M31. We find
that the radial distribution of satellites around M31 is fairly typical of
those predicted for subhalos, while the Milky Way's distribution is more
centrally concentrated that any of our simulated LCDM halos. One possible
explanation is that our census is bright (> 10^5 Lsun) MW dwarf galaxies is
significantly incomplete beyond ~ 100 kpc of the Sun. If there were ~8 - 20
more bright dwarfs orbiting undetected at 100 - 400 kpc, then the Milky Way's
radial distribution would fall within the range expected from subhalo
distributions and alos look very much like the known M31 system. We use our
simulations to demonstrate that there is enough area left unexplored by the
Sloan Digital Sky Survey and its extensions that the discovery of ~10 new
bright dwarfs is not implausible given the expected range of angular anisotropy
of subhalos in the sky.Comment: 10 pages, 8 figures, submitted to MNRA
The SPLASH Survey: Kinematics of Andromeda's Inner Spheroid
The combination of large size, high stellar density, high metallicity, and
Sersic surface brightness profile of the spheroidal component of the Andromeda
galaxy (M31) within R_proj ~ 20 kpc suggest that it is unlike any subcomponent
of the Milky Way. In this work we capitalize on our proximity to and external
view of M31 to probe the kinematical properties of this "inner spheroid." We
employ a Markov chain Monte Carlo (MCMC) analysis of resolved stellar
kinematics from Keck/DEIMOS spectra of 5651 red giant branch stars to
disentangle M31's inner spheroid from its stellar disk. We measure the mean
velocity and dispersion of the spheroid in each of five spatial bins after
accounting for a locally cold stellar disk as well as the Giant Southern Stream
and associated tidal debris. For the first time, we detect significant spheroid
rotation (v_rot ~ 50 km/s) beyond R_proj ~ 5 kpc. The velocity dispersion
decreases from about 140 km/s at R_proj = 7 kpc to 120 km/s at R_proj = 14 kpc,
consistent to 2 sigma with existing measurements and models. We calculate the
probability that a given star is a member of the spheroid and find that the
spheroid has a significant presence throughout the spatial extent of our
sample. Lastly, we show that the flattening of the spheroid is due to velocity
anisotropy in addition to rotation. Though this suggests that the inner
spheroid of M31 more closely resembles an elliptical galaxy than a typical
spiral galaxy bulge, it should be cautioned that our measurements are much
farther out (2 - 14 r_eff) than for the comparison samples.Comment: Accepted for publication in Ap