The initial mass function (IMF) parameterizes the relative number of stars formed as a function of stellar mass. The IMF is fundamental to all calculations of star formation rates and galaxy stellar masses. We measure the IMF in two ultra-faint galaxies in the mass range 0.5-0.7M_sun, finding a shallower than expected slope based on directly counting stars.
Across the handful of galaxies with directly measured IMFs, power-law slopes become shallower (more bottom-light) with decreasing galactic velocity dispersion and metallicity. This trend is qualitatively consistent with results in elliptical galaxies inferred via indirect methods and is direct evidence for IMF variations with galactic environment.
The Stellar Initial Mass Function of Ultra-Faint Dwarf Galaxies: Evidence for IMF Variations with Galactic Environment (arXiv)
M. Geha, T.M. Brown, J. Tumlinson, J. Kalirai, J.D. Simon, E. Kirby, D. VandenBerg, R.M. Munoz, R. Avila, P. Guhathakurta, H. Ferguson, 2013, astro-ph/1304.7769
The nearest massive galaxy beyond our own, M31, hosts many faint dwarf spheroidal (dSph) satellites. In this work, we spectroscopically observed individual stars in two recently-discovered satellite candidates of M31, And XXVIII & And XXIX. Our observations reveal they are genuine satellite galaxies of M31 with kinematics dominated by dark matter. Furthermore, despite being far from M31 relative to many of its other satellites, these galaxies show internal motions similar to the other satellites of both M31 and the Milky Way.
The Outer Limits of the M31 System: Kinematics of the Dwarf Galaxy Satellites And XXVIII & And XXIX (ADS | arXiv)
Tollerud, E., Geha, M., Vargas, L., and Bullock, J., 2013, ApJ, 768, 50
Map of substructure in the Milky Way halo, color-coded by distance and intensity.
Stellar streams are found throughout the galactic halos of the Milky Way, Andromeda and other nearby massive galaxies. They are presumed to originate from disrupting dwarf galaxies or globular clusters, providing irrefutable evidence that galaxies are formed, at least in part, hierarchically. Furthermore, their orbits can be used to constrain the shape of galactic potentials at a range of distances. Most Galactic streams have been discovered in the Sloan Digital Sky Survey (SDSS), e.g. Monoceros stream, GD-1 and Orphan stream.
For the first time, we searched both northern and southern Galactic hemisphere coverage of the SDSS for cold stellar streams. Matched filtering technique was used to filter out most of the Milky Way foreground stars. We found a new cold stellar stream in the direction of Triangulum. The stream signal is strongest at 26 kpc. It is very linear and narrow, spanning 0.2 deg by 12 deg. A color magnitude diagram of the stream region shows an overdensity which, if identified as a main sequence turn-off, corresponds to an old and metal-poor stellar population. Kinematic studies of this and similar streams will provide tight constraints on the strength, shape and lumpiness of the Galactic gravitational potential.
A Cold Milky Way Stellar Stream in the Direction of Triangulum (ADS | arXiv)
Bonaca, A., Geha, M., and Kallivayalil, N., 2012, ApJ, 760, L6