The IMF in Ultra-Faint galaxies

imfThe 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