STScI Preprint #1254

The Stellar Initial Mass Function in Local Dwarf Galaxies

The stellar initial mass function (IMF) for dwarf galaxies in the local universe is reviewed. These galaxies are at distances between 50 kpc (LMC) and tens of Mpc. This distance range allows IMF determinations from star counts in conjunction with photometry/spectroscopy, color-magnitude diagrams, and integrated spectral energy distributions. There is no observational evidence for a significant IMF variation among local dwarf galaxies. Small variations, however, could be hidden by systematic and experimental uncertainties. Environment and metallicity in particular have little observable effect on the IMF. The high-mass end of the IMF above ~10 M follows a power-law slope with a Salpeter exponent. At intermediate masses between 3 and 10 M the slope is close to Salpeter, or somewhat steeper (richer in less massive stars). This trend is reversed at lower masses where a flattening of the IMF or even a truncation may occur, somewhat similar to the solar neighborhood case. Extrapolating the Salpeter slope to the hydrogen-burning limit of ~0.1  M would lead to an overestimate of the star-formation rate by a factor of several. A power-law fit with a Salpeter slope truncated at 1 and 100  M is an acceptable approximation for the IMF and gives reasonable star-formation rates.

Status:

To appear in: the proceedings of the 23rd Moriond Astrophysics Meeting, Dwarf Galaxies and Cosmology, held 14-20 March 1998 in Les Arcs 1800, France

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Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA