The Initial Mass Function of the Galactic Bulge Down to ~0.15 Msun

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  • 28 June 1999
Abstract
We present a luminosity function (LF) for lower main sequence stars in the Galactic bulge near (l,b)=(0,-6) to J=24, corresponding to M_J~9.3. This LF is derived from HST + NICMOS observations with the F110W and F160W filters. Our derived mass function (which is not corrected for unseen companions) is the deepest measured to date, and extends to 0.15 M_\odot with a power law slope of alpha=-1.33\pm0.07; a Salpeter mass function would have alpha=-2.35. We also combine our J band LF with previously published data for the evolved stars to produce a LF spanning 15 magnitudes. We show that our derived mass function has negligible dependence on metallicity and adopted bulge distance modulus. The main sequence locus in the infrared shows a strong change in color at M_J=+5.5 which is well fit by new low mass models that incorporate atmospheres with water and molecular hydrogen opacity. Although shallower than the Salpeter slope, our slope is steeper than has been recently found for the Galactic disk (alpha=-0.8 and alpha=-0.54 from the data of Reid & Gizis, 1997, and Gould et al. 1997, respectively, in the same mass interval). Our mass function also is in good agreement with deep mass functions derived for those globular clusters which are believed to have experienced little or no dynamical evolution. The new mass function may account for the number and duration of bulge microlensing events using normal low mass stars, without requiring an extraordinarily large brown dwarf population.

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