  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. The
main sequence locus in the infrared shows a strong change in slope at J~20.5
(M_J~5.75) which is well fit by new low mass models that include water and
molecular hydrogen opacity. Our derived mass function (which is not corrected
for binary companions) is the deepest measured to date in the bulge, 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 bulge LF
spanning ~15 magnitudes. We show that this mass function has negligible
dependence on the adopted bulge metallicity and distance modulus. Although
shallower than the Salpeter slope, the slope of the bulge IMF is steeper than
that 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), but is virtually identical to the disk IMF derived by
Kroupa et al. (1993). The bulge IMF is also quite similar to the mass functions
derived for those globular clusters which are believed to have experienced
little or no dynamical evolution. Finally, we derive the $M/L_J$ ratio of the
bulge to be $\sim 0.9\pm0.1$ $M_\odot/L_\odot$, and briefly discuss the
implications of this bulge IMF for the interpretation of the microlensing
events observed in the direction of the Galactic bulge.

Astrophysics