The Initial Mass Function of the Galactic Bulge down to ~0.15 M☉

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 MJ ~ 9.3. This LF is derived from Hubble Space Telescope (HST) near infrared camera and multiobject spectrometer (NICMOS) observations of a region of 22.″5 × 22.″5, with the F110W and F160W filters. The main-sequence locus in the infrared shows a strong change in slope at J ~ 20.5 (MJ ~ 5.75) that is well fitted 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☉, with a power-law slope of α = -1.33 ± 0.07; a Salpeter mass function would have α = -2.35. We also combine our J-band LF with previously published data for the evolved stars to produce a bulge LF spanning ~15 mag. 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 initial mass function (IMF) is steeper than that recently found for the Galactic disk (α = -0.8 and α = -0.54 from the data of Reid & Gizis and Gould et al., respectively, in the same mass interval) but is virtually identical to the disk IMF derived by Kroupa and coworkers. The bulge IMF is also quite similar to the mass functions derived for those globular clusters that are believed to have experienced little or no dynamical evolution. Finally, we derive the M/LJ ratio of the bulge to be ~0.9 ± 0.1 and briefly discuss the implications of this bulge IMF for the interpretation of the microlensing events observed in the direction of the Galactic bulge.