New insights on the Galactic Bulge Initial Mass Function
  We have derived the Galactic bulge initial mass function of the SWEEPS field
in the mass range 0.15 $< M/M_{\odot}<$ 1.0, using deep photometry collected
with the Advanced Camera for Surveys on the Hubble Space Telescope.
Observations at several epochs, spread over 9 years, allowed us to separate the
disk and bulge stars down to very faint magnitudes, F814W $\sim$ 26 mag, with a
proper-motion accuracy better than 0.5 mas/yr. This allowed us to determine the
initial mass function of the pure bulge component uncontaminated by disk stars
for this low-reddening field in the Sagittarius window. In deriving the mass
function, we took into account the presence of unresolved binaries, errors in
photometry, distance modulus and reddening, as well as the metallicity
dispersion and the uncertainties caused by adopting different theoretical
color-temperature relations. We found that the Galactic bulge initial mass
function can be fitted with two power laws with a break at M $\sim$ 0.56
$M_{\odot}$, the slope being steeper ($\alpha$ = -2.41$\pm$0.50) for the higher
masses, and shallower ($\alpha$ = -1.25$\pm$0.20) for the lower masses. In the
high-mass range, our derived mass function agrees well with the mass function
derived for other regions of the bulge. In the low-mass range however, our mass
function is slightly shallower, which suggests that separating the disk and
bulge components is particularly important in the low-mass range. The slope of
the bulge mass function is also similar to the slope of the mass function
derived for the disk in the high-mass regime, but the bulge mass function is
slightly steeper in the low-mass regime. We used our new mass function to
derive stellar M/L values for the Galactic bulge and we obtained 2.1
$<M/L_{F814W}<$ 2.4 and 3.1 $< M/L_{F606W}<$ 3.6 according to different
assumptions on the slope of the IMF for masses larger than 1 $M_{\odot}$.
