The Molecular Gas Density in Galaxy Centers and How It Connects to
  Bulges
  In this paper we present gas density, star formation rate, stellar masses,
and bulge disk decompositions for a sample of 60 galaxies. Our sample is the
combined sample of BIMA SONG, CARMA STING, and PdBI NUGA surveys. We study the
effect of using CO-to-H_2 conversion factors that depend on the CO surface
brightness, and also that of correcting star formation rates for diffuse
emission from old stellar populations. We estimate that star formation rates in
bulges are typically lower by 20% when correcting for diffuse emission. We find
that over half of the galaxies in our sample have molecular gas surface density
>100 M_sun pc^-2. We find a trend between gas density of bulges and bulge
Sersic index; bulges with lower Sersic index have higher gas density. Those
bulges with low Sersic index (pseudobulges) have gas fractions that are similar
to that of disks. We also find that there is a strong correlation between
bulges with the highest gas surface density and the galaxy being barred.
However, we also find that classical bulges with low gas surface density can be
barred as well. Our results suggest that understanding the connection between
the central surface density of gas in disk galaxies and the presence of bars
should also take into account the total gas content of the galaxy and/or bulge
Sersic index. Indeed, we find that high bulge Sersic index is the best
predictor of low gas density inside the bulge (not barredness of the disk).
Finally, we show that when using the corrected star formation rates and gas
densities, the correlation between star formation rate surface density and gas
surface density of bulges is similar to that of disks.
