Bulges Of Nearby Galaxies With Spitzer: The Growth Of Pseudobulges In
  Disk Galaxies And Its Connection To Outer Disks
  We study star formation rates (SFR) and stellar masses in bulges of nearby
disk galaxies, using SFRs and stellar masses derived from Spitzer and GALEX
data. At present day SFR the median pseudobulge could have grown the present
day stellar mass in 8 Gyr. In almost all galaxies in our sample the specific
SFR (SFR per unit stellar mass) of the bulge is higher than that of the outer
disk, suggesting that almost all galaxies are increasing their B/T through
internal star formation. In pseudobulges, SFR density correlates, positvely,
with mass density, this is consistent with that stellar mass being formed by
moderate, extended star fromation. As well, SFR density and stellar mass of
pseudobulges are shown to be correlated with the stellar mass of the outer
disk. Classical bulges have the lowest specific SFR implying a growth times
that are longer than a Hubble time. We identify a class of bulges that have
nuclear morphology similar to pseudobulges, significantly lower specific SFR
than pseudobulges, and are closer to classical bulges in structural parameter
correlations. Our results are consistent with a scenario in which bulge growth
via internal star formation is a natural, and near ubiquitous phenomenon in
disk galaxies. Some disk galaxies with out a large classical bulge, over long
periods of extended star formation are able to growth a pseudobulge. In this
sense, galaxies with pseudobulges may very well be bulgeless (or
"quasi-bulgeless") galaxies, and galaxies with classical bulges are galaxies in
which both internal evolution and hierarchical merging are responsible for the
bulge mass by fractions that vary from galaxy-to-galaxy. [Abridged]
