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]