Central Star Formation in Pseudobulges and Classical Bulges

Abstract

I use Spitzer 3.6-8.0 μm color profiles to compare the radial structure of star formation in pseudobulges and classical bulges. Pseudobulges are ``bulges&#39;&#39; which form through secular evolution, rather than mergers. In this study, pseudobulges are identified using the presence of disk-like structure in the center of the galaxy (nuclear spiral, nuclear bar, and/or high ellipticity in bulge); classical bulges are those galaxy bulges with smooth isophotes which are round compared to the outer disk, and show no disky structure in their bulge. I show that galaxies structurally identified as having pseudobulges have higher central star formation rates than those of classical bulges. Further, I also show that galaxies identified as having classical bulges have remarkably regular star formation profiles. The color profiles of galaxies with classical bulges show a star forming outer disk with a sharp change, consistent with a decline in star formation rates, toward the center of the galaxy. Classical bulges have a nearly constant inner profile (r < 1.5 kpc) that is similar to elliptical galaxies. Pseudobulges in general show no such transition in star formation properties from the outer disk to the central pseudobulge. Thus I conclude that pseudobulges and classical bulges do in fact form their stars via different mechanisms. Further, this adds to the evidence that classical bulges form most of their stars in fast episodic bursts, in a similar fashion to elliptical galaxies; whereas, pseudobulges form stars from longer lasting secular processes.