The Key role of the Bulge Compactness in Star-forming Activity in Late-type Galaxies

Which mechanism governs star-formation activity in galaxies is still one of the most important, open questions in galactic astronomy. To address this issue, we investigate the specific star formation rate (sSFR) of late-type galaxies as functions of various structural parameters including the morphology, mass, radius, and mass compactness (MC). We use a sample of ~200,000 late-type galaxies with z = 0.02 ~ 0.2 from SDSS DR7 and a catalog of bulge-disk decomposition (Simard et al. 2011; Mendel et al. 2013). We find a remarkably strong correlation between bulge's MC and galaxy's sSFR, in the sense that galaxies with more compact bulge tend to be of lower sSFR. This seems counter-intuitive given that galactic sSFR is driven predominantly by disks rather than bulges and suggests that the central mass density plays a key role in recent star-forming activity. We discuss the physical cause of the new findings in terms of the bulge growth history and AGN activities.