Bulges and disks in the local Universe. Linking the galaxy structure to star formation activity

L. Morselli, P. Popesso, G. Erfanianfar, A. Concas

Published 2016-07-26Version 1

Galaxy morphology and star formation activity are strictly linked, in the way that bulge-dominated galaxies are in general quiescent, while disk dominated galaxies are actively star-forming. In this paper, we study the properties of bulges and disks as a function of the position of galaxies in the star formation rate (SFR) - stellar mass ($M_{\star}$) plane. Our sample is built on the SDSS DR7 catalogue, and the bulge-disk decomposition is the one of Simard et al. (2011). We find that at a given stellar mass the Main Sequence (MS) is populated by galaxies with the lowest B/T ratios. The B/T on the MS increases with increasing stellar mass, thus confirming previous results in literature. In the upper envelop of the MS, the average B/T is higher than that of MS counterparts at fixed stellar mass. This indicates that starburst galaxies have a significant bulge component. In addition, bulges above the MS are characterised by blue colours, whereas, if on the MS or below it, they are mostly red and dead. The disks show blue colours above and on the MS despite the color of the bulge. They become redder below the MS and in the quiescence region. This would suggest that galaxies above the MS have nuclear and disk star formation activity. The nuclear activity is the first to be suppressed, moving the galaxies on the MS. Once also the disk stops forming stars, the galaxy moves below the MS and eventually on the quiescence region. This is confirmed by a large fraction ($\sim50\%$) of passive galaxies with a secure two component morphology coexisting with a population of pure spheroidals.