SDSS IV MaNGA - sSFR profiles and the slow quenching of discs in green valley galaxies

TL;DR

This study uses spatially-resolved spectroscopy to analyze star formation suppression in galaxies, revealing that green valley galaxies experience uniform quenching across their entire structure, not just their centers.

Contribution

It provides new insights into the quenching mechanisms in green valley galaxies by examining radial sSFR profiles and their relation to galaxy mass and structure.

Findings

sSFR decreases with stellar mass even within the star-forming main sequence

Green valley galaxies show suppressed sSFR at all radii compared to star-forming counterparts

High central mass density alone does not determine central quiescence

Abstract

We study radial profiles in H$\alpha$ equivalent width and specific star formation rate (sSFR) derived from spatially-resolved SDSS-IV MaNGA spectroscopy to gain insight on the physical mechanisms that suppress star formation and determine a galaxy's location in the SFR-$\rm M_\star$ diagram. Even within the star-forming `main sequence', the measured sSFR decreases with stellar mass, both in an integrated and spatially-resolved sense. Flat sSFR radial profiles are observed for $\rm log(M_\star/ M_\odot) < 10.5$, while star-forming galaxies of higher mass show a significant decrease in sSFR in the central regions, a likely consequence of both larger bulges and an inside-out growth history. Our primary focus is the green valley, constituted by galaxies lying below the star formation main sequence, but not fully passive. In the green valley we find sSFR profiles that are suppressed with respect to star-forming galaxies of the same mass at all galactocentric distances out to 2 effective radii. The responsible quenching mechanism therefore appears to affect the entire galaxy, not simply an expanding central region. The majority of green valley galaxies of $\rm log(M_\star/ M_\odot) > 10.0$ are classified spectroscopically as central low-ionisation emission-line regions (cLIERs). Despite displaying a higher central stellar mass concentration, the sSFR suppression observed in cLIER galaxies is not simply due to the larger mass of the bulge. Drawing a comparison sample of star forming galaxies with the same $\rm M_\star$ and $\rm \Sigma_{1~kpc}$ (the mass surface density within 1 kpc), we show that a high $\rm \Sigma_{1~kpc}$ is not a sufficient condition for determining central quiescence.