Implications of Increased Central Mass Surface Densities for the Quenching of Low-mass Galaxies

Abstract

We use the Cosmic Assembly Deep Near-infrared Extragalactic Legacy Survey (CANDELS) data to study the relationship between quenching and the stellar mass surface density within the central radius of 1 kpc ($\hat{I}{\pounds }_1$) of low-mass galaxies (stellar mass $M_{\ast }\lesssim 10^{9.5}{M}_{\odot }$) at $0.5\le z<1.5$. Our sample is mass complete down to $\sim 10^9{M}_{\odot }$ at $0.5\le z<1.0$. We compare the mean $\hat{I}{\pounds }_1$ of star-forming galaxies (SFGs) and quenched galaxies (QGs) at the same redshift and $M_{\ast }$. We find that low-mass QGs have higher $\hat{I}{\pounds }_1$ than low-mass SFGs, similar to galaxies above $10^{10}{M}_{\odot }$. The difference of $\hat{I}{\pounds }_1$ between QGs and SFGs increases slightly with $M_{\ast }$ at $M_{\ast }\lesssim 10^{10}{M}_{\odot }$ and decreases with $M_{\ast }$ at $M_{\ast }\gtrsim 10^{10}{M}_{\odot }$. The turnover mass is consistent with the mass where quenching mechanisms transition from internal to environmental quenching. At $0.5\le z<1.0$, we find that the $\hat{I}{\pounds }_1$ of galaxies increases by about 0.25 dex in the green valley (i.e., the transitioning region from star forming to fully quenched), regardless of their $M_{\ast }$. Using the observed specific star formation rate (sSFR) gradient in the literature as a constraint, we estimate that the quenching timescale (i.e., time spent in the transition) of low-mass galaxies is a few ($\sim 4$) Gyrs at $0.5\le z<1.0$. The mechanisms responsible for quenching need to gradually quench star formation in an outside-in way, i.e., preferentially ceasing star formation in outskirts of galaxies while maintaining their central star formation to increase $\hat{I}{\pounds }_1$. An interesting and intriguing result is the similarity of the growth of $\hat{I}{\pounds }_1$ in the green valley between low-mass and massive galaxies, which suggests that the role of internal processes in quenching low-mass galaxies is a question worthy of further investigation.

12 pages, 11 figures. Accepted by ApJ