Implications of Increased Central Mass Surface Densities for the
  Quenching of Low-mass Galaxies
  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 ($\Sigma_1$) of low-mass
galaxies (stellar mass $M_* \lesssim 10^{9.5} M_\odot$) at $0.5 \leq z < 1.5$.
Our sample is mass complete down to $\sim 10^9 M_\odot$ at $0.5 \leq z < 1.0$.
We compare the mean $\Sigma_1$ of star-forming galaxies (SFGs) and quenched
galaxies (QGs) at the same redshift and $M_*$. We find that low-mass QGs have
higher $\Sigma_1$ than low-mass SFGs, similar to galaxies above $10^{10}
M_\odot$. The difference of $\Sigma_1$ between QGs and SFGs increases slightly
with $M_*$ at $M_* \lesssim 10^{10} M_\odot$ and decreases with $M_*$ at $M_*
\gtrsim 10^{10} M_\odot$. The turnover mass is consistent with the mass where
quenching mechanisms transition from internal to environmental quenching. At
$0.5 \leq z < 1.0$, we find that the $\Sigma_1$ 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 $M_*$. 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 ($\sim4$) Gyrs at $0.5 \leq z < 1.0$. 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 $\Sigma_1$.
An interesting and intriguing result is the similarity of the growth of
$\Sigma_1$ 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.
