The Main Sequence at $z\sim 1.3$ contains a sizable fraction of galaxies with compact star formation sizes: a new population of early post-starbursts?

Abstract

ALMA measurements for 93 $Herschel$-selected galaxies at $1.1⩽z⩽1.7$ in COSMOS reveal a sizable ($>29$\%) population with compact star formation (SF) sizes, lying on average $>\times 3.6$ below the optical stellar mass ($M_{\star }$)-size relation of disks. This sample widely spans the star-forming Main Sequence (MS), having $10^8⩽M_{\star }⩽10^{11.5}{M}_{\odot }$ and $20⩽SFR⩽680M_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$. The 32 size measurements and 61 upper limits are measured on ALMA images that combine observations of CO(5-4), CO(4-3), CO(2-1) and $\hat{I}{»}_{\mathrm{obs}}\sim 1.1-1.3\mathrm{mm}$ continuum, all tracing the star-forming molecular gas. These compact galaxies have instead normally extended $K_{band}$ sizes, suggesting strong specific $SFR$ gradients. Compact galaxies comprise the $50\pm 18\%$ of MS galaxies at $M_{\star }>10^{11}{M}_{\odot }$. This is not expected in standard bi-modal scenarios where MS galaxies are mostly steadily-growing extended disks. We suggest that compact MS objects are early post-starburst galaxies in which the merger-driven boost of SF has subsided. They retain their compact SF size until either further gas accretion restores pre-merger galaxy-wide SF, or until becoming quenched. The fraction of merger-affected SF inside the MS seems thus larger than anticipated and might reach $\sim 50$\% at the highest $M_{\star }$. The presence of large galaxies above the MS demonstrates an overall poor correlation between galaxy SF size and specific $SFR$.

7 pages, 5 figures. Accepted for publication in ApJ Letters