The Puzzling Origin of Massive Compact Galaxies in MaNGA

Abstract

We characterized the kinematics, morphology, and stellar population (SP) properties of a sample of massive compact quiescent galaxies (MCGs, $10\lesssim \log M_{\star }$/$M_{\odot }\lesssim 11$ and $r_{\mathrm{e}}\sim 1-3$kpc) in the MaNGA Survey, with the goal of constraining their formation, assembly history and assessing their relation with non-compact quiescent galaxies. We compared their properties with those of a control sample of median-sized quiescent galaxies ($r_{\mathrm{e}}\sim 4-8$kpc) with similar effective velocity dispersions. MCGs have elevated rotational support, as evidenced by a strong anti-correlation between the Gauss-Hermite moment $h_3$ and $V/\ddot{I}ƒ$. In contrast, 30$\%$ of control sample galaxies (CSGs) are slow rotators, and fast-rotating CSGs generally show a weak $h_3-V/\ddot{I}ƒ$ anti-correlation. MCGs and CSGs have similar ages, but MCGs are more metal-rich and $\hat{I}\pm$-enhanced. Both MCGs and CSGs have shallow negative metallicity gradients and flat [$\hat{I}\pm$/Fe] gradients. On average, MCGs and CSGs have flat age gradients, but CSGs have a significantly larger dispersion of gradient values. The kinematics and SP properties of MCGs suggest that they experienced highly-dissipative gas-rich events, such as mergers, followed by an intense, short, and centrally concentrated burst of star formation, between 4 to 10 Gyr ago ($z\sim 0.4-2$), and had a quiet accretion history since then. This sequence of events might be analogous to, although less extreme than, the compaction events which formed compact quiescent galaxies at $z\sim 2$. The small sizes of MCGs, and the high efficiency and short duration of their last star formation episode suggest that they are descendants of compact post-starburst galaxies.

19 pages, 18 figures (not including appendices). Accepted for publication in the Monthly Notices of the Royal Astronomical Society