Caught in the act: gas and stellar velocity dispersions in a fast quenching compact star-forming galaxy at z~1.7

Abstract

We present Keck-I MOSFIRE spectroscopy in the Y and H bands of GDN-8231, a massive, compact, star-forming galaxy (SFG) at a redshift $z\sim 1.7$. Its spectrum reveals both H${}_{\alpha }$ and [NII] emission lines and strong Balmer absorption lines. The H${}_{\alpha }$ and Spitzer MIPS 24 $\mu$m fluxes are both weak, thus indicating a low star formation rate of SFR $\lesssim 5-10$ M${}_{\odot }$ yr${}^{-1}$. This, added to a relatively young age of $\sim 700$ Myr measured from the absorption lines, provides the first direct evidence for a distant galaxy being caught in the act of rapidly shutting down its star formation. Such quenching allows GDN-8231 to become a compact, quiescent galaxy, similar to 3 other galaxies in our sample, by $z\sim 1.5$. Moreover, the color profile of GDN-8231 shows a bluer center, consistent with the predictions of recent simulations for an early phase of inside-out quenching. Its line-of-sight velocity dispersion for the gas, ${\sigma }_{{}_{\mathrm{LOS}}}^{\mathrm{gas}}=127\pm 32$ km s${}^{-1}$, is nearly 40% smaller than that of its stars, ${\sigma }_{{}_{\mathrm{LOS}}}^{\star }=215\pm 35$ km s${}^{-1}$. High-resolution hydro-simulations of galaxies explain such apparently colder gas kinematics of up to a factor of $\sim 1.5$ with rotating disks being viewed at different inclinations and/or centrally concentrated star-forming regions. A clear prediction is that their compact, quiescent descendants preserve some remnant rotation from their star-forming progenitors.

12 pages, 7 figures, submitted to ApJ

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