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

G. Barro, S. M. Faber, A. Dekel, C. Pacifici, P. G. Perez-Gonzalez, E. Toloba, D. C. Koo, J. R. Trump, S. Inoue, Y. Guo, F. Liu, J. R. Primack, A. M. Koekemoer, G. Brammer, A. Cava, N. Cardiel, D. Ceverino, C. M. Eliche, J. J. Fang, S. L. Finkelstein, D. D. Kocevski, R. C. Livermore, E. McGrath. 2016. Caught in the act: gas and stellar velocity dispersions in a fast quenching compact star-forming galaxy at z~1.7. Astrophysical Journal 820, 2, DOI: 10.3847/0004-637X/820/2/120

We present Keck I MOSFIRE spectroscopy in the Y and H bands of GDN-8231, a massive, compact, star-forming galaxy at a redshift of z similar to 1.7. Its spectrum reveals both H alpha and [N II]. emission lines and strong Balmer absorption lines. The Ha. and Spitzer MIPS. 24 mu m fluxes are both weak, thus indicating a low star-formation rate of SFR less than or similar to 5-10 M-circle dot yr(-1). This, added to a relatively young age of similar to 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 three. other galaxies in our sample, by z similar to 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, gas sigma(gas)(LOS) = 127 +/- 32 km s(-1), is nearly 40% smaller than that of its stars, sigma(LOS*) = 215 +/- 35 km s(-1). High-resolution hydro-simulations of galaxies explain such apparently colder gas kinematics of up to a factor of similar to 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 starforming progenitors.

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