CAUGHT IN THE ACT: GAS AND STELLAR VELOCITY DISPERSIONS IN A FAST QUENCHING COMPACT STAR-FORMING GALAXY AT z ∼ 1.7

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.