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 at a redshift of z  ∼ 1.7. Its spectrum reveals both H α  and [N ii ] emission lines and strong Balmer absorption lines. The H α  and Spitzer MIPS 24 μ m fluxes are both weak, thus indicating a low star-formation rate of SFR  yr −1 . This, added to a relatively young age of ∼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  ∼ 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,  = 127 ± 32 km s −1 , is nearly 40% smaller than that of its stars,  = 215 ± 35 km s −1 . High-resolution hydro-simulations of galaxies explain such apparently colder gas kinematics of up to a factor of ∼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.