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 ∼ 1.7. Its spectrum reveals both Hα and [N II] emission lines and strong Balmer absorption lines. The Hα and Spitzer MIPS 24 μm fluxes are both weak, thus indicating a low star-formation rate of SFR ≳5-10 M_⊙ 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, σ^gas_LOS = 127 ±32 km s^-1, is nearly 40% smaller than that of its stars, σ ∗ _LOS = 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.