Caught in the Act: Gas and Stellar Velocity Dispersions in a Fast Quenching Compact Star-Forming Galaxy at z~1.7
Finkelstein, Steven L.; Barro, Guillermo; Pérez-González, Pablo G.; Kocevski, Dale D.; Koekemoer, Anton M.; Trump, Jonathan R.; Pacifici, Camilla; Brammer, Gabriel; Cava, Antonio; Livermore, Rachael C.; Dekel, Avishai; Koo, David C.; Guo, Yicheng; Primack, Joel R.; Toloba, Elisa; Inoue, Shigeki; Faber, Sandra M.; Fang, Jerome J.; Ceverino, Daniel; McGrath, Elizabeth; Cardiel, Nicolas; Liu, Fengshan; Eliche, Carmen
United States, Israel, South Korea, Spain, China, Switzerland
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 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 }{{{LOS}}}{{gas}} = 127 ± 32 km s-1, is nearly 40% smaller than that of its stars, {σ }{{{LOS}}}\star = 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.