The Ages, Metallicities, and Element Abundance Ratios of Massive Quenched Galaxies at z ≍ 1.6

Abstract

We investigate the stellar population properties of a sample of 24 massive quenched galaxies at 1.25< z<SUB>spec</SUB>< 2.09 identified in the COSMOS field with our Subaru/Multi-object Infrared Camera and Spectrograph near-IR spectroscopic observations. Tracing the stellar population properties as close to their major formation epoch as possible, we try to put constraints on the star formation history, post-quenching evolution, and possible progenitor star-forming populations for such massive quenched galaxies. By using a set of Lick absorption line indices on a rest-frame optical composite spectrum, the average age, metallicity [Z/H], and α-to-iron element abundance ratio [α/Fe] are derived as log(age/Gyr)=0.04<SUB>-0.08</SUB><SUP>+0.10</SUP>, [Z/H]=0.24<SUB>-0.14</SUB><SUP>+0.20</SUP>, and [α /Fe]=0.31<SUB>-0.12</SUB><SUP>+0.12</SUP>, respectively. If our sample of quenched galaxies at < z≥1.6 is evolved passively to z = 0, their stellar population properties will align in excellent agreement with local counterparts at similar stellar velocity dispersions, which qualifies them as progenitors of local massive early-type galaxies. Redshift evolution of stellar population ages in quenched galaxies combined with low redshift measurements from the literature suggests a formation redshift of z<SUB>f</SUB>∼ 2.3, around which the bulk of stars in these galaxies have been formed. The measured [α/Fe] value indicates a star formation timescale of ≲ 1 Gyr, which can be translated into a specific star formation rate of ≃ 1 Gyr<SUP>-1</SUP> prior to quenching. Based on these findings, we discuss identifying possible progenitor star-forming galaxies at z≃ 2.3. We identify normal star-forming galaxies, i.e., those on the star-forming main sequence, followed by a rapid quenching event, as likely precursors of the quenched galaxies at < z≥1.6 presented here. <P />Based on data collected at the Subaru telescope, which is operated by the National Astronomical Observatory of Japan. (Proposal IDs: S09A-043, S10A-058, and S11A-075.)