The Ages, Metallicities and Element Abundance Ratios of Massive Quenched galaxies at z~1.6 　　  

　　Speaker: Nobuo Arimoto, Subaru Telescope (NAOJ)　　  

    Time: Monday, 3:00pm, December 1st,　　

　　Location: Lecture Hall, 3rd floor　　  

　　Abstract: We investigate the stellar population properties of a sample of 24 massive quenched galaxies at 1.25 < z < 2.09 identified in the COSMOS field with our Subaru/MOIRCS near-infrared 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 alpha-to-iron abundance ratio [alpha/Fe] are derived as log(age/Gyr) = 0.18 ± 0.05, [Z/H] = 0.08 ± 0.08, and [alpha/Fe] = 0.36 ± 0.09, respectively. Pure passive evolution to z = 0 brings the <z> = 1.6 quenched galaxies to the parameters 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 the low redshift measurement from the literature suggests a formation redshift of z ~ 2.5 around which the bulk of stars in these galaxies have been formed. The measured [alpha/Fe] value indicates a star formation time scale of < 350 Myr, which can be translated into a specific star formation rate of ~3 Gyr -1 prior to the quenching. Based on these findings, we discuss possible progenitor star-forming galaxies at z ~ 2.5 and find normal star-forming galaxies, i.e, those on the star-forming main sequence, followed by quenching event invoked by some internal mechanisms as likely precursors of quenched galaxies at <z> = 1.6 presented here