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

Speaker: Nobuo Arimoto (Subaru Telescope)  

Time & Place: Monday, 3:00pm, December 1st, 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. 

Biog: Prof. Arimoto graduated from Tohoku University (Sendai, Japan) in 1980 and went on to positions at Observatoire de Paris, Universitaet Heidelberg, and Durham University. In 1992 he became an Associate Professor at the University of Tokyo, then Professor at the National Astronomical Observatory of Japan. Since 2012 he has held the role of Director of Subaru Telescope. His research interests include stellar populations, chemical and spectroscopic evolution of galaxies, Galactic archaeology and the origin of elliptical galaxies.