Morphology Dependence Of Stellar Age in Quenched Galaxies at Redshift ~
  1.2: Massive Compact Galaxies Are Older Than More Extended Ones
  We report the detection of morphology dependent stellar age in massive
quenched galaxies (QGs) at z~1.2. The sense of the dependence is that compact
QGs are 0.5-2 Gyr older than normal-sized ones. The evidence comes from three
different age indicators, Dn4000, Hdelta and fits to spectral synthesis models,
applied to their stacked optical spectra. All age indicators consistently show
that the stellar populations of compact QGs are older than their normally-sized
counterparts. We detect weak [OII] emission in a fraction of QGs, and the
strength of the line, when present, is similar between the two samples;
however, compact galaxies exhibit significantly lower frequency of [OII]
emission than normal ones. A fraction of both samples are individually detected
in 7 Ms Chandra X-ray images (luminosities$\sim10^{40}$-$10^{41}$ erg/sec). 7
Ms stacks of non-detected galaxies show similarly low luminosities in the soft
band only, consistent with a hot gas origin for the X-ray emission. While both
[OII] emitters and non-emitters are also X-ray sources among normal galaxies,
no compact galaxy with [OII] emission is an X-ray source, arguing against an
AGN powering the line in compact galaxies. We interpret the [OII] properties as
further evidence that compact galaxies are older and further along into the
process of quenching star-formation and suppressing gas accretion. Finally, we
argue that the older age of compact QGs is evidence of progenitor bias: compact
QGs simply reflect the smaller sizes of galaxies at their earlier quenching
epoch, with stellar density most likely having nothing directly to do with
cessation of star-formation.
