Morphology Dependence of Stellar Age in Quenched Galaxies at Redshift ∼1.2:Massive Compact Galaxies Are Older than More Extended Ones

Abstract

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-D 4000 n , H d ,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 those of their normal-sized counterparts. We detect weak [O II] emission in a fraction of QGs, and the strength of the line, when present, is similar between the two samples; however, compact galaxies exhibit asignificantly lower frequency of [O II] emission than normal ones. Fractions of both samples are individually detected in 7 Ms Chandra X-ray images (luminosities ∼10 40 -10 41 erg s −1 ). The 7 Ms stacks of nondetected galaxies show similarly low luminosities in the soft band only, consistent with a hot gas origin for the X-ray emission. While both [O II] emitters and nonemitters are also X-ray sources among normal galaxies, no compact galaxy with [O II] emission is an X-ray source, arguing against an active galactic nucleus (AGN) powering the line in compact galaxies. We interpret the [O II] properties as further evidence that compact galaxies are older and further along in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.

The size evolution of star-forming and quenched galaxies in the IllustrisTNG simulation

“…with observations (Williams et al 2017), as well as its weakening towards higher redshifts (Whitaker et al 2012). In contrast, Furlong et al (2017) found no such trends of galaxy size, neither with sSFR nor with mean stellar age, for massive quenched galaxies in the EAGLE simulation.…”

The Intrinsic Characteristics of Galaxies on the SFR–M∗ Plane at 1.2 < z < 4: I. The Correlation between Stellar Age, Central Density, and Position Relative to the Main Sequence

“…Lilly and Carollo (2016) suggested that "progenitor bias" (i.e., the fact that galaxies were smaller and denser in the past) is at least in part responsible for the observed differences between quenched and star-forming galaxies at any epoch. Spectroscopic measures of stellar age at redshift < z 1.5 (Belli et al 2014(Belli et al , 2015Fagioli et al 2016;Williams et al 2017) found that compact galaxies are indeed »1 Gyr older than normally sized ones, adding support to the idea that progenitor bias plays at least some role.…”

Massive post-starburst galaxies at z > 1 are compact proto-spheroids

“…At intermediate redshifts (z ∼ 1) previous studies have found conflicting results on the relationship between stellar age and the compactness of passive galaxies, with indications that progenitor bias may be playing a role at this epoch (Keating et al 2015;Williams et al 2017).…”

“…Thus, while the majority of high-mass passive galaxies at z ∼ 2 are likely to have been through the ultracompact post-starburst phase (Wild et al 2016), this may become an increasingly less dominant channel towards lower redshifts, when progenitor bias may play a more significant role in explaining size evolution (e.g. see Fagioli et al 2016, Williams et al 2017). Even at low redshift, however, there is evidence that the most compact quiescent galaxies have evolved from post-starburst progenitors (Zahid et al 2016).…”