On the Evolution of the Central Density of Quiescent Galaxies

Abstract

We investigate the origin of the evolution of the population-averaged central stellar mass density (Σ 1 ) of quiescent galaxies (QGs) by probing the relation between stellar age and Σ 1 at z  ∼ 0. We use the Zurich ENvironmental Study (ZENS), which is a survey of galaxy groups with a large fraction of satellite galaxies. QGs shape a narrow locus in the Σ 1 – M ⋆ plane, which we refer to as Σ 1 ridgeline. Colors of ( B − I ) and ( I − J ) are used to divide QGs into three age categories: young (<2 Gyr), intermediate (2–4 Gyr), and old (>4 Gyr). At fixed stellar mass, old QGs on the Σ 1 ridgeline have higher Σ 1 than young QGs. This shows that galaxies landing on the Σ 1 ridgeline at later epochs arrive with lower Σ 1 , which drives the zeropoint of the ridgeline down with time. We compare the present-day zeropoint of the oldest population at z  = 0 with the zeropoint of the quiescent population 4 Gyr back in time, at z  = 0.37. These zeropoints are identical, showing that the intrinsic evolution of individual galaxies after they arrive on the Σ 1 ridgeline must be negligible, or must evolve parallel to the ridgeline during this interval. The observed evolution of the global zeropoint of 0.07 dex over the last 4 Gyr is thus largely due to the continuous addition of newly quenched galaxies with lower Σ 1 at later times (“progenitor bias”). While these results refer to the satellite-rich ZENS sample as a whole, our work suggests a similar age–Σ 1 trend for central galaxies.