The origin of ionized gas in retired galaxies: dynamical clues

ABSTRACT We investigate the kinematical and dynamical properties of quiescent cluster galaxies with weak emission lines, referred to as retired (R), and those without emission lines, dubbed passive (P), to better understand the origin of the ionized gas in R galaxies and what drives the differences between these populations. We stack 2907 P and 2387 R galaxies from 336 relaxed galaxy clusters to build an ensemble cluster and estimate their projected number density and velocity dispersion profiles, $\sigma _P(R)$, as well as their projected phase-space (PPS) distributions. Additionally, we apply the MAMPOSSt code and the Jeans equation inversion technique to constrain the velocity anisotropy profiles, $\beta (r)$. We find that P galaxies tend to reside closer to the cluster centres than R galaxies, and that both populations exhibit similar $\sigma _P(R)$ and $\beta (r)$ profiles, regardless of their stellar mass, stellar age, or morphology. The only exception is elliptical R galaxies, which are marginally more concentrated and display more radial orbits than their P counterparts. PPS analyses indicate that R galaxies were, on average, accreted later than P galaxies, except for those with $D_n4000 > 1.86$ or elliptical morphology. These results suggest that R galaxies, though accreted more recently, have already had enough time to evolve towards a dynamical state more consistent with that of the dynamically relaxed P population. Finally, our findings suggest that the ionized gas in early-type R galaxies likely originates from accretion from their own hot gas haloes, and that its removal triggers the transition towards the P phase over relatively long time-scales.