The origin of ionized gas in retired galaxies: dynamical clues

We investigate the kinematical and dynamical properties of quiescent cluster galaxies with weak emission lines, referred to as retired ®, 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 2,907 P and 2,387 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 $D_{n}4000>1.86$ 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 toward the P phase over relatively long timescales.