The origin of ionized gas in retired galaxies: dynamical clues

TL;DR

This study explores the origins and dynamical behavior of ionized gas in retired galaxies, revealing that it likely results from accretion of hot halo gas and that these galaxies evolve dynamically to resemble passive galaxies over time.

Contribution

It provides new insights into the dynamical states and accretion histories of retired galaxies, linking ionized gas origins to their evolutionary processes.

Findings

Retired galaxies are less concentrated and have more radial orbits than passive galaxies.

Retired galaxies were accreted later than passive galaxies, except for certain types.

Ionized gas in retired galaxies likely originates from hot halo gas accretion.

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 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 $D_n4000 > 1.86$ 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 toward the P phase over relatively long timescales.