The origin of ionized gas in retired galaxies: dynamical clues

TL;DR

This study dissects the kinematic and dynamical states of passive (P) and retired (R) quiescent galaxies in a large ensemble of relaxed clusters to understand the origin of ionized gas in R systems. By stacking 336 clusters and applying MAMPOSSt, Jeans inversion, and PPS analyses, the authors find that P galaxies are more centrally concentrated but P and R share similar $\sigma_P(R)$ and $\beta(r)$ profiles, with elliptical R galaxies showing modestly more radial orbits. PPS reveals that R galaxies were, on average, accreted later than P galaxies, yet both populations are near dynamical equilibrium, suggesting gas removal occurs on longer timescales. The results support a scenario where the ionized gas in early-type R galaxies arises from accretion of hot halo gas, and its gradual removal triggers the P transition, while in late-type R systems the gas is likely a remnant of the pre-infall cold phase. Overall, the work links gas content, orbital evolution, and environmental processing to explain the P versus R dichotomy in cluster environments.

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, $σ_P(R)$, 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, $β(r)$. We find that P galaxies tend to reside closer to the cluster centres than R galaxies, and that both populations exhibit similar $σ_P(R)$ and $β(r)$ 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.

Figures (15)

• Figure 1: Distributions of virial radius ($r_{200}$, left panel), velocity dispersion ($\sigma_v$, middle panel), and redshift ($z$, right panel) for the 336 relaxed galaxy clusters that compose the ensemble cluster.
• Figure 2: Projected number density profiles $I(R)$ of the P (red dots, left panel) and R (blue dots, right panel) galaxy populations. The fitted NFW profile is represented by the dashed black line in each panel. The $I(R)$ and the NFW profiles are normalized by the value of the fitted NFW profile at $R/r_{200} = 0.8$. The P NFW profile is exhibited in the right panel (dotted red line) to better compare the slope of the profiles. In each panel, the horizontal axis provides the projected radial distance normalized by the $\langle r_{200} \rangle$ value given in Section \ref{['sec:Galaxy_populations']}, while the horizontal values indicate the central value of each bin.
• Figure 3: Velocity anisotropy profiles $\beta(r)$ estimated from MAMPOSSt (coloured solid line) and IJE (dashed black line) for the P (left panel) and R (right panel) galaxy populations. The uncertainties on MAMPOSSt and IJE $\beta(r)$ profiles are given by the coloured and grey shaded regions, respectively. The $\beta(r)$ profiles are displayed only in the regions where their uncertainties are lower than 0.5. The horizontal dotted black line indicates $\beta = 0$ (isotropic orbits). Purely radial and tangential orbits correspond to $\beta = 1$ and $\beta = -\infty$, respectively. In each panel, the horizontal axis provides the radial distance normalized by the $\langle r_{200} \rangle$ value given in Section \ref{['sec:Galaxy_populations']}.
• Figure 4: Observed LOS velocity dispersion profiles $\sigma_P(R)$ of the P (red dots, left panel) and R (blue dots, right panel) galaxy populations. The MAMPOSSt (coloured solid line) and IJE (dashed black line) $\sigma_P(R)$ profiles are shown (see text for details), with their uncertainties represented by coloured and grey shaded regions, respectively. In each panel, the horizontal axis provides the projected radial distance normalized by the $\langle r_{200} \rangle$ value given in Section \ref{['sec:Galaxy_populations']}, while the horizontal values indicate the central value of each bin.
• Figure 5: Normalized density distributions of P (left panel) and R (middle panel) galaxies in the PPS. The Rhee zones are represented by the dotted grey lines (see Section \ref{['subsec:PPS']}). The right panel exhibits the fractions of P (red dots) and R (blue triangles) galaxies relative to the total number of galaxies in each of the Rhee zones, along with their respective uncertainties represented by the coloured shaded regions.