X‐Ray Thermal Coronae of Galaxies in Hot Clusters: Ubiquity of Embedded Mini–Cooling Cores

Abstract

We present a systematic investigation of X-ray thermal coronae in 157 early-type galaxies and 22 late-type galaxies from a survey of 25 hot (kT > 3 keV), nearby (z < 0.05) clusters, based on Chandra archival data. Cool galactic coronae (kT = 0.5 -1.1 keV generally) have been found to be very common, > 60% in NIR selected galaxies that are more luminous than 2 L * , and > 40% in L * < L Ks < 2 L * galaxies. These embedded coronae in hot clusters are generally smaller (1.5-4 kpc radii), less luminous ( < ∼ 10 41 erg s −1 ), and less massive (10 6.5 -10 8 M ⊙ ) than coronae in poor environments, demonstrating the negative effects of hot cluster environments on galactic coronae. Nevertheless, these coronae still manage to survive ICM stripping, evaporation, rapid cooling, and powerful AGN outflows, making them a rich source of information about gas stripping, microscopic transport, and feedback processes in the cluster environment. Heat conduction across the boundary of the coronae has to be suppressed by a factor of > ∼ 100, which implies the X-ray gas in early-type galaxies is magnetized and the magnetic field plays an important role in energy transfer. Stripping through transport processes (viscosity or turbulence) also needs to be suppressed by at least a factor of ten at the coronal boundary. The stellar mass loss inside the corona is key to maintaining the gas balance in coronae. The luminous, embedded coronae, with high central density (0.1 -0.4 cm −3 ), are mini-versions of group and cluster cooling cores. As the prevalence of coronae of massive galaxies implies a long lifetime ( > ∼ several Gyr), there must be a heat source inside coronae to offset cooling. While we argue that AGN heating may not generally be the heat source, we conclude that SN heating can be enough as long as the kinetic energy of SNe can be efficiently dissipated. We have also observed a connection between radiative cooling and the SMBH activity of their host galaxies as many coronae are associated with powerful radio galaxies. Cooling of the coronal gas may provide fuel for the central SMBH and nuclear star formation in environments where the amount of galactic cold gas is otherwise at a minimum. Diffuse thermal coronae have also been detected in at least 8 of 22 late-type (Sb or later) galaxies in our sample. Evidence for enhanced star formation triggered by the ICM pressure has been found in four late-type galaxies. The fraction of luminous X-ray AGN (> 10 41 ergs s −1 ) is not small (∼ 5%) in our sample.

The Massive Survey. Iv. The X-Ray Halos of the Most Massive Early-Type Galaxies in the Nearby Universe

“…This apparent paucity of objects is expected assuming a virial origin for the heating of the gas. By comparing our sample to the simplest theoretical prediction for the virial gas temperature, such that b m s = m kT p spec 2 gas , where m p is the mass of a proton, mm p is the mean particle mass with m = 0.62, and k is the Boltzmann constant (e.g., Loewenstein & White 1999;Loewenstein 2000;Sun et al 2007;Jeltema et al 2008;Pellegrini 2011), we show that within the measurement uncertainties, no galaxies considered here have b > 1 spec , suggesting that a virial model provides a lower boundary to the observed gas temperature in early-type galaxies (e.g., Cox et al 2006). Hence, we find good evidence that in all systems, the minimum gas temperature, T gas,min , goes as s e 2 .…”

“…The L X,gas -T gas relation may further steepen in the poorest groups and individual galaxy halos as the hot gas surrounding the galaxies is no longer virialized and/or is dominated by other effects. Previous studies have sought to link these differing structural scales to provide a coherent picture of the X-ray properties of the most massive systems in the universe (e.g., O'Sullivan et al 2003;Khosroshahi et al 2007;Sun et al 2007Sun et al , 2009Diehl & Statler 2008;Mulchaey & Jeltema 2010;Vajgel et al 2014). However, observational evidence for the hot-gas origin is still in question.…”

Feedback and environmental effects in elliptical galaxies

“…Some studies, mainly using ROSAT data, found no correlation e.g., O'Sullivan et al 2001. More recent Chandra studies of groups and clusters (e.g, Jeltema et al 2008;Sun et al (2007)) seem to confirm a general anti-correlation of local density and X-ray luminosity. Observations of several galaxies in nearby clusters show evidence for ram pressure stripping; examples include M86 (Randall et al 2008) and NGC 4552 (Machacek et al 2006).…”

“…Observations of several galaxies in nearby clusters show evidence for ram pressure stripping; examples include M86 (Randall et al 2008) and NGC 4552 (Machacek et al 2006). However, despite the efficiency of stripping, most bright early-type galaxies in clusters do retain small coronae (Sun et al (2007)). …”

Scaling properties of the hot gas in early-type galaxies

“…However, recent studies with Chandra, which is better able to resolve galaxy emission, show that LX /LB is suppressed by a factor ∼3-4 in non-central galaxies within both clusters (Sun et al (2007)) and groups (Jeltema et al (2008)). This presumably results from stripping of hot gas within dense environments.…”