Feedback, scatter and structure in the core of the PKS 0745−191 galaxy cluster

Abstract

We present Chandra X-ray Observatory observations of the core of the galaxy cluster PKS 0745-191. Its centre shows X-ray cavities caused by AGN feedback and cold fronts with an associated spiral structure. The cavity energetics imply they are powerful enough to compensate for cooling. Despite the evidence for AGN feedback, the Chandra and XMM-RGS X-ray spectra are consistent with a few hundred solar masses per year cooling out of the X-ray phase, sufficient to power the emission line nebula. The coolest X-ray emitting gas and brightest nebula emission is offset by around 5 kpc from the radio and X-ray nucleus. Although the cluster has a regular appearance, its core shows density, temperature and pressure deviations over the inner 100 kpc, likely associated with the cold fronts. After correcting for ellipticity and projection effects, we estimate density fluctuations of ∼ 4 per cent, while temperature, pressure and entropy have variations of 10 − 12 per cent. We describe a new code, MBPROJ, able to accurately obtain thermodynamical cluster profiles, under the assumptions of hydrostatic equilibrium and spherical symmetry. The forward-fitting code compares model to observed profiles using Markov Chain Monte Carlo and is applicable to surveys, operating on 1000 or fewer counts. In PKS0745 a very low gravitational acceleration is preferred within 40 kpc radius from the core, indicating a lack of hydrostatic equilibrium, deviations from spherical symmetry or non-thermal sources of pressure.

ALMA observations of cold molecular gas filaments trailing rising radio bubbles in PKS 0745−191

“…Fig. 9 shows that the molecular gas filaments also trace the brightest regions of optical line emission and this gas is coincident with low temperature X-ray gas (Sanders et al 2014). The time taken for the radio bubbles to rise at the sound speed to the observed projected radii is ∼ 10 Myr for the E bubble and ∼ 20 Myr for the NW bubble.…”

“…This more diffuse structure observed in PKS 0745-191 in 1−2 GHz VLA images extends towards two X-ray surface brightness depressions, each ∼ 20 kpc across, located to the N and SE in Chandra observations ( Fig. 10; Rafferty et al 2006;Sanders et al 2014). The AGN has inflated two large radio bubbles detected as cavities in the X-ray emission where the radio lobes have displaced the cluster's hot atmosphere (McNamara & Nulsen 2007;Fabian 2012;.…”

“…Sanders et al (2014) found evidence for non-thermal pressure within a radius of 40 kpc in PKS 0745-191 and showed that the coolest Xray emitting gas is offset by around 5 kpc from the nucleus. This asymmetry could be related to sloshing of the cool core (Sanders et al 2014) or, for a magnetically-dominated core, it could indicate clumping of the hot gas as the ICM is squeezed by the magnetic pressure. The hot gas in the cluster centre will therefore have a low filling factor and a lower mean density compared to a uniformly distributed medium.…”

“…Using a total molecular gas mass of 5 × 10 9 M , a gas velocity of 100 km s −1 and velocity dispersion of 100 km s −1 , the energy required to accelerate the molecular gas is several ×10 57 erg. By calculating the 4PV work done inflating the two radio bubbles in the hot atmosphere, Sanders et al (2014) estimated that the AGN outburst supplies ∼ 3 × 10 60 erg, which is energetically sufficient to uplift the molecular gas. Buoyant bubbles cannot lift more weight than they displace.…”

“…This would however require catastropic cooling of the hot gas around the radio bubbles. From Sanders et al (2014), the hot gas density profile within ∼ 20 kpc radius can be approximated by a powerlaw ρ ∝ r −1 . Extrapolating into the cluster centre, the hot gas mass is ∼ 4 × 10 9 M within a sphere of radius 5 kpc, which covers the extent of the molecular filaments.…”