The unusual X-ray morphology of NGC4636 revealed by deep Chandra observations: cavities and shocks created by past AGN outbursts

TL;DR

Deep Chandra observations of NGC4636 reveal complex X-ray structures including bubbles and shocks, indicating past AGN activity that shaped the galaxy's hot gas environment.

Contribution

This study provides detailed imaging and analysis of X-ray bubbles and shocks in NGC4636, linking them to past AGN outbursts and jet activity, which was not previously well characterized.

Findings

Detection of symmetric X-ray bubbles with sharp edges and temperature jumps.

Evidence of shocks driven by past AGN jets shaping the galaxy's X-ray morphology.

Presence of a dense, possibly long-lived, bright core at the galaxy center.

Abstract

We present Chandra ACIS-I and ACIS-S observations (~200ks in total) of the X-ray luminous elliptical galaxy NGC4636, located in the outskirts of the Virgo cluster. A soft band (0.5-2 keV) image shows the presence of a bright core in the center surrounded by an extended X-ray corona and two pronounced quasi-symmetric, 8 kpc long, arm-like features. Each of this features defines the rim of an ellipsoidal bubble. An additional bubble-like feature, whose northern rim is located ~2 kpc south of the north-eastern arm, is detected as well. We present surface brightness and temperature profiles across the rims of the bubbles, showing that their edges are sharp and characterized by temperature jumps of about 20-25%. Through a comparison of the observed profiles with theoretical shock models, we demonstrate that a scenario where the bubbles were produced by shocks, probably driven by energy deposited off-center by jets, is the most viable explanation to the X-ray morphology observed in the central part of NGC4636. As a confirmation to this scenario, radio jets extending towards the bubbles and a central weak X-ray and radio source are detected and are most likely the signs of AGN activity which was more intense in the past. A bright dense core of ~1 kpc radius is observed at the center of NGC4636. A sharp decline in surface brightness from the core to the ambient gas is observed and is not accompanied by a variation in the temperature and thus could not be in thermal pressure equilibrium. However the bright core could be a long lived feature if the radio jets are acting as a balancing factor to thermal pressure or if the bright core is produced by steep abundance gradients.