THE UNUSUAL X-RAY MORPHOLOGY OF NGC 4636 REVEALED BY DEEP CHANDRA OBSERVATIONS: CAVITIES AND SHOCKS CREATED BY PAST ACTIVE GALACTIC NUCLEUS OUTBURSTS

We present Chandra ACIS-I and ACIS-S observations (similar to 200 ks in total) of the X-ray luminous elliptical galaxy NGC 4636, 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 these features defines the rim of an ellipsoidal bubble. An additional bubble-like feature, whose northern rim is located similar to 2 kpc south of the northeastern 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 NGC 4636. As a confirmation to this scenario, radio jets extending toward the bubbles and a central weak X-ray and radio source are detected and are most likely the signs of active galactic nuclei activity which was more intense in the past. A bright dense core of similar to 1 kpc radius is observed at the center of NGC 4636. 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.