Extended X-ray Emission in the HI Cavity of NGC 4151: Galaxy-scale AGN Feedback?

TL;DR

This study reports the discovery of extended soft X-ray emission in NGC 4151's HI cavity, suggesting recent AGN activity that impacts the galaxy's interstellar medium and possibly influences galaxy evolution.

Contribution

First detection of diffuse X-ray emission filling the HI cavity in NGC 4151, indicating recent AGN feedback on galaxy-scale gas structures.

Findings

X-ray emission extends ~2 kpc from nucleus

Emission consistent with thermal plasma or photoionized gas

AGN activity episode occurred within last 10^4 years

Abstract

We present the Chandra discovery of soft diffuse X-ray emission in NGC 4151 (L[0.5-2keV]~10^{39} erg s$^{-1}$), extending ~2 kpc from the active nucleus and filling in the cavity of the HI material. The best fit to the X-ray spectrum requires either a kT~0.25 keV thermal plasma or a photoionized component. In the thermal scenario, hot gas heated by the nuclear outflow would be confined by the thermal pressure of the HI gas and the dynamic pressure of inflowing neutral material in the galactic disk. In the case of photoionization, the nucleus must have experienced an Eddington limit outburst. For both scenarios, the AGN-host interaction in NGC 4151 must have occured relatively recently (some 10^4 yr ago). This very short timescale to the last episode of high activity phase may imply such outbursts occupy $\gtrsim$1% of AGN lifetime.