Black hole accretion and star formation as drivers of gas excitation and chemistry in Mrk231

TL;DR

This study analyzes the gas excitation and chemistry in Mrk231, revealing that star formation and black hole activity both significantly influence the observed molecular line emissions and the galaxy's interstellar medium properties.

Contribution

It demonstrates that X-ray heating from the supermassive black hole explains high-J CO line excitation, providing new insights into AGN-starburst interactions in ultraluminous infrared galaxies.

Findings

Star formation accounts for CO excitation up to J=8.

X-ray heating from the black hole explains high-J CO lines.

OH+ and H2O+ lines indicate X-ray driven chemistry.

Abstract

We present a full high resolution SPIRE FTS spectrum of the nearby ultraluminous infrared galaxy Mrk231. In total 25 lines are detected, including CO J=5-4 through J=13-12, 7 rotational lines of H2O, 3 of OH+ and one line each of H2O+, CH+, and HF. We find that the excitation of the CO rotational levels up to J=8 can be accounted for by UV radiation from star formation. However, the approximately flat luminosity distribution of the CO lines over the rotational ladder above J=8 requires the presence of a separate source of excitation for the highest CO lines. We explore X-ray heating by the accreting supermassive black hole in Mrk231 as a source of excitation for these lines, and find that it can reproduce the observed luminosities. We also consider a model with dense gas in a strong UV radiation field to produce the highest CO lines, but find that this model strongly overpredicts the hot dust mass in Mrk231. Our favoured model consists of a star forming disk of radius 560 pc, containing clumps of dense gas exposed to strong UV radiation, dominating the emission of CO lines up to J=8. X-rays from the accreting supermassive black hole in Mrk231 dominate the excitation and chemistry of the inner disk out to a radius of 160 pc, consistent with the X-ray power of the AGN in Mrk231. The extraordinary luminosity of the OH+ and H2O+ lines reveals the signature of X-ray driven excitation and chemistry in this region.