Black hole accretion and star formation as drivers of gas excitation and chemistry in Markarian 231
Veilleux, S.; Sanders, D. B.; Savini, G.; Isaak, K. G.; Armus, L.; Greve, T. R.; Walter, F.; Martín-Pintado, J.; Harris, A. I.; Spinoglio, L.; Charmandaris, V.; van der Werf, P. P.; Evans, A.; González-Alfonso, E.; Aalto, S.; Mazzarella, J.; Henkel, C.; Kramer, C.; Xilouris, E. M.; Güsten, R.; Stacey, G.; Lord, S.; Spaans, M.; Wiedner, M. C.; Gao, Y.; Meijerink, R.; Weiß, A.; Loenen, A. F.; Fischer, J.; Israel, F. P.; Papadopoulos, P. P.; Dasyra, K. M.; Naylor, D.; Smith, H. A.; Fulton, T.; Vlahakis, C.; Rykala, A.
Netherlands, United Kingdom, Canada, Germany, United States, Spain, Sweden, Greece, France, China, Denmark, Italy
Abstract
We present a full high resolution SPIRE FTS spectrum of the nearby ultraluminous infrared galaxy Mrk 231. 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 Mrk 231 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 Mrk 231. 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 Mrk 231 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 Mrk 231. The extraordinary luminosity of the OH+ and H2O+ lines reveals the signature of X-ray driven excitation and chemistry in this region.