High-excitation OH and H_2O lines in Markarian 231: the molecular signatures of compact far-infrared continuum sources

TL;DR

This study analyzes the far-infrared spectrum of Markarian 231, revealing high-excitation molecular lines that indicate a compact, AGN-dominated core with specific physical and chemical properties, and discusses implications for galaxy luminosity sources.

Contribution

It provides the first detailed modeling of high-excitation OH and H2O lines in Mkn 231, constraining the properties of the compact continuum source and its dominant energy source.

Findings

High-excitation OH and H2O lines are radiatively pumped by the far-infrared continuum.

The continuum source is a warm, optically thick region of 200-400 pc with a luminosity dominated by an AGN.

The [C II] deficit can be explained by high C+ abundance and high luminosity-to-gas-mass ratio.

Abstract

The ISO/LWS far-infrared spectrum of the ultraluminous galaxy Mkn 231 shows OH and H_2O lines in absorption from energy levels up to 300 K above the ground state, and emission in the [O I] 63 micron and [C II] 158 micron lines. Our analysis shows that OH and H_2O are radiatively pumped by the far-infrared continuum emission of the galaxy. The absorptions in the high-excitation lines require high far-infrared radiation densities, allowing us to constrain the properties of the underlying continuum source. The bulk of the far-infrared continuum arises from a warm (T_dust=70-100 K), optically thick (tau_100micron=1-2) medium of effective diameter 200-400 pc. In our best-fit model of total luminosity L_IR, the observed OH and H2O high-lying lines arise from a luminous (L/L_IR~0.56) region with radius ~100 pc. The high surface brightness of this component suggests that its infrared emission is dominated by the AGN. The derived column densities N(OH)>~10^{17} cm^{-2} and N(H_2O)>~6x10^{16} cm^{-2} may indicate XDR chemistry, although significant starburst chemistry cannot be ruled out. The lower-lying OH, [C II] 158 micron, and [O I] 63 micron lines arise from a more extended (~350 pc) starburst region. We show that the [C II] deficit in Mkn 231 is compatible with a high average abundance of C+ because of an extreme overall luminosity to gas mass ratio. Therefore, a [C II] deficit may indicate a significant contribution to the luminosity by an AGN, and/or by extremely efficient star formation.