The role of molecular gas in the nuclear regions of IRAS 00183-7111. ALMA and X-ray investigations of an ultraluminous infrared galaxy

Aims: We present a multi-frequency study of the ultraluminous infrared galaxy (ULIRG) IRAS 00183-7111 (z = 0.327), selected from the Spoon diagnostic diagram as a highly obscured active galactic nucleus (AGN) candidate. ALMA millimetre and X-ray observations are used; the main aim is to verify at what level the molecular gas, traced by the CO, may be responsible for the obscuration observed at X-ray energies. Theory and observations both suggest that galaxy-scale absorption may play a role in the AGN obscuration at intermediate (i.e. Compton-thin) column densities.
Methods: We calibrated and analysed ALMA archival Cycle 0 data in two bands (Bands 3 and 6). The X-ray properties of IRAS 00183-7111 were studied by reducing and analysing separately archival Chandra and XMM-Newton data; recently acquired NuSTAR spectra were first examined individually and then added to the Chandra and XMM spectra for the broad-band (0.5 - 24 keV, observed frame) analysis.
Results: We derived a molecular gas column density of (8.0 ± 0.9) × 10²¹ cm^-2 from the ALMA CO_(1-0) detection, while the best-fit column density of cold gas obtained from X-ray spectral fitting is 6.8_-1.5^+2.1×10²² cm^-. The two quantities suggest that the molecular gas may contribute only a fraction of the AGN obscuration; however, the link between them is not straightforward. The nuclear regions of IRAS 00183-7111 are likely stratified into different layers of matter: one inner and highly ionized by the strong radiation field of the AGN (as inferred from the high-ionization iron line found in the X-ray spectra), and one outer and colder, extending more than 5 kpc from the nucleus (as traced by the molecular gas observed with ALMA). The molecular gas regions also give rise to a vigorous starburst with SFR 260 ± 28 M_⊙ yr^-1. The complexity of this nuclear environment makes it difficult to identify the origin of the AGN obscuration given the quality of the data currently available. Higher resolution observations in the millimetre regime are needed to deeply investigate this issue.