The role of molecular gas in the nuclear regions of IRAS 00183-7111.
  ALMA and X-ray investigations of an ultraluminous infrared galaxy
  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 Cycle 0
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. We derived a molecular gas
column density of $(8.0\pm0.9)\times10^{21}$ 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 $6.8^{+2.1}_{-1.5}\times10^{22}$ cm$^{-2}$. 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$\sim260\pm28$ M$_{\odot}$
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.
