Physical condition of the molecular gas at the centre of NGC1097

Abstract

We have used the XCO conversion factor, local thermodynamic equilibrium and large velocity gradient approximation to parametrize the cold and warm phases of the interstellar medium from five different low transitions of the CO molecule in the central 21arcsec (kpc) region of NGC1097. We have applied a one-component model and derived a typical kinetic temperature of about 33 K, a molecular hydrogen density of 4.9 × 103M⊙pc-3 and a CO column density of 1.2 × 10-2M⊙pc-2. A two-component model results in 85 per cent cold-to-total gas fraction in the presence of a 90 K warm counterpart. Furthermore, we 'resolve' the spatially unresolved single-dish observations by selecting velocity channels that in an interferometric velocity map correspond to specific regions. We have selected five such regions and found that the physical properties in these regions are comparable to those derived from the full line profile. This implies that the central kpc of NGC1097 is rather homogeneous in nature and, although the regions are not uniquely located within the ring, the star formation along the ring is homogeneously distributed (in agreement with recent Herschel observations). We have further revised the mass-inflow rate on to the supermassive black hole in this prototype low-ionization nuclear emission-line region/Seyfert 1 galaxy and found that accounting for the total interstellar medium and applying a careful contribution of the disc thickness and corresponding stability criterion increases the previous estimations by a factor of 10. Finally, we have calculated the XCO conversion factor for the centre of NGC1097 using an independent estimation of the surface density to the CO emission and obtained at a radius 10.5arcsec and at a radius 7.5arcsec. With the approach and analysis described in this paper, we have demonstrated that important physical properties can be derived to a resolution beyond the single-dish resolution element; however, caution is necessary while interpreting the results. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.