Origin of warm high-velocity dense gas in ultraluminous infrared galaxies

Possible origins of molecular absorption discovered in some ultraluminous infrared galaxies (ULIRGs) are investigated, based on a 3-D hydrodynamic model of star-forming interstellar gas in a galactic central region. The blueshifted, warm (similar to 200-300 K), dense (> 106 cm(-3)) molecular gas suggested by the CO absorption in IRAS 08572+3915 could be caused by the innermost region of the inhomogeneous interstellar medium (ISM) around a supermassive black hole. The infrequent observations of dense. gas with absorption in ULIRGs and type-2 Seyfert galaxies could simply suggest that the high-density regions occupy only a very small volume fraction of the obscuring material. This is naturally expected if the inhomogeneous structure of the ISM is caused by a nonlinear development of instabilities. Since the model predicts a turbulent velocity field in the obscuring material, blue- and red-shifted gases should be observable with nearly the same probability for sufficiently large statistical samples.