Origin of Warm High-Velocity Dense Gas in ULIRGs

TL;DR

This paper investigates the origins of warm, high-velocity dense molecular gas in ULIRGs using 3-D hydrodynamic simulations, suggesting inhomogeneous ISM structures around black holes cause observed absorption features.

Contribution

It introduces a 3-D hydrodynamic model to explain the origin of dense, warm molecular gas in ULIRGs and predicts observable turbulent velocity fields.

Findings

Dense gas absorption is linked to inhomogeneous ISM near black holes.

High-density regions occupy small volume fractions, explaining infrequent observations.

Turbulent velocity fields cause nearly equal probabilities of blue- and red-shifted gas observations.

Abstract

Possible origins of the molecular absorption discovered in some ULIRGs are investigated, based on a 3-D hydrodynamic model of star-forming interstellar gas in a galactic central region. The blue-shifted, warm ($\sim 200-300$ K), dense ($>10^6$ cm$^{-3}$) molecular gas suggested by CO absorption in IRAS 08752+3915 could be caused by the innermost region of the inhomogeneous inter-stellar medium (ISM) around a supermassive black hole. The infrequent observations of the dense gas with absorption in ULIRGs and Seyfert 2 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 non-linear development of instabilities. The model predicts a turbulent velocity field in the obscuring material, therefore blue- and red-shifted gases should be observable with nearly the same probability for the large enough statistical samples.