High-velocity-dispersion cold gas in ULIRG Outflows – I. Direct simulations

Abstract

Observations (e.g. Phillips, Heckman et al., Martin) have revealed cold gas with large velocity dispersions (≈300 km s^−1) within the hot outflows of ultraluminous infrared galaxies (ULIRGs). This gas may trace its origin to the Rayleigh–Taylor (RT) fragmentation of a superbubble or may arise on smaller scales. We model a ULIRG outflow at two scales to recreate this gas in three-dimensional hydrodynamic simulations using FLASH. Although resolution is limited, these models successfully produce cold gas in outflows with large velocity dispersions. Our small-scale models produce this cold gas through RT fragmentation of the super-bubble wall, but the large-scale models produce the cold gas after hot bubbles fragment the disc's gas into cold clouds which are then accelerated by thermal pressure, and supplemented by cooling within the outflow. We produce simple mock spectra to compare these simulations to observed absorption spectra and find line-widths of ≈250 km s^−1, agreeing with the lower end of observations.