Multiphase Gas in Elliptical Galaxies: The Role of Type Ia Supernovae

Abstract

Massive elliptical galaxies harbor large amounts of hot gas ( T ≳ 10 ^6 K) in their interstellar medium (ISM) but are typically quiescent in star formation. The jets of active galactic nuclei (AGNs) and Type Ia supernovae (SNe Ia) inject energy into the ISM, which offsets its radiative losses and keeps it hot. SNe Ia deposit their energy locally within the galaxy compared to the larger few ×10 kiloparsec-scale AGN jets. In this study, we perform high-resolution (512 ^3 ) hydrodynamic simulations of a local (1 kpc ^3 ) density-stratified patch of the ISM of massive galaxies. We include radiative cooling and shell-averaged volume heating, as well as randomly exploding SN Ia. We study the effect of different fractions of supernova (SN) heating (with respect to the net cooling rate), different initial ISM density/entropy (which controls the growth time t _ti of the thermal instability), and different degrees of stratification (which affect the freefall time t _ff ). We find that SNe Ia drive predominantly compressive turbulence in the ISM with a velocity dispersion of σ _v up to 40 km s ^−1 and logarithmic density dispersion of σ _s ∼ 0.2–0.4. These fluctuations trigger multiphase condensation in regions of the ISM, where $\min ({t}_{\mathrm{ti}})/{t}_{\mathrm{ff}}\lesssim 0.6\exp (6{\sigma }_{s})$ , in agreement with theoretical expectations that large density fluctuations efficiently trigger multiphase gas formation. Since the SN Ia rate is not self-adjusting, when the net cooling drops below the net heating rate, SNe Ia drive a hot wind which sweeps out most of the mass in our local model. Global simulations are required to assess the ultimate fate of this gas.

Keywords

• Early-type galaxies
• Interstellar medium
• Type Ia supernovae
• Cooling flows