The Impact of Type Ia Supernovae in Quiescent Galaxies. I. Formation of the Multiphase Interstellar Medium

A cool phase of the interstellar medium has been observed in many giant elliptical galaxies, but its origin remains unclear. We propose that uneven heating from Type Ia supernovae (SNe Ia), together with radiative cooling, can lead to the formation of the cool phase. The basic idea is that since SNe Ia explode randomly, gas parcels that are not directly heated by SN shocks will cool, forming multiphase gas. We run a series of idealized high-resolution numerical simulations and find that cool gas develops even when the overall SN heating rate H exceeds the cooling rate C by a factor as large as 1.4. We also find that the time for multiphase gas development depends on the gas temperature. When the medium has a temperature T = 3 × 10⁶ K, the cool phase forms within one cooling time t_c,0; however, the cool phase formation is delayed to a few times t_c,0 for higher temperatures. The main reason for the delay is turbulent mixing. Cool gas formed this way would naturally have a metallicity lower than that of the hot medium. For constant H/C, there is more turbulent mixing for higher-temperature gas. We note that this mechanism of producing cool gas cannot be captured in cosmological simulations, which usually fail to resolve individual SN remnants.