Can supernova from runaway stars mimic the signs of absorbing `super-virial' gas?

TL;DR

This paper proposes that supernovae from runaway stars above the Galactic disk can produce high-temperature, high-ionization gas that mimics the signs of super-virial gas in the circumgalactic medium, offering an alternative explanation for recent observations.

Contribution

It introduces a novel hypothesis that supernova ejecta from runaway stars can account for observed high-ionization absorption lines, challenging the traditional CGM origin interpretation.

Findings

Supernovae from runaway stars can produce gas at temperatures >10^7 K.

The model explains high column densities and super-solar abundances observed.

Covering fraction of such supernovae is less than 0.7%.

Abstract

The recent detection of large column density absorption lines from highly ionized gas in a few directions through the circumgalactic medium (CGM) of the Milky Way (MW) has been puzzling. The inferred temperature from these absorption lines far exceeds the virial temperature of the MW, and the column densities are also too large to be easily explained. In this paper, we propose a novel idea to explain these observations and claim that they may not have originated from the CGM, but from a totally different type of source, namely, stellar ejecta from supernovae (SNe) above the Galactic disk that happen to lie in the line of sight to the background quasars. About $\sim 20\%$ of massive OB stars (progenitors of core-collapse supernovae) are known to be runaway stars that have high ejection velocities near the Galactic plane and can end up exploding as SNe above the Galactic disk. We show that the associated reverse shock in the supernova remnant in the early non-radiative phase can heat the ejecta to temperatures of $\gtrsim 10^7\,{\rm K}$ and can naturally explain the observed high column density of ions in the observed `super-virial' phase along with $\alpha$-enriched super-solar abundance that is typical of core-collapse supernovae. However, SNe from runaway stars has a covering fraction of $\lesssim 0.7 \%$ and thus can only explain the observations along limited sightlines.