Simulations of Supernova Remnants in Diffuse Media III. The Population of Buoyant Remnants Above the Milky Way's Disk

Abstract

We model SNRs at a variety of heights above the disk. Our detailed numerical simulations include non-equilibrium ionization and recombination and follow the remnants' evolution until their hot bubbles have cooled. We analytically calculate the bubbles' buoyant acceleration. From the results, we estimate the time and space average O VI, N V, and C IV column densities and emission intensities, 1/4 keV soft X-ray surface brightness, area coverage, and volume occupation of the population of isolated SNRs above the Galaxy's HI layer. Irrespective of assumed supernova explosion energy, ambient nonthermal pressure, or frictional drag coefficient, the predicted O VI column density matches the observed distribution between 130 pc and 2000 pc. The SNRs' O VI intensity is a significant fraction of the average observed intensity. Within the range of uncertainty in the SN rate, such SNRs can explain all of the observed 1/4 keV surface brightness attributed to the extraplanar gas beyond the H I layer in the southern hemisphere (~400 x 10^-6 counts/s/arcmin^2). Thus, extraplanar SNRs could be the most important source of hot gas between the Local Bubble and z ~ 2000 pc in the relatively quiescent southern hemisphere. These results stand whether the remnants are assumed to be buoyant or not. The population of old extraplanar SNRs should cover most of the high latitude sky, but bright young extraplanar SNRs should cover less than 1% of the sky. Perhaps the l=247, b=-64 crescent in the 1/4 keV X-ray maps could be a young remnant.