Missing Iron Problem and Type Ia Supernova Enrichment of Hot Gas in Galactic Spheroids

TL;DR

This paper investigates the missing iron problem in hot gas of galactic spheroids, proposing that buoyant, high-temperature iron ejecta from Type Ia supernovae are difficult to detect in X-ray observations, explaining the observed abundance gradients.

Contribution

The study models the evolution and distribution of supernova iron ejecta in galactic hot gas, revealing how buoyancy and mixing processes cause the observed iron abundance gradients and measurement biases.

Findings

Iron ejecta are hot and buoyant, escaping detection in X-ray observations.

Radial iron abundance gradients can be explained by ejecta dynamics and mixing.

X-ray measurement biases underestimate true iron abundance in hot gas.

Abstract

Type Ia supernovae (Ia SNe) provide a rich source of iron for hot gas in galactic stellar spheroids. However, the expected super-solar iron abundance of the hot gas is not observed. Instead, X-ray observations often show decreasing iron abundance toward galactic central regions, where the Ia SN enrichment is expected to be the highest. We examine the cause of this missing iron problem by studying the enrichment process and its effect on X-ray abundance measurements of the hot gas. The evolution of Ia SN iron ejecta is simulated in the context of galaxy-wide hot gas outflows, in both supersonic and subsonic cases, as may be expected for hot gas in galactic bulges or elliptical galaxies of intermediate masses. SN reverse-shock heated iron ejecta is typically found to have a very high temperature and low density, hence producing little X-ray emission. Such hot ejecta, driven by its large buoyancy, can quickly reach a substantially higher outward velocity than the ambient medium, which is dominated by mass loss from evolved stars. The ejecta is gradually and dynamically mixed with the medium at large galactic radii. The ejecta is also slowly diluted and cooled by {\sl insitu} mass injection from evolved stars. These processes together naturally result in the observed positive gradient in the average radial iron abundance distribution of the hot gas, even if mass-weighted. This trend is in addition to the X-ray measurement bias that tends to underestimate the iron abundance for the hot gas with a temperature distribution.