Investigating the Potential Dilution of the Metal Content of Hot Gas in Early-Type Galaxies by Accreted Cold Gas

TL;DR

This study investigates the metal content of hot gas in early-type galaxies, finding that cold gas does not dilute hot gas metallicity as previously thought, but molecular gas may have an anti-correlating effect.

Contribution

It provides new evidence that hot gas metal abundance is not significantly diluted by atomic gas and suggests molecular gas may influence metal content.

Findings

Most galaxies have Fe abundance ≥ 0.3 Z⊙, contradicting previous low estimates.

No correlation between hot gas Fe abundance and atomic gas content.

Negative correlation between Fe abundance and molecular-to-hot gas mass ratio.

Abstract

The measured emission-weighted metal abundance of the hot gas in early-type galaxies has been known to be lower than theoretical expectations for 20 years. In addition, both X-ray luminosity and metal abundance vary significantly among galaxies of similar optical luminosities. This suggests some missing factors in the galaxy evolution process, especially the metal enrichment process. With {\it Chandra} and {\it XMM-Newton}, we studied 32 early-type galaxies (kT $\lesssim$ 1 keV) covering a span of two orders of $L_{X,\rm gas}/L_{K}$ to investigate these missing factors. Contrary to previous studies that X-ray faint galaxies show extremely low Fe abundance ($\sim 0.1$ Z${_\odot}$), nearly all galaxies in our sample show an Fe abundance at least 0.3 Z${_\odot}$, although the measured Fe abundance difference between X-ray faint and X-ray bright galaxies remains remarkable. We investigated whether this dichotomy of hot gas Fe abundances can be related to the dilution of hot gas by mixing with cold gas. With a subset of 24 galaxies in this sample, we find that there is virtually no correlation between hot gas Fe abundances and their atomic gas content, which disproves the scenario that the low metal abundance of X-ray faint galaxies might be a result of the dilution of the remaining hot gas by pristine atomic gas. In contrast, we demonstrate a negative correlation between the measured hot gas Fe abundance and the ratio of molecular gas mass to hot gas mass, although it is unclear what is responsible for this apparent anti-correlation. We discuss several possibilities including that externally originated molecular gas might be able to dilute the hot gas metal content.