Diffusion of Elements in the Interstellar Medium in Early-Type Galaxies

TL;DR

This study investigates how diffusion affects element distribution in the hot interstellar medium of early-type galaxies, revealing significant helium enrichment that impacts metallicity measurements and stellar evolution.

Contribution

It provides the first detailed calculation of diffusion effects in early-type galaxies using observed gas profiles and Burgers' equations, highlighting the potential for helium enrichment.

Findings

Helium abundance can increase by 60% within one billion years.

Diffusion causes significant helium enrichment in galaxy centers.

The effect can bias X-ray metallicity measurements.

Abstract

We consider the role of diffusion in the redistribution of elements in the hot interstellar medium (ISM) of early-type galaxies. It is well known that gravitational sedimentation can affect significantly the abundances of helium and heavy elements in the intracluster gas of massive galaxy clusters. The self-similarity of the temperature profiles and tight mass--temperature relation of relaxed cool-core clusters suggest that the maximum effect of sedimentation take place in the most massive virialized objects in the Universe. However, Chandra and XMM-Newton observations demonstrate more complex scaling relations between the masses of early-type galaxies and other parameters, such as the ISM temperature and gas mass fraction. An important fact is that early-type galaxies can show both decreasing and increasing radial temperature profiles. We have calculated the diffusion based on the observed gas density and temperature distributions for 13 early-type galaxies that belonging to the different environments and cover a wide range of X-ray luminosities. To estimate the maximum effect of sedimentation and thermal diffusion, we have solved the full set of Burgers' equations for a non-magnetized ISM plasma. The results obtained demonstrate a considerable increase of the He/H ratio within one effective radius for all galaxies of our sample. For galaxies with a flat or declining radial temperature profile the average increase of the helium abundance is 60\% in one billion years of diffusion. The revealed effect can introduce a significant bias in the metal abundance measurements based on X-ray spectroscopy and can affect the evolution of stars that could be formed from a gas with a high helium abundance.