Nitrogen abundance in the X-ray halos of clusters and groups of galaxies

TL;DR

This study measures nitrogen abundances in the X-ray halos of galaxy clusters and groups to understand their chemical enrichment, finding higher nitrogen ratios than in the Milky Way and highlighting the need for further radial distribution studies.

Contribution

First detailed constraints on nitrogen abundances in the intracluster medium using high-resolution X-ray spectra, comparing results with stellar yields and Galactic populations.

Findings

Nitrogen abundance is well constrained in only one cluster and seven groups.

ICM [N/Fe] and [N/O] ratios are higher than in the Galaxy.

The [O/Fe] -- [Fe/H] relation in ICM is similar to that of the Galaxy.

Abstract

Chemical abundances in the X-ray halos (also known as the intracluster medium, ICM) of clusters and groups of galaxies can be measured via prominent emission line features in their X-ray spectra. Elemental abundances are footprints of time-integrated yields of various stellar populations that have left their specific abundance patterns prior to and during the cluster and group evolution. We aim to constrain nitrogen abundances in the CHEmical Evolution RGS Sample (CHEERS), which contains 44 nearby groups and clusters of galaxies, in order to have a better understanding of their chemical enrichment. We examine the high-resolution spectra of the CHEERS sample carefully and take into account various systematic effects in the spectral modelling. We compare the observed abundance ratios with those in the Galactic stellar populations, as well as predictions from stellar yields (low- and intermediate-mass stars, massive stars and degenerate stars). The nitrogen abundance can only be well constrained ($\gtrsim3\sigma$) in one cluster of galaxies and seven groups of galaxies. The [O/Fe] -- [Fe/H] relation of the ICM is comparable to that for the Galaxy, while both [N/Fe] and [N/O] ratios of the ICM are higher than in the Galaxy. Future studies on nitrogen radial distributions are required to tell whether the obtained higher [N/Fe] and [N/O] ratios are biased due to the small extraction region ($r/r_{500}\lesssim0.05$) that we adopt here. Since abundances of odd-$Z$ elements are more sensitive to the initial metallicity of stellar populations, accurate abundance measurements of N, Na and Al are required to better constrain the chemical enrichment in the X-ray halos of clusters and groups of galaxies.