Revealing O VII from stacked X-ray grating spectra of clusters, groups and elliptical galaxies

TL;DR

This study detects O VII emission in stacked X-ray spectra of galaxy clusters and elliptical galaxies with low temperatures, revealing insights into cooling processes and elemental abundances in hot gas environments.

Contribution

First detection of O VII in stacked spectra of galaxy groups and ellipticals, providing new constraints on cooling and chemical composition in these systems.

Findings

Detected O VII emission in objects with T<1 keV

O VII/Fe XVII flux ratio indicates suppressed cooling below 0.5 keV

Measured N/O ratio of 4.0 +- 0.6 Solar and C/O ratio of 0.9 +- 0.3 Solar

Abstract

We stack 4.6 Ms of high spectral resolution XMM-Newton Reflection Grating Spectrometer spectra from galaxy clusters, groups of galaxies and elliptical galaxies. For those objects with a central temperature of less than 1 keV, we detect O VII for the first time, with a probability of false detection of 2.5x10^-4. The flux ratio of the O VII to Fe XVII lines is 1/4 to 1/8 of the emission expected for isobaric radiative cooling in the absence of heating. There is either a process preventing cooling below 0.5 keV, anomalous O/Fe abundance ratios, absorbing material around the coolest X-ray emitting gas or non-radiative cooling taking place. The mean N VII emission line is strong in the sub-keV sample. As the ratio of the hydrogenic N and O lines is largely independent of temperature, we measure a mean N/O ratio of 4.0 +- 0.6 Solar. Although the continuum around the C VI lines is difficult to measure we can similarly estimate that the C/O ratio is 0.9 +- 0.3 Solar.