The dark haloes of early-type galaxies in low-density environments: XMM-Newton and Chandra observations of NGC 57, NGC 7796 and IC 1531

TL;DR

This study analyzes X-ray observations of three isolated early-type galaxies, revealing diverse dark matter halo properties and gas characteristics, and highlights the presence of substantial dark matter in some isolated ellipticals.

Contribution

It provides detailed measurements of dark matter and gas properties in three low-density environment galaxies, demonstrating that isolated ellipticals can have significant dark matter haloes.

Findings

NGC 57 has a massive dark halo with a high mass-to-light ratio.

NGC 7796 has a lower mass-to-light ratio, possibly due to galactic wind effects.

IC 1531's halo appears to be affected by AGN heating.

Abstract

We present analysis of Chandra and XMM-Newton observations of three early-type galaxies, NGC 57, NGC 7796 and IC 1531. All three are found in very low density environments, and appear to have no neighbours of comparable size. NGC 57 has a halo of kT~0.9 keV, solar metallicity gas, while NGC 7796 and IC 1531 both have ~0.55 keV, 0.5-0.6 Zsol haloes. IC 1531 has a relatively compact halo, and we consider it likely that gas has been removed from the system by the effects of AGN heating. For NGC 57 and NGC 7796 we estimate mass, entropy and cooling time profiles and find that NGC 57 has a fairly massive dark halo with a mass-to-light ratio of 44.7 (4.0,-8.5) Msol/Lsol (1 sigma uncertainties) at 4.75 Re. This is very similar to the mass-to-light ratio found for NGC 4555 and confirms that isolated ellipticals can possess sizable dark matter haloes. We find a significantly lower mass-to-light ratio for NGC 7796, 10.6 (+2.5,-2.3) Msol/Lsol at 5 Re, and discuss the possibility that NGC 7796 hosts a galactic wind, causing us to underestimate its mass.