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

Abstract

We present analysis of Chandra and XMM-Newton observations of three early-type galaxies, NGC 57, 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 Z ⊙ 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 active galactic nucleus heating. For NGC 57 and 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 (M/L) of 44.7 +4.0 8.5 M ⊙ /L B⊙ (1σ uncertainties) at 4.75r e . This is very similar to the M/L found for NGC 4555 and confirms that isolated ellipticals can possess sizable dark matter haloes. We find a significantly lower M/L for NGC 7796, 10.6 +2.5 -2.3 M ⊙ /L B⊙ at 5r e , and discuss the possibility that NGC 7796 hosts a galactic wind, causing us to underestimate its mass.