XMM-Newton and Chandra observations of three X-ray-faint early-type galaxies

Abstract

We present XMM-Newton observations of three X-ray-underluminous elliptical galaxies, NGC 3585, 4494 and 5322. All three galaxies have relatively large optical luminosities (logL_B= 10.35-10.67 L_solar) but have X-ray luminosities consistent with emission from discrete sources only. In conjunction with a Chandra observation of NGC 3585, we analyse the XMM-Newton data and show that the three galaxies are dominated by discrete source emission, but do possess some X-ray-emitting gas. The gas is at relatively low temperatures, kT~= 0.25-0.44 keV. All three galaxies show evidence of recent dynamical disturbance and formation through mergers, including kinematically distinct cores, young stellar ages and embedded stellar discs. This leads us to conclude that the galaxies formed relatively recently and have yet to build up large X-ray haloes. They are likely to be in a developmental phase where the X-ray gas has a very low density, making it undetectable outside the galaxy core. However, if the gas is a product of stellar mass loss, as seems most probable, we would expect to observe supersolar metal abundances. While abundance is not well constrained by the data, we find best-fitting abundances <0.1 Z_solar for single-temperature models, and it seems unlikely that we could underestimate the metallicity by such a large factor.