Monthly Notices of the Royal Astronomical Society, 370(3):1541-1555, 5, 2006. Paper Website abstract bibtex
Massive haloes of hot plasma exist around some, but not all elliptical\ngalaxies. There is evidence that this is related to the age of the\ngalaxy. In this paper, new X-ray observations are presented for three\nearly-type galaxies that show evidence of youth, in order to investigate\ntheir X-ray components and properties. NGC 5363 and NGC 2865 were found\nto have X-ray emission dominated by purely discrete stellar sources.\nLimits are set on the mass distribution in one of the galaxies observed\nwith XMM-Newton, NGC 4382, which contains significant hot gas. We detect\nthe X-ray emission in NGC 4382 out to 4re. The mass-to-light\nratio is consistent with a stellar origin in the inner regions but rises\nsteadily to values indicative of some dark matter by 4re.\nThese results are set in context with other data drawn from the\nliterature, for galaxies with ages estimated from dynamical or\nspectroscopic indicators. Ages obtained from optical spectroscopy\nrepresent central luminosity-weighted stellar ages. We examine the X-ray\nevolution with age, normalized by B- and K-band luminosities. Low values\nof Log(LX/LB) and Log(LX/LK)\nare found for all galaxies with ages between 1 and 4 Gyr. Luminous X-ray\nemission only appears in older galaxies. This suggests that the\ninterstellar medium is removed and then it takes several gigayears for\nhot gas haloes to build up, following a merger. A possible mechanism for\ngas expulsion might be associated with feedback from an active nucleus\ntriggered during a merger.