X-ray luminosities of galaxies in groups

We have derived the X-ray luminosities of a sample of galaxies in groups, making careful allowance for contaminating intragroup emission. The L X :L B and L X :L FIR relations of spiral galaxies in groups appear to be indistinguishable from those in other environments, however the elliptical galaxies fall into two distinct classes. The first class is central-dominant group galaxies which are very X-ray luminous, and may be the focus of group cooling flows. All other early-type galaxies in groups belong to the second class, which populates an almost constant band of L X /L B over the range 9.8 < log L B < 11.3. The X-ray emission from these galaxies can be explained by a superposition of discrete galactic X-ray sources together with a contribution from hot gas lost by stars, which varies a great deal from galaxy to galaxy. In the region where the optical luminosity of the non-central group galaxies overlaps with the dominant galaxies, the dominant galaxies are over an order of magnitude more luminous in X-rays.We also compared these group galaxies with a sample of isolated early-type galaxies, and used previously published work to derive L X :L B relations as a function of environment. The non-dominant group galaxies have mean L X /L B ratios very similar to that of isolated galaxies, and we see no significant correlation between L X /L B and environment. We suggest that previous findings of a steep L X :L B relation for early-type galaxies result largely from the inclusion of group-dominant galaxies in samples.

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