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Deviations of galaxy groups from cluster scaling relations can be understood in terms of an excess of entropy in groups. The main effect of this excess is to reduce the density and thus luminosity of the intragroup gas. Given this, groups should also should show a steep relationship between X-ray luminosity and velocity dispersion. However, previous work suggests that this is not the case with many measuring slopes flatter than the cluster relation. Examining the group L_X:\sigma relation shows that much of the flattening is caused by a small subset of groups which show very high X-ray luminosities for their velocity dispersions (or vice versa). Detailed Chandra study of two such groups shows that earlier ROSAT results were subject to significant (~30-40%) point source contamination, but confirm that a significant hot IGM is present in these groups, although these are two of the coolest systems in which intergalactic X-ray emission has been detected. Their X-ray properties are shown to be broadly consistent with those of other galaxy groups, although the gas entropy in NGC 1587 is unusually low, and its X-ray luminosity correspondingly high for its temperature, compared to most groups. This leads us to suggest that the velocity dispersion in these systems has been reduced in some way, and we consider how this might have come about., Comment: Accepted for publication in ApJ

We have examined the morphological make-up of X-ray bright groups. The brighter galaxies in these groups exhibit clear morphology-density and morphology-radius relations. The group morphology-density relation is offset from the cluster relation in the sense that at a given surface density, X-ray bright groups have a lower spiral fraction. After correcting for projection effects the morphology-3D density relation is still shifted towards fewer spirals for a given 3D density, in comparison with clusters. A simple model which corrects the group data for the effects of projection and for the expected higher merging rate in groups, brings the morphology-density relation into good agreement with that of clusters, suggesting that the relation may be driven by two-body interactions. The fraction of S0 galaxies in these X-ray bright groups is at least as high as that observed in nearby clusters. Given the low velocity dispersion of groups, this indicates that ram pressure stripping is not the dominant mechanism for S0 formation., Comment: 5 pages, accepted for publication in MNRAS Letters

Combining X-ray data from the ROSAT PSPC and optical data drawn from the literature, we examine in detail the relationship between the X-ray and optical properties of X-ray bright galaxy groups. We find a relationship between optical luminosity and X-ray temperature consistent with that expected from self-similar scaling of galaxy systems, L_B \propto T^{1.6 +/- 0.2}. The self-similar form and continuity of the L_B : T relation from clusters to groups and the limited scatter seen in this relation, implies that the star formation efficiency is rather similar in all these systems. We find that the bright extended X-ray components associated with many central galaxies in groups appear to be more closely related to the group than the galaxy itself, and we suggest that these are group cooling flows rather than galaxy halos. In addition we find that the optical light in these groups appears to be more centrally concentrated than the light in clusters. We also use the optical and X-ray data to investigate whether early or late type galaxies are primarily responsible for preheating in groups. Using three different methods, we conclude that spiral galaxies appear to play a comparable role to early types in the preheating of the intragroup medium. This tends to favour models in which the preheating arises primarily from galaxy winds rather than AGN, and implies that spirals have played a significant role in the metal enrichment of the intragroup medium., Comment: 17 pages, accepted for publication in MNRAS

We present multiband optical/infrared photometry of V4580 Sgr, the optical counterpart of the accretion-powered millisecond pulsar SAX J1808.4-3658, taken during the 1998 X-ray outburst of the system. The optical flux is consistent with emission from an X-ray-heated accretion disk. Self-consistent modeling of the X-ray and optical emission during the outburst yields best-fit extinction Av=0.68 and inclination cos(i)=0.65, assuming a distance of 2.5 kpc. The allowed inclination range requires that the pulsar's stellar companion has extremely low mass, 0.05-0.10 Msun. Some of the infrared observations are not consistent with disk emission and are too bright to be from either the disk or the companion, even in the presence of X-ray heating., Comment: 4 pages including 1 figure. Accepted for publication in ApJ Letters

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., Comment: 18 pages, 8 figures. Accepted for publication in MNRAS