SIMBAD references

We analyse the L_x-σ_v relation for the new Mulchaey et al. group Atlas. We find that once we take into account the possible statistical bias introduced by the cutoff in luminosity, we recover a relation that is consistent with that of clusters, i.e., L_x ∝σ^4.1. The larger scatter of this relation for groups of galaxies could be attributed to an orientation effect, due to which the radial velocity dispersion of groups oriented close to orthogonal to the line of sight, would be underestimated. This effect could also contribute to flattening the slope of the group L_x-σ_v relation.