The baryon budget on the galaxy group/cluster boundary

We present a study of the hot gas and stellar content of five optically selected poor galaxy clusters, including a full accounting of the contribution from intracluster light (ICL) and a combined hot gas and hydrostatic X-ray mass analysis with XMM-Newton observations. We find weighted mean stellar (including ICL), gas and total baryon mass fractions within r(500) of 0.026 +/- 0.003, 0.070 +/- 0.005 and 0.096 +/- 0.006, respectively, at a corresponding weighted mean M-500 of (1.08(-0.18)(+0.21)) x 10(14) M-circle dot. Even when accounting for the intracluster stars, four out of five clusters show evidence for a substantial baryon deficit within r(500), with baryon fractions (f(b)) between 50 +/- 6 and 59 +/- 8 per cent of the universal mean level (i.e. Omega(b)/Omega(m)), the remaining cluster having f(b) = 75 +/- 11 per cent. For the three clusters where we can trace the hot halo to r(500) we find no evidence for a steepening of the gas density profile in the outskirts with respect to a power law, as seen in more massive clusters. We find that in all cases, the X-ray mass measurements are larger than those originally published on the basis of the galaxy velocity dispersion (sigma) and an assumed sigma-M-500 relation, by a factor of 1.7-5.7. Despite these increased masses, the stellar fractions (in the range 0.016-0.034, within r(500)) remain consistent with the trend with mass published by Gonzalez et al., from which our sample is drawn.