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 r₅₀₀ of 0.026 ± 0.003, 0.070 ± 0.005 and 0.096 ± 0.006, respectively, at a corresponding weighted mean M₅₀₀ of (1.08_-0.18^+0.21) × 10¹⁴ M_⊙. Even when accounting for the intracluster stars, four out of five clusters show evidence for a substantial baryon deficit within r₅₀₀, with baryon fractions (f_b) between 50 ± 6 and 59 ± 8 per cent of the universal mean level (i.e. Ω_b/Ω_m), the remaining cluster having f_b = 75±11 per cent. For the three clusters where we can trace the hot halo to r₅₀₀ 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 (σ) and an assumed σ-M₅₀₀ relation, by a factor of 1.7-5.7. Despite these increased masses, the stellar fractions (in the range 0.016-0.034, within r₅₀₀) remain consistent with the trend with mass published by Gonzalez et al., from which our sample is drawn.