We present a study of the hot gas and stellar content of 5 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 _ { \mathrm { 500 } } of 0.026 \pm 0.003 , 0.070 \pm 0.005 and 0.096 \pm 0.006 , respectively , at a corresponding weighted mean M _ { \mathrm { 500 } } of ( 1.08 _ { -0.18 } ^ { +0.21 } ) \times 10 ^ { 14 } M _ { \odot } .
Even when accounting for the intracluster stars , 4 out of 5 clusters show evidence for a substantial baryon deficit within r _ { \mathrm { 500 } } , with baryon fractions ( f _ { \mathrm { b } } ) between 50 \pm 6 to 59 \pm 8 per cent of the Universal mean level ( i.e . \Omega _ { b } / \Omega _ { m } ) ; the remaining cluster having f _ { \mathrm { b } } = 75 \pm 11 per cent .
For the 3 clusters where we can trace the hot halo to r _ { \mathrm { 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 _ { \mathrm { 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 _ { \mathrm { 500 } } ) remain consistent with the trend with mass published by , from which our sample is drawn .