标题： 星系团的重子分数来自X射线测量 显示英文标题

标题： The Baryonic Fraction in Groups of Galaxies from X-Ray Measurements

摘要： 本文回顾了近期 ROSAT 系统对三组星系中热星系际气体的 X 射线探测，并重新评估了这些星系群中重子分数的计算结果。 我们表明，假设流体静力平衡，所获得的重子分数应该（或许是敏感地）取决于探测到 X 射线的半径以及气体的温度分布。 我们发现 NGC 2300 星系群在 25 度以内的重子分数至少为 20%，因此比 Mulchaey 等人 (1993) 的原始分析结果高出五倍多，也远高于从大爆炸核合成中获得的结果，但与其他两组以及富星系团相似。 如果 $\Omega h_{50}^2 = 0.3$，则在此重子分数下，星系群可以很好地示踪宇宙中重子的分布。 重子分数随半径增加而增加，这与所有三个星系群的X射线数据一致。 然而，对NGC 2300星系群的详细分析表明，重子分数对半径的依赖性并没有受到数据的很好约束，部分原因是估计背景的不确定性。 显示英文摘要

摘要： The recent {\sl ROSAT \/} X-ray detections of hot intergalactic gas in three groups of galaxies are reviewed and the resulting baryonic fraction in these groups is reevaluated. We show that the baryonic fraction obtained, assuming hydrostatic equilibrium, should depend, perhaps sensitively, on the radius out to which the X-rays are detected, and the temperature profile of the gas. We find that the NGC 2300 group has a baryonic fraction out to $25'$ of at least 20\%, thus over five times higher than in the original analysis of Mulchaey \etal\ (1993), and also much higher than one would obtain from big-bang nucleosynthesis, but similar to the other two groups as well as rich clusters. With this baryonic fraction, groups would be fair tracers of the distribution of baryons in the Universe if $\Omega h_{50}^2 = 0.3$. A baryonic fraction that increases with radius is consistent with the X-ray data from all three groups. However, a detailed analysis of the NGC 2300 group shows that the dependence of baryonic fraction on radius is not well constrained by the data, in part because of uncertainties in the estimated background.