标题： 星系团中的缺失重子在哪里？ 显示英文标题

标题： Where are the missing baryons in clusters?

摘要： 星系团的观测表明，它们所含的重子（气体加恒星）显著少于宇宙的重子比例。 对于最巨大的星系团来说，这一“丢失重子”的谜题尤其令人惊讶，因为这些星系团预计能代表宇宙物质含量（重子和暗物质）。 在这里，我们显示重子实际上可能并没有从星系团中消失，而是分布到了比通常观测到的更大的半径范围。 重子的不足通常是在星系团的中心区域（约0.5倍的virial半径）被观测到的。 然而，观测到的气体密度分布比质量密度分布要浅得多，这意味着气体分布得比质量更广，并且气体比例随着半径增加而增加。 我们利用星系团中气体和质量的观测密度分布来外推测量的重子比例作为半径以及星系团质量的函数。 我们发现，对于所有超过5e13太阳质量的星系团，重子比例在接近virial半径处达到宇宙值。 这表明重子并没有丢失，只是位于星系团的外围。 导致气体膨胀的加热过程（星系团内气体的激波加热，加上超新星和活动星系核反馈）可能是这些结果的合理解释。 即将进行的观测应该能够探测到这些重子。 显示英文摘要

摘要： Observations of clusters of galaxies suggest that they contain significantly fewer baryons (gas plus stars) than the cosmic baryon fraction. This `missing baryon' puzzle is especially surprising for the most massive clusters which are expected to be representative of the cosmic matter content of the universe (baryons and dark matter). Here we show that the baryons may not actually be missing from clusters, but rather are extended to larger radii than typically observed. The baryon deficiency is typically observed in the central regions of clusters (~0.5 the virial radius). However, the observed gas-density profile is significantly shallower than the mass-density profile, implying that the gas is more extended than the mass and that the gas fraction increases with radius. We use the observed density profiles of gas and mass in clusters to extrapolate the measured baryon fraction as a function of radius and as a function of cluster mass. We find that the baryon fraction reaches the cosmic value near the virial radius for all groups and clusters above 5e13 solar masses. This suggests that the baryons are not missing, they are simply located in cluster outskirts. Heating processes (shock-heating of the intracluster gas, plus supernovae and AGN feedback) that cause the gas to expand are likely explanations for these results. Upcoming observations should be able to detect these baryons.