CHANDRASTUDIES OF THE X-RAY GAS PROPERTIES OF GALAXY GROUPS

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Abstract
We present a systematic analysis of 43 nearby galaxy groups (kT 500 = 0.7-2.7 keV or M 500 = 1013-1014 h –1 M ☉, 0.012 <z< 0.12), based on Chandra archival data. With robust background subtraction and modeling, we trace gas properties to at least r 2500 for all 43 groups. For 11 groups, gas properties can be robustly derived to r 500. For an additional 12 groups, we derive gas properties to at least r 1000 and estimate properties at r 500 from extrapolation. We show that in spite of the large variation in temperature profiles inside 0.15 r 500, the temperature profiles of these groups are similar at > 0.15 r 500 and are consistent with a "universal temperature profile." We present the K-T relations at six characteristic radii (30 kpc, 0.15 r 500, r 2500, r 1500, r 1000, and r 500), for 43 groups from this work and 14 clusters from the Vikhlinin et al. (2008) sample. Despite large scatter in the entropy values at 30 kpc and 0.15 r 500, the intrinsic scatter at r 2500 is much smaller and remains the same (~ 10%) to r 500. The entropy excess at r 500 is confirmed, in both groups and clusters, but the magnitude is smaller than previous ROSAT and ASCA results. We also present scaling relations for the gas fraction. It appears that the average gas fraction between r 2500 and r 500 has no temperature dependence, ~ 0.12 for 1-10 keV systems. The group gas fractions within r 2500 are generally low and have large scatter. This work shows that the difference of groups from hotter clusters stems from the difficulty of compressing group gas inside of r 2500. The large scatter of the group gas fraction within r 2500 causes large scatter in the group entropy around the center and may be responsible for the large scatter of the group luminosities. Nevertheless, the groups appear more regular and more like clusters beyond r 2500 from the results on gas fraction and entropy. Therefore, mass proxies can be extended into low-mass systems. The M 500-T 500 and M 500-Y X,500 relations derived in this work are indeed well behaved down to at least 2 ×1013 h–1 M ☉.

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