CHANDRA STUDIES OF THE X-RAY GAS PROPERTIES OF GALAXY GROUPS

We present a systematic analysis of 43 nearby galaxy groups (kT(500) = 0.7-2.7 keV or M-500 = 10(13)-10(14) h(-1) M-circle dot, 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 (similar to 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, similar to 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,Y-500 relations derived in this work are indeed well behaved down to at least 2 x 10(13) h-1 M-circle dot.