Chandra studies of the X-ray gas properties of galaxy groups
  We present a systematic analysis of 43 galaxy groups (kT_500=0.7-2.7 keV or M_500=10^13-10^14 h^-1 M_solar, 0.012<z<0.12), based on Chandra archival data. With robust background subtraction and modeling, we trace gas properties to >r_2500 for all 43 groups. For 23 groups, gas properties can be robustly derived to or extrapolated to r_500. We show that in spite of the large variation in T profiles inside 0.15 r_500, the T profiles of these groups are similar at >0.15 r_500 and are consistent with a "universal temperature profile". We present the entropy-T relations at six characteristics radii (30 kpc - r_500), for 43 groups from this work and 14 clusters from Vikhlinin et al. (2008). Despite large scatter in the entropy values at <0.15 r_500, the intrinsic scatter from r_2500 is much smaller and remains the same (~10%) to r_500. 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 to inside 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 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-T and M-Y relations derived in this work are indeed well behaved down to at least 2E13 h^-1 M_solar. 
M. Sun, G. M. Voit, M. Donahue, C. Jones, W. Forman, A. Vikhlinin