2009ApJ...693.1142S

We present a systematic analysis of 43 nearby galaxy groups (kT₅₀₀ = 0.7-2.7 keV or M₅₀₀= 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 r₂₅₀₀ for all 43 groups. For 11 groups, gas properties can be robustly derived to r₅₀₀. For an additional 12 groups, we derive gas properties to at least r₁₀₀₀ and estimate properties at r₅₀₀ from extrapolation. We show that in spite of the large variation in temperature profiles inside 0.15 r₅₀₀, the temperature profiles of these groups are similar at > 0.15 r₅₀₀ and are consistent with a "universal temperature profile." We present the K-T relations at six characteristic radii (30 kpc, 0.15 r₅₀₀, r₂₅₀₀, r₁₅₀₀, r₁₀₀₀, and r₅₀₀), 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 r₂₅₀₀ is much smaller and remains the same (∼ 10%) to r₅₀₀. The entropy excess at r₅₀₀ 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 r₂₅₀₀ and r₅₀₀has no temperature dependence, ∼ 0.12 for 1-10 keV systems. The group gas fractions within r₂₅₀₀ 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 r₂₅₀₀. The large scatter of the group gas fraction within r₂₅₀₀ 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 r₂₅₀₀ from the results on gas fraction and entropy. Therefore, mass proxies can be extended into low-mass systems. The M₅₀₀-T₅₀₀ and M₅₀₀-Y _X,500 relations derived in this work are indeed well behaved down to at least 2 x10¹³.h^–1 M_☉.