2003ApJ...598..250S

We present a detailed Chandra study of the galaxy group NGC 1550. For its temperature (1.37±0.01 keV) and velocity dispersion (∼300 km/s), the NGC 1550 group is one of the most luminous known galaxy groups (L_bol=1.65x10⁴³ ergs/s within 200 kpc, or 0.2r_vir). We find that within ∼60 kpc, where the gas cooling time is less than a Hubble time, the gas temperature decreases continuously toward the center, implying the existence of a cooling core. The temperature also declines beyond ∼100 kpc (or 0.1r_vir). The temperature profile of NGC 1550 is remarkably similar to those of two other 1 keV groups with accurate temperature determination. The temperature begins to decline at 0.07r_vir-0.1r_vir, while in hot clusters the decline begins at or beyond 0.2r_vir. Thus, there are at least some 1 keV groups that have temperature profiles significantly different from those of hot clusters, which may reflect the role of nongravitational processes in intracluster medium/intergalactic medium evolution. NGC 1550 has no isentropic core in its entropy profile, in contrast to the predictions of ``entropy floor'' simulations. We compare the scaled entropy profiles of three 1 keV groups (including NGC 1550) and three 2-3 keV groups. The scaled entropy profiles of 1 keV groups show much larger scatter than those of hotter systems, which implies varied preheating levels. We also discuss the mass content of the NGC 1550 group and the abundance profile of heavy elements.