Chandra OBSERVATIONS OF THE NGC 1550 GALAXY GROUP — IMPLICATION FOR THE TEMPERATURE AND ENTROPY PROFILES OF 1 KEV GALAXY GROUPS

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−1), the NGC 1550 group is one of the most luminous known galaxy groups (Lbol = 1.65×1043 erg s−1 within 200 kpc, or 0.2 rvir). 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 0.1 rvir). There is a remarkable similarity of the temperature profile of NGC 1550 with those of two other 1 keV groups with accurate temperature determination. The temperature begins to decline at 0.07 0.1 rvir, while in hot clusters the decline begins at or beyond 0.2 rvir. Thus, there are at least some 1 keV groups that have significantly different temperature profiles from those of hot clusters, which may reflect the role of non-gravitational processes in ICM/IGM 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 pre-heating levels. We also discuss the mass content of the NGC 1550 group and the abundance profile of heavy elements. Subject headings: galaxies: groups: individual (NGC 1550) — hydrodynamics — X-rays: galaxies: clusters