ChandraObservations 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 itstemperature (1.37$\pm$0.01 keV) and velocity dispersion ($\sim$ 300 kms$^{-1}$), the NGC 1550 group is one of the most luminous known galaxy groups(L$_{\rm bol}$ = 1.65$\times10^{43}$ erg s$^{-1}$ within 200 kpc, or 0.2 \rv).We find that within $\sim 60$ kpc, where the gas cooling time is less than aHubble time, the gas temperature decreases continuously toward the center,implying the existence of a cooling core. The temperature also declines beyond$\sim$ 100 kpc (or 0.1 \rv). There is a remarkable similarity of thetemperature profile of NGC 1550 with those of two other 1 keV groups withaccurate temperature determination. The temperature begins to decline at 0.07 -0.1 \rv, while in hot clusters the decline begins at or beyond 0.2 \rv. Thus,there are at least some 1 keV groups that have significantly differenttemperature profiles from those of hot clusters, which may reflect the role ofnon-gravitational processes in ICM/IGM evolution. NGC 1550 has no isentropiccore 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 of1 keV groups show much larger scatter than those of hotter systems, whichimplies varied pre-heating levels. We also discuss the mass content of the NGC1550 group and the abundance profile of heavy elements.Comment: emulateapj5.sty, 18 pages, 11 figures (including 4 color), to appear in ApJ, v598, n1, 20 Nov 200