MAPPING THE DARK-MATTER IN THE NGC-5044 GROUP WITH ROSAT - EVIDENCE FOR A NEARLY HOMOGENEOUS COOLING FLOW WITH A COOLING WAKE

The NGC 5044 group of galaxies was observed by the ROSAT PSPC for 30 k s during its reduced pointed phase (1991 July). Due to the relatively cool gas temperature in the group (kT = 0.98 +/- 0.02 keV) and the exc ellent photon statistics (65,000 net counts), we are able to determine precisely a number of fundamental properties of the group within 250 kpc of the central galaxy. In particular, we present model-independent measurements of the total gravitating mass, the temperature and abund ance profiles of the gas, and the mass accretion rate. Between 60 and 250 kpc, the gas is nearly isothermal with T is-proportional-to r-0.13 +/- 0.03. The total gravitating mass of the group can be unambiguousl y determined from the observed density and temperature profiles of the gas using the equation of hydrostatic equilibrium. Within 250 kpc, th e gravitating mass is 1.6 x 10(13) M., yielding a mass-to-light ratio of 130 M./L.. The baryons (gas and stars) comprise 12% of the total ma ss within this radius. At small radii, the temperature clearly increas es outward and attains a maximum value at 60 kpc. The positive tempera ture gradient in the center of the group confirms the existence of a c ooling flow. The cooling flow region extends well beyond the temperatu re maximum with a cooling radius between 100 and 150 kpc. There are tw o distinct regions in the cooling flow separated by the temperature ma ximum. In the outer region, the gas is nearly isothermal with a unifor m Fe abundance of approximately 80% solar, the flow is nearly homogene ous with M = 20-25 M. yr-1, the X-ray contours are spherically symmetr ic, and rho(gas) is-proportional-to r-1.6. In the inner region, the te mperature profile has a positive gradient, the mass accretion rate dec reases rapidly inward, the gas density profile is steeper, and the X-r ay image shows some substructure. NGC 5044 is offset from the centroid of the outer X-ray contours indicating that the central galaxy may ha ve a residual velocity with respect to the center of the group potenti al. There is also a linear X-ray feature with an extent of approximate ly 30 kpc with one end coincident with NGC 5044. The X-ray emisson fro m this feature is softer than the ambient gas. We interpret this featu re as a ''cooling wake'' formed by the accreting gas as it is gravitat ionally focused into the wake of NGC 5044. One of the most surprising results of our PSPC observation is the discovery of a nearly homogeneo us cooling flow. Prior results concerning the mass accretion profile i n cooling flows indicate that M is-proportional-to r. This relation im plies that significant mass deposition occurs at large radii which gen erates an inhomogeneous flow. The mass accretion rate in the NGC 5044 group is essentially a constant beyond 40 kpc (well within the cooling radius). Significant mass deposition (a declining M) does not commenc e until the gas accretes to within 40 kpc of the group center where th e radiative cooling time is almost-equal-to 10(9) yr. This radius also corresponds to the temperature maximum, the break in the gas density profile, and the onset of structure in the X-ray image. A Hubble const ant of H-0 = 50 km s-1 Mpc-1 is used throughout the paper.