Mapping the dark matter in the NGC 5044 group with ROSAT: Evidence for a nearly homogeneous cooling flow with a cooling wake

Abstract

The NGC 5044 group of galaxies was observed by the ROSAT PSPC for 30 ks 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 excellent 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 abundance profiles of the gas, and the mass accretion rate. Between 60 and 250 kpc, the gas is nearly isothermal with T ∝ r-0.13 ± 0.03. The total gravitating mass of the group can be unambiguously determined from the observed density and temperature profiles of the gas using the equation of hydrostatic equilibrium. Within 250 kpc, the gravitating mass is 1.6 × 1013 M⊙, yielding a mass-to-light ratio of 130 M⊙/L⊙. The baryons (gas and stars) comprise 12% of the total mass within this radius. At small radii, the temperature clearly increases outward and attains a maximum value at 60 kpc. The positive temperature gradient in the center of the group confirms the existence of a cooling flow. The cooling flow region extends well beyond the temperature maximum with a cooling radius between 100 and 150 kpc. There are two distinct regions in the cooling flow separated by the tem-perature maximum. In the outer region, the gas is nearly isothermal with a uniform Fe abundance of approximately 80% solar, the flow is nearly homogeneous with Ṁ = 20-25 M⊙ yr-1, the X-ray contours are spherically symmetric, and ρgas ∝ r-1.6. In the inner region, the temperature profile has a positive gradient, the mass accretion rate decreases rapidly inward, the gas density profile is steeper, and the X-ray image shows some substructure. NGC 5044 is offset from the centroid of the outer X-ray contours indicating that the central galaxy may have a residual velocity with respect to the center of the group potential. There is also a linear X-ray feature with an extent of approximately 30 kpc with one end coincident with NGC 5044. The X-ray emisson from this feature is softer than the ambient gas. We interpret this feature as a "cooling wake" formed by the accreting gas as it is gravitationally focused into the wake of NGC 5044. One of the most surprising results of our PSPC observation is the discovery of a nearly homogeneous cooling flow. Prior results concerning the mass accretion profile in cooling flows indicate that Ṁ ∝ r. This relation implies that significant mass deposition occurs at large radii which generates 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 commence until the gas accretes to within 40 kpc of the group center where the radiative cooling time is ≈ 109 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 constant of H0 = 50 km s-1 Mpc-1 is used throughout the paper.