X-RAY-EMISSION FROM THE FORNAX CLUSTER

We have analyzed the ROSAT PSPC observations of the central region of the Fornax cluster, a relatively poor group of galaxies at a distance of about 24 Mpc. The brightest X-ray and optical galaxy in the group i s NGC 1399, an E1 galaxy located near the center of the Fornax cluster . We characterize the hot gas around the galaxy, derived from a 2' to 18' annulus around NGC 1399, as having a mean temperature of 1.30 +/- 0.05 keV and a heavy element abundance of 0.6 +/- 0.1 with respect to solar abundance (Fe/H = 4.68 x 10(-5) by number). Spatially resolved s pectral data provide both gas temperature and gas abundance profiles e xtending to 125 kpc (18') from the galaxy. The temperature distributio n, combined with the X-ray surface brightness profile, yields an accur ate determination of the gravitating mass within 125 kpc, which falls in the range (4.3 - 8.1) x 10(12) M. (95% confidence range, including systematic uncertainties). If we include the extended optical halo aro und NGC 1399, the mass-to-light ratio increases with radius from 33 +/ - 8 M./L. at 18 kpc to 70 +/- 22 M./L. at 110 kpc. We compare the heav y element abundance distribution measured around NGC 1399 with that me asured around the Virgo galaxy NGC 4472, as well as to models for hot coronae. We find that the abundance distribution is in good agreement with that previously measured for NGC 4472 by Forman et al. in 1993. F or both galaxies, the observed abundance profiles require both a weak evolution of the type Ia supernova rate with time and a present epoch rate which agrees with that of Cappellaro et al. We compare mass measu rements in NGC 1399 to those for M87. The similarity of the optical ma sses in these systems and their differences in gas masses and gravitat ing masses lead us to suggest that the optical galaxies formed at an e arly stage when the central potentials of these two systems were simil ar. Subsequent infall of gas and dark matter into the larger, deeper V irgo potential resulted in the greater mass of the Virgo cluster compa red to Fornax. We also report on X-ray properties of thirteen other Fo rnax galaxies. Eight of these were detected in ROSAT images with lumin osities in the 0.2 to 2 keV energy band from 1 x 10(39) to 1.6 x 10(41 ) ergs s(-1) Five galaxies were sufficiently bright to permit spectral analyses and all but one (NGC 1380) had spectra consistent with therm al emission. Two (NGC 1404 and NGC 1387) of the four galaxies with wel l-constrained spectral parameters have hot coronae with characteristic gas temperatures of about 0.5 keV and iron abundances less than that found around NGC 1399 and other bright ellipticals. To maintain these hot coronae, the absolute magnitudes of these galaxies must be brighte r than -19. Thus the distance to Fornax must be at least 18 Mpc, and, if there are no large peculiar velocities, the Hubble constant should be less than 75 km s(-1) Mpc(-1). Since these galaxies are all members of Fornax, distance uncertainties do not affect the relationship betw een their optical magnitude and X-ray luminosity. Analysis of the Forn ax galaxies supports the contention that the scatter in the X-ray and optical relationship is intrinsic and does not arise solely from dista nce uncertainties. For the elliptical galaxy NGC 1404, the X-ray image s show that the hot corona is distorted and likely is being stripped, indicating infall of the galaxy toward NGC 1399 and the cluster center .