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Astron. Astrophys. 318, 361-375 (1997)

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ROSAT PSPC observations of 5 X-ray bright early type galaxies

G. Trinchieri 1, 2, G. Fabbiano 3 and D-W. Kim 4

1 Osservatorio Astronomico di Brera, Via Brera 28, I-20121 Milano, Italy
2 Visiting Astronomer, Max-Planck Institut für extraterrestrische Physik, Garching bei München, Germany
3 Harvard-Smithsonian Center for Astrophysics 60 Garden St., Cambridge, MA 02138, USA
4 Chungnam National University, Dept. of Astron. and Space Sci., 305-764 Daejeon, Korea

Received 4 March 1996 / Accepted 1 July 1996


We report on the ROSAT PSPC observations of 5 X-ray bright early-type galaxies. Their X-ray morphology is more complex than Einstein data had shown, ranging from the ellipsoidal shape of NGC 533 to the [FORMULA] 200 kpc tail of NGC 7619. Hot gas at an average temperature of 0.8-1 keV dominates their X-ray emission. The spectral analysis is based on the assumption of a hot thin plasma at cosmic abundances, since the spectral resolution of the PSPC does not allow unambiguous measures of the model parameters (temperatures, low energy cut-off, metal abundance). However, the estimated temperature are not strongly affected by this choice. A temperature distribution of the hot interstellar medium is also derived. Higher temperatures are in some cases observed at larger radii, while the innermost [FORMULA] region is cooler in all of the objects studied. Due to the large range in distances however this corresponds to significantly different galaxy radii, from 5 kpc in NGC 4649 to 30 kpc in NGC 533.

The temperature and density distributions derived are used to estimate the total mass of these systems, which span from [FORMULA] [FORMULA] 10 [FORMULA] [FORMULA] and corresponding mass-to-light ratios M/L [FORMULA] 10-150 in solar units. Formal errors on the derived masses are estimated to be [FORMULA] 25%.

Care must be taken in comparing results for different systems, since some of the emission attributed to these objects could suffer from the contribution for the groups and clusters they belong to. Accordingly, our mass estimates could reflect the effect of the cluster/group potential.

Key words: galaxies: individual: NGC 533 – galaxies: individual: NGC 2563 – galaxies: individual: NGC 4649 – galaxies: individual: NGC 7619 – galaxies: individual: NGC 7626 – X-rays: galaxies

Send offprint requests to: G. Trinchieri

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998