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Astron. Astrophys. 330, 423-434 (1998)

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Testing the physical reality of binaries and compact groups

Properties of early-type galaxies in groups with diffuse X-ray emission * ** ***

R. Rampazzo 1, S. Covino 1, G. Trinchieri 1, 2 and L. Reduzzi 1

1 Osservatorio Astronomico di Brera, Via Brera 28, I-20121 Milano, Italy
2 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, D-85740 Garching, Germany

Received 6 February 1997 / Accepted 20 August 1997


We present data on the stellar kinematics of the brightest ellipticals in HCG 62, HCG 68, NGC 2300 groups. Moreover, we report on ASCA GIS as well as optical observations of the early-type member of the pair K 416 (NGC 5480/5481) similar, in many respects, to the dominant pair in the NGC 2300 group.

The kinematics of HCG 62a/b and HCG 68a/b indicate that they are not interacting pairs. HCG 62a is instead possibly interacting with HCG 62c, as suggested by their morphology (see Mendes de Oliveira and Hickson 1994),

In contrast to the results for the NGC 2300/2276 group, ASCA observations indicate a significant absence of hot gas in the K 416 system. Whether the NGC 2300 multiplet is viewed as a loose group or as a massive E+S pair, it is clear that similar morphological entities do not always show similar X-ray properties. Under the hypothesis that diffuse X-ray emission marks the group potential, we consider the possibility that K 416 is an unbound encounter. In this scenario, morphological distortions are indicative of the ongoing interaction, but are only circumstantially correlated with the physical reality of a pair/multiplet as a bound system.

Key words: galaxies: elliptical and lenticular – galaxies: interactions – galaxies: kinematics and dynamics – galaxies: photometry – X-rays: galaxies

* Based on observations obtained at 1.93m telescope of Observatoire de Haute Provence operated by INSU.
** Based on ASCA observations under ESA observing time.
*** Original tables are available at CDS

© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998