On the Characteristic Isolation of Compact Subgroups within Loose Groups of Galaxies

ABSTRACT

We have explored the hypothesis that a compact subgroup lying within a dense environment as loose group of galaxies, at a certain stage of its evolutionary history, could be influenced by the action of the tidal field induced by the gravitational potential of the whole system. We argue that the empty ring observed in projection around many compact subgroups of galaxies embedded in larger hosts originates around the spherical surface drawn by the tidal radius where the internal binding force of the compact subgroup balances the external tidal force of the whole system. This effect would tear apart member galaxies situated close to this region determining a marked isolation of the subgroup from the rest of the host group. If so, subsequent evolution of these subgroups should not be affected by external influences as the infall of new surrounding galaxies on them. Following this idea we have developed a statistical method of investigation and performed an application to point out such an effect studying a loose group of galaxies hosting a compact group in its central region. The system UZC 578 / HCG 68 seems to be a fair example of such hypothesized process.

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