Evidence for Mergers in a Cluster at High Z

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Session 30 -- Clusters of Galaxies
Display presentation, Tuesday, 9:30-6:30, Pauley Room

[30.14] Evidence for Mergers in a Cluster at High Z

R.D.McClure (DAO/HIA/NRC), M.J.Pierce (NOAO), R.J.Lavery (DTM-CIW)

We have done high resolution CCD imaging of GHO 0020+0407, a cluster of galaxies at z = 0.698. The image, taken with the High Resolution Camera (HRCam) on the Canada-France-Hawaii Telescope, has a FWHM = 0.42 arcsec and a limiting magnitude of R $\sim$ 26. The image shows a dramatic incidence of close pairs/triplets of galaxies, with separations of $1 - 1.5$ arcsec. We evaluate the significance of these close pairs using both the angular two-point correlation function ({\sl w}$(\theta)$), and the distribution of nearest neighbors. We find that the number of close pairs is enhanced by a factor of $\sim$ three above that expected for a random distribution and is significant at the 99\% confidence level. The luminosities of a few of the pairs are similar, with the brightest members being about L$^*$. At this redshift the physical separations of these pairs is typically $6 - 12$ kpc, depending on the assumed cosmological model.

Given that models of interacting systems with similar separations invariably merge on timescales of $< 1$ Gyr, we interpret this enhancement of close pairs as strong evidence for mergers occurring in this cluster. We find a similar effect occurring at the present epoch in the Hercules Cluster which is morphologically similar to GHO 0020+0407. We conclude, therefore, that interactions and mergers in rich clusters are not primarily an effect of look-back time, but rather, they are related to the dynamical evolutionary state of the cluster at the particular epoch of observation.

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