The poor cluster of galaxies S639 |
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| Authors: | Inger Jørgensen & Helge Jønch-Sørensen |
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| Institution: | McDonald Observatory, The University of Texas at Austin, RLM 15.308, Austin, TX 78712, USA,;Copenhagen University Observatory, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, Denmark |
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| Abstract: | We have studied the poor southern cluster of galaxies S639. Based on new Strömgren photometry of stars in the direction of the cluster, we confirm that the Galactic extinction affecting the cluster is large. We find the extinction in Johnson B to be AB =0.75±0.03. We have obtained new photometry in Gunn r for E and S0 galaxies in the cluster. If the Fundamental Plane is used for determination of the relative distance and the peculiar velocity of the cluster, we find a distance, in velocity units, of (5706±350) km s?1, and a substantial peculiar velocity, (839±350) km s?1. However, the colours and the absorption line indices of the E and S0 galaxies indicate that the stellar populations in these galaxies are different from those in similar galaxies in the two rich clusters Coma and Hydra I. This difference may severely affect the distance determination and the derived peculiar velocity. The data are consistent with a non-significant peculiar velocity for S639 and the galaxies in the cluster being on average 0.2 dex younger than similar galaxies in Coma and Hydra I. The results for S639 caution that some large peculiar velocities may be spurious and caused by unusual stellar populations. |
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| Keywords: | dust extinction galaxies: clusters: individual: S639 galaxies: distances and redshifts galaxies: elliptical and lenticular cD galaxies: fundamental parameters galaxies: stellar content |
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