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For refcode 1998ApJ...507...84M:
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1998ApJ...507...84M Galaxy Evolution in the z = 0.4274 Cluster MS 1621.5+2640 SIMON L. MORRIS AND J. B. HUTCHINGS Dominion Astrophysical Observatory, National Research Council, 5071 West Saanich Road, Victoria, BC V8X 4M6, Canada; Simon.Morris, John.Hutchings@hia.nrc.ca R. G. CARLBERG AND H. K. C. YEE Department of Astronomy, University of Toronto, Toronto, ON M5S 1A7, Canada; carlberg@moonray.astro.utoronto.ca, hyee@makalu.astro.utoronto.ca ERICA ELLINGSON CASA, University of Colorado, Boulder, CO 80309-0389; e.elling@pisco.colorado.edu MIKE L. BALOGH Department of Physics and Astronomy, University of Victoria, Victoria, BC V8W 3P6, Canada; Balogh@uvphys.phys.uvic.ca R. G. ABRAHAM Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, England, UK; abraham@ast.cam.ac.uk AND TAMMY A. SMECKER-HANE Department of Physics and Astronomy, University of California, Irvine, CA 92797-4575; smecker@carina.ps.uci.edu Received 1998 January 16; accepted 1998 June 10 ABSTRACT We discuss the galaxy population of the rich cluster MS 1621.5+2640 at z = 0.4274, based on spectra and imaging in a field of size 9' x 23' (~2 x 5 h^-1^ Mpc). The sample comprises 277 galaxies, of which 112 are cluster members, 7 are "near-members," and 47 are field galaxies in the redshift range 0.37 <= z <= 0.50. The results are analyzed and compared with the z = 0.2279 rich cluster Abell 2390. MS 1621.5+2640 has a higher blue fraction and a younger stellar population, and it is a less evolved cluster. We do not find strong evidence of significant excess star formation compared with the field, although there is a small population of outlying near-members that is unusually blue and that may be affected by the cluster. There is a substantial population of red galaxies with significant H{delta} absorption, which are not easily explained by any simple form of modeled star formation history. We detect two distinct populations of cluster galaxies: those where star formation ceased some time ago, and those with a gradual decrease over many Gyr. Our data suggests that the cluster formed by accretion from the field, with truncation of the star formation beginning at very large radii (>2 times the virial radius). The truncation process does not seem to be a sharp one though, in that lower luminosity early-type galaxies in the inner core of the cluster are seen with significant H{delta} absorption, indicating some star formation within the last 1-2 Gyr. Some combination of stripping of gas from the outer parts of the galaxy together with a gradual exhaustion of the gas in the inner parts would be consistent with our data. Subject headings: galaxies: clusters: individual (MS 1621.5+2640)- galaxies:distances and redshifts-galaxies: photometry-galaxies: stellar content
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