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We examine the origin of clustercentric gradients in the star formation rates and colors of rich cluster galaxies within the context of a simple model where clusters are built through the ongoing accretion of field galaxies. The model assumes that after galaxies enter the cluster their star formation rates decline on a timescale of a few Gyrs, the typical gas consumption timescale of disk galaxies in the field. Such behaviour might be expected if tides and ram pressure strip off the gaseous envelopes that normally fuel star formation in spirals over a Hubble time. Combining these timescales with mass accretion histories derived from N-body simulations of cluster formation in a Lambda-CDM universe, we reproduce the systematic differences observed in the color distribution of cluster and field galaxies, as well as the strong suppression of star formation in cluster galaxies and its dependence on clustercentric radius. The simulations also indicate that a significant fraction of galaxies beyond the virial radius of the cluster may have been within the main body of the cluster in the past, a result that explains naturally why star formation in the outskirts of clusters (and as far out as two virial radii) is systematically suppressed relative to the field. The agreement with the data beyond the cluster virial radius is also improved if we assume that stripping happens within lower mass systems, before the galaxy is accreted into the main body of the cluster. We conclude that the star formation rates of cluster galaxies depend primarily on the time elapsed since their accretion onto massive virialized systems, and that the cessation of star formation may have taken place gradually over a few Gyrs.Comment: 11 pages emulateapj style, 4 embedded postscript figures Accepted for publication in Ap

We present a very high signal-to-noise ratio (S/N ~ 400) composite spectrum of the rest-frame ultraviolet and optical region of high luminosity quasars. The spectrum is derived from 718 individual spectra obtained as part of the Large Bright Quasar Survey. The moderate resolution, <4 Â, and high signal-to-noise ratio allow numerous weak emission features to be identified. Of particular note is the large equivalent-width of the Fe ii emission in the rest-frame ultraviolet and the blue continuum slope of the composite. The primary aim of this paper is to provide a reference spectrum for use in line identifications, and a series of large-scale representations of the composite spectrum are shown. We also present a measure of the standard deviation of the individual quasar spectra from the composite spectrum.

To re-examine the rich cluster $\Omega$ value the CNOC Cluster Survey has observed 16 high X-ray luminosity clusters in the redshift range 0.17 to 0.55, obtaining approximately 2600 velocities in their fields. Directly adding all the K and evolution corrected $r$ band light to $M_r(0)=-18.5$, about $0.2L_\ast$, and correcting for the light below the limit, the average mass-to-light ratio of the clusters is $283\pm27h\msun/\lsun$ and the average mass per galaxy is $3.5\pm0.4\times10^{12}h^{-1}\msun$. The clusters are consistent with having a universal $M_v/L$ value (within the errors of about 20\%) independent of their velocity dispersion, mean color of their galaxies, blue galaxy content, redshift, or mean interior density. Using field galaxies within the same data set, with the same corrections, we find that the closure mass-to-light, $\rho_c/j$, is $1160\pm130h\msun/\lsun$ and the closure mass per galaxy, $\rho_c/\phi(>0.2L_\ast)$, is $13.2\pm1.9\times10^{12}h^{-1}\msun$. Under the assumptions that the galaxies are distributed like the mass and that the galaxy luminosities and numbers are statistically conserved, which these data indirectly support, $\Omega_0=0.20\pm0.04\pm0.09$ where the errors are, respectively, the $1\sigma$ internal and an estimate of the $1\sigma$ systematic error resulting from the luminosity normalization.Comment: 34 page Latex document (no figures) requiring AAS macros. Postscript document (or uufile) availble at http://manaslu.astro.utoronto.ca/~carlberg/cnoc/general.htm

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Original article can be found at: http://www.iop.org/EJ/journal/apj Copyright American Astronomical Society [Full text of this article is not available in the UHRA