Re: color plot

From: Marc Postman (postman@stsci.edu)
Date: Sat Jan 14 2006 - 07:52:19 PST

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    Saul et al.,

    Remember that the color offset plot includes all galaxies regardless of
    redshift. Your impression is indeed correct - there are as many ellipticals
    outside the red sequence as in it. However, the ones that are not in the red
    sequence are most likely foreground ellipticals at much lower redshifts. If your
    goal is simply to find SNe in ellipticals regardless of their redshift - then
    looking at clusters only gains you a factor of 2. But as we are specifically
    interested in SNe in ellipticals at z > 1, looking at z > 1 clusters gains you a
    much higher factor - the way to see this is simply to extrapolate the average
    number of blue ellipticals per color offset bin into the red sequence region.
    That floor is representative of the number of red field ellipticals. The ratio
    of that floor to the RCS peak is indeed comparable to the enhancement seen in
    the redshift based plot.

    Marc

    >
    >...I'll be curious to hear what other people think, but I suspect that
    >the previous figure you sent (with delta z/1+z on the x-axis) makes the
    >case more strongly. (Actually, why do I get the impression from this
    >plot that there are comparable number of ellipticals+S0 in the cluster
    >and in the field above z>1, while the previous plot suggests that the
    >ratio is more like 3.5 to 1?) If we go with the previous plot, can you
    >redo it with all (and only) galaxies above z=1? --S
    >

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