From: Saul Perlmutter (saul@lbl.gov)
Date: Fri Jan 13 2006 - 09:07:44 PST
Hi Marc,
Nice plot! I think the number we care most about is -- for
ellipticals -- the ratio of the integral under the cluster's spike
compared to the integral under all the rest of the curve that is above
z=1. I can't quite do that calculation exactly from the current plot,
since there may be some below z=1 on the plot (for the z=1.1 cluster)
and some missing above z=1 (for the z=1.24 and 1.27 clusters), but
assuming that these average out it looks like the answer is: ~3.5
times as many z>1 E+S0 in the cluster as in the field. (For Sa+, it
looks like the ratio is more like ~0.75 in cluster to 1 in field.)
Now I'm curious to know if each of these three clusters contributes
simlarly to this overdensity of ellipticals.
Can you make a version of this that shows all galaxies above z=1 and
no galaxies below z=1? Also, could we see what a version of this plot
looks like that uses (GalColor - ClusterMeanColor) for the x axis?
I'm wondering which version of the plot would be better for the proposal
-- one version is closer to the data, and the other is perhaps easier to
understand quickly.
Thanks, --S
Marc Postman wrote:
> Saul et al.,
>
> Here is a plot that may demonstrate the point that early type galaxies
> are strongly localized in the clusters for z > 1. This is a histogram
> of spectroscopic redshifts for the clusters RXJ0910+54, RXJ1252-29,
> and CL0848+44 (which are at z=1.10, 1.24, and 1.27, respectively). The
> red histo is E+S0 galaxies and the blue histo is Sa or later. Relative
> to the spikes seen in the field, the E+S0 overdensity in the cluster
> is about a factor of 10 whereas the Sa+ overdensity is at best a
> factor of 2. There may be some selection effects in the spectroscopic
> selection to slightly favor early types, however. But these effects,
> if corrected for, might only increase the Sa+ peak at dz=0 by about 20
> - 30% (but would also probably increase the spikes in the field as
> well so that the overall enhancement factor would probably still be
> only about a factor of 2). The redshift range covered by this plot is
> 0.9 < z < 1.47.
>
> Marc
>
>
> -----------------------------------------------------------------
> Marc Postman Email: postman@stsci.edu
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