From: Greg Aldering (aldering@panisse.lbl.gov)
Date: Sat Apr 12 2003 - 12:28:21 PDT
Hi Rob,
A few things on the HST paper.
First, it sounds from the report by Ariel that your U-template is
in good shape at the epochs that matter - so we can just say that in
the paper. I suggest adding on page 8, before "Each of these ..."
This template is in good agreement with the composite $U$-band template
shown in Figure~XX of \cite{jhaphd}, as well as more detailed template
analysis which included the additional SNe~Ia from that same work.
Now, regarding the U-B error to quote. First, I would suggest that for
the sole purpose of determining the extinction correction that you *do*
use a stretch-dependent U-B at max even though you did not do this for
the K-corrections. Since the K-corrections are tilted to match the
observed colors, most of the apparent inconsistency is eliminated.
This was the approach used in P99. In this context I suggest you add
a line at some appropriate place saying:
The intrinsic $U$-$B$ depends on lightcurve stretch. We have fit this
dependence for the stretch range of our affected SNe~Ia --- those at the
highest-redshift --- including data from \cite{jhaphd}. This relation
was then applied when determining host galaxy extinction for those
highest-redshift SNe.
(By the way, why don't you quote E(U-B) in Table 3? Is it that you are
taking E(U-B) and converting to E(B-V)? Do you think this is clear to
the reader? Perhaps a footnote for the afffected SNe is in order. Or
maybe I missed something entirely!)
Next, in your email of yesterday, you say that the overall chi-square
doesn't change much when you change the intrinsic dispersion in U-B.
But, can you tell us how does the cosmology change? Also, how is the
chi-square of the individual affected SNe changed? If all of their
chi-squares are too good for sigma_U-B = 0.09 that may indicate that
the intrinsic dispersion being used is too high.
In particular, one point I haven't seen discussed (or maybe I've
forgotten!) is that since U-band maps to F675W at z ~ 0.88, only
SN1997ek is nearly 100% U-B in the F675W-F814W frame. For the other
SNe, you still have B-band light in F675W, so the intrinsic dispersion
used should be a combination of the U-B and B-V intrinsic dispersions,
weighted by the relative fractions of U and B light in the filter.
In my opinion, the main impact of the choice of the color dispersion is
in whether we can say that we have measured host-galaxy extinction for
many more new SNe~Ia. This matters not only because we can generate an
unbiased OM-OL fit, but because it can directly demonstrate for
individual high-redshift SNe that the low extinction measured statistically
for the ensemble of P99 SNe is in fact confirmed. (Based on papers we are
presently being asked to review, this is a big point!)
In P99 there were 5 SNe with sigma_(B-V) < 0.10 mag. In this paper
there will be only 6 more if we adopt sigma_U-B = 0.09, or 11 more if
we adopt sigma_U-B = 0.04. (Granted, it isn't this black and white, as
the 6, above, would be measured about 2x better than the best 5 SNe
colors in P99.)
Finally, I have a few presentation items that I'll pass along since they
might require a little time for you to implement:
In Table 6, could you please provide columns giving the stretch and
host E(B-V) (or A_B) that was assumed?
In Table 8, could you add P(Omega_Lambda > 0) for each of your fits?
Cheers,
Greg
P.S. BTW, I invite everyone to scan the paper for missing references and
then provide Rob with the bibtex entry (from ADS), as currently the
scholarship is a bit light in a few places. Those types of things
annoy some reviewers, and may make them question our analysis because
they may think we haven't done our homework.
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