From: Alex Conley (AJConley@lbl.gov)
Date: Tue Sep 07 2004 - 15:29:10 PDT
Hi Greg,
Thanks for the comments. They were very useful. Some
will take me a while to answer (not that I'm complaining;
these were exactly the sort of comments I was hoping for
before I unblinded), but I can try to respond to some of them.
Analysis questions:
\sigma_int comes only from the low redshift SNe, although
the high redshift SNe are consistent with the same value.
Does more dust imply hihger R_B? Maybe, maybe not. But
R_B is usually larger in dense clouds, which is thought to
arise because of UV shielding. So
more dust->UV shielding->different grain size distribution,
which suggests to me that more dust -> different R_B.
When points with large uncertainties are allowed (the
magerror cut is relaxed) the contours on the cosmological
parameters are still well behaved.
I did not exclude SN1997O in the first cosmology fits that
I did. At the time I was not aware that it had been
excluded from P99.
I have not gone back and investigated the SNe that fail the
probcut test in any detail.
I have not looked at the P(chisq|N) distribution for the CMAG
fits, but the expectation is that it will be far too good.
This is what led Wang '03 to rescale the photometric errors --
the error bars on the low redshift sample are almost certainly
both overstated and correlated. However, I will try to put
this plot together for you. I'll have to think about how
to deal with the fact that N varies over the sample, so the
expected width of the probability distribution is different
for different SN. Perhaps I can scale by the width of the
distribution, kind of like dividing by sigma.
The value of the blindness point (om = 1, ol = 1.1) was chosen
for two reasons. My initial blindness point was 1, 1.5.
Reynald pointed out that this was very similar to the expected
result, so there was psychological advantage to moving away
from the expected value and forcing readers of my draft to
psychologically confront the blindness scheme. On the other
hand, the size of the contours depends somewhat critically on
the om, ol values, so if I chose extremely different values the
contours would look huge, and people would dismiss the analysis
as not providing meaningful constraints. I will add a footnote
to this effect.
Values of S -- is it different from 1, etc. : There is a
slightly newer version of the paper than the one you looked
at (1.27, as opposed to 1.26 which I think is the one you saw)
which includes error bars in the residual plots. When I
performed fits including the error bars I get S=1.11+-0.22 (low-z)
and 0.96+-0.19 (high-z). I am going to try and improve this
by attempting to include some covariance information. Mark
Strovink also pointed out that I should include the errors in
the Pearson's correlation coefficients, which I have not done.
Figuring out if SN2001fo is a Ia:
Type Ic SN have CMAG diagrams that look like low stretch Ia
CMAG diagrams, so unfortunately it can't easily be used to
test for Ia-ness.
I have tried to investigate differences between the Riess/Hamuy/Jha
sample. I refer you to sections 7.1, 7.5, and 8.4 and figure 14 of
the method document linked to off of my web site. To summarize,
however, the Riess and Hamuy samples seem consistent, but the
Jha sample has a larger dispersion and a mean offset from the
other samples. When the Jha sample is removed the intrinsic
scatter drops to 0.08. However, the high redshift sample does
not seem to be consistent with this small of a value of sigma_int.
The effects on the final cosmological fit are not large.
I will look into breaking up the sample into various subsets. The
one you suggest (R->B, I->B, Z->B) is essentially by redshift.
Color at B max (fig 4): The newer version of the paper has a much
nicer version of this figure thanks to a suggestion by Chris.
I haven't performed an F-test, but I would certainly expect the
samples to fail the KS test. The KS test effectively compares
means and variances of two populations, and the high redshift
sample will have a higher variance because of measurement errors.
The same can be said for the beta (cmag slope) fits. I will
add a note about the mean values of the colors.
The presence of a bump is not correlated with the slope, nor is
the stretch. These plots are in my thesis, but not in the
paper. The second (at least) was shown in Wang '03.
The first three high redshift SNe being high on the Hubble diagram:
These are SN1995ba (P99), SN1998as (K03) and SN1996K (Riess '98).
I have no idea what the galaxy density around these SNe was.
The most likely explanation is dust -- these are the lowest
redshift end of whatever survey they came from, so are the least
affected by dust related Malmquist bias, and therefore should
have higher mean extinction values than the rest of the sample.
I will look into it.
Matched residuals decreasing with faint resids: I will investigate.
Other questions:
The script form of R_B is used by some authors and not by
others. There doesn't seem to be any clear standard.
On the qustion of what plausibly a Ia means in section 3:
Yes, that wording was very vague. I'm not sure how to
make it much more precise. Chris also commented on this,
so I changed the text in version 1.26 to read:
'must be at least plausibly a Ia based on either lightcurve
shape, spectroscopic ID, or host galaxy morphology.'
Basically the criteria here is meant to be something like
that we use when deciding whether or not to follow a
SNe candidate. Any suggestions for a better way to
explain this are welcome, but I don't think I could ever
put a percentage number on it.
Covariance matricies: No, I didn't ask for anybody else's
covariance matricies. Ours have never been
published. I will try to think of a good way to reword
my statement to avoid the implication that we are the only
people who bother with calculating them.
Script K in equation (4) (luminosity distance equation):
Goobar and Perlmutter '95 used the script K. Also,
people frequently restrict k to -1,0,+1 by a coordinate
rescaling. What you may mean is 'Should script k be
\Omega_k?', which is true except when \Omega_k = 0.
Infrared extinction laws of LMC/SMC: In the far IR
the LMC and SMC have substantially more extinction
than one might expect. In the MW people usually assume
that the number of graphite and silicate grains is about
the same, but the MCs seem to require more silicates.
See Pei ApJ 395, 130 (1992).
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