From: Greg Aldering (aldering@panisse.lbl.gov)
Date: Mon Jan 05 2004 - 02:02:35 PST
Hi Serena,
I found your draft paper on the I-band Hubble diagram quite
interesting. It was generally quite easy to read and understand. I do
have some more extensive commments which I will send along as soon as I
can, but let me mention the main points regarding the analysis which
are of interest to me based on my understanding of the paper so far.
It is probably fair to say that in this paper we have a difficult time
drawing conclusions with the high-redshift data because larger
uncertainties have been assigned in this analysis compared to the Riess
analysis. It is interesting to see how this arises. It seems that the
main components are that a large statistical error is assigned to 99Q,
and no stretch correction is applied in the high-z analysis so a large
intrinsic dispersion applies.
Now, the large uncertainty for 99Q arises from the fact that you have
added a "systematic" error because you believe that the statistical
error is underestimated due to the limited number of data points.
However, by taking the systematic error as the range from fits with
delta_chi^2 < 3 you are more than double-counting the statistical error
itself. delta_chi^2 includes fits that are at 1.7 sigma, so you have
included this statistical part and called it a systematic. In
particular, this blows-up the uncertainty on 99Q quite a bit. While
you may be correct in worrying about the reported statistical errors
for 2000fr and 1999ff, which have fewer points, I don't think that we
want to claim that 99Q's uncertainty has to be as large as the one that
you have quoted because we don't believe the fitted errors.
Instead, it seems that you need to Monte-Carlo the data points and see
whether the observed scatter is larger than the expected statistical
scatter. Even if it is, you still have to remove the statistical part
of the scatter before assigning a systematic error. I bet you will find
that 99Q is good to about 0.1 mag. If my guess is right, this may allow
for stronger constraints in this paper.
Next, you decided to not stretch-correct, so the intrinsic scatter is
larger. This is because you don't seem to have a stretch for 99Q.
However, Riess published the B-I colors for 99Q, from which you might
be able to constrain the stretch well enough so that the error in using
it still leads to a net gain for the overall sample. Note also that at
least the HST R and I data for 99Q are public and Riess says this SN
has no host. So, we can certainly measure a lightcurve from that.
On a related theme, in the past we have argued that it is always better
to stretch correct even if the result is noisier because this
eliminates a systematic trend in the data - you seem to make the
opposite arguement here.
Because Riess has made strong claims using just 99Q, I believe it is
incumbent on us to try to squeeze all we can out of the available
data. This is also true because we promised the TAC's that we could do
so much with this NIR data. If people can see that we have tried to do
this, it will make the paper much stronger. Conversely, it would be bad
for it to appear that we are just inflating the uncertainties to
undermine Riess (not that we are, but then sometimes people don't read
as carefully as they should). This further reinforces the need to
take another look at the uncertainties which have been assigned in
your paper.
Finally (for now), I believe it is possible that the CT data are offset
from the CfA data. Examining figure 10, it appears by eye that the
offset could be as much as 0.10 mag. If this is confirmed it would have
a big effect on this paper (for the better). Since you have the data in
hand, could you examine the mean, and error in the mean, for CT and CfA
separately? It would also be interesting to test CT versus CfA/Jha and
CfA/Riess, as well as CfA with the two different filters that you said
were used. This could help explain why I-band has more scatter than
other optical and NIR bands.
I would be happy to further discuss these and other points with you.
Right now I'm just trying to meet the deadline imposed by the Exec,
so have had to be rather brief. As I mentioned, I will also send
some more specific comments on the text and figures, most of which
should be rather straightforward.
All the best in the New Year,
Greg
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