From: Serena Nobili (serena@physto.se)
Date: Thu Mar 11 2004 - 04:35:44 PST
On Wed, 10 Mar 2004, Alex Conley wrote:
>> For the difference you found between the results obtained using Rob's
>> templates and mine, they agree with what have been already pointed out by
>> Chris for K_JI. These differences are in fact within the quoted uncertainty
>> of 0.05. I believe also that Rob's work in the I-band is probably not as
>> accurate as in B,V and R, nor as in the Nobili et al. 2003 paper, since it
>> was not a relevant band for the Knop et al paper (Rob, do you agree with
>> this?). I any case, the quoted uncertainty (0.05) takes care of these
>> differences, giving us confidence in what we are doing.
>
>Things are not so simple. I'm willing to grant that you may have better I
>band, but the B and V band don't agree either. Attached is a B->B band
>comparison plot, which also disagrees by about 0.05 magnitudes (stretch 1,
>redshift 0.05).
>
>This is not a minor difference. Using your B and V K-corrections when
>fitting cosmologies results in a shift along the short axis of the
>cosmological parameters by around 1 sigma. If we have to double the
>error bar size of all of our cosmological results that we have ever
>published, then people probably wouldn't pay that much attention to
>our results.
>
>Alex
>
>
Hi Alex,
I have done some checks now. You are perfectly right about the difference
in K-corrections. I tell you what I think the story is. Both Rob and I
have started with the spectral template provided by Peter, and modified
its colors according to our (different) studies of average color
evolution. I did not modify the B-band color, but only V,R and I. Rob did
modify also the U-band. I believe this is the origin of the discrepancy. I
attach a plot to illustrate this. Both Rob's and my template at day 0 are
plotted together with the restframe B-band and the red-shifted (z=0.05)
one, and with the restframe U-band. As you see the two templates differ
most in the blue edge of the B-band filter, where there is some influence
of the U-band. This means that Rob's template should be used any time the
U-band could play any role (even marginal) in the computation of
K-corrections. If you trust one of the two method for estimating the color
evolution more than the other, then you should use the corresponding
template. However, I think in general they are both valid and I don't see
mine as better than Rob's (or viceversa), unless it involves the U-band.
This is why I decided to add an uncertainty on the K-corrections in the
paper where only I-band is involved. K-corrections are still an open
issue, I am afraid.
I really hope this will help the discussion.
Cheers,
Serena
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