From: Gabriele Garavini (garavini@in2p3.fr)
Date: Tue Jan 25 2005 - 12:58:34 PST
> What Eric is currently seeing with code that we *think* reproduces what
> you've done is that the high-z supernovae he has from Keck are showing
> up with EW's a little bit lower than the ones you have-- sort of at the
> ridgeline of your distribution. It is possible that this results from
> systematics in different galaxy subtraction. Eric is using Andy's fit
> program for galaxy subtraction; what are you using?
This is in principle possible. We allowed for this source of systematic
and extra uncertainties of 10% in the galaxy subtraction. This is
included in the paper and added to the statistical error. The procedure
used to subtract the galaxy is that written by Gregory Sainton, and
uses a
similar approach as that of Andy, but with a different minimization
algorithm.
>
> Can you send us the galaxy-subtracted spectra from which you made your
> high-z measurements? (The original galaxy unsubtracted ones are all
> the
> ones on Isobel's spectrum page, yes? If not, please send us the
> location where we can find those.) That way, Eric can try his galaxy
> subtraction and compare it to your galaxy subtraction on the *same*
> spectra to see if that is the systematic. It may also allow us to
> diagnose if Eric's procedure really is the same as your procedure
> without having to wait for Gaston's numbers (although we *still* *need*
> *those*, so please get them to us!)
please find the tar file with the vlt_hz spectra at
supernovae.in2p3.fr/~garavini/papers/vlt_hz.tar
>
>> Rob do you find the new section of the Evolution paper where I explain
>> the technique and possible systematic, clear enough?
>
> A couple of outstanding issues:
>
> * What procedure do you do to identify the maximum, i.e. the center
> of
> the 20-30 angstrom window on each side? In low-z spectra, it's
> visually obvious, but it's less so with high-z spectra. You should
> comment how you choose it. Is it just by eye? Are you smoothing
> and doing it by eye? Are you fitting a parabola to the local data?
> Or something else? (Eric speaking: this is currently my biggest
> question about the procedure.)
Unfortunately I'm still waiting for the exact number of the window size
in
the high and low S/N ratio cases, but what is written in the paper right
now should be reasonable. Still I'd like to have the final number from
Gaston. For the procedure: I think Gaston was actually looking at the
unsmoothed spectra by eye and was not fitting any parabola.
>
> * Can we confirm the 20-30 angstrom region? Why not just pick 25
> angstroms and always use that for noisy spectra? (This is what
> Eric
> has done so far.) If sometimes you use one window and sometimes
> another window, what is it that guides the choice?
As before, this is a detail Gaston need to sort out.
> * It's still a little unclear exactly what you're doing. One
> paragraph
> says straight line fit. The next paragraph says the wavelength
> regions selected. I would suggest the following wording for
> Section
> 3.2.1, starting with paragraph 2:
> In order to measure an equivalent width (EW), the wavelength
> range of the line and the underlying continuum must be
> identified. In the case of narrow emission or absorption
> lines,
> this procedure is straightforward; however, SN spectra show
> very
> broad features bounded not by smooth continua, but mearly by
> local maxima between them. The wavelength range and continua
> used for EW measurements are defiend as follows.
> First, on either side of the feature a local maximum is
> identified; these local maxima are used as the edges of the
> wavelength range in which the equivalent width is measured.
> Table 4 lists the range of wavelengths in which the local
> maxima
> on either side of each feature can be found. In order to
> specify the location of the local maximum, we DO WHATEVER IT IS
> THAT WE DO TO FIGURE THIS OUT.
> Once the positions of the local maxima are identified, we
> extract all of the data within a small window around both
> maxima. To those data, we perform a chi-square fit of a
> straight line. This fit is used to define the effective
> continuum for the equivalent width measurement. (This is an
> empirical definition used for purposes of the measurement;
> given
> the complicated blended nature of the SN spectra, it almost
> certainly does not represent a "real" continuum.) The
> wavelength span of these windows depends on the SNR of the
> spectrum. In high SNR spectra, a typical window has a width of
> 10 A (?????). In low SNR spectra, where the position of the
> maximum is less easily identified, a larger region (typically
> 20-30 angstroms, depending on WHAT DOES IT DEPEND ON?) is used.
>
> The systematic effect of low signal to noise... (((last two
> paragraphs as is for now))).
>
>
> * Please explicitly say that you're using the original resolution of
> the spectrum without smoothing or binning, if that is the case.
>
> -Rob & Eric
>
I think the rewording sounds good to me. I'll put it in as soon as
possible.
Thanks
Gabriele
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