From: Robert A. Knop Jr. (robert.a.knop@vanderbilt.edu)
Date: Tue Jan 25 2005 - 12:14:38 PST
On Thu, Jan 20, 2005 at 02:50:00AM +0100, Gabriele Garavini wrote:
> I would like to know were we stand with the checks that
> we set up to do during the last phone-conference before Xmas.
Just to summarize the phone conversation we had a few minutes ago --
Eric and I are stuck at the moment, because we *still* do not have the
numbers and spectra for the low-z equivalent widths. We need this to
verify that Eric's procedure really is the same as what Gaston has done,
and any statements we make before we have those numbers to verify may be
meaningless (cf: the long conversation at the meeting in December).
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?
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!)
> 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.)
* 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?
* 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
-- --Prof. Robert Knop Department of Physics & Astronomy, Vanderbilt University robert.a.knop@vanderbilt.edu
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