From: Robert A. Knop Jr. (robert.a.knop@vanderbilt.edu)
Date: Thu Dec 11 2003 - 08:56:10 PST
I think that Serena's paper is ready to go out to the collaboration. By
and large, I think it's a good piece of work. It's too bad that the
grey dust conclusion is "we can't conclude anything", but I like to see
this out there. Two big worries:
(1) The punt on the difference between Serena's K-correction and the
Tonry/Riess K-correction. Puting is probably the right thing to
do, but this will raise huge red flags and objections from referees
and from our competitors.
(2) The lightcurve fitting method for high-z supernovae that Chris is
debating. See my comments below. I think that the method used is
probably fine as it is, but that a lot more supporting
documentation and cross checks are needed (many of which I suspect
Serena has already done, but just hasn't written into the paper).
In particular, I'd like to see the answer to two questions. (1)
How much different is the fit I-band magnitude when a different
low-z supernova's template are used for a given high-z supernova?
Perhaps some statistics and a plot showing the dispersion you get
when you use different templates. (2) Do the templates chosen
match the stretch of the high-redshift supernova where that
information is available? I understand you can't *choose*
templates based on this, as one high-z supernova doesn't have a
stretch availble, but you could comment on how well the stretch
matches. Given that there is a correlation between stretch and
time of second peak, it is a major warning sign if the stretch of
the low-z supernova used as a template doesn't match the B stretch
of a high-z supernova.
This one thing-- more documentation on consistency and variation
with templates-- might be worth putting in before the paper goes
out to the whole collaboration. If you want to go on to the next
step now, I won't object, but I think that the next step might go
more smootly if that analysis is put into the paper before the
paper is released to the ravaging hoards of collaborators.
General comments on the paper:
* Section 2.2: Is there any correlation between the time offset from
Bmax to Imax and Bmax to t2? (I probably asked this before... If
there is no correlation, mention that.) (Does the last sentence of
section 2 refer to this?)
* Section 2.3: I think this could be clarified a bit further. Before
the sentence starting "However, on a total of 42 supernovae, 2
cases..." add the following sentence: "This lends confidence that
the fitting procedure is robust, and given the model of the
lightcurve template will not yeild biased estimations of the
parmeters."
* Section 3, second paragraph: what was alpha_I? I think you may have
mentioned it earlier (1.19 if memory serves), but it woudl be worht
putting it in again here and referring back to the earlier section.
Did you use an error on alpha_I? Or was alpha_I a fit parameter?
* Section 3, top of page 8: mentioning the RMS is 0.19 is, I believe,
redundant with putting in 0.19 as the scatter; didn't the latter come
from the former prveiously?
* Figure 10 is pretty
* End of section 4.1, where does this 0.05 magnitude estimate come
from?
* Figure 11 : if it doesn't make the figure too complicated, it might
be nice to add a supernova spectrum at some relevant redshift here,
pointing out the Ca triplet feature. This might help lend confidence
in the K-corrections for those who worry about it.
* Section 4.4 : did you allow for any intrinsic dispersion in the date
of I max relative to B max, given that earlier in the paper you
report that low-redshift supernovae show such a thing? I'd like to
see more details on the fitting method-- explicitly stating what was
not done or what was assumed, rather than leaving it unstated.
* Section 4.5: "randomlly distributed around the data points". Does
this mean at the time of the data points, and starting with the data
fluxes distributed by a gaussian with a sigma of the data errors?
Please be more explicit about this.
It might be worth reporting more results of this. E.g., given that
you've fit all high-redshift supernovae with all 42 low-redshift
templates, what is the typical dispersion in Imax you get from the
different fits? I.e., how robust is your parameter estimation to
choice of template? It might be worth discussing this in some depth,
and perhaps shoring it up with a plot or two.
Also, comment on how similar the low-z supernova you chose for a
template is to the high-z supernova based on stretch and spectrum,
where each is avilable. Especially given Figure 8 (stretch is
correlated with time delay to second peak), use of the stretch ought
to allow you to constrain which low-redshift supernovae are
reasonable for selecting a template for the high-redshift supernovae.
* I'm very happy to see you taking Riess to task for not publishing his
B-band data. Just because I'm nasty and mean and tired of him taking
us to task for that same thing.
* Consider doing Figure 15 in color.
* Section 5 : "possible systematic differeences cannot be excluded and
could result in measuring an artifically large dispersion". Might
this be worth some more analysis/discussion?
* Table 12 : clarify the first column:
no dust, Omega_M=0.3, Omega_Lambda=0.7
dust Rv=9.5, Omega_M=1, Omega_Lambda=0
dust Rv=4.5, Oemga_M=1, Omega_Lambda=0
(I belive that's what you're saying!) This will make the table more
immediately transparent.
* Last sentence of paragraph 6.0: "These results do not allow to reach
any definitive conclusions". (I think there's an "us" missing.)
Perhaps expand on this a bit? Is there any hope that further
statistics in rest-frame I-band will allow us to limit grey dust, or
are the intrinsic uncertainties and systematics big enough that it's
hopeless at the moment?
* Figure 6.1: the first data point of B-I color lies well above the
other models. Does it? It only looks like 2-sigma to me in the
plot, which really isn't all that huge. I do agree that the B-I
colors appear to have correlations (too red early, too blue late)
compared to the models.
-Rob
-- --Prof. Robert Knop Department of Physics & Astronomy, Vanderbilt University robert.a.knop@vanderbilt.edu
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