From: Gabriele Garavini (garavini@in2p3.fr)
Date: Fri Mar 04 2005 - 09:09:28 PST
Dear Chris, Dear All,
I've implemented the latest comments from Chris, Andy and Rob into a
new draft of the paper and made some more modifications
(supernovae.in2p3.fr/~garavini/papers)
The most relevant news are:
Section 3.3
I've completely redone the statistics. I've included in the chi^2 test
also the error on the x axis into. It was not there before. This was
done propagating it into an y axis error according to the ew model of
the low redshift SNe. This needed to be done for correctness, and has
some nice consequences especially for MgII. Since the ew evolution of
this feature has a big derivative wrt time the difference in the
probability in table 5 is significant. Now the hypothesis that low and
high redshift SN have the same ew trend is true for both Fe and Mg.
Also, I've included in table 5 the same chi^2 test considering in the
uncertainties also the systematic due to the misidentification of the
fitting regions.
Section 3.2.2
The last paragraph about the systematic due to host galaxy removal was
not exhaustive. I've expanded it explaining in some more details where
the 10% of uncertainty comes from.
Section 3.2.3
In the second last paragraph of page 8 we commented on the higher
than average values of ew for Mg before max light (a 2-sigma effect).
The previous version of the paper included an explanation that did not
convince neither me nor Chris. So I've done some thinking and some
checks. Looking at the spectra and trying to identify the feature I
noticed that in the early epochs since this feature is shallow it is
easy to misidentify the maxima perhaps including some of the neighbor
absorptions. This would bias the ew measurements toward higher values.
This effect is taken into account by the misidentification systematic
uncertainties. In the current version of this paragraph I explain that
this could be the origin of the problem but that anyway it is not
statistical significant.
Section 3.2.1
I included the latest comments from Rob. We now agreed on a description
that hopefully will allow the reader to measure ew as was done for this
paper.
This should cover all the comment and worries raised so far.
Please let me know what you think and how we proceed from here?
I think now the paper can safely state that we do not see evolution.
The new version includes all the authors that were included into the
VLT paper. I know some of the affiliation are wrong. I'll check them
out.
Thank you very much
Cheers
Gabriele
On Feb 23, 2005, at 4:48 PM, Chris Lidman wrote:
> Hi Gabriele,
> I've given some more thought about the possibility of doing a 2D KS
> test with the data in figure 5. It seems to me that we cannot do
> this test because one is not sampling the nearby and
> distant samples randomly.
>
> The alternative is the chi-sq test, as you have done. The
> resulting probability (0.15) is low. The question we must now ask
> ourselves is do we believe this result. If we do, then we cannot
> say that there is no evidence for evolution, as we do in the abstract.
>
> There could be reasons for the difference. The example you give
> is systematic errors in host galaxy subtraction. If this is the case,
> why would if affect the the Mg II index but not the FeII index.
>
> Regards, Chris.
>
> On Tue, 2005-02-22 at 20:56, Chris Lidman wrote:
>> Hi Gabriele,
>> Thanks for the reply. I forwarded your paper to Andy and he has
>> provided some additional comments. Andy's comments consist of
>> grammatical corrections, which I reproduce below, and an important
>> question regarding host galaxy subtraction.
>>
>> Before I list Andy's comments, I'd like to add to my point
>> concerning
>> the significance of the difference seen in figure 5.
>>
>> The test you have done is to test whether high redshift SNe are more
>> similar to low redshift normal SNe or to low redshift underluminous
>> SNe.
>> You have found the former to be true. However, this does not mean that
>> they are the same as low redshift normal supernova. Hence, you cannot
>> conclude that there is no evolution. For this, you need to conduct a
>> more sophisticated test.
>>
>> It may not be easy to do this in 2 dimensions. In 1D, the
>> obvious tests would be the KS test and Student's t-test.
>> However, using a web search, I found the following web page
>> which describes a 2D KS test.
>>
>> http://beowulf.lcs.mit.edu/18.337-2002/projects-2002/ianchan/KS2D/
>> Project%20Page.html
>>
>> Have a look and let me know what you think.
>>
>> In addition to Andy's comment regarding host galaxy subtraction, one
>> should note that the bias could go either way. In some cases, you
>> might
>> subtract too much host and in other cases you might subtract too
>> little.
>> Currently, the paper assumes that the error in the subtraction is 10%.
>> Where does this percentage come from? I think that this needs more
>> justification.
>>
>> Cheers, Chris.
>>
>> Start of Andy's comments
>> ========================
>>
>> Comment #1
>>
>> The largest source of uncertainty is the host galaxy subtraction. Yet
>> this is swept under the rug. If this was done using the determination
>> from my program, for example, you can't trust any of the conclusions
>> from the paper. My program simultaneously fits host galaxy and SN
>> templates. In other words, it *assumes* that the high-z spectra is
>> similar to a low-z one and subtracts an amount of host light such that
>> the high-z and low-z spectra will look similar. The proper thing to
>> do
>> would be to use the photometry (e.g. percent increase) to constrain
>> the
>> amount of host galaxy light, and to use imaging and spectral features
>> to
>> constrain the host characteristics. You might be able to say some
>> spectrum has 30% host +/- 10%, and is either an E, SO, or Sa. Then
>> you
>> could do different subtractions in that parameter space and test the
>> results. Defining a technique like this would be a great addition to
>> this paper because any time people use high-z spectra they are going
>> to
>> have to do something like this.
>>
>> Comment #2
>> ==========
>>
>> One other point -- in the SNLS we have many 91T/99aa SNe at high
>> redshift. So there is no need to make a big deal out of having one in
>> the sample, as this paper does.
>>
>> Minor comments
>> ==============
>>
>> p. 1
>> Intro, par. 1 -- plural of supernova is supernovae
>> tidly ????
>> par. 4 it should be pointed out that the effect metallicity has
>> on the spectrum pointed out by Hoeflich and Lentz are only for
>> early-time
>> spectra, of which there aren't many here.
>>
>> par. 6 -- I have submitted a paper on SNLS gemini spectra -- Howell et
>> al.
>> 2005, ApJ, submitted
>>
>> p. 2, section 2.1 last sentence, One SNe -> One supernova
>>
>> last paragraph of section -- two days is no longer the uncertainty for
>> the
>> spectroscopic epoch determination in Hook et al.
>>
>> section 2.3, par 1 -- you say EWS but it is not defined yet.
>>
>> p. 4
>> table 2 'Si' should be 'Si 4000' since you say it is absent in
>> underluminous SNe.
>>
>> first paragraph on page - there is a period missing before SN 2002fd
>>
>> p. 5
>> 3.1, 3rd par. "uncertainty in the redshifts is taken to be 300 km/s if
>> the
>> redshift was estimated from galaxy lines..."
>> shouldn't this be proportional to the redshift?
>>
>> 3.2 first sentence -- delete ", they" and second comma.
>>
>> p. 6
>> "possible involved" delete involved
>>
>> p. 7 last par in 3.2.1 delete space in "spectra ,"
>>
>> 3.2.3, 4th par -- delete space: "( with"
>>
>> p. 8 3.3 miss-identification -> misidentification
>> this whole parenthetical expression should be made into a new sentence
>> because it is confusing with "uncertainties" written twice in a row.
>>
>>
>>
>> On Tue, 2005-02-22 at 05:24, Gabriele Garavini wrote:
>>> I'm very sorry chris I sent by mistake the wrong file.
>>> Here is the one I meant to send.
>>> Thanks
>>> Gabriele
>>>
>>> On Mon, 21 Feb 2005, Chris Lidman wrote:
>>>
>>>> Hi Gabriele,
>>>> Could you resend the attachment. The one I received seems to be
>>>> chopped at the end.
>>>>
>>>> Regards,
>>>>
>>>> Chris.
>>>>
>>>> On Mon, 2005-02-21 at 13:29, Gabriele Garavini wrote:
>>>>> Dear Chris
>>>>>
>>>>> thanks you very much for the careful reading of the paper. I've
>>>>> implements most of you comments. Please find all my answers in the
>>>>> attached file. I also changed extensively the section about EW
>>>>> (3.2) I
>>>>> hope this will allow us to go to submission shortly.
>>>>>
>>>>> Thanks again
>>>>> Hasta la vista.
>>>>> Cheers
>>>>> Gabriele
>>>>
>>>>
>>>>
>>
>>
>
>
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