From: Reynald Pain (Reynald.Pain@in2p3.fr)
Date: Fri May 23 2003 - 07:02:55 PDT
Rob et al.,
I did not follow in detail the discussions on the extinctions so I
apologies if the questions below have trivial answers.
page 10 (last paragraph)
intrinsic B-V, V-R, R-I colors
- is there any reason why you do not give the values b(t) and m(t)
It would be interesting to add a table with these values
- also what is the reason for 1/s3 and not another s dependency
can you justify the choice of 1/s3 ?
page 13 (top)
can you explain why 0.08 was added in quadrature ? (to get a X2=1?).
Isnt this the instrinsic color dispersion (the expression in not used) ?
Any reason why you use the same in B-V, V-R and R-I ?
How does this "intrinsic color" relate to the additional U-B color
dispersion ?
Also why dont you also add a stretch dependence in U-B ? (see my question
on the values of alpha in table 8)
Now if this 0.08 is the intrinsic (B-V) color dispersion, how can it be
"included in principle" in the 0.11 intrinsic mag of Phillips 99 (para 2.4
page 17 colomn 2) ?. Wouldnt then the intrinsic mag dispersion be
dominated by the color dispersion ?
or am I totally confused ?
Table 8:
Am I right that when correcting for extinction alpha changes mostly
because of the introduction of an intrinsic color stretch dependency
(your 1/s3) ?
If that is so the difference between the values of alpha in fits (1-3)
should be approximatively constant compared to fit (4-6). Fit 5 seems
odd in that respect.
Could that be related to the fact that you do not introduce any strech
dependency in the U-B color (or are alpha_fit2 and alpha_fit5 swapped ?)
one last question : Am I correctly guessing that a U-B=-0.3 (instead of
-0.4) would get us (the SN best value) right on the flat universe line ?
Cheers
Reynald
_____________________________________________________________________
R. Pain - LPNHE, Universites Paris VI & VII, 4 Place Jussieu - T33 RdC
75252 Paris cedex 05 - Tel: +33 1 44 27 72 53 Fax: +33 1 44 27 46 38
On Fri, 23 May 2003, Ariel Goobar wrote:
> Hi Greg,
> that sounds interesting. Have you tried what happens when
> adding a Gaussian intrinsic spred in B-V around 0.05 mag?
> As you point out the effect will be highly dependent on the
> the flux limit you use in your simulations. I am not sure
> there is a perfect 1-to-1 correspondence with the
> parameter you used (dim-cut). There are a few steps between
> candidate discovery (a different stages in the I-band LC,
> stretches, spectroscopy screening, K-corrs, LC fit, etc) and
> the residual showing up in Rob's Hubble diagram.
> Ariel
>
> On Thu, 22 May 2003, Greg Aldering wrote:
>
> >
> > I have simulated the behavior of our low-extinction method. I started
> > with the models of Hatano, Branch and Deaton, which provide the
> > probability distribution function of B-band extinction, A_B, for SNe
> > observed through spiral and elliptical galaxies viewed at various
> > orientations. I generated random values of A_B following this
> > distribution function, but adding a flux-limit bias which depresses the
> > probability of finding extincted SNe in proportion to the volume of
> > space in which they would be accessible in a flux-limited sample. I
> > next "observed" them by randomly chosing an E(B-V) error bar and then
> > generating a Gaussian deviate with this error bar, and adding the
> > result to the generated value of A_B. I then applied the cuts a SNe
> > currently must pass to be included in the low-extinction subset in the
> > HST paper:
> >
> > sigma R-I < 0.25
> > E(B-V) < 0.1 or E(B-V) < 2*(sigma E(B-V))
> >
> > I also added a too-faint cut, requiring that SN not be fainter than N
> > times its magnitude uncertainty (including an intrinsic error of
> > 0.17). In Rob's last fits, all the residuals were within 3-sigma for
> > the low-extinction subset, even though no cut on deviation was
> > applied. Thus, my cut considers the likelihood that if there were a
> > significantly dim outlier we might have chosed to reject it even
> > without a well-measured color to prove that it is reddened.
> >
> > I find that our method does have a bias, but that the bias is small
> > compared to our other systematics. However, it might not be small
> > relative to the Riess prior (that has to be checked).
> >
> > Here is what I get:
> >
> > N-sigma <A_B> <A_B> Bias
> > dim cut low-z high-z low-high
> > -------------------------------------
> > 2.0 0.068 0.081 0.013
> > 2.5 0.075 0.094 0.019
> > 3.0 0.080 0.104 0.024
> >
> > All mean A_B values are in magnitudes. Basically what happens is that
> > the requirement that E(B-V) < 2*(sigma E(B-V)) lets in SNe which are
> > more reddened than E(B-V) < 0.1. This effect is larger at high redshift
> > because the errors are larger. Note also that the average is not zero,
> > and in fact the mode is not zero either. For our current method, I
> > estimate that the bias is about 0.024 magnitudes in the sense that the
> > high-redshift SNe are dimmer. This depresses Omega_M by a comparable
> > amount.
> >
> > Note that if I do not include the flux-limit suppression, our bias is
> > worse. So, we would expect the low-extinction technique to perform
> > worse in a volume-limited sample.
> >
> > - Greg
> >
> >
>
> --
> ___________________________________________________________________
> Ariel Goobar (www.physto.se/~ariel)
> Department of Physics, Stockholm University
> AlbaNova University Center, SE-106 91 Stockholm, SWEDEN
> tel: +46 8 55378659 fax: +46 8 55378601
>
>
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