From: Ariel Goobar (ariel@physto.se)
Date: Fri May 23 2003 - 09:58:38 PDT
Hi Greg,
Have you tried to reproduce Phillips' result? As you
know, he got a spread in the fitted max(B-V)~0.03 only after we selected
the SNe that had extremely small scatter in B-V at late times... Clearly
a "special" subsample of SNe. I don't think I know enough to exclude
the possibility I suggested, but maybe you know more.
Cheers,
Ariel
On Fri, 23 May 2003, Greg Aldering wrote:
>
> Hi Ariel,
>
> You're right that the other steps subsequent to discovery will have
> important effects. In fact, I think that the reason why we see relatively
> more extincted SNe at low redshift from the 1998 search is not only due to
> the survey flux limit allowing them to be found, but also because their
> observed magnitudes were similar to those of unextincted high-redshift SNe
> - which were the ones we were really after - and we didn't have time (due
> to weather) to check out all the candidates. So, these additional affects
> can both help and hurt us in terms of allowing extincted SNe into the
> sample. Unfortunately, I have no means of recreating these in a
> simulation because of the important subjective component. This was also a
> limitation in what could be said about Malmquist bias for this dataset.
>
> As for the intrinsic color dispersion of 0.05 mag, I haven't tried that.
> I've read Serena's paper, but have to say I'm not really convinced of that
> color dispersion, since if it were present one could not get an intrinsic
> scatter of 0.11 mag about the Hubble diagram after extinction correction.
> However, I will grant that there many be bonifide color outliers, since
> Phillips had to cut out objects with B-V > 0.2 to get his nice dispersion.
> So, a Gaussian core with sigma < 0.05, with some non-Gaussian outliers -
> that I would believe. But then throwing those all together and calling it
> a Gaussian dispersion doesn't seem the thing to do. The fact that Phillips
> could make a color cut for extreme color and then get a small scatter
> suggests to me that the outlier population gives itself away, and is not
> hiding within a Gaussian distribution.
>
> Cheers,
>
> Greg
>
>
> 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
> >
>
>
-- ___________________________________________________________________ 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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