comments on May 7th draft - round 1

From: Greg Aldering (aldering@panisse.lbl.gov)
Date: Tue May 13 2003 - 11:15:03 PDT

  • Next message: Greg Aldering: "May 7 comments - round 2"

    Hi Rob,

    I've read the May 7th draft, but found it difficult to assess in detail
    because the analysis is in an intermediate state. This is not a criticism,
    it is just to let you know that I do have some questions which I will
    not go into yet, pending the updated analysis.

    Let me start by discussing the new analysis with R_B = 2.X. You of
    course realize that the minimization I performed is valid only if all
    the other steps - the photometry, K-corrections, fiducial color vs
    stretch - are correct. While they may be, I believe the read may
    equally well take our modified R_B as a signal that those other steps
    are causing systematic errors that force a low R_B. The fact that
    a change in R_B covaries with alpha (as can be seen in looking at your
    fits with different R_B's) also means that the minimization that I
    performed is only very approximate. It also makes me concerned about
    the fiducial color vs stretch. We have not yet seen your color vs
    stretch relation - it seems prudent to show that relation (internally)
    so it can be compared with the relation used in P99. It would also
    help to see that relation with data only from E/S0 hosts (which is
    how the P99 relation was set).

    To avoid a quagmire, I suggest the following solution:

     a) Follow Phillips 1999 in throwing out very red objects even though
        you are going to extinction correct. I believe that Phillips
        threw-out objects with Bmax-Vmax > 0.2 mag. (Talk about dirty
        laundry!)

     b) Use the Phillips 1999 R_B, and note that that value is consistent
        with both our low- and high-redshift SNe.

    This way, 96bo and 98aw will be eliminated, and our procedure with be
    based on the literature. Otherwise I believe we could be forced to spend
    pages defending a new R_B, all the while undermining confidence.

    Next, on a related front, there are a few unsettling changes wrt to
    earlier draft. Whereas in the early version OM was lower by 0.02 for
    the extinction-corrected HST sample *relative to* the uncorrected HST
    sample, it is now relatively 0.04 higher for the HST
    extinction-corrected sample (see Table 8). In addition, in Table 7 one
    now sees that different SN subset have become redder by different
    amounts following the change in the fiducial color by 0.02 mag. (By
    the way, could you let us know how you performed the shift in the
    fiducial color?) The differences aren't huge, but when multiplied by
    4.1 (or whatever) they start to matter. (Note that since we are
    discussing a shift using the same data, the significance is much larger
    than the quoted statistical uncertainties might lead one to think.)

    Here is what I mean:

           Table 7 Table 7 Diff
           Earlier May 7 New-Old
           Version Version
         Low-extinction Samples
                 E(B-V)
    ---------------------------------------
    H96 -0.021 -0.002 +0.019
    R99 -0.011 +0.002 +0.013
    P99 -0.008 -0.013 -0.005
    HST -0.008 +0.022 +0.030

    This has broadened the gap between H96 and HST from 0.013 to 0.024 in
    E(B-V), which is a *change* in the difference by 0.045 mag in A_B.

    I guess I'm just suprised that a color shift of 0.02 mag is amplified
    by so much. It is not only shifting the colors by the expected 0.02
    mag, but is also increasing the H96-HST difference by 0.01 mag at the
    same time.

    Let me close this e-mail by noting one other theme which needs to be
    handled more carefully. When you discuss the Sullivan et al results,
    and when you discuss the Bayesian prior bias, you focus on the
    differences relative to the true brightnesses of the SNe. However, as
    you know, it is only differences between high and low redshift which
    matter. So, for example, the fact that Sullivan et al find a fainter
    script-M in spirals than in E/S0's doesn't affect the cosmology. (It
    is of course interesting in that we do expect more extinction in
    sprials.) Likewise, the DC bias from the Bayesian is irrelavent. It is
    only the change in this bias between low and high redshift, which can
    occur if the *uncertainties* are different, that matters. This is a
    very important point; misunderstanding of this point is a major source
    of papers assailing the SN results. We can not afford to muddy the
    waters ourselves.

    Cheers,

    Greg

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