U-B dispersion from our own SNe

From: Greg Aldering (aldering@panisse.lbl.gov)
Date: Mon Apr 14 2003 - 18:28:48 PDT

  • Next message: Greg Aldering: "Chi-squared of current HST paper OM,OL fit"

    Hi everyone,

    First, I encourage everyone to look at Rob's recent results showing
    that the choice of U-B instrinsic color dispersion has little
    impact on either OM,OL or OM,w. This is very nice.

    Since we need to quote a value in the paper nonetheless, I examined the
    residuals of the z > 0.638 SNe in our sample. In this sample are 5 P99
    SNe and 5 new HST SNe. I examined the chi-squared/DOF for both
    populations for intrinsic U-B color dispersions of 0.0, 0.043, and
    0.095 mag. I found that 0.0 mag was too small, 0.095 mag was too
    large, but 0.043 was juuuust right. 0.043 mag gives chi-squared/DOF
    very near 1 for both datasets. Conversely, 0.0 gives too large
    a value for well-measure HST SNe, suggesting that a little color
    uncertainty does exist. These results show no redshift dependence
    of the range 0.638 < z < 0.863 covered by our data. (Note that
    I have not calculated the significance of the chi-squared/DOF for
    the less optimal values, since as I argue below, what I've done
    is good enough for this paper.)

    One word of warning, however, is that the residual distribution for
    these SNe is strongly bimodal. Half the SNe have very low residuals,
    peaking near 0.3, and half have residuals peaking near 1.5. The P99 and
    new HST SNe are evenly spread between the two peaks, with both samples
    having 2 SNe in the lower peak and 3 SNe in the larger peak. There is
    no redshift dependence. This a little weird given that the error bars
    are so totally different for these two datasets.

    In conclusion, an intrinsic color dispersion in U-B of 0.043 mag
    best describes our own observations. (One caveat is that the residual
    distribution distribution is not Gaussian for any of the above
    choices of intrinsic color dispersion.) An intrinsic color dispersion
    of 0.043 is consistent with Jha's thesis over the stretch range of
    our SNe, after two outliers are removed from Jha's sample. Rob
    has shown that the choice of intrinsic color dispersion for U-B has
    little impact on OM,OL or OM,w. Therefore, I suggest that we use
    0.04 (or 0.043 if you like), and close the book on this chapter.

    Cheers,

    Greg

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