From: Alexander Conley (AJConley@lbl.gov)
Date: Thu Sep 02 2004 - 15:03:22 PDT
Hi Ariel,
I finally managed to investigate your first question -- the one about
not applying a MW extinction correction. Indeed, not applying the
MW extinction correction has a very small effect on the intrinsic
dispersion of B_BV0.6, but has a significant negative effect on
m_B.
With MW extinction correction, sigma_int = 0.1206 for B_BV0.6
(where sigma_int is the error to add in quadrature to get the
chisquare to come out right) and the raw RMS is 0.196. If I
remove the MW extinction correction these values actually
get quite a bit worse: sigma_int = 0.2055 and RMS 0.2825.
However, looking in a little more detail, both increases are
completely dominated by SN1995bd. This has
E(B-V)MW = 0.5, corresponding to 2 mag of B band extinction,
or a full magnitude for CMAGIC. If I eliminate this SNe, the
numbers drop to sigma_int = 0.117 RMS 0.19, which is not
statistically significant from the MW corrected values.
That is, the MW extinction correction has little effect on B_BV0.6
except in the case of 95bd, which has a very large amount of
MW extinction.
When I turn to the m_B values, with MW extinction correction
sigma_int = 0.222 RMS 0.2696. Without MW correction
sigma_int = 0.2637 RMS 0.304. Unlike the B_BV0.6 case,
this increase is not dominated by any one SN. The reduction
in the quality of the fit seems to be spread along the sample.
This is a significant change in sigma_int, by around 4 sigma.
Also note that SN1995bd is not in the m_B sample. In addition
to having a large amount of MW extinction, it also has a lot of
host galaxy extinction (0.24 from Phillips). In my analysis I
discovered that a relatively strict color cut is necessary in m_B
analyses (assuming that you aren't performing an extinction
correction) or the fits are very poor, and 95bd fails the level of
cut that I have been using for the maxmag fits.
To summarize: As expected, a Milky-Way extinction correction
is of great benefit to a maximum magnitude fit, but has a much
smaller effect (too small for me to detect, in fact) on a CMAGIC
analysis.
Alex
> 1) As I understand it, you correct for MW extinction. Have you
> considered
> the idea of using the "known" MW extinction for tests the CMAGIC
> method
> versus the standard analysis? Eg, one could perhaps look for what
> the
> MW dust does to the low-z dispersion in both methods and thus check
> that the lower sensitivity to extinction for CMAGIC holds. Maybe
> this
> could be extended to the best measured high-z SNe.
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