From: Alexander Conley (AJConley@lbl.gov)
Date: Mon Nov 08 2004 - 13:11:51 PST
Hi Greg,
I've been looking at the color distribution at maximum. I guess the
first reaction
should be: what in the world was I thinking, saying that those two
distributions
were compatible -- they don't even really look like it to the eye.
And, in fact, I did discover than using the very weak color cut (0.6)
for
the max mag fits was disastrous -- the Hubble diagram looked terrible,
and
the results were way off of expectation. I ended up using a cut of 0.2,
and things seemed to work well.
So I tried to look at the effect of various color cuts on the mean
colors.
A complicating factor is that there are two high-z SNe with rather
anomalous
colors: 01iy, and 97af (-0.32, -0.24). Now, 97af has huge errors, so
may
not be a big deal, but 01iy claims to have good errors. Looking at the
actual lightcurve, it is rather poorly sampled. I'm not sure it's
wrong, but
I think I can categorically state that this is one of those cases where
snmin
radically underestimates the errors. Incidentally, removing either of
these
SNe from the sample does not particularly affect the cosmological
results
(see the jackknife tests).
Okay -- so here's how it plays out in mean color:
E(B-V) < 0.6 : low-z: 0.09417 +- 0.0248 high-z: 0.0229
+- 0.0287
+01iy removed: low-z: 0.09417 +- 0.0248 high-z: 0.0378 +-
0.0256
+01iy,97af removed: low-z: 0.09417 +- 0.0248 high-z: 0.0504 +-
0.0234
E(B-V) < 0.4 : low-z: 0.07 +- 0.0191 high-z:
0.0229 +- 0.0287
+01iy removed: low-z: 0.07 +- 0.0191 high-z: 0.0378
+- 0.0257
+01iy, 97af removed: low-z: 0.07 +- 0.0191 high-z: 0.0504 +-
0.0234
E(B-V) < 0.25 : low-z: 0.04516 +- 0.0140 high-z:-0.0009
+- 0.0257
+01iy removed: low-z: 0.04516 +- 0.0140 high-z: 0.0143 +-
0.0217
+01iy, 97af removed: low-z: 0.04516 +- 0.0140 high-z: 0.0270 +-
0.0185
E(B-V) < 0.2 : low-z: 0.03933 +- 0.0132 high-z:-0.0124
+- 0.0241
+01iy removed: low-z: 0.03993 +- 0.0132 high-z: 0.0030 +-
0.0195
+01iy,97af removed: low-z: 0.03993 +- 0.0132 high-z: 0.0158 +-
0.0152
E(B-V) < 0.1 : low-z: 0.04091 +- 0.0097 high-z:-0.0344
+- 0.0243
+01iy removed: low-z: 0.04091 +- 0.0097 high-z:-0.0176 +-
0.0187
+01iy, 97af removed: low-z: 0.04091 +- 0.0097 high-z:-0.0038 +-
0.0133
Whew!
Okay -- ignoring 97af, 01iy, what does this say?
Well, with E(B-V) < 0.6, the low-z sample is about 0.043 mag redder,
which
translates into 0.18 mag for Bmax and 0.09 for CMAGIC. This reddening
is detected at 1.28 sigma.
With a slightly tighter cut, E(B-V) < 0.4, the low-z sample is 0.02
redder,
or 0.08 for Bmax and 0.04 for CMAGIC. This reddening is detected at
0.64 sigma.
Tighter still, E(B-V) < 0.25, the low-z sample is also about 0.02
redder, with
the same consequences. This is at 0.78 sigma.
Tightening to E(B-V) < 0.2, the value used for the maxmag fits, the
low-z sample
is also 0.02 redder, at 1.18 sigma. Again, this is 0.08 in Bmax and
0.04 in CMAGIC.
Finally, going to E(B-V) < 0.1, the low-z sample is redder by 0.008
at 0.47 sigma.
This is negligible.
There is a difference, but it seems to be heavily driven by the two
really blue points,
01iy and 97af.
One may view this as an argument that a tighter extinction correction
should be used.
I have already done this experiment in the method paper. Here are the
results,
quoted in the principle axis frame of the error ellipse. Recall that
the error along
the short axis is about 0.11, and along the long axis is about 0.82.
Given in terms of shifts relative to the primary fit
E(B-V) < 0.4: -0.007 short, +0.033 long -> negligible
E(B-V) < 0.25 : -0.038 short, +0.0426 long -> about 1/3 sigma short,
negligible long
E(B-V) < 0.1 : -0.0018 short, -0.547 long -> about 1/2 sigma long
The interesting one is the really tight cut, which moves the error
ellipse strongly
along the long axis. This is one of the major long-axis systematics
left in this study.
Bottom line:
The low-z sample is redder than the high-z sample. This messes up the
maximum
magnitude fit, but doesn't do nearly as much to the CMAGIC fit.
Alex
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