From: Robert A. Knop Jr. (robert.a.knop@vanderbilt.edu)
Date: Thu Mar 20 2003 - 13:25:31 PST
The "extinc" column from the old gersontable. I am having trouble
getting that number out of the current FSD E(B-V) values that I have.
In particular, there is a statement in P99 that I have questioned before
and which I shall question again, as I simply do not believe it:
These values of R_R range from 2.56 at z=0 to 4.88 at z=0.83
This is A_R/E(B-V) due to various different supernova spectra, used for
calculating Galactic extinction.
This CAN NOT be right, and I hope it's not what we really did -- if so,
then we biased ourself seriously at high redshift.
Think about it. We're always talking R-band light... whatever the
sueprnova's redshift, by the time it reaches our galaxy, it's done
redshifting and is at whatever wavelength we will observe it at. As
such, the light being extincted by the galaxy's dust is all red light.
As such, you wouldn't expect a value too different from 2.56. Now, it
can vary, because the shape of the spectrum (i.e. the weighting of the
different parts of the R filter) will vary a lot depending on how much
it's been shfited to the red. BUT, it's completely ridiculous that you
might get an R_R from *any* weighting across the R band which is
*greater* than the quoted R_B=4.14. Unless extinction laws are highly
non-monotonic, there is *no* weighting of the effective R_lambda across
the wavelenghts of the R filter that will give you a larger R_R than
what you get from taking any reasonable average across the B filter.
The only way I can see this huge range happening is if you *deredshift*
the filter before calculating the extinction, rather than *redshifting*
the spectrum. Physically, though, the latter is what happened, and the
latter is the right way to calculate your extinction. We are observing
red light, and it's red as it goes through our galaxy; the dust in our
galaxy does not care that that light may have been blue when it was
first emitted five billion years ago.
(Obviously, host galaxy extinction corrections should be done using the
z=0 R_B value, since there the light *is* blue, and the dust in *that*
galaxy doesn't care if by the time we observe it it's red.)
If we did what we said we did, we've got pretty serious problems in P99.
At least some of the differences I'm seeing in my mb numbers compared to
the P99 mb numbers come from the galactic extinction, and the difference
between O'Donnell and Cardelli laws aren't enough to explain it. The
error in the procedure I describe would make a pretty big difference,
however.
-Rob
-- --Prof. Robert Knop Department of Physics & Astronomy, Vanderbilt University robert.a.knop@vanderbilt.edu
This archive was generated by hypermail 2.1.4 : Thu Mar 20 2003 - 13:25:46 PST