Tuesday, November 4, 2014

4 years in the making...SNIa and their host extinction properties

Back in South Africa (a postdoc, a kid, two moves, a continent, and a lifetime ago) I worked with a summer student, Adam Reynolds, on the possibility of using the SDSS-III data to use Supernovea Type Ia (SNIa) as probes of the ISM. The idea worked in principle on the --then public-- SDSS-III data but to be really convincing, we'd need their final data-release.
This was a few years in the making but finally arrived in 2014 (huzzah!). I dug out Adam's report and converted it to a small paper (script everything folks...). My interest is of course the distribution of extinction values, could not give a fig about SNIa...

The results are interesting. SNIa effectively "see" a dust screen (inclination dependence is cos(i), nothing more fancy) and their distribution is roughly exponential. The latter is weird, based on everything I've seen (PHAT survey results, occulting galaxy pairs), one would expect a log-normal one. I suspect that is what it really is but right now an exponential is used in SNIa measurements.

The gist of the current paper is Host Galaxy Inclination Matters!

So constructing the next version of SNIa cosmology, should start by taking disk inclination into account as the first host observable to be included. My second guess is host mass. But that is the second paper (already submitted...)

After that, one would need to redo the lightcurves with different extinction priors. To provide the templates for that, I got my STARSMOG HST SNAP program. The ultimate goal is here to reduce the errors in SNIa to below 1%. That'll keep them competitive as a cosmological tool.

That'll keep me busy. All that from a side-interest...

you can find the paper here:

SNIa Host Galaxy Properties and the Dust Extinction Distribution

Supernovae Type Ia display a complex relation with their host galaxies. An important prior to the fit of the supernovae's lightcurve is the distribution of host galaxy extinction values that can be encountered. The SDSS-SN project has published light curve fits using both MLCS2k2 and SALT2. We use the former fits extinction parameter (AV) to map this distribution of extinction values.
We explore the dependence of this distribution on four observables; the inclination of the host galaxy disk, radial position of the supernova, redshift of the supernova and host, and the level of star-formation in the host galaxy. The distribution of AV values encountered by supernovae is typically characterised by: N0 eAV/τ, with τ= 0.4 or 0.33.
We find that the inclination correction using an infinitely thin disk for the SNIa is sufficient, resulting in similar exponential AV distributions for high- and low-inclination disks. The AV distribution also depends on the radial position in the disk, consistent with previous results on the transparency of spiral disks. The distribution of AV values narrows with increased star-formation, possibly due to the destruction or dispersion of the dusty ISM by stellar winds prior to the ignition of the supernova.
In future supernova searches, certainly the inclination of the host galaxy disk, should be considered in the construction of the \av \ prior with τ=0.4/cos(i) as the most likely prior in each individual host galaxy's case.

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