forward scattering by dust

Is the Universe transparent? This issue has been interesting me for many years; the answer is "yes" of course, but just how transparent? There are many ways to look at transparency, but a talk by Corrales last week got me thinking about it again: Take some point sources, regress their images against brightness, color, and line-of-sight dust amplitude. Do you see a scattering halo that is correlated with dust? In the optical and UV, I think you won't, based on unpublished work I have done previously. However, there might be a tiny signal. Also, there is more likely to be an effect in the x-ray, which (with Chandra) is accessible. And there is abundant archival data for this project in every waveband from infrared to X-ray.

what is the spectrum of dust attenuation?

The SDSS has taken spectra of thousands of F-type stars, at different distances and through different amounts of interstellar dust. These stars were chosen for calibration purposes; they were chosen because they have very well-understood and consistent spectra. These have been used to calibrate the SDSS telescope, but they can also be used to calibrate interstellar dust.

The general procedure would be to start by measuring the equivalent widths of a few absorption lines—preferably a couple of Balmer lines and a couple of metal lines— consistently for all F-stars. These line EWs would provide a dust-indpendent temperature and metallicity indicator for all the stars. Compare the spectra of the F-stars at different reddening but fixed absorption-line equivalent widths (and therefore fixed temperature and metallicity) to get the dust attenuation at resolution of a few thousand. There probably isn't anything interesting there, but if there is it would be a valuable discovery.

The easiest way to do this project is by spectral stacking, but there might be methods that build a non-linear model of the stellar spectrum with three controlling parameters: Balmer EW, metal EW, and SFD-dust-map amplitude. I started discussing this project many years ago with Karl Gordon (STScI); if you want to give it a shot, send us both email for ideas (if you want to; otherwise do it and surprise us!).