transparency

Brice Ménard (CITA) gave a nice seminar at Columbia about angular correlations between background (redshifts one and higher) quasars and foreground (redshifts one third) galaxies. The correlations are dominated by lensing, but also have a small color term which is consistent with absorption by dust. His results on dust compare favorably to my results with Jo Bovy and John Moustakas, although he worked entirely in angular units; it is somewhat easier to interpret and model in projected transverse distance units (a simple modification to their current strategies). He had not done any de-projection or halo modeling of the results, so he couldn't precisely say what dust is associated with galaxies of each individual type.

After the talk Lam Hui (Columbia) and I discussed various matters transparent, including tests for the monopole (unclustered) term in attenuation, and possible other explanations for chromatic effects in photon propagation. For example, if the dark matter is made of substantial millilensing or microlensing lumps, and if quasars have a wavelength–size relation, there might be chromatic effects in the angular correlations.