It will be with horror that my reader learns that Lang and I spent part of the morning in a pair-code session binning down SDSS data to larger (less informative) pixels. We had to bin down everything: The data, the error model, the point-spread function, the photometric calibration, the astrometric calibration, etc. Why did we do it, you may ask? Because for the Sloan Atlas project, we are encountering galaxies that are so large on the sky (think
M101) that we can't—without major code changes asap—fit the data and model and derivatives into (our huge amount of) memory, even in our (fairly clever) sparse implementation of the problem. The amazing thing is that by the end of the day we (meaning Lang) got something that works: We can run The Tractor on the original data or the rebinned data and it seems to give very similar results. Testing tomorrow!
In the afternoon, Andrew MacFadyen (NYU) gave the Physics Colloquium, about ultrarelativistic plasma problems, motivated by gamma-ray bursts. The most interesting things to me in this business are about universality: In the non-relativistic limit there is the Sedov–Taylor scale-free expanding explosion model. In the ultra-relativistic limit there is the Blandford–McKee jet model. However, on the latter, the different parts of a collimated jet can't actually communicate with one another laterally (for relativistic causality reasons), so there is no possibility of homogeneity. In other words, the jet must be a heterogeneous mixture of jets, in some sense. The mixture fuses together into one jet continuously over time. That seems like a very interesting physics problem. MacFadyen and his group have been doing fundamental work, with awesome visuals to boot.