Dennis Zaritsky (Arizona, on sabbatical at NYU) gave the astro seminar today, about measuring the IMF for the purposes of understanding the mass-to-light ratios of stellar populations. He is using bound stellar clusters, measuring kinematic masses and visible and infrared photometry. He finds that there seem to be two different IMFs for stellar clusters, one for the oldest clusters and another for those less than 10 Gyr in age. But this difference also more-or-less maps onto metallicity (the older clusters are more metal poor) and onto environment (the younger clusters are Magellanic-Cloud disk clusters, the older clusters are Milky-Way bulge and halo clusters). So it is hard to understand the causal relationships in play. Zaritsky is confident that near-future observations will settle the questions.
At lunch, Fadely proposed that we enter the Strong-Lens Time Delay Challenge. We imagined an entry that involves multi-band Gaussian Processes (like those worked out by Hernitschek, Mykytyn, Patel, Rix, and me this summer) added to multi-band Gaussian Processes. Time to do some math.