modeling galaxies, flat-fields

Larry Widrow (Queen's) gave a very nice informal talk about his work modeling galaxies as self-consistent dynamical systems, with (axisymmetric) disk, bulge, and halo. He has a nice framework in which he can use all the (data) constraints and do MCMC fast, because his models are flexible but not N-body. He then takes a subsample of the acceptable galaxy models from the posterior PDF and integrates them with an N-body code; sure enough they are all unstable to spiral-arm and bar formation, but probably not inconsistent with the observed bar and arm sizes and amplitudes. He also showed some nice data on the M31 group, and indicated that recent results on non-sphericity of the Milky Way halo are almost certainly over-estimates.

Holmes and I made the flat-field in our Euclid simulations far more pessimistic (or conservative, if you prefer), giving it low-level power on a range of scales. We also switched our uber-calibration code so that it has slightly wrong estimates of the true noise variances affecting each star (and systematically wrong), and we made it such that the function space in which we are fitting does not match the function space in which the true flat-field was generated. That is, we made the whole system much more realistic. Results are even more stark that random observing strategies crush grid-like strategies for calibration.

No comments:

Post a Comment