theories and measurements of galaxies

Annalisa Pillepich (UCSC) gave a seminar in the morning about the effects of baryons on galaxy formation, comparing dark-matter-only and more realistic simulations. She finds (as does Zolotov and collaborators) that baryons can have a big effect on the structure of dark-matter halos. She finds that baryons have a big enough effect to resolve many of the remaining issues in the comparison of simulations and observations of low-mass galaxies in the Local Group. Really her conclusion (and my reading of her and Zolotov's work) is that—though there are large uncertainties—the magnitudes of the effects of baryons are large enough that they could resolve most current issues, and other things might help resolve them too. So once again the dark matter could be vanilla CDM. That's bad!

In the afternoon, Stephane Courteau (Queens) argued similarly that theories of galaxy evolution have enough uncertainties in them that they don't really predict the properties of the galaxy population well, but from the opposite perspective: He argued that no simulations get all the observed properties of the disk-galaxy population right at the present day. He argued that there needs to be very strong feedback to explain the population in current semi-analytic-style models. He also showed some amazingly good scaling relations for disk galaxies, measured carefully and lovingly by his group.

Late in the day, he argued with me that doing galaxy photometry by fitting simple parameterized models is always wrong. That's what I am doing with my Atlas project with Mykytyn and Patel. I don't agree with Courteau here—my view is that all photometry is in effect model fitting—but his points are good and need to be addressed. Translated into my language, effectively he argues for very flexible photometric models of galaxies; non-parametric models if you will. I agree but only on the condition that those flexible models are regularized so that in effect galaxies observed at different angular resolutions are being photometered equivalently. Well, at least for my purposes I need that (I want to find similar galaxies at different distances).


  1. What's the goal of the galaxy photometry? Just to measure total flux, or other things too?

    I have many thoughts on this (and a summer student looking into some of them). Would be good to have a chat about it.

  2. Brendon: Different people have different goals, which is one of the things that leads to strong, conflicting opinions. Would love to talk about this. I think a lot of the procedures that have evolved have probably evolved for reasons that can be well understood in inference terms.