Local Group on the Local Group

Today, Kathryn Johnston (Columbia) organized a “Local Group on the Local Group” meeting at Columbia. Here are some highlights:

Lauren Anderson (Flatiron) gave an update on her data-driven model of the color–magnitude diagram of stars. This led to a conversation about which features in her deconvolved CMD are real? And are there too many red-clump stars given the total catalog size?

Steven Mohammed (Columbia) showed our GALEX Galactic-Plane survey data on the Gaia TGAS stars. The GALEX colors look very sensitive to metallicity and possibly other abundances. The audience suggested that we look at the full dependences on metallicity and temperature and surface gravity to see if we can break all degeneracies. This led to more discussion of the use of the Red Clump stars for Galactic science.

Adrian Price-Whelan (Columbia) presented a puzzle about the Galactic globular cluster system, which he has been thinking about. Are the distant clusters accreted? The in-situ formation hypothesis is unpalatable (it had to be many clusters at early times; should be many thin streams); the accreted hypothesis over-produces the smooth component of the stellar halo (unless dwarf galaxies had far more GCs per unit stellar mass in the past). These problems can be resolved, but only with strong predictions.

Yong Zheng (Columbia) spoke about the gaseous Magellanic stream and associated (or plausibly associated) high-velocity clouds. Many of the challenges in interpretation connect to the problem that we don't know where the gas is along the line of sight. She showed really nice data on something called Wright’s Cloud. For this huge structure—and for the stream as a whole—there is little to no associated stellar component.

Nicola Amorisco (Harvard) Showed theoretical simulations of the accreted part of the MW (and MW-like-galaxy) halo, with the goal of finding stellar-halo observables that strongly co-vary with the assembly history of the dark-matter halo. Both theory and observations suggest large scatter in halo properties at Milky-Way-like masses, and much less scatter at higher masses (because of central-limit-like considerations). His results are promising for understanding the MW assembly history.

Glennys Farrar (NYU) spoke about the MW magnetic field, using rotation measures and CMB to constrain the model. She showed UHECR deflections in the inferred magnetic field, and also discussed implications of her results for electron and cosmic-ray diffusion. There are also tantalizing implications for the synchrotron spectrum and CMB component separation. One interesting comment: If her results are right for the scale and amplitude of the field, there are serious questions about origin and generation; is it primordial or generated on scales much larger than the galaxy?