#GaiaSprint, day 3

Boris Leistedt (NYU) and I have been talking for a while about a set of subjects related to the point that proper motions and parallaxes are both inversely related to distance, so you can use them to inform one another. This is a covariance induced by the geometry! Today he got this all working, along with a hierarchical inference of the velocity distribution in the Milky-Way halo. It is early days, but it looks like he substantially improves the parallax estimates for most stars. And, importantly, he can produce improved parallax likelihoods not just improved parallax posteriors. That is, they have wider use in downstream inference than, say, the Bailer-Jones et al distances. But still they will be hard to use absolutely correctly.

Andy Casey (Monash) and I discussed a possible a non-parametric model for the radial-velocity scatter delivered in Gaia DR2. This model would compare any star to its neighbors in relevant parameters (like color and apparent magnitude and housekeeping flags) to establish whether it has enough of a RV excess to be considered a likely binary.

Ana Bonaca (Harvard) showed me maps of the Jhelum stellar stream which make it look (to my eye) like a fold caustic! Many moons ago, Scott Tremaine (IAS) asked me if we could find various kinds of catastrophes in the stellar density, and I (and friends) responded with this paper on the cusp catastrophe. Maybe Bonaca has found one, but a fold! (And folds should be more common than cusps.)

Christina Eilers (MPIA) and I temporarily paused our methodological developments on our spectroscopic-parallax project and made maps of the Milky-Way disk. We tried plotting velocities, abundances, and vertical distortions (warps). Getting good visualizations is hard because the APOGEE selection function is so featured. That reminds me of why I am such a big fan of SDSS-V!

Many interesting things were shown in the afternoon check-in, but incredibly Sihao Cheng (JHU) and Sergey Koposov (CMU) found that galaxies appear in the Gaia DR2 data as variable stars! Why? Because the asymmetric Gaia point-spread function projects onto the complex galaxy morphology differently at different s/c orientations. That rocks! In principle the galaxy morphologies could be inferred from the time-variable data...

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