# Talk:Milky Way astrometry

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- First paragraph: " a stellar halo with streams and "clouds" "

 "Clouds" is a bit misleading, should perhaps be replaced with "overdensities"


Section on Optical Catalogs:

Please, include the Southern Proper Motion Catalog (a major 47-year effort, with better accuracy and precision than other catalogs of its class). The reference is Girard et al 2011, AJ 142, 15. The latest installment of the catalog is SPM4, has ~103 million objects down to V~18.

Other efforts that should be mentioned are the more recent catalogs based on 2MASS and POSSI (e.g. PPMXL, Roeser et al. 201, aj 139, 244).

Section on Spiral Arms

To my knowledge the Monoceros ring was seen only in the stellar component. There is still controversy about its origin: a perturbation of the outer disk, or debris from an accreted satellite. Both scenarios should be mentioned.

Section on Thick and Thin Disks

The radial migration mechanism is a rather novel idea that has some potential for explaining the mixing of populations; however it's efficiency is very dependent on the size, strength of spiral arms (which we know rather poorly). For instance, the eccentricity distribution of stars in the thick disk does not support the radial migration (e.g. Casetti-Dinescu et al. 2011, Wilson et al. 2011, Dierickx et al 2010)

Thus, the other ideas of forming the thick disk via accretion of a satellite (or merging of early gaseous clumps) are not at all obsolete, and I would like to see these mentioned.

Section on Streams

There is a lot more recent work on proper motions in stellar streams. See e.g., the Casetti-Dinescu et al 2010 on the Virgo stellar stream, Carlin et al. 2010 on the Anticenter/Monoceros stream, Carlin Ph D thesis on the trailing tail of Sagittarius. All these are based on a high precision, deep proper-motion survey in Kapteyn areas (Casetti-Dinescu et al .2006). I doubt the Munn et al. 2008 catalog could be used for such purposes.