A Lagrangian View on the Relation between Galaxy and Halo Angular Momentum

Observations combined with basic theoretical considerations suggest that disk galaxies have approximately the same specific angular momentum (sAM) as a typical dark matter halo with the corresponding host mass. The most simplistic interpretation of this newly established result is that the baryons that make up the disks have retained the sAM they obtained from cosmological tidal torques throughout their evolution from the intergalactic medium down to the present-day stars within the galaxy. There is evidence, however, that reality may be substantially more complex than this simplistic picture. Here is a theoretical discussion of the sAM of such baryons through various stages in their evolution. It is argued that the sAM evolution during many of these stages is still not well known, is expected to be substantial, and is probably interrelated despite the diversity of scales and physical processes involved. A strong interplay is necessary to obtain a result that mimics the simplistic `retention' picture.