What kind of mass distribution would be required to make
the bar in a barred spiral galaxy rotate as a unit? Does it in fact
rotate as a unit? Like a rigid body?
Very timely!! An article: "Ripples in a Galactic Pond" in the October 2005
issue of Scientific American addresses your question in detail. The
article also cites a number of Internet and print sources for further
exploration. The dynamics of galaxies is not simple and is not fully
understood. To say much more is way beyond what can be done in a format
like NEWTON BBS.