On 18/10/2013 6:07 PM, Steve Willner wrote:
Understanding this in detail would take more work than I'm prepared
to do, but the "Magorrian relation" says that black hole mass
correlates with _spheroid mass_ (i.e., bulge mass for a spiral
galaxy), not with total galaxy mass. In other words, the galaxy disk
mass doesn't count towards expected black hole mass. There's also a
fair bit of scatter around the mean relation, though not usually as
much as indicated above.

There's some indication that the MW is a barred spiral galaxy, rather
than a standard spiral. That might indicate that it's got an extended
central bulge. Is it possible that there are other mid-sized blackholes
(i.e. 100,000 Msun) anchoring the outer parts of the bar?

Another thing to check is where that M31 BH mass value comes from,
though the first source I glanced at gives that value. (The Milky
Way BH mass comes from stellar orbits and should be reliable.)

Even if M31's BH mass isn't completely accurate, I'm sure it's within
50% of the final determination. I remember before the MW's BH was
well-established, they were guessing between 3 million and 6 million
Msun, which is remarkably close to the finally established 4 million
Msun mass.

And anyways, I might be wrong, but M31's BH mass was apparently recently
fully established. I think they got stellar velocity data around its BH too.

Yousuf Khan