The central object

High energy Radiation

Sgr A* is also an X-ray source, with an X-ray luminosity of ~ 10⁵ L_sun.

The X-ray luminosity is variable , on a timescale of a few months. This sets an upper limit on the size of the X-ray source. Because of causality, the fastest an object can change brightness is simply given by the light-crossing time of the object:

Since the Sgr A* X-ray source is variable on the timescale of a few months, it is (at most) a few light months across in size. In other words, the size of Sgr A* is < 0.1 pc .

(An outburst viewed in the infrared, from the UCLA Galactic Center Group)

Radio Interferometry

Using radio interferometry, we can study the structure of Sgr A* down to a few milliarcseconds.

On these scales it is still unresolved, meaning that it must have a physical size of ~ 20 AU or less -- the size of the solar system!

Kinematics of stars around the galactic center

Let's look at the velocities of stars around the galactic center. How would we expect the velocities of stars to change?

So we should expect the velocities of stars to not change much as we get close to the galactic center.

But they do change. The stellar velocities rise rapidly in the inner few parsecs. What does this mean?

Over the past 10 years, two research groups (Ghez etal; Eckart etal) have been studying the proper motion of stars at the galactic center. To detect the transverse motion of stars 8 kiloparsecs away, some of these stars must be moving very fast: ~ 1500 km/s!

The velocity dispersion of these stars show a Keplerian falloff (v~1/sqrt(r)) from r=0.01 pc to r=0.1 pc:

This argues there is a lot of mass well inside a radius of 0.01 parsecs.

Actual Keplerian orbits can be derived for individual stars passing w/in a few hundred AU of the galactic center (Schodel etal 2002; Ghez etal 2002).

More stars have been tracked since then (from the UCLA Galactic Center Group):

Keplerian orbits -- object is a point mass (or at least much smaller than the orbit size). In mid-2014, one of the orbiting stars (G2) passed within 130 AU of the central source and remained on a Keplerian orbit.

All this data is consistent with a supermassive black hole at the galactic center with a mass of ~ 4x10⁶ M_sun.

Questions:

How did it form? How did it grow?
Why is it at the center?
How has it affected the galactic center?
It's quiet now, but what happens when it feeds?
Is there one of these at the center of every galaxy?