Zooming in on Sgr A*

High energy Radiation

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

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 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.

More recently, orbits were derived for individual stars passing w/in a few hundred AU of the galactic center (Schodel etal 2002; Ghez etal 2002).

How are these images possible? Laser Guide Star Adaptive Optics.

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

All this data is consistant with a massive object at the galactic center with a mass of (Ghez et al 2005):

M = (3.7 +/- 0.2) x10⁶ M_sun

And we don't "see" any of it.... What the hell is this thing?