Re: What is the escape velocity of our galaxy

Good question. We wish we knew.

The problem is that we don't have a good handle on the total mass of the Galaxy. The orbital velocity at the outer edge of the Disk, as measured using millimeter-wavelength radio measurements of the gas clouds out there, is about 250 kilometers per second. (We measure these rotation curves for other galaxies both in optical and radio wavelengths, but the dust in our Galaxy makes the optical part difficult for our measuring that in our own Galaxy.) The rotation speed at each point in a galaxy is set by the amount of mass interior to that point. That means that rotation curves can be turned into traces of how mass is distributed in galaxies.

The problem is that when you get to the edge of the Galaxy in terms of what we can see, there's no sign that you are starting to run out of matter. We know what the shape of a rotation curve should be once you're sampling places where you're outside all, or even most, of the matter in a system: it should start falling off, with rotation speed dropping with distance from the center of the motion. In fact, you almost never see this fall-off when you look at a galaxy, including our own. This means that out to very last places you can check, you are still including appreciable amounts of mass to the total in the Galaxy. That means you can't have any confidence that that last point you can measure includes all the mass in the Galaxy. In fact, studies of the velocities of the Local Group galaxies suggest that it doesn't.

This leads off to the general problem of "missing matter" or "dark matter", a bugaboo for astrophysics on all scales larger than our Solar System. There's a page here that starts with rotation curves for galaxies and leads off to the global dark matter problem.

The result is that we don't know what the escape velocity of the Galaxy is.