Galaxies Lecture 17

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Radio HI

When we make observations through the Milky Way with a radio telescope, we are trying to sample and use that information to tell us how gas is moving. However, our radio telescope has a beam width, and our spectrometer has bins of a certain width, so we essentially pixel-ize this data at some . Changing to cylindrical coordinates centered on the center of the galaxy, we can translate our pixels into . Now we are in physical coordinates, and we can use the equation:

Now , or we can flip around and consider columns in . If we want to find the thickness of the column of hydrogen along the z axis, we can just compute the moment:

Using data gathered with these z measurements, we can analyze the warping modes of the plane of our galaxy. We find that the warping of our galaxy is dominated by m=0 (bowl shaped), m=1 (integral shaped), and m=2 (saddle shaped). Weinberg at UMass makes an argument that the warping is the gravitational effect of satellite galaxies (like the Large Magellenic Cloud) on the dark matter halo of our galaxy, where distortions in the dark matter halo are giving rise to coherent, low-order, modal structure in our disk.

On to a different topic: we can determine the velocity dispersion of gas in the plane of our disk in our neighborhood by looking at some direction where we shouldn’t see any postive-shifted gas, and attributing any positive velocities we see to the tail of a gaussian velocity distribution. Fitting for this, we find a dispersion in our neighborhood of about 7 km/s.

Suppose we would like to solve for some of the rotation constants of the Milky Way in the neighborhood of our sun–i.e.:

and we want for small . To do this, we’ll Taylor expand :

Now , so we have

Changing variables from to , we have:

This is called the Oort A constant. The first measurement of this constant confirmed Shapely’s theory that we are not in the center of the galaxy.