Arecibo Observatory

Estimating the dynamical mass of a galaxy from the HI line width

The HI 21 cm line profile as detected by ALFALFA is spread over frequency/velocity by Doppler broadening. The principal cause of the velocity spread is rotational motion in the galaxy's HI disk. We can use the HI line width as an indicator of the rotational speed of the galaxy, and in combination with an estimate of the extent of the HI disk, R, we can derive the dynamical mass contained within radii r < R, via:

where Vrot is the rotational velocity, R is a measure of the outermost radius at which Vrot is measured and G is the gravitational constant. Needless to say, you have to use the right units to get the answer to work out in solar masses!

We will add more here to describe how we get from V to Vrot, what we use for R, etc.

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Last updated Mon Oct 8 09:08:17 EDT 2007 by martha