Example: Galaxy Rotation Curve

In this example, we show you the bright line (emission line) spectrum obtained along the major axis of a spiral galaxy in Palomar 5m Hale telescope by Nicole Vogt*, Martha Haynes and Terry Herter.

On the left is an image of the galaxy which is viewed nearly edge-on. The slit of the spectrograph is aligned so that it is oriented along the major axis of the galaxy. In the image, the slit is shown artificially by the thin red rectangle. The light from the galaxy within the box is what the spectrograph detects.

The basic principle is to measure the Doppler shift of the spectral lines at different positions along the slit, hence on both sides of the center of the galaxy.

The rotation curve is obtained from the two-dimensional "spectral image" seen at the right. The horizontal axis is the distance along the major axis from one side of the slit to the other. The vertical axis is wavelength. This spectral image was obtained with the red camera of the Palomar Double Spectrograph.

The white features running horizontally trace emission in the hydrogren H-alpha line (lower and stronger feature, rest wavelength 6563 Angstroms), and in nitrogren ([NII], upper feature, rest wavelength 6583 Angstroms). These emission lines arise from hot gas surrounding hot, young, massive O and B-type stars.

The final rotation curve is obtained by folding the two sides (approach and receding) around the center. It is shown below.


* Nicole Vogt received her Ph.D. at Cornell University in August 1995, spent several years as a postdoctoral fellow at the Lick Observatory at UC Santa Cruz and at the Institute of Astronomy at Cambridge, England, but is now an assistant professor at New Mexico State University. There is life after graduate school (and after winters in Ithaca)!

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