WHAM First Light
January 24, 1996
North American Nebula
The first spectrum of the North American Nebula (NGC 7000) taken by WHAM.
On a bitterly cold Wisconsin night, the persistent clouds cleared out for eight hours to let WHAM view the galaxy for the first time. This image comes from the first object observed with WHAM, the bright North American Nebula in the constellation Cygnus.
In WHAM's spectrometer, different wavelengths of light fall at different distances from the center of the image. Moving from the center of the image toward the edge, the wavelength decreases by about 4Å. The spectrum below is formed by averaging over concentric rings (i.e. pixels at the same wavelength) in the image above.
When viewing an emission spectrum from a single atomic transition, it is very useful to use the Doppler equation to convert wavelength into velocity. The resulting plot shows us the motion of gas along our line of sight. The zero point of the velocity axis below is arbitrary. When calibrated correctly, the bright Hα line from the nebula appears at vLSR = -8 km/s.
A spectrum from WHAM toward the pulsar PSR 1237+25.
The spectrum at the right was taken in the direction of a pulsar on this same night. The bright ring in this 10 minute exposure is Hα emission from the earth's atmosphere. The geocoronal line is undetectable in the North American Nebula image above since the nebula is about 85 times brighter. In this case, because the emission from the Galaxy toward PSR1237+25 is 25 times fainter than the geocoronal emission, it is only barely visible in the image to the right.
But after "ring-summing" the image into a spectrum, a bump near +10 km/s reveals the galactic emission and confirms the excellent sensitivity of WHAM. Other measurements of pulsar timing (dispersion measure) can be combined with our Hα observations (emission measure) to obtain important information about the electron density and distribution of ionized gas in the interstellar medium.