The Illkirch Surveys of Galactic Hydrogen emission

Joachim Köppen Strasbourg 2011

Since the application of a band-pass filter in spring 2010 ensured interference-free observations, we started a complete survey of the Milky Way's hydrogen emission, covering all galactic longitudes accessible from Illkirch (about 0 to 250 in steps of 2) and the latitude band between -30 and +30 also in steps of 2. Data was taken in the form of vertical scans at each longitude.

Although the system temperature was still rather high (between 700 and 1000 K), the first trial survey showed all the essential features. Since then, we have continually improved the system temperature, and repeated the survey several times. Here is a short history of events:

Note that the vertical axis shows simply the baseline-subtracted flux in arbitrary units, the improved system temperature results in higher values.

In summary, we may conclude that even with a poor system temperature of 1000 K one may successfully map the hydrogen emission in the Milky Way, by averaging over a sufficient number of spectra for each position. Below we show from Survey0 a raw single spectrum, a single baseline-subtracted spectrum, and the average over all 56 spectra:

For comparison, here is a raw spectrum from Survey5:
While the improvement in system temperature accounts for a three-fold increase of the signal-to-noise ratio, averaging over 56 spectra gives an eight-fold increase, albeit at the expense of observation times 56 times longer! Having both options in place is of course even more advantageous ...

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last update: July 2011 J.Köppen