Survey of the Galactic Plane

Joachim Köppen Strasbourg 2010

As the ESA-Haystack telescope operates on 21 cm, its principal application is the investigation of the distribution of neutral hydrogen gas in the Milky Way, and to study its kinematics. This is done by systematic observations of the spectra taken in the Galactic Plane. From our geographical position, it is possible to cover Galactic Longitudes between l=0° and l=250°, almost 70 percent of the Galactic Equator. As the antenna's beamwidth is about 6°, if suffices to record spectra every 5° in longitude - although it is no waste of time to do in with a finer spacing! Either in manual mode or in batch mode one records at each position the spectra, say for a couple of minutes, during which a sufficient number is obtained to get a good average spectrum.

An example of such a batch observation (during the night of 15 july 2009) is shown below, first in the form of a waterfall map of the raw data

Each horizontal line is a false colour representation of a single spectrum. The first spectra (bottom) are taken at the Galactic Centre, the last are from l=110°; the longitudes were scanned in 1° steps, staying at each position for 5 minutes, and recording about 25 spectra. One notes that during the observations, there are several deep gaps with very low signals, due to interference. But already in the raw data one can distinguish the broadish galactic features which give several velocity components (see also here). There appears also a narrow emission at 1419.9 MHz from some human-made transmission.

If one averages all the spectra taken at each position, and then creates a false colour plot of the radial velocities and galactic longitudes:

the structure of the Milky Way's gas disk becomes apparent. We notice that the emission components seen at one longitude can be traced to the others as well. Thus each of the velocity components constitutes a separate spiral arm. Due to the interference, there are gaps in the data, and the intensities of the components seem to vary more strongly with longitude as they do in reality. Nontheless, one can identify several spiral arms: the component near to 0 km/s is the spiral arm in which our Sun is located. The two further components seen distinctly at l=90° at negative radial velocities show the presence of two arms further away from the Centre than the Sun. For longitudes below 90°, one can use the maximal radial velocities to derive the rotation curve of the Milky Way.

If one uses a reasonable approximation for the rotation curve, one may use the radial velocities of each component as an indicator of the distance of the emitting region from the Sun, and reconstruct the true distribution of neutral hydrogen in the Galactic Plane with its spiral arms:

Following the successful use of band-pass filters in spring 2010 we started a complete survey of hydrogen emission in the Milky Way.

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last update: Nov. 2010 J.Köppen