Illkirch Survey of Galactic Hydrogen: : Data Cube

Joachim Köppen Strasbourg Oct.2012/Jan.2018

Some brief explanations

This tool shows the results of a data cube survey at the radio line of atomic hydrogen at 21 cm wavelength, done with the ESA-Haystack radio telescope at the International Space University in Illkirch between February and October 2012.

The initial objective of this survey was to observe the Galactic Plane near longitude 90° in greater detail, to show how the outer spiral arms are located above the Plane, and how their position changes with longitude. Albeit the step of 1° taken in latitude is much less than the width of the antenna beam (about 6°), these oversampled data provide a better view of the spiral arms. From these data, it is possible to construct a view of the spiral arms in three dimensions.

Eventually, this survey was extended to the entire Milky Way Plane visible to us. Data were taken by staying 5 min at each galactic longitude (from 0° to 250° in steps of 2°) and galactic latitude from -10° to +10° (in steps of 1°), with a coverage in radial velocity from -150 to +150 km/s.

The system temperature was about 300 K or below..

The results are shown as:

• one may chose the longitude range between 0°..360°, -90°..270°, -180°..180°, and -270°..90°


• false colour map of antenna temperature (i.e. the measured intensity) for galactic longitude and latitude, at a fixed value of the radial velocity. The colour bar at the right hand side gives the relation between colour and the value varying linearly between minimum (violet) and maximum (red).
• false colour map of antenna temperature for galactic longitude and radial velocity, for a given value of the galactic latitude. For glat=0° one obtains a map from which the spiral arms of the Milky Way become apparent and the galactic rotation curve, too (the white curve is the limit of the hydrogen emission for a constant rotation velocity of 230 km/s).
• false colour map of antenna temperature for radial velocity and galactic latitude, for a given value of the galactic longitude. From the map at glong=90° one can see that the outer spiral arms lie above the Galactic Plane, i.e. that the disc of the Milky Way is warped.
• the spectrum at the specified galactic longitude and latitude is a plot of antenna temperature as a function of radial velocity. The bump near v_rad = 0 km/s is the emission from neutral hydrogen clouds in the local spiral arm in which the Solar System is embedded. Other bumps are from other spiral arms.
• the deprojected view of the data for a fixed value of the galactic latitude shows how the Milky Way would look like when seen from the Northern Galactic Pole. Similarly, a view with fixed galactic longitude shows a cut through the disk, perpendicular to the Plane. By assuming a constant rotation velocity, we can computing from the galactic coordinates and the radial velocity of every pixel the true distance from the sun of the emitting parcel of gas. By simply associating the pixel's observed antenna temperature with this true position in the Galaxy the false colour map is generated.


• The values for the fixed parameter(s) can be changed by the user: enter a new value in the associated field and hit the return key. NOTE: if the display does not change, there's simply no data available for that position.
• Mouse position gives the values of all coordinates of the current position of the mouse.

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