Observations of the interstellar hydrogen line of wave length 21 cm made at Kootwijk, Netherlands.
Abstract
The change of energy effected in the ground state of a hydrogen atom by reversing the electron spin relative to the proton spin corresponds to a frequency of 1420.4 Mc/sec, or a wave length of 21 cm. This line may be emitted by magnetic 4ipole radiation. The computed life time of the upper level, i.e. the inverse transition probability, is I I million years. The discovery of this line as an emission line from interstellar space was made by Ewen on March 25, 1951, about seven years after its prediction. It was confirmed on May I I by Mr. C. A. Muller of the Radio Astronomy Research Group in the Netherlands. A later confirmation was obtained early in July by Christiansen and Hindman of the Australian group. The Dutch measurements were made with a movable mirror of 7 meter diameter and 2?8 beam width. Scans were made across the Milky Way at many longitudes with continuous switching from a frequency in the line to a frequency outside it. The intensity on the galactic equator in Cygnus is somewhat larger than in the direction towards the galactic center. Half the maximum intensity is reached at latitudes +40 to 80, depending on longitude. The large width in latitude suggests that the emission comes from relatively nearby clouds, which is confirmed by a computation of opacity depth based on assumed values of the transition probability, temperature and density. The shape of the line toward the center (1 = 3250) is symmetrical and yields an average cloud velocity of 5 km/sec in one Co- ordinate. At 1 = 3550 the line is broadened by galactic rotation. A scan across the Milky Way at that longitude and at a frequency 250 kc/sec below normal, i.e. in the wing of the line, gave a much sharper drop of intensity with latitude, showing that more distant parts of the galaxy are observed. The importance of this discovery for investigations of galactic structure is very great. The Doppler displacement will reveal for the first time the rotation of the inner parts of our galaxy, which are hidden from sight by obscuring clouds. And the vast regions of interstellar space where hydrogen is neutral become for the first time accessible to observation. A full survey will require three-dimensional scanning, in longitude, latitude and frequency. The equipment at Kootwijk is now disassembled for improvements. More details of these results are described in a letter to Nature. Leiden Observatory, Leiden, Netherlands
- Publication:
-
The Astronomical Journal
- Pub Date:
- October 1951
- DOI:
- 10.1086/106564
- Bibcode:
- 1951AJ.....56..144V