Harvard 21-Cm Maser Observations of Galaxies.

Abstract

A three-level solid- state ruby maser has been installed on the 60-foot Harvard radio telescope. The maser was constructed primarily for a study of the 21-cm hydrogen-line radiation. The sensitivity of the maser has permitted a study of galaxies which are relatively nearby in space. The maser radiometer is a switched or Dicke type that measures the difference in total power available from the antenna and a reference load. In this case, the reference load is maintained at liquid-helium temperature. The antenna and reference-load temperatures can be balanced by adding noise to the reference load from an argon noise source. The maser radiometer is balanced at the beginning of each observation in order to minimize the effects of radiometer gain changes. The total noise temperature of the radiometer is approximately 1000K, made up of 700K because of ohmic losses in the switch, circulator, and coaxial cables, 200K because of the side lobes of the antenna, and 100K because of the ~emainder of the receiver. The input switch is a Y-type circulator with a reversible magnetic field; the phase-sensitive detector uses a relay with gold-plated contacts. Both the switch and the detector operate at 3.5 cycles/second. The radiometer bandwidth is 1 Mc for continuum observations and 200 kc for observations on the line; the integration time is 50 sec, and the antenna beamwidth is 55 minutes of arc. Each of the observations is a drift curve, half an hour long for sources near the celestial equator and longer at higher declinations. For each source, a minimum of three reliable drift curves were obtained at a frequency corresponding to the uncorrected red- shift given by Humason, Mayall, and Sandage. A minimum of three drift curves were obtained at a frequency substantially removed from the redshift frequency. Three of the galaxies observed by the Leiden group, NGC 3031, 4258, and 6822, are alike in that their redshifts are within the range of velocities of hydrogen in our own galaxy. Detectable signals for each of these sources exist at the optical position and velocity.. However, only in the case of NGC 6822 does the radiation have a small enough angular size to make it certain that the external galaxy is the source. Confirmation of Leiden observations on N'GC 5194,5457, and 6822 was obtained. Hydrogen emission from NGC 2403, 4214, 4631, 5236, and IC 10 has been observed. Continuum emission has also been detected for NGC 5236. The galaxies in our list not previously observed were selected on the basis of large angular size in the Shapley-Ames catalogue or from the presence of emission regions reported by Humason, Mayail, and Sandage. This program is continuing and is supported by the National Science Foundation.