Abstract
view Abstract Citations (1) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Attempt to Observe 21-cm Line Emission from Intergalactic Hydrogen. Goldstein, S. J., Jr. Abstract In an expanding universe containing a uniform distribution of optically thin neutral hydrogen atoms, the brightness temperature of the sky has a step in its spectrum at 1420.4 Mc. The amplitude of this step is proportional to the density of neutral hydrogen; for a Hubble parameter 100 km/sec/Mpc, a density of 10-~ atoms/cm3 gives a step of 1.70K, if the gas is in thermal equilibrium. Field (Astrophys. J. 135, 684, 1962) has attempted to observe 21-cm absorption in the spectrum of Cyg A. Observing emission has the advantage that the measurement is independent of the spectra of sources and that the observed brightness temperature can be expressed as a density without knowledge of the excitation temperature of the gas. A horn antenna with half-power beamwidths of 24 x 27 deg, directed at the celestial north pole, fed one input of a switched maser radiometer. A coaxial resistor immersed in liquid nitrogen fed the other input. The difference in available power of these sources was measured at 2.0-Mc intervals with 1 .0-Mc bandwidth over a 16-Mc range centered at 1420.4 Mc. The system noise temperature was 1900K including 1 100K from the antenna and feed line. Test spectra of the temperature difference between two coaxial resistors clamped together in an aluminum block at room temperature show a maximum variation of 0.40K at the switch terminals (0.60K in brightness temperature). Observations of the north pole show an increase in brightness temperature of 20K at the center frequency with respect to the adjoining frequencies. I conclude that a step as large as 1.1 0K is not present, if the random motions of the integalactic gas have a standard deviation of 400 km/sec or less. Hence, subject to the same condition, the average density of optically thin neutral hydrogen atoms in the intergalactic space observed is less than 7x 10-~ atoms/cm3. This research was supported by the National Science Foundation. Publication: The Astronomical Journal Pub Date: June 1962 DOI: 10.1086/108873 Bibcode: 1962AJ.....67R.577G full text sources ADS |
Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
