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Title:
New upper limits on the local metagalactic ionizing radiation density
Authors:
Vogel, Stuart N.; Weymann, Ray; Rauch, Michael; Hamilton, Tom
Affiliation:
AA(University of Maryland, College Park, MD, US), AB(University of Maryland, College Park, MD, US), AC(Observatoires of the Carnegie Inst. of Washington, Pasadena, CA, US), AD(Observatoires of the Carnegie Inst. of Washington, Pasadena, CA, US)
Publication:
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 441, no. 1, p. 162-169 (ApJ Homepage)
Publication Date:
03/1995
Category:
Astrophysics
Origin:
STI
NASA/STI Keywords:
H Alpha Line, Hydrogen Clouds, Intergalactic Media, Ionization, Radiant Flux Density, Universe, Cassegrain Optics, Fabry-Perot Spectrometers, Telescopes
DOI:
10.1086/175346
Bibliographic Code:
1995ApJ...441..162V

Abstract

We have obtained H-alpha observations with the Maryland-Caltech Fabry-Perot Spectrometer attached to the Cassegrain focus of the 1.5 m telescope at Palomer Observatory in order to set limits on the number of ionizing photons from the local metagalactic radiation field. We have observed the SW component of the Haynes-Giovanelli cloud H I 1225+01, an intergalactic cloud which should be optimum for measuring the metagalactic flux because it is nearly opaque to ionizing photons, it does not appear to be significantly shielded from the metagalactic radiation field, and the limits on embedded or nearby ionizing sources are unusually low. For the area of the cloud with an H I column density greater than 1019/sq cm we set a 2 sigma limit of 1.1 x 10-19 ergs/sq cm/s/sq arcsec (20 mR) for the surface brightness of diffuse H-alpha. This implies a 2 sigma upper limit on the incident one-sided ionizing flux of Phiex is less than 3 x 104/sq cm/s. For a radiation field of the form Jnu is approximately nu-1.4, this yields a firm 2 sigma upper limit on the local metagalactic photoionization rate of Gamma is less than 2 x 10-13/s, and an upper limit for the radiation field Jnu at the Lyman limit of Jnu0 is less than 8 x 10-23 ergs/sq cm/Hz/sr. We discuss previous efforts to constrain the metagalactic ionizing flux using H-alpha surface brightness observations and also other methods, and conclude that our result places the firmest upper limit on this flux. We also observed the 7 min diameter region centered on 3C 273 in which H-alpha emission at a velocity of approximately 1700 km/s was initially reported by Williams and Schommer. In agreement with T. B. Williams (private communication) we find the initial detection was spurious. We obtain a 2 sigma upper limit of 1.8 x 10-19 ergs/sq cm/s/sq arcsec (32 mR) for the mean surface brightness of diffuse H-alpha, about a factor of 6 below the published value.

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