The strong southern radio source Centaurus A (lAU 13S4A) was one of the first to be identified with an optically observable object. Bolton, Stanley, and Slee (1949) suggested that the radio emission was associated with the peculiar galaxy NGC 5128, and this identification has been strengthened by more recent positional measurements (Mills 1952). NGC 5128 is a most unusual object, appearing to be a spheroidal galaxy transected by a heavy belt of obscuring matter (Baade and Minkowski 1954a; Sersic 1958). Its distance is uncertain, but it is likely to be about 1 megaparsec away (Baade, personal communication).
Observations of two strong southern radio sources, the Eta Carinae Nebula (NGC 3372) and Centaurus A (13S4A), at a frequency near 1400 Mc/s, are described. The observations were made with a 36 ft transit-mounted paraboloidal aerial and a 21 cm hydrogen-line receiver modified for the reception of continuum radiation. The flux density of the Eta Carinae Nebula near 1400Mc/sis5� 82 X 10-24Wm-2 (c/s)-\ with an estimated uncertainty of less than �20 per cent. This source appears to be fairly symmetrical, with a strong central condensation.
SummaryThe interpretation of radio-frequency observations of H II regions is discussed with particular regard for the possible effects of random variations in the electron density and electron temperature through the nebulae. It is shown that such variations serve to alter the optical depth and that the conventional definition of the" emission measure " requires modification if it is to be considered an observable quantity. The radio emission of Stromgren spheres is discussed, and a means of determining their electron temperatures is described. An empirical method for the determination of Stromgren's constant defining the ionized volume as a function of the spectral type and luminosity of the exciting star is described.1. INTRonUCTION A number of galactic H II regions have been observed at radio frequencies.The observations published to date cover frequencies ranging from 19·7 to 9375 Mc/s. S-qch observations can provide information about the temperatures and densities of the nebulae, and about the far ultraviolet radiation of the stars exciting them. The objective of the present paper is to examine in some detail the problem of deriving physical data on the nebulae from radio observations;The H II regions constitute a special class of radio source characterized by their spectra, which are "fiat" except at the lower frequencies where they become optically thick. That is, their fiux densities are nearly constant over a very wide range of frequencies. This is what is expected if the nebulae are radiating by the thermal process of free-free transitions in an ionized gas. They are readily distinguished from the" non-thermal" radio sources, whose spectra show a strong frequency dependence.The radio emission of H II regions is well understood theoretically (e.g. Piddington 1951). Discussions of the nebulae as radio sources have generally assumed for simplicity that the objects are uniform throughout, although it has been known that a non-uniform distribution of the nebular gas would tend to increase the radio emission because the emissivity of an ionized gas depends on the square of the electron density. In the present paper we shall give particular attention to the consequences of such a non-uniform distribution.Section II is devoted to a discussion of the directly observable properties of H II regions. Section III considers the effect of density and temperature variations on the optical depth, and the magnitude of the effect is estimated for some particular nebular models. A modification of the definition of the term
The 1400 Mcls observations of NGC 3372 described in Paper I (Hindman and Wade 1959) are discussed. Comparison of the flux densities at 85· 5 Mcls (Mills, Little, and Sheridan 1956) and at 1400 Mcls leads to a value of 10,000±1000 OK for the electron temperature of the nebula. Unpublished optical measurements of the distribution of surface brightness across the object, made by Gum, indicate that there is a dense core about 24 min of arc in diameter, surrounded by a much less dense region with a diameter of 120 min of arc. Adopting the optically determined distance of 1400 parsecs (Hoffieit 1953), we find r.m.s. densities of 71 ions cm-3 in the core and 11 ions cm-3 in the outer region. The total mass of the object is not more than 25,000 solar masses. It is shown that several O·stars probably are needed to maintain the ionization of the nebula.
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