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3C 273 : A Star-Like Object with Large Red-Shift

THE only objects seen on a 200-in. plate near the positions of the components of the radio source 3(7 273 reported by Hazard, Mackey and Shimmins in the preceding article are a star of about thirteenth magnitude and a faint wisp or jet. The jet has a width of 12 and extends away from the star in position angle 43. It is not visible within 11" from the star and ends abruptly at 20" from the star. The position of the star, kindly furnished by Dr. T. A. Matthews, is R.A. 12h26m33-35s ±0-04s, Decl. +2 19'42-0" ±0-5" (1950), or 1" east of component B of the radio source. The end of the jet is 1" east of component A. The close correlation between the radio structure and the,star with the jet is suggestive and intriguing.

Spectra of the star were taken with the prime-focus spectrograph at the 200-in. telescope with dispersions of 400 and 190 A per mm. They show a number of broad omission features on a rather blue continuum. The most prominent features, which have widths around 50 A, are, in order of strength, at 5632, 3239, 5792, 5032 A. These and other weaker emission bands are listed in the first column of Table 1. For three faint bands with widths of 100-200 A the total range of wave-length is indicated.

The only explanation found for the spectrum involves a considerable red-shift. A red-shift AX/X0 of 0-158 allows identification of four emission bands as Balmer lines, as indicated in Table 1. Their relative strengths are in agreement with this explanation. Other identifications based on the above red-shift involve the Mg II lines around 2798 A, thus far only found in emission in the solar chromosphere, and a forbidden line of [O III] at 5007 A. On this basis another [O III] line is expected at 4959 A with a strength one-third of that of the line at 5007 A. Its detectability in the spectrum would be marginal. A weak emission band suspected at 5705 A, or 4927 A reduced for red-shift, does not fit the wave-length. No explanation is offered for the three very wide emission bands.

It thus appears that six emission bands with widths around 50 A can be explained with a red-shift of 0-158. The differences between the observed and the expected wave-lengths amount to 6 A at the most and can be entirely understood in terms of the uncertainty of the measured wave-lengths. The present explanation is supported by observations of the infra-red spectrum communicated by Oke in a following article, and by the spectrum of another star-like object associated with the radio source 3C 48 discussed by Greenstein and Matthews in another communication.

Table 1. WAVE-LENGTHS AND IDENTIFICATIONS

(1) The stellar object is a star with a large gravitational red-shift. Its radius would then be of the order of 10 km. Preliminary considerations show that it would be extremely difficult, if not impossible, to account for the occurrence of permitted lines and a forbidden line with the same red-shift, and with widths of only 1 or 2 per cent of the wavelength.

(2) The stellar object is the nuclear region of a galaxy with a cosmological red-shift of 0-158, corresponding to an apparent velocity of 47,400 km/sec. The distance would be around 500 megaparsecs, and the diameter of the nuclear region would have to be less than 1 kiloparsec. This nuclear region would be about 100 times brighter optically than the luminous galaxies which have been identified with radio sources thus far. If the optical jet and component A of the radio source are associated with the galaxy, they would be at a distance of 50 kiloparsecs, implying a time-scale in excess of 105 years. The total energy radiated in the optical range at constant luminosity would be of the order of 1059 ergs.

Only the detection of an irrefutable proper motion or parallax would definitively establish 3(7 273 as an object within our Galaxy. At the present tune, however, the explanation in terms of an extragalactic origin seems most direct and least objectionable.

I thank Dr. T. A. Matthews, who directed my attention to the radio source, and Drs. Greenstein and Oke for valuable discussions.

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SCHMIDT, M. 3C 273 : A Star-Like Object with Large Red-Shift. Nature 197, 1040 (1963). https://doi.org/10.1038/1971040a0

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