Photometric Selection of Quasi-Stellar Objects

Abstract

Five QSO candidates have been s~1ected from fifty-seven Tonantzintla blue stars by a red-excess criterion. A spectroscopic investigation of the candidates has shown that one is a white dwarf and the remaining four are QSOs. One of the most distinctive characteristics of quasi-stellar objects (QSOs) is the un- usual nature of their spectral-energy distributions. Johnson (1964) has found that 3C 273 has an essentially constant flux level from the ultraviolet to the I-band in the near- infrared and that there is an increasing flux for longer wavelengths. Similarly, Oke (1966) has found that the flux for several QSOs generally increases in going from 3000 to 9000 A. The shapes of these spectral-energy distributions cannot be readily understood as black-body radiation or radiation from a stellar constituent. They seem, in some cases, to resemble more closely power-law spectra of a type to be expected from a source of synchrotron radiation. This suggests that it might be possible to distinguish the QSOs from other objects photometrically. Even in the case of ordinary UBV photometry there is a partial separa- tion of QSOs from known stellar types (Sandage and Luyten 1967). About one-third of the QSOs fall completely outside the region occupied by single stars in the U - B, B - V diagram. However, a longer base line in wavelength than that provided by UBV photometry seems desirable. The validity of this idea appears to be borne out by the discovery of QSOs, using infrared photometry, reported by Braccesi (1967) and by Braccesi, Lynds, and Sandage (1968). An independent attempt to extend the photometry into the near-infrared has been made by one of us (Janes) using the 61-inch reflector of the U.S. Naval Observatory. The telescope was equipped with a camera that reduces the f/1O focal ratio to f/2. Most of the work was accomplished using this camera. As a compromise between the desired range of wavelength and the slow speed of infrared-sensitive emulsions, lO3aE plates were used. (Braccesi used I-N emulsion for his infrared exposures.) In the initial study, twenty objects-ten stars selected from a list by Eggen and Greenstein (1965) and ten QSOs-were studied photographically. The stars are mostly white dwarfs and were selected because they are indistinguishable from QSOs on the basis of UBV photometry alone. Each of the twenty objects was photographed on lO3aE emulsion, once through an RG1 filter and again through a BG12 filter, with a slight shift of the telescope between exposures so that the two images were adjacent. The effective wavelengths for the two exposures are approximately 6500 and 4200 A, respectively. The * Contributions from the Kitt Peak National Observatory, No. 380

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