Astronomy and Astrophysics – Astrophysics
Scientific paper
1998-11-25
Astronomy and Astrophysics
Astrophysics
23 pages Latex, 5 figures. Accepted for publication in ApJ
Scientific paper
We have performed a successful targetted search for a population of red radio-quiet, and probably absorbed, quasars. Radio-quiet, optically-red ROSAT PSPC X-ray sources brighter than 10^{-13} erg cm^{-2} s^{-1} were searched for red (O-E > 2.0, O < 20) counterparts in the APM catalog of Palomar Sky Survey objects. Of 45 objects for which we obtained adequate follow-up optical spectroscopy, we have found 7 red quasars, 5 with alpha(opt) < -2. Their redshifts range from 0.06 to 0.31, and their luminosities are moderate, lying on the Seyfert/Quasar boundary. These red quasars strengthen the case for a radio-quiet population that is the counterpart of the radio-loud red quasars found by Smith and Spinrad (1980), and Webster et al. (1995). Unidentified, fainter, sources could increase the fraction of red quasars by up to a factor 7. For the red quasars found here, the H-alpha/H-beta ratios, optical slope and X-ray colors all indicate that they are absorbed by A(V) ~ 2, rather than having intrinsically red spectra. This amount of obscuration seems to hide ~1-7% of quasars at a given observed flux, or ~3-20% when their fluxes are corrected to their intrinsic values. This size of population is consistent with earlier limits, with predicted values from Comastri et al. (1995), and is comparable to the rate found among radio-loud quasars. A large population of more heavily absorbed (A(V)=5), fainter, quasars equal in size to the blue population could exist, without violating existing upper limits, in accord with the Comastri et al. (1995) predictions.
Elvis Martin
Kim Dong-Woo
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