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S.G. Djorgovski, R. Brunner, F. Harrison, R.R. Gal (Caltech), S. Odewahn (Caltech and ASU), R.R. de Carvalho (ON/CNPq Brasil), A. Mahabal, S. Castro (Caltech)
Unified models of AGN, whereby the observed phenomenology is dependent on the viewing angle are among the central ideas of modern extragalactic astronomy. There is one glaring missing link in this field: the existence of a radio-quiet population of type 2 quasars. These objects are inevitable if unification schemes are correct, and may make up a substantial fraction of the diffuse X-ray background.
We have identified a population of emission-line objects in the Digital Palomar Observatory Sky Survey (DPOSS), which can be plausibly interpreted as the long-sought type 2 quasars. While a few individual objects of this type have been discussed in the literature, this is the first time that an entire population of them has possibly been identified.
The objects we have discovered Seyfert 2 type spectra, with strong, high-ionization narrow lines and very weak continua. They are selected as as outliers in the DPOSS gri color space, because of the strong [O III] lines in the r band, in the typical redshift range z ~0.31 - 0.36. The standard emission line diagnostics show that these objects must be powered by AGN. In some cases we also see a broad component in the H\alpha line, but not in the H\beta, consistent with a partial obscuration of the central engine. The [O III] emission line fluxes are comparable to those of the bright PG quasars in the same redshift range, yet the equivalent widths are much higher (~ a few hundred Å), ostensibly due to the obscuration of the central ionizing continuum source. The observed surface density is ~0.01 deg-2 in this redshift range, within our selection limits. For comparison, down to Blim \approx 17 mag, there are ~0.04 quasars deg-2, but only ~5-10% of them are in this redshift interval. Thus, our candidate type 2 quasars are at least a few times more numerous than the unobscured (type 1) quasars with comparable narrow-line luminosities. This is consistent with estimates in various AGN unification schemes, and the numbers required in some models of the diffuse hard X-ray background.