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5.1. The relative number of Seyfert 1 and 2 galaxies

We have seen that Seyfert 1s are objects seen at a relatively narrow angle to the axis of the dusty torus, while Seyfert 2s are seen at larger angles such that the nucleus is hidden from view. In Seyfert 1.8s and 1.9s, the line of sight is more or less tangent to the torus and the nucleus is seen through a moderate amount of absorption. The determination of the relative number of Seyfert 1s to Seyfert 2s per unit volume would be a measurement of the covering factor of the torus, i.e. of its opening angle.

For many years Seyfert 1s, which were largely UV excess selected, were thought to outnumber Seyfert 2s. More recently, a number of studies of complete samples of bright galaxies have allowed to derive an estimate of the ratio of the space density of Seyfert 2s to Seyfert 1s. [386] and [446] found that the space densities of Seyfert 2s and Seyfert 1s are about equal; [276] found that the relative number of Seyfert 2s to Seyfert 1s is ~ 1.5, while [336] and [200] found that the space density of Seyfert 2s is approximately twice as large as the space density of Seyfert 1s. [327] obtained a space density about three times larger for Seyfert 2s than for Seyfert 1s.

The thermal infrared photons are emitted isotropically so that both Seyfert types should have the same IR luminosities. [254] have identified 530 IRAS sources from a complete flux limited sample (S60 µm > 0.5 Jy), which contains 10 galaxies tentatively classified as Seyfert 1s and 17 which are most probably Seyfert 2s, giving a ratio of Seyfert 2s to Seyfert 1s ~ 1.7. [441], in a complete sample of 108 ULIGs making up an unbiased flux-limited (S60 µm > 1 Jy) infrared sample, have found 10 broad line and 23 narrow line AGNs so, in this case, Seyfert 2s outnumber Seyfert 1s by ~ 2.3.

In a complete sample of 49 FR IIs, [184] have found 15 LERGs, 9 BLRGs and 25 NLRGs. Discarding the low-excitation objects, the proportion of NLRGs to BLRGs is ~ 2.8.

Several bias could affect these results; in weak line objects, broad lines could escape detection while the narrow lines suggest a Seyfert 2; on the other hand, in magnitude limited samples, galaxies with a bright Seyfert 1 nucleus will be selected even if the parent galaxy is weaker than the magnitude limit of the sample.

All these studies give results which, although not in perfect agreement, are rather similar: the ratio of the volume density of Seyfert 2s to Seyfert 1s is probably somewhere between 1.5 and 2.

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