The blue continuum in Seyfert 2 galaxies

Koski (1978) and Kay (1994) found that all Seyfert 2 galaxies show an ultraviolet excess and weak absorption lines when compared with galaxies with no emission lines, indicating the presence of a blue featureless continuum. Boisson & Durret (1986) and Vaceli et al. (1997) suggested that this continuum is a non-thermal power-law continuum. Kinney et al. (1991) argued that most of the Seyfert 2s in which a blue continuum has been observed are of type Sb or earlier, suggesting that it is truly associated with the Seyfert nucleus. Shuder (1981) showed that its strength and the $\rm H\alpha$ luminosity are strongly correlated suggesting that a direct physical connection exists between the two; studying a sample of 28 Seyfert 2s, Yee (1980) found that the $\rm H\beta$ and continuum fluxes (rather than luminosities) are proportional over two orders of magnitude, with, however, a relatively large dispersion; but a number of those objects are now known to be Seyfert 1 galaxies.

Martin et al. (1983) discovered that a small fraction of all Seyfert 2 galaxies have a highly polarized continuum. Subsequently, Antonucci & Miller (1985), Miller & Goodrich (1990) and Tran et al. (1992) showed that these objects harbour a hidden Seyfert 1 nucleus, the observed polarized continuum arising from scattering of the nuclear continuum by dust or warm electrons. But most Seyfert 2s have very little polarization (Martin et al. 1983), much less than expected in the reflection model (Miller & Goodrich 1990).

On the other hand, Terlevich et al. (1990) showed that in Seyfert 2 galaxies, the IR Caii triplet is equal or, in some cases, higher than in normal elliptical galaxies, which is most naturally explained by the presence of young stars contributing heavily to the nuclear light at near-IR wavelengths.

 

Figure:  Histogram of log($\lambda$6300$\lambda$5007) for the 159 Seyfert 2 and Liners plotted in Fig. .

Heckman et al. (1995) used International Ultraviolet Explorer (IUE) spectra of 20 of the brightest type 2 Seyfert nuclei to build an ultraviolet template for this class; while the continuum was well detected in the template, there was no detectable broad line region (BLR), implying that no more than 20% of the template continuum could be light from a hidden Seyfert 1 nucleus scattered by dust; they suggested that either most of the nuclei in their sample were ``pure'' type 2 Seyfert galaxies for which we have a direct view of the central engine and which simply lack of BLR, or that most of the observed ultraviolet continuum is produced by starbursts. From the absence of polarization of the continuum of most Seyfert 2 galaxies and of broad Balmer lines, Cid Fernandez & Terlevich (1995) concluded that, most probably, this continuum was due to a population of young stars in the vicinity of the nucleus. Colina et al. (1997) obtained ultraviolet HST images of four nearby Seyfert 2 galaxies known to have circumstellar star-forming rings, providing direct empirical evidence that the UV flux emitted by these galaxies is dominated by radiation coming from clusters of young hot stars distributed along the star-forming ring. If similar rings are a common characteristic of Seyfert 2 galaxies, the large IUE aperture would include both the Seyfert 2 nucleus and the rings for distances larger than 25 Mpc. Gonzalez Delgado et al. (1998) presented HST images and ultraviolet spectra of three Seyfert 2 nuclei (IC 3639, NGC 5135 and IC 5135); the data show the existence of nuclear starbursts (with absorption features formed in the photosphere of late O and early B stars) dominating the ultraviolet light. It is remarkable that, of the three observed galaxies, two (NGC 5135 and IC 5135) have a ``composite'' nuclear emission spectrum, while the third (IC 3639), which has the largest UV nuclear flux (associated with the Seyfert nucleus) relative to the total UV flux, has a pure Seyfert 2 spectrum due to the relative weakness of the starburst emission component.

We conclude that there is ample evidence for the presence of young, hot stars in the nuclear region of many Seyfert 2 galaxies. When the continuum is relatively bright, the associated H II region could be strong enough to displace the object into the ``transition'' zone in the diagnostic diagrams.

AGNs are more frequent in early type galaxies while starbursts are more often found in late-type galaxies (Véron & Véron-Cetty 1986; Ho et al. 1997b; Vaceli et al. 1997). It is therefore rather surprising to find almost systematically a population of young stars in Seyfert 2 galaxies; perhaps the nuclear activity triggers the star formation?

 

Figure:  Histogram of log($\lambda$6583/$\rm H\alpha$) for the 131 Seyfert 2 galaxies plotted in Fig. .