f’;‘is/m ill Aswonon~~ Vol. 40, No. I, pp. 17 -22. 1996

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STAR-FORMINGTORIINSEYFERTNUCLEI ROBERTO TERLEVICH

Royal Greenwich Observatory, Cambridge, U.K.

Abstract- In the simple unified model for Seyfert nuclei, type 2 Seyferts are those where direct view of the central source is blocked by a dusty molecular torus. This simple model is able to explain many observed properties of Seyfert nuclei but fails to explain the UV excess observed in the continuum of many type 2 Seyferts, the relatively low level of continuum polarization, and the undiluted near IR stellar absorptions. The simplest solution is to postulate a continuum source on top of the scattered nuclear one. I discuss here some of the implications of having young stars in the torus providing the additional continuum. Copyright @ 1996 Published by Elsevier Science Ltd.

1. INTRODUCTION

A remarkable property of active galactic nuclei (AGN) is their remarkable similarity over several decades in total luminosity. AGNs either radio-quiet and spanning from Seyfert galaxy nuclei to quasi-stellar objects (QSOs), or radio loud and spanning from radio galaxies to quasi-stellar radio sources (quasars), seem to form a single family of objects. In recent years the so-called unified models have attempted to explain all the variety of types as due solely to differences in the orientation of the central object and/or its nearby environment. There are two branches of unification, the radio-quiet and the radio-loud. In the radio-quiet unification, nuclei dominated by narrow emission lines, i.e. type 2 Seyferts are interpreted as normal Seyfert 1 nuclei or QSOs whose central regions, containing the ionising source and the Broad Line Region (BLR), are obscured by an edge-on opaque dusty torus. Reflection of the nuclear light by electrons and/or dust in the extra nuclear regions provides an indirect view into the hidden active nucleus. Seyfert 1 nuclei are seen directly without obstruction by the dusty torus.

Convincing observational support for the radio-quiet unified scenario has been gathered during the last few years. Following the discovery of broad permitted lines in the polarized light of NGC 1068 by Antonucci and Miller (1985), Miller and Goodrich (1990) and Tran et al. (1992) reported the discovery of broad permitted lines in the polarized spectra of another eight Seyfert 2’s, thus strengthening the idea that Seyfert 2’s harbour obscured BLR’s. Several workers reported the existence of “ionization cones” in Seyfert 2’s (Wilson et ai. 1988; Pogge, 1989; Tadhunter and Tsvetanov, 1989; Storchi-Bergmann and Bonatto. 1991; Storchi-Bergmann et al. 1992; Schmitt et of. 1993). These cones are generally aligned

17

18 R. Terlevich

with the radio axis and perpendicular to the polarization vector (Antonucci, 1984). These facts agree with the unified model, which predicts that only gas within a biconic region (defined by the opening angle of the obscuring torus) directly “sees” the nuclear ionizing continuum, whereas the line of sight to the nucleus is blocked and only a fraction of the central source’s continuum is actually scattered towards the observer.

There is, therefore, a strong case for the unified model. However, several important problems remain to be solved (see Cid Fernandes and Terlevich, 1994, 1996)

A serious problem with this simple unified model is that many of the known Seyfert galaxies were found searching for galaxies with UV excess in their nuclei. Follow-up spec- troscopy indicated that UV excess nuclei are of two types, with broad permitted lines (Syl) or without broad permitted lines (Sy2). The presence of a strong UV excess in these Seyfert

nuclei therefore precedes classification as type 1 or 2. If the UV excess comes from the nucleus, how is it possible in Sy2 to obscure the broad lines without obscuring the UV continuum? Thus, while the unified model is able to explain the weakness or absence of the

broad lines in Sy 2, it does not explain the origin of their strong UV excess. A related problem is that the polarization level of the continuum is generally low, (typically

O-2%). The highest observed value is P = 16% for NGC 1068. In the simple unified model, polarization levels as high as 50% are expected to be common in type 2 Seyferts.

The detection of undiluted stellar features in the IR nuclear spectrum of type 2 Seyferts with large UV excess is another important problem. The continuum of these nuclear regions show weak Mgz, weak Ca H and K, and weak G band stellar absorption, but strong to very strong IR CaII triplet lines. In many cases the strength of the CaII triplet is larger than in normal galaxies.

There is also a strong possibility that some Seyfert 2’s are not obscured Seyfert I’s at all. Several low luminosity AGN are known to have undergone type transitions from Seyfert 1 to Seyfert 2 and/or vice vemu (see Aretxaga and Terlevich, 1993 for a compilation of cases). Clearly, such nuclei are not obscured Seyfert 1’s. suggesting that BLR evolution is playing

an important role at least in some Seyferts. In a recent paper Cid Fernandes and Terlevich (1994) proposed that these problems faced

by the unified model can be explained if young stars embedded in the dusty molecular torus are the source of the observed blue featureless continuum (BFC; see also Cid Fernandes in this proceedings). Here I will describe some of the properties and expected consequences of young stars in the central regions of Seyfert galaxies.

2. THE STRENGTH OF THE STELLAR ABSORPTIONS

The most convincing evidence for the presence of large numbers of massive main sequence stars is the detection of their spectral signatures. Unfortunately searches for such signatures in the optical spectra of Starbursts failed, mainly for two reasons (Melnick ct nl., 1985).

(1) Main sequence stars more massive than few solar masses are the dominant contributors to the integrated optical continuum of a young cluster and their absorption lines are weak.

(2) The optical spectrum of these intermediate mass stars has only weak metal lines and is dominated by absorption lines of H and He. Thus, in starbursts the main stellar features coincide in wavelength with strong nebular emission lines. Even at high S/N the optical spectrum of a young starburst, i.e. with strong emission lines. is featureless.

Star-forming Tori 19

As a result, there has been no unique detection of spectral lines at optical wavelengths of 0 stars, i.e. stars with masses above 10 Solar masses, in distant Starbursts.

In the UV, P-Cygni type absorptions are expected in the SiIV 1400 and CIV 1550 A lines. These lines should be particularly strong in clusters with ages of less than 20 Myr. Un- fortunately, in Seyfert nuclei these lines appear also in emission making their detection in absorption very difficult.

The only spectral region accessible from ground based observations that shows clear signatures of massive stars is the near IR. The strong absorption lines of the CaII 8498,8542,8662 A absorption triplet should dominate the near IR spectrum of typical starburst for ages between 10 and 20 Myr (Garcia-Vargas et al., 1992; Cervino and Mass- Hesse, 1994). Clusters with ages less than 10 Myr should have no CaII triplet absorption while in those with ages of more than 20 Myr the CaII strength should diminish rapidly with age. This results seem valid for a range of metallicities from about half to twice solar.

Using this approach, the origin of the blue continuum in Sy2 was addressed by Terlevich et al. (1990) using near IR spectroscopy of a sample of Seyfert 2’s. Seyfert l’s, LINERS, Starburst and normal galaxies. They found that the equivalent width of the IR CaII triplet in strong UV excess Seyfert 2’s is not diluted with respect to that of normal galaxies as one would expect if a featureless power-law continuum component matched to the optical spectra extends to the near IR. In general the IR CaII triplet in type 2 Seyferts is similar or stronger than in normal galaxies. Interestingly, the strongest IR CaII triplet of the whole sample of galaxies was found in a Seyfert 2 galaxy.

This result was confirmed by Cid Fernandes and Terlevich (1992). They show that a young stellar population can produce a featureless power-law type spectrum in the optical and also have Ca II absorption with strength comparable to the measurements of Terlevich ef nl. (1990). These findings raise strong doubts about the “featurelessness” and the “non- stellarity” of the BFC in Seyfert 2’s and supports the hypothesis that at least part of this continuum may originate in a young stellar population in the vicinity of the nucleus.

3. THE COLOUR OF A DUST EMBEDDED YOUNG CLUSTER

The stellar origin for the BFC will be feasible if a stellar population can mimic the so-called “non-stellar” BFC observed in Seyfert 2’s. This is actually possible if a young stellar cluster component is present (Cid Fernandes and Terlevich, 1992, 1994). In the few cases where such a blue stellar component has been included in the galaxy template used to isolate the “pure” Seyfert 2 spectrum, no need was found to include any power-law. “non-stellar” component (Storchi-Bergmann et al., 1990; Bonatto et rd.. 1989, Alloin et d., 1992). A young stellar population might well be the source of the extended BFC in Cygnus A (Goodrich and Miller, 1989-observations of the Ca II IR triplet are needed for this object). Recent population synthesis studies of very young systems seem to indicate that a young star cluster may reproduce the continuum of Seyfert 2’s in the UV range as well (Garcia-Vargas et al., 1993).

The slope of the continuum for dusty torus with young stars can be easily estimated under the assumption that dust and stars are well mixed and that the dust optical depth is very large (Cid Fernandes and Terlevich, 1994). In this case, each star has its flux attenuated by a factor that depends of the star’s depth into the cloud. For a simple homogeneous distribution of stars and dust inside the cloud, the change in the cluster spectral shape

20 R. Terlevich

depends only on the exponent of the extinction law. For a normal extinction law F(h) cc h-’ the observed spectral index is one unity steeper than the intrinsic index of the cluster. An intrinsic power-law continuum with F, DC v a is thus seen as FV CC P-l. Young stellar systems like the templates Yl, Y2 and Y3 (ages between 7 x lo6 and 2 x lo* yr) on Bica et a/.‘~ (1990) library have continuum slopes in the - 1 5 o( d 0 range. If embedded in a dusty torus, their observed spectral slopes would be in the -2 to -1 range, independently of the total dust optical depth and indistinguishable from the range of BFC slopes measured in Seyfert 2’s (e.g. Ferland and Osterbrock, 1986; Kimley et al.. 1991).

4. SIZING THE STAR-FORMING TORI

One important aspect refers to the size expected for a toroidal region of star formation in the central regions of a large bulge. For that I will assume that the whole region is the result of a burst of star formation and use scaling laws that are valid for giant HI1 regions to estimate the size of larger examples.

To estimate the size of a star forming region, I will use recent results on the structure of the nearest giant burst of star formation, 30 Doradus. Campbell et al. (1992) and Malamuth and Heap (1994) analysis of HST data indicates that 30 Dor has an optical core radius about 0.2 pc, and a strong mass segregation in the sense that the central region has a higher fraction of massive stars. Mass segregation as detected in 30 Dor and most galactic open clusters implies that massive stars preferentially populate the cote of these young clusters. In the case of equipartition of kinetic energy among the stars, a cluster with a Salpeter IMF will have all the stars more massive than 1OMo confined to a region of about 15 ‘%, of the mass effective radius. The massive stars will have smaller velocity dispersion and therefore will be closer to the cluster centre (Binney and Tremaine, 1987).

I will adopt therefore, 0.2 pc for the core radius of 30 Doradus and 1 pc for its effective of half light radius. The total blue luminosity of 30 Dor is Mb - - 13.5 . Assuming that 30 Dor is close to virial equilibrium, and with stellar velocity dispersion equal to the ionized gas velocity dispersion of 20 km s-’

Using the virial theorem it is possible to scale the observed parameters of 30 Dor to yomig stellar clusters with other masses and velocity dispersion as,

for the case of similar M/L ratio for all young clusters. If most of the blue nuclear luminosity in a type 2 Seyfert is due to the young stars in the torus, the above equation gives an upper limit to its size.

This equation predicts R, < 0.25 pc or less than 2 m.a.s. for a nearby AGN like NGC 4151 (Mb = - 19.5 and w = 200 km s-l). The half mass radius of the cluster or the characteristic size of the mass distribution of the torus, will be about five times.

Other interesting parameters are the stellar density and the relaxation time in the core region. Campbell et al. (1992) estimated a total of 1.6 x lo5 stars of one solar mass in a 0.25 pc radius for the core of 30 Dor. This corresponds to a density no - 2.5 x lo6 Mope-? and a relaxation time of about 2 x 1 O7 yr. The corresponding ones for NGC 4 15 1 are 4 x 1 O7 stars of one solar mass and density of 6 x lo* M 0 PC-’ inside the 0.25 pc core and a relaxation time of 7 x lo7 yr.

Star-forming Tori 21

One large uncertainty in our scaling method refers to the value of the velocity dispersion in 30 Dor. If the velocity dispersion of the stellar population in 30 Dor is factor of two smaller than the velocity dispersion of the ionized gas as suggested by Chu and Kennicutt (1994) the predicted sizes will be smaller by a factor of four.

5. DISCUSSION AND CONCLUSION

The unified model for Seyfert galaxies states that the blue and featureless continuum in type 2 Seyferts comes from reflection of a hidden Seyfert 1 “non-stellar” continuum. If the occulted continuum source and the BLR are scattered in the same way, this implies that broad lines should also be scattered towards the observer. Seyfert 2’s, however, do not show obvious broad lines in their spectra, but do have a conspicuous BFC.

We believe that a source of blue continuum is required to better miderstand the properties of Seyfert 2’s inside the unified model. We propose that a young stellar population, prob- ably embedded in the dusty torus produces most of the BFC observed; i.e. that the dusty molecular tori in Seyfert nuclei are actively forming stars, and that only a small fraction of the BFC is actually scattered radiation from a hidden nucleus.

This modification is able to explain, at least qualitatively, most of the problems faced by the reflection model: the simultaneous presence of a strong blue continuum and absence of conspicuous broad emission lines, and the low polarizations of the continuum (see Cid Fernandes. 1996). The strength of the near IR Ca II absorption lines in the near IR combined with the weakness of the optical metal absorption suggest that young stars are an important ingredient in the BFC. Furthermore, the observed slope of the BFC is consistent with that of a dust embedded young stellar population.

What are the next steps? Although their strength depend on the age of the cluster, detection of UV stellar features will reinforce the case for stars. If detected, the UV lines together with the measurements of optical and IR stellar features will provide good age estimates for the stellar population.

AS a final comment, I would like to point that a considerable number of Seyfert galaxies change types. This implies that on top of geometrical effects, evohtion qf’the central object must pluy an important role.

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