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Astron. Astrophys. 363, 455-475 (2000)

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9. Conclusions or starting point? Our wind model and other components of a radio-quiet AGN

We have built a model AGN nuclear wind trying to account for the specific characteristics and physical processes relevant to the AGN phenomenon. Our model wind is of course quite schematic and in the present work we did not model explicitly its relation and interactions with other known AGN components. In Sect. 2, we discussed the basic hypotheses and requirements we have taken into account for the construction of this wind-type outflow in the AGN context; we refer to that section for details and here we just briefly recall them. Since we are interested in radio-quiet AGNs, we assume the flow must be sub-relativistic; also, the wind mass flux, [FORMULA], is supposed to be much lower than the accretion mass flux, and, as a consequence, much lower than the critical accretion rate [FORMULA]. Moreover, we have required the wind to be optically thin to scattering, so that we can neglect the effects of Compton interactions between the wind plasma and the photons of the central source radiation field on the radiation field itself (although, as we have seen, the same Compton interactions are energetically very significant to the wind plasma); still with the aim of neglecting any possible variation of the central radiation field due to the existence of the wind, another request we have considered is that the wind's own emission (that is basically by bremsstrahlung, due to the high temperature of the plasma) is such to be negligible in luminosity with respect to that of the central source.

We found out that a stationary, non-magnetized wind-type outflow, satisfying the conditions above, can exist in an AGN under rather specific conditions, that is, although we have several parameters that come into play for the model definition, the range of parameters allowing for a complete and physically consistent solution of the wind problem is always quite narrow.

However, our results do seem to be encouraging, since there are several observational hints for the actual presence of material outflowing from the central regions of radio-quiet AGNs, and we have started to define more precisely under what physical conditions such outflows can be expected.

We have analyzed hydrodynamic and energetic conditions allowing for the resolution of the stationary wind problem.

Recalling briefly Sect. 8.1 results, one of these conditions is the necessity of a heating source for the wind plasma, distributed in radial distance and proportional to the plasma density; we have introduced it in terms of a parameterized heating rate [FORMULA], and we have shown its importance from the energetic balance point of view both for the transonic region of the wind, where the solution is the result of the complex balance of energy deposition/loss and momentum deposition as well, and for the outer supersonic region, where the wind behaviour is more similar to that of a polytropic-like wind, for which the energetic balance must be [FORMULA]. We have modeled [FORMULA] in a parameterized form, but we have not yet identified its physical origin, that is certainly an issue to be clarified in the context of radio-quiet AGN physics (see Sect. 4).

In the chosen descriptive framework, rather strict requirements turn out to be imposed on the parameters defining this heating function [FORMULA].

We have found that we can build model winds that are characterized by very high temperatures (up to relativistic values for the wind plasma electrons close to the wind origin) in the inner regions, and external (supersonic) zone temperatures that can be easily maintained high enough to ensure complete ionization of the wind plasma ([FORMULA]K). Also, the wind turns out to be rather tenuous, with plasma number density at the wind origin that is typically around a few[FORMULA] cm-3 and then decreases so that, for a constant mass flux wind, the outer supersonic regions of the wind are extremely rarefied, what makes the wind almost unsubstantial.

To circumvent this problem so as to account for possible interactions of the wind with other physical components of the AGN central region, leading to external mass entrainment in the outflow, we have devised a simple treatment for the inclusion of a distributed mass source along the wind way. We have thus built up wind models with non-constant mass flux, increasing with radial distance in a given region, whose extension and location we can appropriately define. The resulting outer plasma density can be therefore maintained at larger values.

The consideration of mass-loaded wind models also allows us to attack the issue of the relationship between the nuclear hot wind, as a kind of background, and other interesting phenomenological components of the AGN central region. In fact, on one hand we can relate the origin of the external mass input for the wind to the presence of a clumpy line emitting component as the BLR, with which the wind interacts, somehow entraining part of its material; on the other hand, wind models with non-negligible density values at "large" distances, that is distances comparable with the inferred estimates of the position of outflowing UV absorbers, allow us to examine the possible relation of our wind with these UV(-X-ray) absorbers as well.

As for the relation with the BLR, we postpone this study to a forthcoming paper (Torricelli & Pietrini 2000); here we just mention that we are going to study this problem within the framework of those models that structure the BLR with a central compact star cluster, whose evolved stars (the so-called "bloated" stars) originate gas envelopes and stellar winds that can be both considered the site of the line emission and the source of mass for wind entrainment [see Korista (1999) for a general review, and Alexander & Netzer (1994), Alexander & Netzer (1997) and Alexander (1997) for a recent model of "bloated"-star BLR].

The relation with UV and X-ray absorbers, quite commonly present in Seyfert galaxies (Crenshaw et al. 1999) should be explored as well. We find encouraging the fact that the physical properties of our mass-loaded wind models at the estimated (model dependent) distances of these AGN components are such that, for example, the wind thermal pressure ([FORMULA]) is comparable or anyway within the range of estimated values of the thermal pressure of these absorbers. Moreover, at these same distances our wind-type outflow models can have a velocity quite similar to the values of outflow velocity of the UV-absorbers (at least for what regards Seyfert 1s) estimated from spectroscopic analysis of the blueshift of the observed absorption lines, i.e. [FORMULA].

These results would suggest a possible relation between a background nuclear wind as the one we have modeled and these phenomenological components of AGNs. The nature of this relation is at present not defined. However, in the framework of models in which UV absorbers are due to clumpy material embedded in a surrounding medium, a possible speculation, suggested by the order of magnitude pressure equilibrium between our wind-type outflow and the absorbing material, could be that the absorbing clumps are somehow dragged along by the wind itself, identified with the background medium, and they are essentially comoving with the wind, thus avoiding the disrupting effects of hydrodynamical instabilities. In this case, substantial thermal pressure equilibrium would be achieved thanks to the conspicuous local values of the wind temperature and to the fact that the input of external mass, that we suppose to take place at BLR distances, guarantees appropriate (sufficiently large) wind density at UV absorber distances. This would be obtained without requiring too large mass loss rate from the very central region (i.e. , close to the wind origin) (see de Kool 1997).

To be more specific, we have to recall, first of all, that, apart from this spectroscopic determination of the outflow velocity of the UV-absorbers, the estimates of the distance and of other physical properties of the absorbing material, such as density and temperature, that are found in literature do depend on the assumed photoionization model through which the authors analyse the observations. The estimated distances typically range from [FORMULA]ld to [FORMULA]1 pc, and the order of magnitude of the thermal pressure, given as nT, is around [FORMULA]Kcm-3 and [FORMULA]Kcm-3, assuming temperatures in the range [FORMULA]K; see, for example, the studies on NGC3516 by Mathur et al. (1997) and on NGC5548 by Mathur et al. (1995), and by Crenshaw & Kraemer (1999). From these same authors and references therein, an estimate of the outflow velocity of the UV absorbing material of Seyfert galaxies gives [FORMULA] km s-1, and, more specifically, for the two AGNs mentioned above, it is [FORMULA] km s-1 (for NGC3516) and [FORMULA] km s-1 (for NGC5548).

An inspection of the two example solutions for the case of mass-loaded wind models shown in Fig. 5, both corresponding to [FORMULA] erg s-1, allows us to verify that for both the solutions nT is in the range of estimated order of magnitude of the UV-absorber thermal pressure mentioned above between [FORMULA] and [FORMULA], corresponding to the interval between [FORMULA]ld and [FORMULA]ld, which is right within the range of estimated distance of the UV-absorber component. Also, it is interesting to notice that, in this range of distances, the wind outflow velocity is between [FORMULA] km s-1 for the solution in panel (A) of Fig. 5, whereas for the one shown in panel (B) it is [FORMULA] km s-1; again these values seem to match rather well the observed values of [FORMULA].

Of course, the considerations above are purely speculative at present, but they are stimulating to start the analysis of the possible role of a nuclear wind such as the one we have studied in the present paper in understanding the scenario of UV-X-ray absorbers in AGNs. This is postponed to future work.

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Online publication: December 11, 2000