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Gas and Dust Emission from the Nuclear Region of the Circinus Galaxy

Simultaneous modeling of the line and continuum emission from the nuclear region of the Circinus galaxy is presented. Composite models which include the combined effect of shocks and photoionization from the active center and from the circumnuclear star forming region are considered. The effects of dust reradiation, bremsstrahlung from the gas and synchrotron radiation are treated consistently. The proposed model accounts for two important observational features. First, the high obscuration of Circinus central source is produced by high velocity and dense clouds with characteristic high dust-to-gas ratios. Their large velocities, up to 1500 km\s, place them very close to the active center. Second, the derived size of the line emitting region is well in agreement with the observed limits for the coronal and narrow line region of Circinus.


Introduction
Multiwavelength continuum and line data information for a key number of active galactic nuclei (AGN) in the closer Universe is rapidly growing up and becoming more and more complete. The availability of such multiwavelength dataset allows a more detailed and consistent study of the nuclear emitting regions and is definitively demanding more elaborated spectral modeling approaches.
The Circinus galaxy (A1409-65) was first reported by Freeman et al. (1977) with coordinates (1950)  Circinus shows a spectacular, one side [OIII] 5007 ionization cone with apex at the nucleus of the galaxy (Marconi et al. 1994). This asymmetry is probably due to extinction by the galaxy disk which is inclined by about 65 degrees, hiding the counter ionization cone to the observer. Circinus also shows a prominent dust lane South-West of its nucleus (Marconi et al.) which might be causing the obscuration of the intrinsic nuclear light, in particular considering the non detection of Circinus nucleus in the UV light.
In most known Seyfert galaxies residing in spirals, the AGN light is contaminated by circumnuclear star forming regions which largely complicates the interpretation of the line and continuum spectra from the active nucleus. Circinus is not an exception: its Seyfert 2 nucleus resides on a Sb-d system and several pieces of evidence indicate outgoing star formation in the nuclear vicinity. Resolved star formation is seen up to ∼10 arcsec from the centre, where a ring shaped region is detected (Marconi et al. 1994), whereas CO band absorptions -most probably from red supergiants -are 3 seen within the inner 0.75 arcsec region (Maiolino et al. 1998). Thus, lines from low ionization states in the nuclear spectrum might include substantial contribution from the stellar activity. On the other hand, lines from highly ionized species, essentially above ion IV, are expected to be produced by radiation from the active center (AC).
Besides the starburst contribution, the AGN emission can itself be dominated by two competing mechanisms, namely, the ionizing radiation from the AC and the shocks. Line and continuum emissions from Circinus indicate that both mechanisms are active. They also reflect the different physical conditions of the clouds from which they arise, including the effect of dust.
Therefore, if a meaningful representation of the nuclear region is sought, any modeling of the continuum and of the line emission should be consistent with each other.
Previous modeling of Circinus by Oliva et al. (1994), Moorwood et al. (1997), and Binette et al. (1997) provides a first notion of the complex structure of Circinus nuclear region. In all of the three cases the analysis and results are exclusively based on the modeling of the high ionization line spectrum. However, the available data from Circinus span from radio to Xray, which requires a self consistent model to explain the complete dataset.
Gas emission accounts for most of the continuum and for the lines from high and low ionization levels, whereas dust emission accounts mostly for the IR continuum. Thus, the emission from gas and dust should be calculated consistently, and their effects properly weighted in the different regions of the electromagnetic spectrum. In the modeling of Circinus, several models are used: some provide a better fit to the line spectrum, whereas, others fit better the continuum.
Modeling is based on hypothesis about the symmetry of the region, stellar emission, central source energetics, etc., which added to the input parameters (velocity field, density, dust characteristics) lead to a description of the real galaxy complex. It is difficult to establish errors of the theoretical results since they depend on atomic and molecular data, some of them being still roughly estimated. Therefore, the most probable model should be selected among the best fitting to the observational data throughout consistency.
This paper is devoted to a self consistent treatment of the continuum and narrow emission line spectra from an AGN. A multiwavelength modeling of the nuclear and extended emission region in the Seyfert 2 NGC 5252 was presented in a similar way (Contini, Prieto & Viegas, 1998). The modeling approach is based on previous work by Viegas & Contini (1997 review and references therein) which focuses on the coupled effect of photoionization and shocks.
The paper is organized in the following way. In §2 the observational dataset for Circinus is given. The general model and the input parameters are presented in §3. A grid of single-cloud models for the Circinus galaxy and the fit to the observed line spectra by multi-cloud models are discussed in §4. The spectral energy distribution (SED) of the continuum given by these models as well as by other models necessary to explain specific features of the continuum appear in §5. The consistent fit of selected multi-cloud models to the line and continuum spectra are discussed in §6. Conclusions follow in §7.

Observational dataset for Circinus
Continuum and line data for Circinus were collected from the literature.
The continuum emission is well sampled from the X-ray to radio waves with the exception of the UV region; line emission was collected from the optical to the middle IR. In all cases, data from the smallest available aperture were considered in order to minimize the stellar light contribution. Data sources are listed below.

5
Circinus was detected in the ROSAT "all sky survey" with a flux of about 10 −11 erg cm −2 s −1 in the 0.1-2.4 keV (Brinkmann et al. 1994). The X-ray spectrum of Circinus was obtained by ASCA (Matt et al. 1996). The authors derived a 2-10 keV flux of ∼ 10 −11 erg cm −2 s −1 , and a spectral index of -0.5 ± 0.5. Matt et al. argue that above 2-3 keV, the ASCA emission is mainly reflected X-ray radiation, at the level of a few percent, from an otherwise invisible nucleus even at 10 keV. Moorwood et al. (1997) suggest that the intrinsic ionizing spectrum has the same power-law as the reflected one, and that the observed X-ray emission at 10 keV is about 1% of the intrinsic AGN luminosity at that frequency. We thus adopt the intrinsic nuclear emission radiation at 10 keV to be ∼ 2 × 10 −9 erg cm −2 s −1 as in Moorwood et al. (1997).
The optical continuum and line spectra are taken from Oliva et al. (1994). These data cover the 4000-10000Å region and are integrated over the central 1.5x4.5 arcsec region.
The near-IR line spectrum between 1 and 4 µm is also taken from Oliva et al. These data are integrated within a rather large aperture, 4.4x6.6 arcsec, and so, only the coronal line fluxes are used for the modeling purposes. The data were cross-checked with much higher spatial resolution data collected in the K band by Maiolino et al. (1998). In this case, the K nuclear emission lines were integrated within a 0.75 arcsec aperture. The coronal line in common to the two dataset, [SiIV] 1.96 µm, measures ∼15% less in the 0.75 arcsec aperture, which is consistent within the errors with a point like origin for the coronal lines.
Middle IR line data between 3 and 40 µm are provided by ISO . ISO apertures are large, above 20 arcsec, and so, emission from the circumnuclear star-forming ring is included.
For the IR continuum between 1 and 10 µm two different sources were used. Moorwood & Glass (1994) data from a 5 arcsec aperture were com-6 plemented with those from Siebenmorgen et al. (1997)  Below 2.2 µm, the stellar contribution in Circinus is very significant. Maiolino et al. (1998) provide a stellar-corrected flux at 2.2 µm derived from a 0.15 arcsec aperture which turns to be an order of magnitude lower than the integrated value by Moorwood & Glass (1994) in the 5 arcsec aperture. Both set of values shall be considered in the modeling as reference limits to the stellar and pure AGN contribution.
Data at 60, 100 and 150 µm were taken from the spatial resolution analysis by Ghosh et al. (1992), and correspond to an aperture of about 40 arcsec, which is the deconvolved size of Circinus at λ > 50 µm. Circinus was not detected at 150 µm and so we used the 5 sigma upper limit suggested by Ghosh et al.
Radio data at 5, 2.7 and 1.4 GHz were taken from the Parkes catalogue (Wright & Otrupcek 1990). The data point at 4.85 GHz was taken from Wright et al. (1994), and corresponds to a beam size of 5x5 arcsec and agrees with the 5 GHz Parkes data within less than 1%. Radio emission at 408 MHz was taken from Large et al. (1981) and corresponds to a circular beam size of 43 arcsec.

7
The nebular spectrum was corrected for internal extinction by Av∼ 5 mag as determined from the H recombination lines. The continuum data were corrected for interstellar extinction in the direction of Circinus by Av∼1.5 mag.
The nuclear line profiles in the optical and IR are so far unresolved at the limit of the available observations. FWHM of less than ∼ 150 km s −1 are derived from Oliva et al. (1994) for the optical and near-IR coronal lines and in the range 150-300 km s −1 from Moorwood et al. (1997) for the ISO lines. Prominent blue wings are seen in the high excitation lines from the nucleus and within the ionization bicone (Vielleux & Bland-Hawtorn 1997) suggestive of outflowing gas motions.

The general model
A description of the methodology and application of the modeling approach can be found in Contini et al. (1998) and a more extended discussion in Viegas & Contini (1994, 1997. A model for the narrow-line region (NLR) is proposed. In the model, the circumnuclear clouds move radially outward from the AC and a shock-front is formed on the outer edge of the clouds.
The photon flux from the active source reaches the clouds on the inner edge opposite the shock front. Black body radiation from the stars reaches either the inner or the outer edge of the clouds depending on location of the stars.
The SUMA code (Viegas & Contini 1994) is adapted to calculate the emission spectra (lines and continuum) from a shocked cloud which may also be photoionized by a power-law radiation from the central source and a black body stellar radiation. The code consistently calculates shock and radiation effects in a plane-parallel geometry. The calculation of dust reradiation is also included.

8
The input parameters to the models are the shock velocity, V s , the preshock density, n 0 , the magnetic field , B 0 , the ionizing radiation flux from the AC which is characterized by the flux at the Lyman limit F H (in number of photons cm −2 s −1 eV −1 ) and the spectral index, α. Black body radiation from the starburst is characterized by the temperature of the stars, T * , and the ionization parameter, U. The amount of dust is given by d/g, the dust-to-gas number ratio. D is the geometrical thickness of the clouds.
Cosmic gas abundances from Allen (1973) are used. The values of F H and V s determine the type of prevealing model, i.e., radiation dominated or shock dominated.

Observational constrains to the input parameters
A grid of models characterized by different values of the input parameters is calculated for Circinus. The ranges of the input parameters are selected to match the observed values as follows : • The intensity of the radiation flux, F H (in number cm −2 s −1 eV −1 ) is in the range 10 9 -10 12 , depending on the distance of the illuminated clouds from the AC. These values are derived from the diagnostic diagrams presented in Viegas-Aldrovandi & Contini (1989) and are consistent with Lyman photon number estimates (∼ 10 53.5 s −1 derived from Moorwood et al. 1997) if the emitting clouds are at 30 pc to 1 kpc from the nucleus.
• A composite ionizing spectrum with spectral index α U V = 1, and α X = 0.4 is assumed for all models. In the UV region, α U V = 1 was adopted on the basis of the observed He II 4686 / Hβ ratio. The choice of the X-ray spectral index is based on the ASCA spectrum of Circinus (see §2).
• The shock velocities, V s , are assumed to be less than or about 100 km s −1 , as deduced from the FWHM of the emission line profiles (see §2).
Larger velocities will be further discussed when modeling the SED of Circinus.
Values between 100 and 300 cm −3 are assumed. Models with higher densities shall also be considered and discussed in relation to the continuum emission.
• The geometrical thickness of the clouds within the NLR region, D, is defined by the best fit of the spectrum and ranges between 5 × 10 17 and 10 19 cm. This range of values is consistent with the observed size of the coronal line region, <10 pc (Maiolino et al. 1998;Oliva et al. 1994), and of the NLR, ∼ 30 pc (Marconi et al. 1994).
• The range of dust-to-gas ratio, d/g, is taken in the range 10 −14 ≤ d/g ≤ 5 × 10 −12 . These values are essentially determined by the shape of the middle and far IR continuum respectively. This will be discussed in §5.

The line spectrum
On the basis of the above observational constraints a grid of suitable singlecloud models for Circinus is proposed. The input parameters characterizing each model are presented in Table 1. As a first step, we shall focus on the modeling of the line spectrum; the continuum analysis will follow as a second step. Notice, however, that modeling the line and continuum spectra simultaneously implies cross-checking of one another until a fine tuning of the models is obtained.

Grid of single-cloud models
The line ratios calculated for each of the models in Table 1 are presented in Table 2. They are given relative to Hβ= 1.
Model BD is the only one intrinsically different from the rest of the models presented in Table 1. It represents a cloud with V s = 100 km s −1 and n 0 = 100 cm −3 photoionised by a black body. Models with different values for the T * and U were calculated. Among those, a model (BD) with T * = 7 ×10 4 K and U = 0.01 was selected as it was found to provide the best fit to the optical-near IR SED (cf Fig. 1a) and, as much as possible, to the line ratios. Maiolino et al. suggest that Circinus could be experiencing an outward propagating starburst with the current activity being occurring mainly at the 200 pc radius starburst ring (Maiolino et al. 1998). Therefore, model BD is calculated assuming that radiation reaches the very shock front edge of the clouds. Table 1 also includes two shock-dominated models, SD1 and SD2, characterized by both large shock velocities and large densities, and a radiationdominated model, DD, with high density. They shall be discussed when modeling the continuum SED of Circinus ( §5).
The lines in Table 2 are grouped following ionization levels, and line ratios lower than 5 10 −4 are omitted. In this way it is easy to realise that a spectrum characterized by both strong low (≤II) and high (≥VII) ionization lines cannot be fitted by single models. this model provides also strong lines from species ≥ IV, but, being matter bound, negligible neutral and singly ionized lines.
Besides, the black body model BD produces relatively strong lines of levels II and III but negligible highly ionized ions or singly ionized and neutral species.
The trends shown in Table 2  normalised to the [OIII] 5007 flux are also given in Table 1.

Power-law radiation dominated models: MA and MB
The main difference between MA and MB lies on the emission contribution from larger clouds (model RD4) included in the MB spectra. Let us recall that, although the best fit to the line spectra was searched throughout a wide range of the input parameters, these were restricted by the observational data as described in §3.

Relative abundances
The models in Table 1 are all run with cosmic abundances from Allen (1973), with the exception of S/H which is lower by a factor of 1.4. These assumptions are based on previous modeling of AGN (e.g., Contini et al. 1998

Continuum radiation
Modeling the continuum energy distribution is not always as straightforward as it is for the line spectrum. Bremsstrahlung from very dense gas is absorbed in the radio-far IR (FIR) range. FIR and middle IR emissions are produced by dust grains at different temperatures. In the near IR-optical range black body radiation from old stars in the galaxy prevails in some galaxies. Correction for stellar emission is a delicate issue and can mislead the data. In the hard X-ray domain the flux from the AC is directly observed, while bremsstrahlung from heated gas is seen in the UV and soft X-ray ranges.
Gas at very high (> 10 5 K) and very low temperatures (< 10 4 K) is clearly recognized in the SED of the continuum. Particularly, high temperatures are responsible for soft X-ray emission and for grain reradiation in the mid IR. Low temperature gas refers to the radio range where bremsstrahlung and synchrotron radiations are of competing strengths.
Therefore, models which explain the line spectra are not always sufficient to fit the entire SED of the continuum.
In modeling the line spectrum of Circinus it was found that clouds

Continuum emission components in Circinus
The available data for the SED of Circinus continuum are shown in Fig. 1a.
Depending on the spectral range, different emission components contribute to the observed continuum with different importance.
First of all, the emission from the underlying old stellar population associated with Circinus host galaxy contributes mainly in the optical domain.
A black body spectrum with T = 2000 K (model BB hereafter) is found to provide a reasonably fit to the observed SED in the 0.4 to 2 µm range (dash-dotted line in Fig. 1a). From hereafter, in all proposed models this component shall be fixed and normalised to the observed emission at about 2 µm. The derived stellar-subtracted emission at 2 µm by Maiolino et al. (1998) is represented by an asterisk in Fig. 1a. We will use this value as a reference for the non-stellar contribution to the continuum emission.
Following Matt et al. (1996) arguments, the intrinsic X-ray AGN continuum source is assumed to be totally absorbed below 10 keV. In Fig 1a, the cross in the X-ray domain represents the intrinsic 10 keV flux derived under the assumption that the observed ASCA flux at 10 keV is 1% percent of the intrinsic AGN flux. The dotted line is the absorbed ionizing continuum.
In the current modeling any UV to soft X-ray emission is assumed to be due to bremsstrahlung from shock-heated gas. The strong obscuration towards Circinus nucleus prevents the detection of Circinus in the UV, but ROSAT observations provide a 0.1 -2.4 keV integrated flux. This information shall be used to constrain the bremsstrahlung component, and will be discussed in connection with the shock-dominated models ( §5.4).
The second important bremsstrahlung component is due to gas heated by radiation (AGN and starburst) to temperatures of about 10 4 K. This component extends its contribution from the optical -where it peaks -to the radio domain.
The mid and far IR continuum radiation is assumed to be due to re-radiation by dust at different temperatures. Dust is heated by both radiation and shocks. The modeling of the mid-to-far IR SED essentially determines the dust-to-gas ratio parameter, d/g, which in turn is used as an input value for the modeling of the line spectrum. Notice that in the process of fitting the various continuum emission components, and, in the end, the continuum and line spectra together, an overall consistency is sought. In particular, the bremsstrahlung emission components should be consistent with the position of the IR bumps (cf Appendix 1) since the temperature of the dust grains follows the temperature of the gas (Viegas & Contini 1994).
Radio emission is coming from synchrotron and bremsstrahlung processes. We shall see that synchrotron emission (cf Appendix 2) appears to be the dominant component.

The SED of radiation dominated single-cloud models
The SED associated with each of the models RD1 to RD5 (labeled 1,2,3,4,5) and BD are shown in Fig. 1a. Their relative constributions is defined by their weights (cf. W in Table 1) (Table 1). Also negligible is the contribution of the high ionization model RD5 because of its low weight.
Model BD, representing the starburst contribution, is also plotted in Fig.   1a and shifthed according to its weight (Table 1) in the multi-component model MC. As mentioned above, BD was selected among several black body radiation dominated models because of best fitting the SED of the continuum in the optical range.
Due to the high extinction in Circinus, no UV data of the galaxy are available. We thus assume that the starburst ionizing continuum represented by a T * = 7 10 4 K black-body is fully absorbed.
The dust emission component associated with RD and BD models peaks in the far-IR region (Fig. 1a). The maximum temperature of the grains is about 30 K for model RD1, and 50 K for model RD2 (cf Figs. 3a and 3b).
The continuum emission beyond 2 10 12 Hz readily constrains the dust-to-gas ratio to d/g < 10 −14 . Note that due to its larger weight, the contribution of model BD to the optical and near-IR is more important than that due to the RD models.
In the radio domain the main contribution to the emission is due to synchrotron radiation calculated by models BD and RD4 (straight lines in Fig. 1a). The other models give a negligible contribution and are not represented. The corresponding bremsstrahlung components associated with models BD and RD are far less important and furthermore show different trends than observed (Fig. 1a).

High density clumps : model DD
The sharp cut-off observed in the SED at 2 10 12 Hz strongly constrains the d/g parameter. Absorption by high density matter may be causing the fall of the emission at this low frequencies. Following Kirchoff's law, the absorption coefficient, κ, can be expressed as: with T, the temperature, in K, ν in GHz and E = n + n e ds in cm −6 pc (Osterbrock 1989). Adopting T = 10 4 K, ds = 100 pc and ν = 2 10 3 GHz, values of τ ν > 1 are obtained for densities n > 5 10 5 cm −3 .
Such high densities could result from compression downstream of shocked gas with densities n 0 ≥ 5000 cm −3 . This section investigates the role of a model, DD, characterized by n 0 = 5000 cm −3 . DD is a radiation-dominated model also characterised by cloud velocities V s = 250 km s −1 and Lyman flux F H =10 12 (Table 1). In the downstream region the densities rise to ∼ 5 10 5 cm −3 , and radiation is absorbed for ν ≤ 2.10 12 Hz. The line spectrum from model DD is included in can account for the far-IR and radio SED, but the mid-IR SED is still not reproduced. Furthermore, DD bremsstrahlung component is overpredicting the stellar-subtracted emission at 2 µm (asterisk in Fig. 1 b). To fill up the middle IR region, hotter dust is required. Higher temperatures of both gas and dust can be reached via stronger shocks. Therefore, shock-dominated models are introduced in the SED modeling (next section).

Shock-dominated spectra from high velocity clouds: models SD1, SD2
In previous sections we have modeled the line spectra and part of the SED of the continuum. Still the data in the soft X-ray range and in the middle IR are unfit. We recall that the temperature of the grains follows the temperature of the gas, therefore, models characterized by higher temperature should be included in the final average. This section analyses the impact of shockdominated high velocity models on the overall spectrum.
Two shock-dominated models, SD1 and SD2, are proposed. Model SD1 is characterised by a cloud velocity V s = 400 km s −1 and a density n 0 = 300 cm −3 , and model SD2 by V s = 1500 km s −1 and n 0 = 1000 cm −3 . The SED generated by these models are represented by dashed lines in Fig. 1b. The respective bremsstrahlung components calculated with these models peak in the soft X-rays. These models are normalised as to account for the observed 0.2-2.4 keV integrated flux, F X = 1.3 10 −11 erg cm −2 s −1 (cf §2), and the mid-IR continuum emission. These in turn impose d/g ∼ 5 10 −12 .
Regarding dust emission, the adopted value of V s = 1500 km s −1 is a lower limit to shock velocities which could be deduced from the location of the IR bump. Indeed, due to sputtering the grains entering the shock front are rapidly destroyed and have no time to cool. However, they survive long enough because their lifetime is τ = 3 10 12 a/n ∼ 10 4 years (cf Osterbrock 1989), where a is the grain radius (≤ 0.2µm) and n the gas density (3000-5000 cm −3 ). Such life times in the present conditions correspond to a maximum distance from the shock-front of ∼ 3 10 18 cm, which is reduced by sputtering. In AGN, large velocities generally correspond to large densities, and in turn, to large compression downstream, so, the sputtering of grains entering the shock front will be more effective for large V s and the grains will be completely destroyed.
The line ratios calculated by models SD1 and SD2 are presented in Table 2. Model SD1 is characterised by weak line fluxes (notice that Hβ= 5.5 10 −4 erg cm −2 s −1 ), as compared with the radiation-dominated models.
Downstream compression is strong for model SD2. The high density and high d/g ratio cause rapid cooling in the emitting line region (dominated by T e ∼ 10 4 − 10 5 K). Therefore, compared with radiation-dominated models, SD2 line spectrum is relatively weak for species below level IV but strong for levels above.
6 Line and continuum spectra from multi-cloud models We emphasize that the single models represent prototype clouds in the different regions of the galaxy. The multi-cloud models imply the relative importance (and position) of the single cases and give a more realistic picture of the AGN. Both MA+ and MC+ still fail to reproduce the overall SED of the continuum, even if the weights of the single-cloud models are substantially changed. A further possibility is to include the contribution of the high density radiation dominated model DD which was found to provide an improved fit to the low frequency continuum beyond 2 ×10 12 Hz. This corresponds to the multi-cloud model MD.
The relative contribution of the different components to MD is given in Table 1. The main differences from model MC are the increase of the contribution of model BD from 31 % to 47 %, the corresponding decrease of the radiation dominated models (RD) by about the same amount, and the relative minor contribution of model DD. As expected, MD provides a substantial improvement to the fit of the SED over the entire energy range (Fig. 2c), and, relative to the emission line spectra (Tables 3a and 3b) We recall that model DD, characterized by V s = 250 km s −1 , implies line profiles with FWHM larger than those predicted by radiation dominated RD models. Considering that model DD only affects the low ionization levels lines (see Table 2), profiles of low ionization lines slightly broader than those from high ionization lines are predicted. In addition to those low density clouds, a small fraction of radiation dominated clouds are characterised by very high densities, n 0 ∼ 5000 cm −3 , and d/g ∼ 4 10 −12 . The contribution of these clouds to the line spectrum is very small; yet, they dominate the low-frequency end of the mid-IR continuum emission and the far-IR emission.
Shock dominated clouds are characterised by velocities up to 1500 km s −1 , densities up to 1000 cm −3 , and are highly dusty with d/g > 10 −12 . Their large velocities place them close to the AC; their large d/g ratios make them consistent with the obscuration of the central source. The contribution of these shock-dominated clouds to the line spectrum is negligible, but they fully account for the observed soft X-ray and mid-IR continuum emission.
The radio continuum in Circinus is mainly synchrotron emission due to Fermi mechanism acting in the shock front edge of the clouds. The total bremsstrahlung component due to the shock-dominated and radiationdominated clouds is insufficient to account for the observed trend and intensity in the radio waves. The high density clumps provide the strongest contribution to the synchrotron emission.
Finally, a rough estimate of the average distance of the emitting clouds from the AC can be derived by comparing the observed [OIII] flux (1.95 10 −11 erg cm −2 s −1 at earth, Oliva et al. 1994) with that derived from model MD at the nebula (13.4 erg cm −2 s −1 ). As the larger contribution to the [OIII] flux arises from the highest ionization models, RD4 and RD5 -as it can be easily deduced from Table 1 Appendix 1 : The heating of dust grains We have shown in previous sections that the SED of the continuum in the IR range is determined by the temperature of the grains. In this section, dust heating mechanisms are discussed and the distribution of dust temperature throughout the clouds is presented for the most significative models presented in Table 1. The dust temperature is higher in the immediate postshock region rather than in the photoionized edge, even for a relatively weak shock as for model RD1 (Fig. 3 a). Dust grains are collisionally heated as they pass through the shock front. Therefore, the location of the dust emission peaks in the SED of the continuum (cf. Figs. 1 a and b) are determined by the temperature of the gas in the postshock region, and consequently depends on V s .
The dust and gas temperatures drop faster downstream for model DD ( Fig. 3c) because the density of the gas is high. The temperatures reached in model DD are higher than in model RD2 (Fig. 3b) because V s is higher, so the SED peaks at a higher frequency. Notice that for model DD collision heating of the grains always prevails.
Finally, the results of model BD are plotted in Fig. 3d. Radiation heating of the grains prevails through most of the cloud, but collision heating prevails in the region of gas at T e ∼ 10 4 − 10 5 K. We recall that in this case radiation reaches the very shock front edge.
The other models of Table 2 are more predictable. For models RD3, RD4, and RD5 radiation heating prevails. However, the temperature of the grains is not higher than that calculated for models which show the same V s . Therefore, reradiation peaks in the FIR. Regarding shock dominated models (SD1 and SD2) collision heating prevails. Particularly, for SD2, the grain temperature reaches a maximum value of 275 K downstream and the grains survive up to a distance from the shock front of 10 15 cm. Therefore, reradiation peaks at about 9 µm.
In conclusion, the presence of shocks is important in the heating of gas and dust to high temperatures. Radiation heating and collision heating are both effective, but collision heating of the grains prevails in the hot region, therefore, the location of the peak corresponding to reradiation by dust in the SED of the continuum depends strongly on the shock velocity.
Notice that in all the models an initial grain size a = 0.2 µm is assumed.
A different size could change the temperature results, but not the main conclusions.

Appendix 2 : Synchrotron radiation
Radio emission in Circinus clearly departs from the calculated bremsstrahlung trend (see Figs. 1a and 1b) and suggests that synchrotron emission could be the main component. The case of Circinus is in marked contrast with that of the Seyfert 2 galaxy NGC 5252 for which simple bremsstrahlung could nicely account for the radio SED (cf. Contini et al. 1998, Fig. 1).
Synchrotron radiation due to Fermi mechanism arises from reacceleration of particles through the shock-front, this being located at the outer edge of the cloud facing the observer, while radio and IR bremsstrahlungs are emitted by relative cold gas from the inner region of the cloud and more easily absorbed (cf §5.3). Synchrotron emission is included in the SUMA code, and is consistently calculated with that of the bremsstrahlung. Two power-law spectral index, -0.35 and -0.75, which define the limits between the nonrelativistic and the relativistic case respectively, are considered. Among the various models discussed, only the synchrotron radiation associated with model DD provides sufficient emission to account for the observed radio flux ( Fig. 1 b), whereas, the other models give negligible contributions. In this case, the -0.75 power-law index reasonably fits the radio trend. If the emission at 2.3 10 11 Hz were associated with the synchrotron component, then a much steeper spectral index had to be considered. As shown in Fig 1b, a power-law spectral index of about -0.35 gives a crude approximation to the data.
The cutoff of the synchrotron emission at lower frequencies calculated by model DD must be consistent with the data. Following Ginzburg & Syrovatskii (1965) the presence of a dense plasma reduces and even stops synchrotron radio emission for ν ≤ 20n e /B. As B (downstream) = B 0 (n e / n 0 ) the cutoff depends on the preshock magnetic field and on the preshock density. Adopting B 0 = 10 −4 gauss, the cutoff is at ν ∼ 10 8 Hz for low density models, and at ν ∼ 10 9 Hz for high density models. The latter is larger than it is observed as emission is still observed up to at least frequencies of 4 10 8 Hz. However, we recall that the magnetic field can be amplified at the shock front by turbulence which is created by collision and fragmentation of high density clumps through R-T and K-H instabilities (Jun & Norman 1995). Consequently, the cutoff could be shifted to lower frequencies.        Line ratios are from Oliva et al. (1994) and are corrected by Av∼5.8 mag.

Figure Captions
We assume 10% uncertainty for the stronger lines, 30% for weak and IR lines. Optical lines are from the central 1.5x4.5 arcsec 2 region, IR lines from the central 4.4x6.6 arcsec 2 , thus only the IR coronal lines are modelled. 1 Flux measured within the 15 pc region by (Maiolino et al. (1998) with 10% quoted error. The value measured in a 90x40 pc region by Oliva et al. differs by 15%.