µ±Ç°Î»ÖÃ£ºÊ×Ò³ >> >> The signature of high velocity gas in the spectra of NGC 4151

The signature of high velocity gas in the spectra of NGC 4151

A&A manuscript no. (will be inserted by hand later) Your thesaurus codes are:

ASTRONOMY AND ASTROPHYSICS

The signature of high velocity gas in the spectra of NGC 4151
M. Contini1,2 , S. M. Viegas2 , and M.A. Prieto3
1 2

arXiv:astro-ph/0202146v1 7 Feb 2002

3

School of Physics & Astronomy, Tel Aviv University, 69978 Tel Aviv, Israel Instituto Astron?mico e Geof? o isico, USP, Av. Miguel Stefano, 4200,04301-904 S?o Paulo, Brazil a European Southern Observatory, D-85748 Garching, Germany

Abstract. The multiwavelength emission spectrum and associated velocity ?eld of the Seyfert prototype NGC 4151 is modeled. NGC 4151 has been since the thirties subject of extensive modeling, nuclear photoionization being the basic approach considered by all authors. HST data has impressively revealed the existence of a large range of velocities (100 - 1500 km s?1 ) dominating the emitting clouds in the extended emission line region of the galaxy. Following this observational result, a revision of the photoionization modeling approach applied to NGC 4151 is presented. It is concluded that a mixture of radiation dominated clouds and shock dominated clouds are required to explain the multiwavelength line and continuum spectra of the galaxy. The relative contribution of shock excitation versus photoionization is consistently modeled along the nebulae taking into account the spatial variation of both ?ux and velocity of main optical and UV lines. The multiwavelength continuum spectrum of NGC 4151 is then nicely accounted for by a combination of nuclear emission at high energy, gas Bremsstrahlung, and dust emission. The last two phenomena are directly linked to the composite shock and photoionization excitation of the gas. The radio SED is found however dominated by synchrotron emission created by Fermi mechanism at the shock front. In addition, a 3x103 K black body component accounts for the host galaxy contribution. As a result of the modeling, silicon is found depleted by a high factor and included in dust grains, while N/C abundance ratio is found compatible with cosmic values. Key words: galaxies : nuclei - galaxies : Seyfert - shock waves - galaxies : individual : NGC 4151 - X-ray : galaxies

1. Introduction Detailed imaging of the narrow-line region (NLR) of Seyfert galaxies with the Hubble Space Telescope (HST) has revealed its complex morphology and velocity ?eld.
Send o?print requests to: M. Contini, contini@ccsg.tau.ac.il

Recently, observations of NGC 4151 indicated the presence of emitting clouds with velocities ranging from +846 to -1716 km s?1 (Winge et al. 1997, Kaiser et al. 1999). NGC 4151 is a nearby barred galaxy, usually classi?ed as a Seyfert type 1, although it has already been considered as a Seyfert 1.5 (Osterbrock & Koski 1976) and has also shown the characteristics of a Seyfert 2 galaxy (Penston & P?rez 1984). NGC 4151 is one of the most obe served active galactic nuclei (AGN), from radio to X-rays. The recent HST longslit emission-line data (Nelson et al. 2000), coupled to ISO data (Alexander et al. 1999), o?er an excelent opportunity to improve our understanding of the NLR physical conditions with the added e?ect of the velocity ?eld. The kinematics derived from the long slit observations (Nelson et al. 2000, Crenshaw et al. 2000) show evidence of three components: a low velocity system, consistent with normal disk rotation, a high velocity system in radial out?ow at a few hundred km s?1 and an additional high velocity system with velocities up to 1400 km s?1 , as previously found from STIS slitless spectroscopy (Hutchings et al. 1998, 1999, Kaiser et al. 1999). The authors see the signature of a radial out?ow, with no interaction with the radio jet. However, high velocity components, shifted up to about 1500 km s?1 from the systemic velocity, are also seen by Winge et al (1997) associated with individual clouds located preferentially along the edges of the radio knots. Such association suggests a cloud-jet interaction, which de?nitively may in?uence the morphology and the physical conditions of the NLR. The high spatial resolution permits to see that the emission line ratios vary substantially on scales of a few tenth of an arcsecond, indicating that the density and ionization state of the emitting gas are strongly in?uenced by the local conditions, hence suggesting that shock fronts may be at work. The presence of high velocity clouds in the NLR of active galaxies has been predicted from the modeling of the emission-line and continuum spectra of the Seyfert 2 objects NGC 5252 and Circinus (Contini, Prieto & Viegas 1998a, 1998b), as well as suggested from the more gen-

2

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151

eral analysis of the optical-ultraviolet continuum of a relatively large sample of Seyfert 2 galaxies (Contini & Viegas 2000). High velocity clouds are also invoked in an alternative model based on a multiwavelength analysis of the emission-line and continuum spectra of the source (Contini & Viegas 1999), which explains the soft X-ray excess in NGC 4051. In all of these models the physical conditions of the high velocity clouds are usually shock-dominated. The shocks contribute to the high ionization emission-lines and to the continuum emission, mainly by providing e?cient heating of dust, contributing to reproduce the observed middle-infrared emission, and to the observed soft X-ray spectrum which originates in the high temperature postshock zone. Yet, the interpretation of the NGC 4151 emission-line and continuum spectra is so far residing exclusively on photoionization models. All the authors agree that, given the simplicity of the models, the observed line ratios can be reproduced. In NGC 4151 the high velocity clouds are directly observed. Velocities of ? 1500 km s?1 could be theoretically explained extrapolating to the NLR the arguments about high velocity clouds raised by Weymann et al. (1997). Here, however, in order to obtain a large range of velocities collisions are invoked. Although photoionization models reproduce within better than a factor of 2 the most important line ratios (Schulz 1988, 1990; Kraemer et al. 2000; Alexander et al. 1999), some questions remain still open, as for example the mere fact of the in?uence of the velocity ?eld in the cloud spectrum. Thus, in this paper we will look for the signature of the high velocity gas in the emission-line spectrum, as well as in the radio to X-rays continuum spectral energy distribution (SED). We present a new type of single-cloud modeling based on the spatial distribution of the observational data in the optical range; then, a multi-cloud model is proposed to explain the emission lines in the IR. As was done previously in modeling other objects (NGC 5252, Circinus), the multi-cloud model issued from the analysis of the emission lines is then constrained by ?tting the continuum SED in a large frequency range. This is possible only with composite models (shock + photoionization), because the gas heated only by the radiation from the active center cannot reach temperatures high enough to ?t the Bremsstrahlung emission and dust reradiation in the large range of the observed frequencies. A review focused in the successes and problems of photoionization models is presented in Sect. 2. The composite models, coupling photoionization and shocks, are described in Sect. 3. The optical emission-line spectrum is discussed in Sect. 4, while the infrared lines are presented in Sect. 5, and the results for the ultraviolet lines appear in Sect. 6. The observed and calculated SED are compared in Sect. 7, and the conclusions appear in Sect. 8.

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151

3

36.0 which are underpredicted, and for [S III] 9069, [Ne V] 14.3, [S III] 18.7, [Ne V] 24.3, and [S III] 33.5 that are overpredicted. Regarding the SED implied by models, the presence of a big blue bump in the ionizing radiation spectrum, peaking at 50 eV, is excluded by their best-?t model because its e?ect would be to overproduce the low-ionization lines and underproduce the high-ionization lines. However, this is the net e?ect produced by imposing a ?lling factor less than unity, as adopted in their best-?t model. Kraemer et al. models (type B ) use HST/STIS lowdispersion long-slit data at position angle P.A. = 221o . Thus, their data set includes UV and optical lines, but no infrared lines that are the basis of type A models by Alexander et al. (1999). Their model results are compared with observed emission-line ratios derived at di?erent positions along the slit. The large majority of the lines are reproduced within a factor of 2. Divergencies larger than 2 are often found for the high ionization lines: [Fe VII] lines, and the UV lines N V 1240, [Ne IV] 2423, and NIV] 1486, although in the latter case the signal-to-noise is very low. At some locations of the emitting nebulae, the calculated C III] 1909, CII] 2326, [O III] 4363, and the [S III] lines also largely diverge from the data. Notice, however, that CIII] and CII] are blended and that, in the case of the [S III] lines the discrepancy may be due to an instrumental e?ect as pointed out by Kraemer et al. (2000). Several interpretations for the above discrepancies are discussed by Kraemer et al. (2000). Our assumption is that those discrepancies may be revealing the presence of an additional ionizing mechanism, which is not the dominant process, but that shows its signature through particular observational features. This point of view is adopted here assuming that the additional mechanism is due to the presence of shocks. 3. Single-cloud models Faint high velocity emission regions intermingled with brighter emission clouds are shown in NGC 4151 imaging (Hutchings et al. 1999). We consider this observational fact as an indication that the extended NLR of NGC 4151 is a mixture of low velocity radiation-dominated clouds and high velocity shock-dominated clouds, all contributing to the emission-line spectrum. Accordingly, composite models accounting for the coupled e?ect of the central ionizing radiation and shock excitation due to cloud motions are assumed. Numerical simulations for single clouds are obtained with the SUMA code (see, for instance, Viegas & Contini 1994). Notice that the simulations apply whether the shocks originate from an interaction of the emitting clouds with the radio jet or from a radial out?ow of the clouds. The input parameters are the shock velocity, Vs , the preshock density, n0 , the preshock magnetic ?eld, B0 , the ionizing radiation spectrum, the chemical abundances, the

dust-to-gas ratio by number, d/g, and the geometrical thickness of the clouds, D. A power-law, characterized by the power index ¦Á and the ?ux, FH , at the Lyman limit, reaching the cloud (in units of cm?2 s?1 eV?1 ) is generally adopted. For all the models, B0 = 10?4 gauss, ¦ÁUV = 1.5, and ¦ÁX =0.4, and cosmic abundances (Allen 1973) are adopted. The basic models are calculated with d/g = 10?15 , however, this value is changed a posteriori to better ?t the continuum SED. Shock dominated models (SD) are calculated assuming that the e?ects of the shock prevail on radiation (FH =0.). Radiation dominated models (RD), however, are composite, i.e. they account both for photoionization and shocks up to Vs =500 km s?1 , but photoionization dominates the physical conditions of the emitting gas. The grid of models which are actually used for modeling is presented in an accompanying paper by Contini & Viegas (2001, hereafter referred to as CV01). In the following, calculated emission-line ratios from a selected number of models in the grid are compared to the HST log-slit optical data at P.A.= 221o (Nelson et al. 2000 and Kraemer et al. 2000) and to the ISO integrated aperture SWS data by Sturm et al. (1999). Models are selected on the basis of the physical conditions of the emitting gas dictated by the observations, e.g. the FWHM of the line pro?les for Vs . In models which account for shock e?ects, the density downstream is determined by compression, which depends on the shock velocity, and changes considerably with distance from the shock front (CV01, Figs. 5a, 6a, and 7a). Therefore, the preshock density and the shock velocity chosen de?ne a distribution of the density across the cloud, which must be adequate to provide a good ?t to the density sensitive lines, e.g., [OII] 3727, [NI] 5200, [NII] 6548 ([SII] 6716+ is not very signi?cant because S can be locked in dust grains). Moreover, in the NLR the density of the clouds follows the gradient of the cloud velocity, decreasing with the distance to the center. For each cloud, the observed [S II] 6717/6730 line ratio (which does not depend on S/H) is used as a ?rst test for the choice of n0 and Vs . Then, the intensity of the ionizing radiation, the physical conditions calculated by the model, and the geometrical thickness of the cloud are deduced from the line spectrum, as a whole. The [O III]/H¦Â line ratio is an indicator for FH . On the other hand, wide clouds are optically thick, leading to stronger low ionization level lines, while narrow clouds are matter-bounded, with fainter low-ionization lines. Thus the choice of D is then constrained by the best ?t of a large number of line ratios, particularly, the ratio of the low ionization lines to H¦Â.

4

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151

4. The optical emission-lines Modeling the spatial distribution Because of the availability of spatially resolved emissionline spectra in di?erent regions of NGC 4151, a new modeling approach is followed in this work. The most significant optical emission lines are modeled accounting for their spatial distribution across the NLR. In each position we check the consistency of the prevailing models explaining several line ratios, including those that are poorly reproduced by the photoionization models. The observed and calculated emission-line intensities relative to H¦Â are presented in a series of ?gures (Figs. 14), where the emission-line ratios are shown as a function of the projected nuclear distance, including both the SW data (on the left) and the NE data (on the right) at P.A.= 221o. In order to determine the intensity of the power-law radiation ?ux in the NLR edge closer to the nucleus a preliminary estimate is made by comparing model results with the observed line ratios as presented in Fig. 1. Filled squares refer to Nelson et al. 2000 (not reddening corrected) and ?lled triangles to Kraemer et al. 2000 (reddening corrected) data, respectively. The scales for log FH are indicated upon the upper horizontal axis. Each curve represents RD models calculated with di?erent FH but corresponding to the same Vs - n0 : Vs =100 km s?1 and n0 =100 cm?3 (dotted lines), Vs =200 km s?1 and n0 =200 cm?3 (dash-dotted lines), Vs =300 km s?1 and n0 =300cm?3 (short-dash lines), Vs =500 and n0 =300 cm?3 , (long-dash lines). Thin lines refer to a narrow cloud (D=1017?18 cm) and thick lines to wider clouds (D = 1019 cm). A sequence of three di?erent cases (log FH =13, 12, and 11.3) is shown in the top, middle, and bottom diagrams, respectively. We have chosen the [OIII]/H¦Â and [OII]/H¦Â line ratios, because are the most signi?cant. The ?t of both [OIII]/H¦Â and [OII]/H¦Â data is acceptable only in the bottom diagrams (notice that in these diagrams the ?ux is not fully symmetric, indicating that the SW and NE regions are slightly di?erent). So, for consistency we chose the bottom case to model also the other line ratios. Since we are investigating the intensity of ionizing radiation from the active center (AC) and the velocity distribution in the central region of NGC 4151, both FH and Vs are shown in Figs. 2-4 diagrams. In this way, the relative importance of the velocity ?eld and the AC radiation ?eld can be recognized. The scales for log FH and Vs (in km s?1 ) are indicated upon the horizontal upper axis of the top and middle and bottom diagrams, respectively. In the top diagrams of Figs. 2-4 (panels a and d) each line corresponds to RD model results as in Fig. 1. Panels b and e show the solid lines corresponding to SD models (CV01, Tables 1-10) for which the maximum value of the velocity distribution is Vs =700 km s?1 in the central

region, while diagrams c and f correspond to the results with a maximum velocity of Vs =1400 km s?1 . Because shock dominated models for narrow and wide clouds give very similar results, these models are represented by one line (model results overlap), corresponding to one serie of results. We draw attention to the thin and thick lines in the two bottom diagrams which refer to reduced and full intensity line ratios, respectively, and not to models calculated by small and large D, as shown in the top diagrams. The model results shown in Figs. 2-4 cover most of the data consistently for the [OIII] 5007+4959/H¦Â, [OII] 3727/H¦Â, [OI] 6300+6363/H¦Â, [SIII] 9069/H¦Â, [SII] 6713+6731/H¦Â, and [FeVII] 6086/H¦Â line ratios. The ?rst three ratios are related to the ionization state of the gas, while the other three are chosen because they are not well reproduced by pure photoionization models. The ranges and the spatial distributions of FH were chosen phenomenologically by the ?t, particularly, of the [OIII]/H¦Â and [SIII]/H¦Â ratios, while the ranges and the distributions of Vs were chosen by ?tting the low ionization ([OII]/H¦Â, [SII]/H¦Â) and neutral ([OI]/H¦Â) line ratios. The data corresponding to the high ionization line ratio [FeVII]/H¦Â are not enough to constrain the models. We adopt a velocity ?eld decreasing from the center of the galaxy outwards. The observed velocity ?eld is, however, complex, so, two di?erent shock velocity distributions are shown, one with a maximum Vs ? 700 km s?1 in the central region (middle diagrams), and another with a higher maximum Vs , up to 1400 km s?1 (bottom diagrams). Two di?erent lines represent the SD models in both middle and bottom diagrams. The thick one corresponds to the calculated models, the thin one to the models downwards shifted by a factor of 3 (middle diagrams) and of 2 (bottom diagrams). The shift in the middle diagram is dictated by the ?t of the [OI]/H¦Â ratios (see Sect 4.1) and in the bottom diagram by the [FeVII]/H¦Â ratios (see Sect. 4.3). These shifts do not represent lower abundances of the elements relative to H, but they indicate that in a multi-cloud model corresponding to the weighted sum of single-cloud models, the weights of the SD high velocity (> 300 km s?1 ) clouds is reduced (see Sect. 5). Indeed, by modeling the data on a large scale, this solution may look arbitrary, particularly considering faint lines (e.g. [OI]), and lines a?ected by the presence of dust (e.g. [SIII], [SII], etc.). If reducing the weights of the SD models, the ?t of all the emission-lines is consistently improved, we can conclude that this reduction is sound and SD models corresponding to high velocities in the nuclear region have lower weights. The relative weight accounts for the relative number of clouds in the conditions determined by the model, for the dilution factor (i.e., the square ratio between the distance of the cloud from the galaxy center) and the distance of the galaxy from Earth. Notice that the middle and bottom diagrams in Figs. 2-4 refer

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151

5

Fig. 1. The [O III] 5007+4959/H¦Â (left diagrams) and the [O II] 3727/H¦Â (right diagrams) emission-line ratios are shown as a function of the projected nuclear distance.

6

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151

Fig. 2. The [O III] 5007+4959/H¦Â (left) and the [O II] 3727/H¦Â (right) emission-line ratios are shown as a function of the projected nuclear distance. Same notation as in Figure 1.

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151

7

Fig. 3. The [O I] 6300+6360/H¦Â (left) and the [S III] 9069/H¦Â (right) emission-line ratios as a function of the projected nuclear distance. Same notation as in Figure 1.

8

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151

Fig. 4. The [S II] 6713+6731/H¦Â(left) and the [Fe VII]6086/H¦Â (right) emission-line ratios as a function of the projected nuclear distance. Same notation as in Figure 1.

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151

9

to ¡±high velocities¡± in the nuclear region of the galaxy (between 1500-700 km s?1 and about 300 km s?1 ). Indeed, Nelson et al (2000, Fig. 5) show that the bulk of cloud velocities is within 300 km s?1 . So, the reduction of the weights in the SD diagrams indicates that the number of high velecity SD clouds is small. Recall that observational data result from the integration along the line of sight, which may include di?erent clouds. Thus, a more realistic ?t should be obtained by a multi-cloud average (see Sect. 5). A compromise between a consistent picture over a large spatial distribution of several emission lines and the precision of the ?t must be achieved. Observed features In several cases there are two or more emission lines within the passband, so that di?erent velocities are sampled for the di?erent lines (Hutchings et al. 1999). These are the [OII] 3727 and [SII] doublets, as well as [NII] lines plus H¦Á at about 6548 ?. The [OIII] image covers velocA ities between -1200 and -860 km s?1 . High velocity material in this velocity range is much fainter than the main bright clouds showing low velocities. It is seen on both sides of the nucleus and outside the main biconical emission regions. The high velocity gas is weak in H¦Â and [OII] lines and seems to be associated with highly ionized material. Ionization of oxygen is higher along radial locations on both sides of the nucleus. Generally, there is an association of high velocity clouds with high ionization gas, however, there are regions of high ionization (mainly to the E side) with no known high velocity gas. Nelson et al. (2000) claim that the high velocity components generally account for a small fraction of the total ?ux in the [OIII] emission lines. In several cases they ?nd clouds with multiple velocity components. 4.1. Oxygen Lines: [OIII], [OII], and [OI] The results for [O III]/H¦Â and [O II]/H¦Â are shown in Fig. 2. Di?erent models are selected to ?t the data as expected from the multiple structure of the observed lines (Kaiser et al. 2000). As shown in Fig. 2 a, the data are well reproduced by composite RD models, with the intensity of the ionizing radiation FH decreasing by more than two orders of magnitude from the center towards the outskirts of the nuclear region. Focusing on [OIII]/H¦Â, RD clouds (Fig. 2 a) with velocities of 200-300 km s?1 contribute preferentially in the inner 1 arcsec region, while those with velocities of 100 km s?1 appear at larger distances from the center, in agreement with Winge et al. (1999). The contribution to [OIII]/H¦Â line ratios from RD clouds with Vs ? 500 km s?1 is small. Regarding the SD clouds (Fig. 2 b and c), the high velocity ones (Vs > 500 km s?1 ) are responsible for only a few percent of the central region emission, while a larger

Fig. 5. The distribution of the electron temperature and of the ionic fractional abundance of oxygen downstream for a shock-dominated model (FH =0) with Vs =700 km s?1 and n0 =700 cm?3 . The shock front is on the left.

contribution at 3¡± from the center comes from the low velocity clouds (Vs < 500 km s?1 ). The situation is markedly di?erent regarding [OII]/H¦Â (Figs. 2 d, e, and f) as it appears dominated by SD models. The trend of [OII]/H¦Â is nicely explained by SD models with Vs between ? 400 and 700 km s?1 (thick line). Recall that the contribution of high velocity clouds to the [O II]/H¦Â line ratio is due to the di?use radiation generated at the high temperature post-shock zone reaching the low ionization zone. In order to illustrate this, the distribution of the temperature as well as the fractional abundance of oxygen ions downstream are plotted in Fig. 5 for an SD model with Vs = 700 km s?1 . Notice, however, that explaining the [OII]/H¦Â line ratio only with SD clouds may not be acceptable in a global picture for the NLR, since RD clouds must be present and contributing to other emission lines. Indeed, the reduction by a factor of 3 is not only dictated by the ?t of the [OI]/H¦Â data (Fig. 3 b), but is consistent with a general scenario. Regarding the [OI]/H¦Â line ratio, notice that SD models overpredict the observed [O I]/H¦Â data by a factor of ? 3. This indicates that SD models with Vs ? 700 km s?1 in the central region must be taken with a lower weight in an eventual averaged model. The shift of the SD models

10

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151

dictated by the [OI]/H¦Â ratios is consistent with all the other lines. 4.2. Sulphur Lines: [SIII] and [SII] To further investigate NGC 4151 line emission we address a problem raised by Kraemer et al. (2000), namely, the overprediction of the [SIII]9069+9532 /H¦Â line ratio by photoionization models. This ratio is shown in Fig. 3. Di?erent clouds with di?erent Vs coexist in the center. The larger values can be ?tted by RD models with Vs ¡Ý 500 km s?1 . Notice, however, that RD models with Vs = 500 km s?1 and log FH ¡Ü 10.5 overpredict the data in the region beyond 1¡± at both sides of the center. Therefore, for consistency, they were taken out from all ?gures. There is no contribution from SD clouds to the [SIII]/H¦Â ratio in the central region (Fig. 3 e, f). On the other hand, a strong contribution to the [SII]/H¦Â line ratios comes from the SD clouds with rather high velocities (700 km s?1 ) in the central region, and a not negligeble contribution of SD clouds with Vs = 1000 km s?1 (Fig. 4 c), as for the case of [OII]/H¦Â. Here too the signature of the di?use radiation generated at the high temperature post-shock zone is seen. From the [SII]6717/6731 diagrams of Nelson et al (2000, Fig. 10), the gas is rather clumpy and also indicates a decrease in density with distance. The data in the outer region (> 2¡±) are well explained by RD models with Vs = 100 km s?1 and n0 = 100 cm?3 , leading to [SII] 6716/[SII] 6730 ¡Ý 1. Density downstream decreases for lower shock velocities and lower preshock densities (see Contini & Aldrovandi 1986), therefore, [SII] 6716/[SII] 6730 ¡Ý 1 is expected in the regions farther from the center. In fact, we have found that velocities generally decrease with distance from the center. However, some ratios of about 0.5 are given by Nelson et al. They are supported by the fact that some models with Vs of about 500 km s?1 also ?t the data beyond 1¡± (Figs. 4b and 4c). 4.3. The [FeVII] line Fe coronal lines are usually underpredicted by pure photoionization models. In the case of NGC 4151, [FeVII]/H¦Â is underpredicted by a factor of ? 3 by Kraemer et al. (2000) and by Alexander et al. (1999). To explain the discrepancy, an overabundance of Fe is often suggested, which is somewhat surprising since a fraction of Fe may be locked in grains, although sputtering is strong for small grains and high velocity shocks. [FeVII]/H¦Â values for P.A. = 221o are shown in Fig. 4 d, e, and f. The available data are scarce, hence further constraints to the models are really not possible. The models used to ?t the other lines are compared with the available data. As seen in Fig. 4 f high velocity models overpredict the [FeVII]/H¦Â line ratio. A better ?t is obtained reducing the weight of the high velocity models

by a factor of 2. This reduction does not change our discussion above concerning the other lines, because models with Vs =1400 km s?1 contribute by no more than 20 % to [OII]/H¦Â and [SII]/H¦Â line ratios. As we will see over the next sections, a low contribution from high velocity clouds is also needed to explain the IR lines (¡ì6) and the continuum SED (¡ì7). Notice that the [Ne V] 3426 lines, also coming from the high ionization zone in the clouds, are overestimated by high velocity models (see ¡ì5). 5. The Ne and Si infrared lines The ISO coronal line data reported by Sturm et al. (1999) are used to further constrain the models. Since the ISO aperture are of the order of several arcminutes, only the coronal lines are used. Lines from lower ionization stages may include the contribution from star forming regions in the galaxy. Nevertheless, they are included in the discussion in order to complete the data set with information from di?erent ionization stages. Two Ne coronal lines are observed : [NeVI] 7.6 and [NeV] 14.3 in addition to two other strong lines, [NeIII] 15.5, and [NeII] 12.8. Regarding Si, [Si IX] 3.94, [SiVII] 2.48, and [SiII] 34.8 are detected. From all the models plotted in Figs. 2 to 4, we select those which could better reproduce the IR line ratios. The corresponding input parameters are listed in Table 1, as well as the weights (W(Ne)) adopted for the best average model (AV). The weights are relative to that of model 2. The weights adopted for the Si lines (W(Si)) are, however, di?erent because they account for Si depletion from the gas phase (and included in grains), which is di?erent in each model. To account for the Si coronal line emission, Si/H has to be depleted by a factor of about 15. Moreover, the di?erent depletion of Si in the di?erent clouds may be indicating that the dust is not homogeneously distributed in the emitting clouds (see Kraemer et al. 2000). Notice that the weights are very di?erent for di?erent models because they must compensate the di?erence by many orders of magnitude of the line intensity ?uxes, in order to obtain similar contribution to the emission lines from di?erent models (CV01). SD and RD clouds correspond respectively to models 2, 3, 4, 6, and models 1 and 5. The observed IR emission-line ?ux as well as the ratios of the calculated to observed lines are listed in Table 2. This table is organized as follows: (a) Line ?uxes (row 1); (b) the results corresponding to the best ?t model of Alexander et al. (1999) (row 2); (c) single-cloud results (rows 3 to 8); (d) the weighted average model AV (row 9); (e) the contribution to the IR emission lines from each single-cloud model (rows 10 to 15). Actually, the line and continuum spectra must be modeled consistently, so, the results of the AV model in Table 2 are cross-checked by the results of the continuum SED in Sect. 7, until the best tuning for both is achieved.

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151 Table 1 The models for the infrared lines model Vs (km s?1 ) n0 (cm?3 ) log FH D (1017 cm) W(Ne) W(Si) 1 100 100 11 100 5.(-4) 0.114 2 300 300 100 1. 1. 3 500 300 10 6.5(-4) 1.4(-3) 4 500 300 100 7.9(-4) 1.43(-3) 5 500 300 12.7 100 2.(-6) 1.4(-4) 6 1500 300 1 2.9(-8) 1.6(-6)

11

Table 2 The infrared lines relative to the observation data [NeVI] 7.90 0.5 2.30 1.20 1.80 1.80 1.98 2.05 1.30 8.70 88.3 0.01 0.01 3.00 0.0 [NeV] 5.50 1.8 10.3 0.40 0.70 0.70 15.0 1.53 1.24 42.8 32.3 0.0 0.0 24.9 0.0 [NeIII] 20.7 0.7 2.39 0.50 6.80 6.70 27.5 0.16 1.19 10.3 42.0 0.04 0.04 47.5 0.00 [NeII] 11.8 0.5 0.017 0.90 11.2 11.0 0.92 0.01 0.92 0.09 97.7 0.08 0.09 2.05 0.0 [SiIX] 0.41 0.9 0.064 17.6 205. 199. 3.70 1.04e5. 1.44 3.04 72.7 1.21 1.18 21.8 0.06 [SiVII] 1.2 0.4 0.137 3.90 11.1 10.6 5.42 240. 0.80 11.9 29.4 0.12 0.11 58.5 0.0 [SiII] 15.6 3. 80.4 6.6 36.7 27.7 0.11 0.0 1.0 57.3 41.2 0.33 0.25 0.98 0.0 d2 /R2 1011 109 109 109 1013 109 -

?uxes (obs)1 model (calc/obs)2 model 1 (calc3 /obs) model 2 (calc/obs) model 3 (calc/obs) model 4 (calc/obs) model 5 (calc/obs) model 6 (calc/obs) model AV (calc4 /obs) model 1 % model 2 % model 3 % model 4 % model 5 % model 6 %
1

in 10?13 erg cm?2 s?1 Alexander et al. 3 calculated at the nebula 4 To calculate the averaged model from the models given in CV01 : ¦²i [(F(¦Ë)/F(H¦Â))CV 01 F(H¦Â)CV 01 )i W(model i)]/ F(¦Ë)obs
2

Because the theoretical ?uxes are calculated at the nebula and the observed ?uxes are measured at Earth, the ratio of the square distance of the galaxy to Earth (d2 ) to the square distance of the nebula to the galaxy center (R2 ) is given in the last column of Table 2. From Table 2, one sees that model 2 (SD, Vs = 300 km s?1 ) strongly contributes to all the lines. Gas is heated to temperatures between 1.3 106 K and 3.7 106 K for SD clouds with Vs of 300 km s?1 and 500 km s?1 , respectively. Radiation dominated model 1 contributes mainly to [NeV] and [SiII] and model 5 to [NeIII] and [SiVII]. Model 5 is associated with a very high ionizing ?ux: 5 1012 (cm?2 s?1 eV?1 at the Lyman limit). The high velocity clouds (model 6, Vs = 1500 km s?1 ) do have a low contribution. This contribution must remain low as to prevent the increase of [SiIX] ?ux far beyond its observational value.

6. The UV line ratios Ultraviolet lines may provide important information about their origin in a gas either heated by shocks or ionized by a strong radiation. The most signi?cant lines are NV 1240, CIV 1550, and HeII 1640. Other strong UV lines are generally blended (OIV, SiIV 1402, CIII], SiIII] 1909, etc), and have not been used in the modeling. Because the reddening correction in the UV is important, we use the NV/CIV and HeII/CIV line ratios instead of intensities relative to H¦Â. The UV data correspond to P.A.= 221o observations. Model results are presented in Fig. 6 where South-West (SW) data (?lled triangles) are separated from those form the North-East (NE) region (open triangles). The theoretical results indicate that the NE UV data can be explained by low velocity RD clouds (< 300 km s?1 ) reached by an ionizing ?ux FH < 1011 units, while

12

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151

7. The continuum To further constrain our model for NGC 4151, the continuum SED is analysed and modeled using the composite model derived from the Ne and Si lines in ¡ì5. Observations Gamma-rays to the optical continuum emission is taken from the 1993 December multiwavelength monitoring campaign of NGC 4151 (Edelson et al. 1996). The source was near its peak historical brightness during this campaign and showed the strongest variations of its continuum emission at medium energy X-rays (?1.5 keV) with amplitude variations of 24%, weaker variations (6%) at the gamma-ray energies, decreasing variation from ultraviolet (9%) to optical (1%) and ?nally, not signi?cant variation at the soft X-rays (0.1-1 keV). Assuming that the continuum emission variations are within a few percent level beyond optical wavelengths, the 1993 data are combined with IR data taken at di?erent epochs. Since the optical aperture used by Edelson et al. (1996) is 12x3 arcsec, near -IR ?uxes from an aperture size of 10 arcsec is adopted when possible. Sources are as follows: The data for the near-IR come from the NASA Extragalactic Data (NED), J, H and K data are taken from Balzano & Weedman (1981) and L and M data from McAlary et al (1979). The 10 microns emission is from Lebofsky & Rieke (1979) and corresponds to an aperture size of 6 arcsecs. In addition, to trace the non-stellar contribution at the near-IR, Kotilanien¡¯s et al. (1992) data within a 3 arcsec aperture are also considered for comparative purposes. For the far-IR region between 16 and 200 microns, ISOPHOT data from Perez-Garcia et al. (1998) are taken. Due to the large aperture size used in ISOPHOT, those data are integrated over the complete galaxy. For the radio data, values at 1.4 GHz, 4.85 GHz, and 408 MHz were at the NED, and come from Becker et al. (1995), Becker et al. (1991), and Ficarra, Grue? & Tommasetti (1985), respectively, while data at 8.4 and 5 GHz are taken from Pedlar et al. (1993). Compared with other Seyfert galaxies, NGC 4151 is relatively weak in X-rays with L(2-10 keV) ? 7 1042 erg s?1 (Weaver et al. 1994). The soft X-ray emission is extended (Morse et al. 1995). Recent Chandra data resolve up to ? 70 % of the 0.4-2.5 keV emission (Ogle et al. 2000). This emission appears to be associated with the optical narrow line gas (NLR) extending asymmetrically to the South-West of the nucleus. On the nature of the soft X-ray emission Both, Weaver et al. (1994) and Ogle et al.(2000) provide similar estimates for the plasma pressure of the hot gas of about 3 ¡Á 107 cm?3 K. This is one order of magnitude larger than the pressure derived from the cold NLR

Fig. 6. He II 1640/C IV 1550 versus N V 1240/C IV 1550. SW UV data and NE UV data correspond, repectively, ?lled and empty triangles. The RD results for Vs = 100 km s?1 and n0 =100 cm?3 (dotted line), and Vs =200 km s?1 and n0 =200 cm?3 (dash-dotted line) are labelled by the values of log(FH ), and the SD results (solid line) by Vs in km s?1 .

the SW UV data come from the RD clouds reached by a stronger radiation ?eld, with some contribution from SD clouds with Vs > 500 km s?1 . However, these results are only indicative (see the discussion by Kraemer et al. 2000). It is well known that the HeII 1640 line is strongly dependent on the spectral index of the ionizing radiation, and the NV/CIV ratio depends on the adopted N/C abundance ratio. Regarding the HeII lines, if ¦ÁUV < 1.5 the theoretical results may vertically shift to the upper part of the diagram, and the derived velocities would be larger. Regarding the N/C abundance ratio, the cosmic values is adopted. For consistency, we present in Table 3 the observed/calculated UV-optical line ratios obtained with the same AV model as for the IR lines (see Table 2). To avoid the uncertainty of the dilution factor the ratios of the AV model are calculated between the line ratios to H¦Â. The ?t is within 1.6 and the trend is always to overestimate the values. This is consistent with reduction of the SD models which was adopted in Figs. 2-4 b, c, and e, f .

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151 Table 3 The UV-optical lines

13

?uxes (obs)1 model (calc/obs)2 model (calc/obs)3 model AV (calc/obs)
1 2 3

[NeIV]2423 1.77 0.4 1.3

[NeV]3426+ 1.56 1.5 1.5

[NeIII]3869+ 1.81 0.8 0.5 1.5

HeII 4686 0.31 1.1 1.1 1.1

[OIII]4363 0.35 0.7 1.6

[NII]6584+ 2.16 0.8 1.8 1.4

in 10?15 erg cm?2 s?1 (Kraemer et al 2000, Table 1, 0¡±.1-0¡±.3) Kraemer et al. (2000, Table 1, 0¡±.1-0¡±.3) Alexander et al. (1999)

gas (Penston et al. 1990). The di?erence strongly argues against the soft X-ray emission being associated with the NLR con?ning medium (Weaver et al. 1994, Ogle et al. 2000). Weaver et al. (1994) also found that the Fe K edge energy in their 0.4 - 11 keV data (from the BBXRT mission) is inconsistent with an origin in a gas with the same ionization parameter as the low-energy absorber. This allows the authors to rule out a line-of-sight ionized absorber as the sole source of the soft X-ray excess in NGC 4151 (see also Contini & Viegas 1999 for the case of NGC 4051). George et al. (1998) propose a model for the 0.2-10 keV spectrum where the underlying power law nuclear component is partially absorbed by an ionized absorber and partially scattered. However, it requires an additional component due to Bremsstrahlung thermal emission from an extended photoionized gas at T? 6 106 K. Recent Chandra data provide direct evidence of X-ray line emission gas at T ? 107 K. The strength and ionization potential of the X-ray narrow emission lines indicate a composite spectrum in which both photoionization and shocks are at work (Ogle et al. 2000). The fair spatial association between the central NLR optical region and the soft X-ray emission indicated that both mechanisms are contributing to the multi-wavelength SED and to the line spectrum of NGC 4151 (cfr. Komossa 2001). Taking into account these observational constraints, the origin of this soft X-ray excess is evaluated in the next section assuming high velocity models dominated by shocks. Model Results NGC 4151 shows the characteristic of Seyfert galaxies rather than starbursts. Therefore, we consider that the continuum SED is Bremsstrahlung radiation from clouds ionized and heated by the radiation from the AC and by shocks. We refer to previous studies (e.g Contini & ViegasAldrovandi 1990, Contini & Viegas 1991, Viegas & Contini 1994, Contini & Viegas 2000, etc) on this subject which could lead to a better understanding of the results. Particularly, dust and gas are coupled entering the shock front and mutually heat each other. The grains are collisionally heated to the highest temperatures (¡Ý 300 K) leading to emission in the near-IR. On the other hand, heating by

the central radiation is not e?cient enough, so dust in the radiation dominated zone does not reach such high temperatures. Before discussing model results, let us notice that below the Lyman limit, up to say 0.2 keV, we have very little observational information: this is the ¡±unknown window¡± where observations are di?cult because of the heavy absorption by our Galaxy. However, in the 0.2 to 2 keV region, NGC 4151 shows extended emission, spatially correlated with the optical NLR gas. If due to Bremsstrahlung, the tail of this X-ray emission should somehow show up in the far UV. However, no detected extended emission appears in the UV HST images of NGC 4151 (Boksenberg et al. 1995). Therefore, the emission from the NLR gas should be lower than that of the UV nucleus of NGC 4151. We will use this observational fact as a constraint in the proposed modeling in the sense that the observed nuclear UV emission can only be associated with model emission within a nuclear region of outmost 3 pc size. Observational data and model results are plotted in Figs. 7 and 8, which refer to models and relative weights adopted to ?t the Ne infrared lines (Table 1, row 5). Notice that Bremsstrahlung peaks at high frequencies depending on Vs (see Viegas & Contini 1994). The large geometrical thickness of the emitting clouds and the high densities downstream due to compression (n/n0 ¡Ý 10, depending on Vs , n0 , and B0 ) lead to high optical thickness of the emitting gas with column densities of the order of 1021 -1024 cm?2 . This is within the range of column densities estimated from the X-ray data, ? 5 1022 cm?2 (Weaver et al. 1994, George et al. 1998, and Ogle et al. 2000). Accordingly, the emitted radiation gets absorbed in the cloud itself between ? 13.6 eV and 500 eV, hence the emission gap in this region shows up in Figs. 7 and 8. The high velocity model 6 has however a lower column density (< 1021 cm?2 ) because is radiation-bounded. In fact, due to strong compression, the temperature rapidly decreases downstream at a distance < 1016 cm from the shock front. Thus, the contribution of model 6 to the SED extends into the far UV range (Fig. 7 long-dashed line). However, the X-ray data in the frequency range 1015 and 1017 Hz impose a limit on the relative constribution of this

14

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151

Fig. 7. Di?erent components contributing to the SED of the continuum of NGC 4151, corresponding to the results of several models, scaled following the weights used to ?t the IR lines. The models are the following: 1 (dash-dotted line), 2 (dotted line), 3 (short dash-dotted line),4 (shortdashed line), 5 (solid line), 6 (long dashed line). The ?ux from the AC in the far X-rays, which is seen through the clouds, is represented by a thick dotted line, while the continuum from the old stellar population is indicated by a thick long dash-dotted line. The theoretical results are compared to observational data taken from the NED (?lled squares) and from other sources described in Sect. 7 (open squares).

Fig. 8. The continuum spectrum of NGC 4151. The thin solid lines correspond to the summed theoretical contributions from dust emission, gas Bremsstrahlung and radio emission as shown in Fig. 7. The thick solid line corresponds to the summed contributions from dust emission, gas Bremsstrahlung and the old stellar population continuum, while the dotted line corresponds to the AC continuum seen through the clouds. The observational data are taken from the NED (?lled squares) and from other sources described in Sect. 7 (open squares). gion of a Sa galaxy to get a more realistic ?t of the stellar contribution, it would not change our conclusions. For instance, Malkan & Filippenko (1983) ?nd that about 1/10 of the emission at 5500 ?and ? 1/3 at 9000 A ?are due to stars (in aperture of 10¡±, just the same aperA ture of the optical and IR data used in this paper), namely they estimated the stellar emission to be 14 mJy at 5500 ?. We have added the corresponding value to Fig. 7 (star). A Accounting for the errors in the observations and models, notice the good agreement (within 30%) with the black body emission curve. The bump in the IR is rather wide, representing the sum of contributions from di?erent models. In previous papers (e.g. Contini et al. 1999b, Appendix A) it was explained that the bump in the IR due to each model depends on the shock velocity. Particularly, the high velocity model (model 6) has its maximum at the near-IR. To ?t the near-IR data, a high d/g (2 10?12 ) is assumed in model 6, indicating that high velocity material is rather dusty. A shock velocity of 1500 km s?1 produces a post shock region at a temperature of 3.4 107 K, in agreement with

model 6 (see below). All together, the ?t of the continuum at the UV range is obtained with model 2. The dust-togas ratio that characterizes this model is d/g = 10?14 . The other models (models 3, 4, 5, and 6) underpredict the data. The summed SED which better explains the observations is shown in Fig. 8. The main components are summed separately: radio emission, reradiation by dust in the IR, gas Bremsstrahlung, and the ?ux from the active source. Moreover, as was found for many Seyfert galaxies (see Contini & Viegas 2000) a black-body with temperature of ? 3000 K is used to roughly represent the optical continuum due to the stellar population (thick solid line). Notice that we are interested in the bulk contribution of the stars rather than the detailed shape of their continuum emission. Thus, although we could use the spectrum of an early-type galaxy or the spectrum of the central re-

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151

15

the hot plasma temperatures derived from the X-ray data. indicating that high velocity material is rather dusty. Shocked clouds with Vs ? 700 km s?1 produce lower temperatures, ? 106 K, which are still within the range of values estimated from X-ray data (e.g. George et al. 1998). These velocities are also revealed by optical lines (¡ì4). Yet, the contribution of a model with Vs = 1500 km s?1 in the SED should be taken with a relative small weight (¡Ü 2.9 10?8 ), because the data in the soft X-ray range constrain the model. As a consequence, the contribution of this model to the IR lines is very low. Thus, a signature of these high velocity clouds is not expected to be seen in the infrared emission-line spectrum. In the radio range, Pedlar et al. (1993) derive an average spectral index for the entire source of ¦Á = -0.87 from their 8.4 and 5 GHz data. They interpreted the emission as due to thermal free-free emission from the NLR. In the present modeling, the radio data are instead well explained by synchrotron radiation with spectral index -0.75, generated by Fermi mechanism at the shock front (Bell 1978). Notice that self-absorption of the Bremsstrahlung emission at these radio wavelengths may be important, hence its contribution is negligeble (see, for instance Contini et al 1998b). 8. Discussion and Conclusions In this paper we have modeled the narrow emission-line and continuum spectra of NGC 4151 with particular attention to the large range of velocities indicated by the line pro?les. The analysis of the line pro?les is complex and clouds in many di?erent conditions can contribute to each line. By modeling the NLR to analyse the high spatial resolution data (Nelson et al. 2000, Kraemer et al. 2000), it is found that the contribution of each cloud to a given line may show a large variation from line to line. In the central region high velocity clouds revealed by the observations (Hutchings et al. 1999, Winge et al. 1997, etc) are shock dominated (SD). There is a strong contribution of SD clouds with shock velocity Vs ? 700 km s?1 to [OII] 3727/H¦Â, and [OI] 6300/H¦Â. Radiation dominated (RD) clouds with Vs ? 500 km s?1 are necessary to explain [SIII]/H¦Â and [OI]/H¦Â, while clouds with low Vs (100-200 km s?1 ) and reached by a relatively strong ionizing radiation (log FH ? 11) contribute to the [OIII]/H¦Â, [OII]/H¦Â, and [SII]/H¦Â line ratios. Beyond 2¡± from the center, RD clouds, photoionized by a weaker radiation ?ux are responsible for the observed line emission. Quantitative modeling of the Ne and Si infrared lines from di?erent ionization levels leads to more results. Notice, however, that the observations give the integrated

?ux from all the galaxy, so that it is not possible to understand which type of clouds prevail in di?erent regions. SD clouds with Vs = 300 km s?1 strongly contribute to all Ne lines corresponding to di?erent ionization levels (Table 2), while RD clouds with velocities of 500 km s?1 and reached by a strong radiation ?ux (log FH = 12.7) contribute particularly to the [NeIII] and [SiVII] IR lines. Due to the high postshock temperature, high velocity clouds (Vs = 1500 km s?1 ) would overpredict the [SiIX] 3.94 line. Therefore, the contribution of these clouds is low. A large contribution to [NeV] and [SiII] lines come from low velocity (100 km s?1 ) clouds reached by an ionizing radiation characterized by log FH = 11. Modeling of the IR lines also shows that silicon is depleted by a factor of 15 because included in dust grains, while N/C abundance ratio is found compatible with cosmic values from the modeling of the UV lines. Comparing the results of the present work with those of other authors, notice that our results are slightly better. Indeed, better ?ts could be obtained with a di?erent choice of weights. However, the results presented in Tables 2 and 3 are consistent with the ?t of the continuum SED (¡ì7) and were chosen accordingly. More particularly, it seems that the three Ne emission lines are higher than observed by factors of 1.3 - 1.5. This may be an indication that the Ne abundance we have used is a factor of 1.3-1.5 too high. So, decreasing the Ne/H abundance by 1.3, we get a better agreement. The HeII 4686 line is slightly higher, but this is mainly due to the power law above 54.4 eV, which we (and the others) could have taken somehow too ?at. The only problem is the [OIII]4363 line. Because oxygen is a coolant, changing the O abundance may not solve the problem. However, a factor of 1.6 higher is still within the limit of a factor of 2 proposed by Alexander et al., who do not give their results for [OIII] 4363. Notice that we based our models on IR lines and we compared with Alexander et al. (1999). So, we should refer only to them. The results of Kraemer et al in row 2 of Table 3 are better than ours only for [OIII] 4363 and [NeIII]. Actually, they come from observations in the region between 0.1¡± and 0.3¡± SW, whereas we average on all the regions. The analysis of the continuum SED leads to the following results: The high velocity material is very dusty (d/g? 10?12 ). The radio emission is synchrotron created by Fermi mechanism at the shock front. Moreover, shocks are important to explain the soft X-rays: shocks with velocities of 1500 km s?1 produce a post shock region at a temperature of 3.4 107 K. This value is in agreement with the temperature found by Weaver et al. (1994) and Ogle et al. (2000) for a non-equilibrium plasma from X-ray observations, while clouds with Vs ? 750 km s?1 produce temperatures of 8. 106 K (George et al. 1998).

16

M. Contini, S.M. Viegas, and M.A. Prieto: High velocity gas in NGC 4151 Contini, M., Prieto, M.A., & Viegas, S.M. 1999b, ApJ, 505, 621 Crenshaw, D.M., Kraemer, S. B., Hutchings, J. B., Bradley, L. D., II, Gull, T. R., Kaiser, M. E., Nelson, C. H., Ruiz, J. R.& Weistrop, D. 2000, AJ, 120, 1731 Ficarra, A., Grue?, G. & Tommasetti, G 1985, A&AS 59, 255 George,I.M., Turner, T.J., Netzer, H., Nandra,K., Mushotzky, R.F., & Yaqoob, T. 1998, ApJS, 114, 73 Hutchings, J.B. et al 1998, ApJ, 492, L115 Hutchings, J.B. et al 1999, AJ, 118, 210 Kaiser. M.E. et al. 2000, ApJ, 528, 260 Komossa, S. 2001, A&A, 371, 507 Kotilanien et al, 1992, MNRAS 256, 149 Kraemer, S.B., Crenshaw, D. M., Hutchings, J. B., Gull, T. R., Kaiser, M. E., Nelson, C. H.& Weistrop, D. 2000, ApJ, 531, 278 Lebofsky, M. J., & Rieke, G. H. 1979, ApJ 229, 111 Malkan, M.A. & Filippenko, A.V. 1983, ApJ 275, 477 McAlary, C.W., McLaren,R.A., and Crabtree, D.R. 1979, ApJ, 234, 471 Morse, J., Wilson, A. Martin, E. & Weaver, K. 1995, ApJ 439, 121 Nelson, C.H., Weistrop, D., Hutchings, J. B., Crenshaw, D. M., Gull, T. R., Kaiser, M. E., Kraemer, S. B.& Lindler, D. 2000, ApJ, 531, 257 Ogle, P. Marshall, H. L., Lee, J. C. & Canizares, C., 2000, ApJ, 545, L810 Osterbrock, D. & Koski, A. 1976, MNRAS, 176, P61 Pedlar, A. et al. 1993, MNRAS, 263, 471 Penston, M.V. & P?rez, E. 1984, MNRAS, 211, 33p e Perez-Garcia et al, 1998, ApJ 500, 685 Rieke, G.H. and Low, F.J. 1972, ApJ, 176, L95 Schulz, H.R. 1988, A&A, 203, 233 Schulz, H.R. 1990, AJ, 99, 1442 Sturm, E. et al. 1999, ApJ, 512, 197 Ulrich, M.H. 2000, The Astronomy & Astrophysics Review, 2000, 7 Viegas, S.M. & Contini, M. 1994, ApJ, 428, 113 Weaver, K.A. et al. 1994, ApJ, 423, 621 Weyman, R.J., Morris, S.L., Gray, M.E., Hutchings, J.B. 1997, 483, 717 Winge, C., Axon, D.J., Macchetto, F.D., & Capetti, A. 1997, ApJ, 487, Winge, C., Axon, D.J., Macchetto, F.D., Capetti, A., & Marconi, A. 1999, ApJ, 519, 324

An estimate of several physical quantities applying to the central AGN region can be derived from the proposed modeling. If d is the distance from Earth (19.8 Mpc) and d2 /R2 = 109 (see Table 2), R ? 0.626 kpc is the average distance of the emitting nebula to the AC. Adopting an average downstream density n=105 cm?3 , and D=1018 ? 1019 cm, the calculated mass is M ? 3.5 108?9 ? M¡Ñ , where ? is the ?lling factor which is likely to be less than unity. In this case our estimation of the NLR mass is lower than the values quoted by Ulrich (2000) for an emitting region closer to the center, i.e. 109 M¡Ñ within 40 pc and 5 107 M¡Ñ within 12 pc (0.15¡±). The corresponding calculated central source luminosity is about 1.6 1043 erg s?1 . The average kinetic energy of the NLR is about 3 1056 ? ergs, assuming an average velocity of the NLR clouds of 300 km s?1 . This would imply that the time scale for the AGN phase in NGC 4151 should be larger than 5 105 years. Summarizing, we provide a self-consistent modeling of the multiwavelength line spectrum and SED of the nuclear region of NGC 4151. The modeling is based on the coupled e?ect of shocks and photoionization operating in the narrow emission line gas. As such, it implies the existence of emitting clouds with velocities and densities in a large range. An emittingcloud distribution in the velocity/density space is precisely what is revealed by the HST observations of this galaxy. This together with the fair ?tting of the line and continuum spectra obtained in this work reinforces our hypothesis that shocks and photoionization are e?ectively coupled in the NLR of AGN.
Acknowledgements. We are grateful to the referee for many helpful comments. This paper is partially supported by the Brazilian agencies: CNPq (304077/77-1), PRONEX/Finep (41.96.0908.00), and FAPESP (00/06695-0).

References
Alexander, T., Sturm, E., Lutz, D., Sternberg, A., Netzer, H. & Genzel, R. 1999, ApJ, 512,204 Balzano, V.A., Weedman, D.W. 1981, ApJ 243, 756 Becker, R., White, R. & Helfand, D. 1995, ApJ, 450, 559 Becker, R., White, R. & Edwards, A. 1991, ApJS, 75, 1 Bell, A.R. 1978, MNRAS, 182, 443 Boksenberg, A., Catchpole, R. M., Macchetto, F., Albrecht, R., Barbieri, C., Blades, J. C., Crane, P., Deharveng, J. M., Disney, M. J., Jakobsen, P., Kampermann, T. M., King, I. R., Mackay, C. D., Paresce, F., Weigelt, G., Baxter, D., Green?eld, P., Jedrzejewski, R., Nota, A.& Sparks, W. B. 1995, ApJ 440, 151 Contini, M. & Aldrovandi, S.M.V. 1986, A&A,168,41 Contini, M. & Viegas-Aldrovandi, S.M. 1990, ApJ, 350,125 Contini, M. & Viegas, S.M. 1991, ApJ, 373, 405 Contini, M. & Viegas, S.M. 1999, ApJ, 523, 114 Contini, M. & Viegas, S.M. 2000, ApJ, 535, 721 Contini, M. & Viegas, S.M. 2001, ApJS, 132, 211 (CV01) Contini, M., Prieto, M.A., & Viegas, S.M. 1999a, ApJ, 492, 511

¸ü¶àÏà¹ØÎÄµµ:

High Velocity Gas in the Halos of Spiral Galaxies_Í¼ÎÄ.pdf

High Velocity Gas in the Halos of Spiral ...velocity diagram along the major axis of NGC ...The signature of high ... ÔÝÎÞÆÀ¼Û 16Ò³ Ãâ·Ñ...

...in the XMM-Newton spectrum of NGC 4151....pdf

NGC 4151 in a substantial (NH ? 1023 cm?2 ) line-of-sight gas column...present in the very high signal-to-noise EPIC spectra of NGC 4151. Our...

The Narrow Line Region of NGC 4151 a Turbulent Caul....pdf

The Narrow Line Region of NGC 4151 a Turbulent ...High velocity components shifted up to ? 1500 km...of gas in the galactic disk by an anisotropic ...

The high velocity outflow in NGC6334I_Í¼ÎÄ.pdf

The high velocity outflow in NGC6334I_×¨Òµ×ÊÁÏ...of deriving the physical parameters of the gas. ...To reduce the noise level in the spectra, we ...

...of the optical spectra of NGC 4151 I. Light curv....pdf

Long-term variability of the optical spectra of NGC 4151 I. Light curves ...Methods. High quality spectra (S/N> 50 in the continuum near H¦Á and ...

The nature of the X-ray source in NGC 4151.pdf

The nature of the X-ray source in NGC 4151_×¨Òµ×ÊÁÏ¡£Analysis of broad-band X/$\gamma$-ray spectra of NGC~4151 from contemporaneous observations by ...

Soft X-ray Spectroscopy of Seyfert 2 Galaxies.pdf

collisionally-driven gas contribution to the soft ...in the helium-like triplets of oxygen and ...Figure 4. Chandra HETG spectra of NGC 4151 and...

...in the Seyfert Galaxy NGC 4151.pdf

Gas Dynamics in the Barr... ÔÝÎÞÆÀ¼Û 13Ò³ Ãâ·Ñ...NGC 4151 based on the high-resolution (R ? 45...the velocity shifts from the recession velocity of...

Shocked molecular hydrogen in the bipolar outflow NGC.pdf

molecular hydrogen in the bipolar outflow NGC_...from, the lobes of high{velocity CO line ...Spectra of the 1{0 S(1), 1{0 S(0) and ...

...the ionized gas in the nuclear region of NGC 421....pdf

Kinematical analysis of the ionized gas in the nuclear region of NGC 4214 ...the spatial variation of different properties (intensity, velocity and width ...

...Model For The Soft X-Ray Spectrum Of NGC 4151_Ãâ....pdf

(2005) in their study of NGC 4151 absorption spectra. Subject headings: ...of the nuclear continuum in the soft Xray spectrum (Kraemer et al. 2005)...

High velocity HI in NGC 6946 and extra-planar gas i....pdf

In the outer parts of NGC 6946 there is also evidence for recent gas ...The signature of high ... ÔÝÎÞÆÀ¼Û 16Ò³ Ãâ·Ñ Measurement and CFD Pr.....

AU Scale Structures in Extra-planar Gas.pdf

of extra-planar gas in the halo of the Milky ...high-velocity clouds - neutral gas clouds embedded...NGC 4151 NGC 3310 HD 93521 PG 1116+215 HD ...

High Resolution X-ray Spectroscopy of Seyfert 2 Gal....pdf

rst high resolution spectra of several of the ...erent from those formed in photoionized gas (Lie...NGC 4151 (Ogle et al. 2000), and the Circinus...

...CO emission from shock-heated gas in the L1157 o....pdf

2 spectra from the blueshifted gas of the ...high-velocity gas in the blue lobe of the L...and NGC7538 IRS1, and was better than 3¡ä¡ä ...

The complex Halpha line profile of the bright compa....pdf

in the Galactic Globular Cluster NGC 6397), exploiting a series of high resolution spectra o....gas resides outside the Roche lobe of COM J...

ORFEUS II echelle spectra The scale height of inter....pdf

NGC 4526 gas, High Veloc... ÔÝÎÞÆÀ¼Û 10Ò³ Ãâ·Ñ...ORFEUS II echelle spectra The scale height of interstellar...3.3.1. LMC stars In the spectra of H...

...in the hard X-ray XMM-Newton spectrum of NGC 415....pdf

Iron K features in the hard X-ray XMM-Newton spectrum of NGC 4151 Recent...2 ) line-of-sight gas column density (Holt et al. 1990; Yaqoob et al...

...in the Antennae Galaxies (NGC 40389) I. High-Res....pdf

Feedback in the Antennae Galaxies (NGC 40389) I...(R ~ 24,600) near-IR spectroscopy of the ...This high-velocity unbound gas is ?owing out in...

...with a central plateau in the gas velocity curve....pdf

in the gas velocity curve along the major axis?...This is also the case of NGC 4178 where the ...the kinematic signature of an inner polar disk (...