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Up: Optical colour maps

5. Results

There is a clear difference between the direct images obtained through the various filters. The I band images display a smooth distribution of mainly evolved red stars, whereas the B and V images reveal the presence of absorbing dust, H II regions and young stars. In the B-I maps of most galaxies, we detect clumpy, ring-like or elongated blue or red structures. Next we present as b/w figures (Plates 1-22) for each galaxy a blue (B band, except V band for NGC 7469) direct image for morphological information and various colour maps.

NGC 1068

The Sb type Seyfert 2 galaxy NGC 1068 is the nearest object displaying both AGN and starburst activity, with 1'' corresponding to a projected size of 110 pc. In Plate 1, we show its B band image. The nucleus exhibits a Seyfert 2 optical spectrum in direct light, but spectropolarimetry has revealed a broad, polarized component to the narrow emission lines (e.g.\ Htex2html_wrap_inline2775), indicating the presence of an obscured Seyfert 1 nucleus (Antonucci & Miller 1985; Miller et al. 1991). UV, optical and near-IR (NIR) observations of the bright inner (tex2html_wrap_inline277715'') disk indicate a young stellar spectrum with strong Balmer absorption and emission lines, suggesting a large population of B type stars and their associated H II regions, typical of a starburst (e.g. Bruhweiler et al. 1991). NGC 1068 is extraordinarily rich in molecular gas, with an estimated total Htex2html_wrap_inline2781 mass of tex2html_wrap_inline2783 tex2html_wrap2791  (Planesas et al. 1991). The MIR and FIR emission is extended (Braatz et al.\ 1993; Cameron et al. 1993) indicating that the IR emission originates in dust not solely heated by the AGN. The complex linear radio source of tex2html_wrap_inline27877'' in extent has two compact jets on opposite sides of the nucleus (, Wilson & Ulvestad 1983, 1987). The NLR is cospatial with the radio emission and displays an outflow in the direction of the jets, driven by interaction with radio plasma (Baldwin et al. 1987), characteristic of AGN activity. Both the NLR (Pogge 1989) and the ENLR (Unger et al. 1992) share a conical shape aligned with the radio axis, indicative of photoionization by a nuclear radiation field collimated by the torus. A ROSAT  X-ray image of NGC 1068 (Wilson et al. 1992) reveals, in addition to a compact central source, larger scale emission, which is coincident with the outer edges of the optical disk and is probably due to SNRs, X-ray binaries or a starburst driven wind, and not to electron scattering of the hidden Seyfert 1 nucleus.

The primary aim of this work is to use the colour maps to investigate the properties of the nucleus, and circumnuclear regions on kiloparsec scales. However, most of the host galaxy is also imaged, so we shall begin by briefly commenting on the large scale structures.

Large scale structures in the B-I map

There are two previous studies of the large scale colour properties of NGC 1068. Schild et al. (1985) obtained CCD images in B, V and R. Ichikawa et al. (1987) published ratios of photographic images, based on plates taken using B, V, and i filters. Our large scale B-I (tex2html_wrap_inline2809; tex2html_wrap_inline2811) map is shown in Plate 2. Our map agrees well with the B-R image of Schild et al. excluding regions within 5'' of the nucleus. The oval distribution of H II regions, centred on the nucleus and with major axis about 30'', is clearly seen in Plate 2. We see much more detail in the structure of the H II regions than Schild et al. because of our deeper imaging and better seeing.

There is an arc of blue resolved features NE-SW through the nucleus roughly parallel to the radio jet axis. These knots (labelled by Schild et al. 1985) are known to be related to sites containing luminous young stars (e.g. Bruhweiler et al. 1991; Truong & Bruhweiler 1991). All the knots have similar spectra, so they should have comparable star formation ages. Our images cover the knots 2 to 4 (see Schild et al.). The colours of these knots within a 3'' aperture are listed in Table 3. As for all the galaxies in Table 3, the colours include contribution both from the knots and the underlying old redder population. Therefore, the real colours of the knots must be even bluer than those reported here. The colours are in good agreement with Schild et al. (who used a 7'' aperture), and confirm that they are sites of recent star formation. Knot 4 is extremely blue and could be a very recent star formation site, as supported by its Wolf-Rayet type optical spectrum (Bruhweiler et al. 1991).

Table 3: Colours of the maxima

The excellent correspondence of the colour properties of large scale structures, gives us confidence concerning the procedures used to produce our maps. We now turn to interpretation of the nuclear features.

Small scale structures in the B-I map

Unlike the previous work mentioned above, our data can be used to study details on scales of a few arcsecs. For this purpose we show the B-I map of the innermost tex2html_wrap_inline2835 (tex2html_wrap_inline2837) in Plate 3. In the nuclear region on scales less than 10'', there is an elongated blue structure, extending along PA 40tex2html_wrap2875  for about 6'' (660 pc). It peaks tex2html_wrap_inline2845 NE of the nucleus and has similar colours to the average of the H II region knots (Table 3), except B-V, which is redder. These colours are much bluer than a typical B-I index for a spiral bulge of T = 2 (tex2html_wrap_inline2853), but redder than the average B-I index for Seyfert 1 galaxies (tex2html_wrap_inline2857). It is likely that the blue elongation is caused by scattered light from the nucleus of NGC 1068, which is superimposed on the stellar population of the nuclear region, which is redder. Therefore the observed colour index is an upper limit to the blueness of the nuclear source whose radiation is scattered.

Pogge & De Robertis (1993) presented narrow band emission line and continuum imaging observations of NGC 1068. In the innermost region of their ``UV/Red'' (3600/6100 Å) colour map, they find a number of blue features. The bluest region is offset from the nucleus by tex2html_wrap_inline28590.8'' in PA tex2html_wrap_inline286333tex2html_wrap2877.  The blue elongation is along the radio axis and roughly coincident with the peak of the [OIII] cone structure (Evans et al. 1991), radio continuum NE peak (Wilson & Ulvestad 1983) and the 10 tex2html_wrap_inline2867m emission peak (Tresch-Fienberg et al. 1987). Additionally, there is an extended, diffuse UV excess continuum component with a conical morphology. More complex situation is revealed by the recent HST UV and optical imaging of NGC 1068 (Macchetto et al. 1994) who confirm the presence of the conical blue structure in their UV/U continuum ratio map extending over 4'' with quite uniform colour. The UV peak is displaced by 0.05'' to north of the [OIII] peak and the UV continuum region envelopes the [OIII] emitting region. Intriguingly, they found a ``twin-crescent'' object SW of the cone. This object is highly polarized (tex2html_wrap_inline287345%) and lies at the center of the overall polarisation pattern of the circumnuclear region and at the apex of the cones (, Capetti et al. 1995a,c). This structure probably straddles the real nucleus, but is not the torus, because it is misoriented with respect to the cones.

The morphology in Plate 3 is in good agreement with the structure observed by Pogge & De Robertis (1993) and Macchetto et al. (1994) and with the structure in the imaging polarimetry of NGC 1068 by Miller et al. (1991) and , Capetti et al. (1995a,c). We shall discuss the origin of this elongation in more detail, in conjunction with other galaxies, in the context of scattering of the nuclear light, in Sect. 6.2.

NGC 3227

The SAB pec galaxy NGC 3227 (a B band image is shown in Plate 4) is interacting with a spheroidal companion NGC 3226. NGC 3227 has Seyfert type 1 spectrum (Salamanca et al. 1994). but NGC 3226 shows no signs of activity, either AGN or star formation. The system is probably in the postcollision phase (Taniguchi et al.\ 1990). The X-ray luminosity of NGC 3227 is low for a Seyfert 1 galaxy (Ward et al. 1987). It has a substantial amount of molecular hydrogen emission, based on NIR spectroscopy (Fischer et al. 1987) as well as strong millimeter CO emission (Blitz et al. 1986; Meixner et al.\ 1990). The resolved CO distribution has two intensity peaks straddling the nucleus, aligned roughly E-W and separated by 2'' (Meixner et al.\). A more extended component (with a diameter 17'') lies SE-NW along a 30tex2html_wrap2912  angle to the galactic major axis, the NW part is the site of most of the emission. In the maps of NGC 3227, 1'' corresponds to 100 pc.

Our full scale (tex2html_wrap_inline2890; tex2html_wrap_inline2892) B-I map (Plate 5) shows a wealth of structure. Especially, there is a band of light (blue) colour, roughly in a shape of Z delineating the nucleus, indicating the geometry of the dust absorption in the galaxy. This geometry agrees well with that seen in the H I 21 cm absorption map by Mundell et al. (1995).

In Plate 6, we show the B-I image of the innermost tex2html_wrap_inline2898 (tex2html_wrap_inline2900) of NGC 3227. The colour map shows two blue maxima on either side of the nucleus (NW and SE) with a separation tex2html_wrap_inline29022''.

Star forming regions are known to be associated with dense molecular clouds and dust. CO mapping indicates the presence of molecular clouds and hence the material available for star formation (Heckman et al. 1989). Therefore, a correlation between the CO distribution and the colour maps would be expected if the blue maxima are caused by recent star formation. Indeed, we find a good correspondence between the CO map (Meixner et al. 1990) and our B-I map (Plate 6), where two blue maxima are coincident with the CO emission peaks. The agreement is especially good for the SE maximum.

However, there is no direct evidence of ongoing star formation in the nuclear region of NGC 3227. On the other hand, the nucleus of NGC 3227 is strongly polarised, with a degree of polarisation rising smoothly from tex2html_wrap_inline2908 in the red to nearly 3% in the blue (Schmidt & Miller 1985), and essentially constant PA, both for the continuum and the broad lines. This polarisation can be best explained by interstellar dust scattering, where the dust is intermixed with the narrow line region (NLR) gas (Schmidt & Miller). The PA of the polarisation is, interestingly, roughly parallel to the nuclear radio source axis (Ulvestad et al. 1981) and the optical major axis, suggesting that the dust clouds are distributed in the same plane as the overall galactic structure. Indeed, the double maxima are orientated along the major axis, and it is plausible that they represent scattering of the nuclear light in unobstructed directions.

The colours of the blobs in a 1'' aperture are given in Table 3. They are quite red, and not typical of young hot stars or scattered light, but the real colours must be bluer, since they include also the underlying structure, which is redder than the actual blue maximum. Note that the aperture used is very small (to avoid contamination from the other maximum and the nucleus) and the colours therefore prone to photon statistics errors.

NGC 4151

NGC 4151 is an SABab galaxy (de Vaucouleurs et al. 1991) and it is the best studied bright Seyfert 1 galaxy. A B band image of NGC 4151 is shown in Plate 7. Its redshift is 0.0033 and 1'' corresponds to 100 pc on the sky. The BLR is point-like but the NLR extends in a biconical structure tex2html_wrap_inline29164'' at PA tex2html_wrap_inline292060tex2html_wrap2970  (Evans et al. 1993) and the ENLR is resolved into a jet-like feature with a series of high-ionization blobs covering tex2html_wrap_inline292415'' SW and a fainter plume 5'' NE at PA tex2html_wrap_inline293050tex2html_wrap2972  (e.g. Heckman & Balick 1983; Perez et al. 1989). Extended X-ray emission at the brightest parts of the ENLR was discovered by Elvis et al. (1983). The radio structure is a collection of resolved sources (Harrison et al. 1986). The nuclear jets are at arcsec scale at tex2html_wrap_inline2934 and misaligned with the ENLR by tex2html_wrap_inline293630tex2html_wrap2976, whereas at smaller scales, the radio component is elongated at PAtex2html_wrap_inline294057tex2html_wrap2978,  similar to the NLR and ENLR (Heckman & Balick 1983; Pedlar et al. 1993).

In Plate 8, we plot the B-I map of the innermost tex2html_wrap_inline2946 (tex2html_wrap_inline2948) of NGC 4151. There is a strong colour gradient with the circumnuclear region being bluer than the rest of the bulge. This elongated blue structure is at PAtex2html_wrap_inline295060tex2html_wrap2980  extending over 7'' in diameter. The elongation is parallel to the small scale radio components and the NLR (Evans et al. 1993). In the nuclear region we find two unresolved blue components symmetrically located each side of the nucleus (at PAtex2html_wrap_inline295680tex2html_wrap2982)  separated by tex2html_wrap_inline29602''. They are misaligned by tex2html_wrap_inline296420tex2html_wrap2984  with respect to the ionisation cones (Evans et al. 1993) and the small scale radio axis. This structure is in good correspondence with the knots in the similar resolution radio map by Johnston et al. (1982).

The colours of the two maxima are given in Table 3. The B-V colours are surprisingly red; the other colours are consistent with normal spiral bulges. Since the real colours (without the underlying structure) must be bluer, these colours could be caused by a young stellar population or by scattered nuclear light. Again, with no firm evidence of ongoing star formation in the nucleus of NGC 4151, we prefer the latter possibility, but discuss the problems associated with this interpretation in Sect. 6.2.

NGC 7469

NGC 7469 (a V band image is shown in Plate 9) is a well studied Seyfert 1 galaxy of Hubble type SAB(rs)a and redshift 0.0167 (1'' corresponds to 490 pc on the sky) exhibiting a circumnuclear starburst ring. It shows strong 3.3 tex2html_wrap_inline2991m emission feature from dust heated to tex2html_wrap_inline2993, in a region between 1 and 3'' from the nucleus, with the heating of the dust grains probably due to radiation from hot young stars in the circumnuclear H II regions (Cutri et al. 1984). NGC 7469 also exhibits the PAH emission features in the MIR spectrum (Roche et al. 1991), common for starbursts. The 12.5 tex2html_wrap_inline2997m emission is elongated 3'' along PA = 78tex2html_wrap3057  probably representing unresolved integrated emission from the nucleus and two brightest starburst ring features (Keto et al. 1992). NGC 7469 has strong emission in the tex2html_wrap_inline3003CO J= 1-0 (Meixner et al. 1990) and Htex2html_wrap_inline3007 tex2html_wrap_inline3009 = 1-0 S(1) (Genzel et al. 1995) lines within a few arcsec from the nucleus, providing evidence for a concentration of molecular gas.

A high resolution CO map of NGC 7469 (Meixner et al. 1990) shows, in addition to a CO peak coincident with the nucleus, a SW component tex2html_wrap_inline30133.5'' from the nucleus and an extension to the NW. In Plate 10, we present our V-I map of the innermost tex2html_wrap_inline3019 (tex2html_wrap_inline3021). The V-I map shows an irregular ring of blue emission around the nucleus with a diameter tex2html_wrap_inline30253''. There are three notable blue maxima, in the S and in the NE and NW quadrants. The overall dimensions agree quite well with the CO emission (Meixner et al.\) although there is not a one-to-one correspondence. The SW CO plume is not reproduced, neither the NE maximum in the colour map.

A high resolution (0.4'' FWHM) radio map at 6 cm of NGC 7469 (Wilson et al. 1991) is dominated by a strong compact source centered on the Seyfert nucleus. Much of the remaining radio halo (tex2html_wrap_inline3031 8'' diameter) is concentrated into an annular structure of diameter tex2html_wrap_inline30353'', with bright maxima to the NE and SW. An [OIII]/Htex2html_wrap_inline3039 image of Wilson et al. shows that Htex2html_wrap_inline3041 is extended around a bright peak coincident with the continuum nucleus. The [OIII] emission is much more centrally concentrated with a plume to the N.

Mauder et al. (1994) presented high resolution, speckle masking images at three optical wavelengths (5400 - 6700 Å) with the central source subtracted. The images show a ring shaped emission region with a radius of 1.3'' and a lumpy structure with three main maxima at PA tex2html_wrap_inline304740, 245 and 320tex2html_wrap3059.  The maxima in our V-I image (Plate 10) correspond well with those in the radio, emission line and red continuum images. There is an especially good coincidence for the NE peak. The agreement for the SW peak is less good, but we have to remember the much higher resolution in the radio map. Also, the overall dimensions are in exact agreement. This similarity between the radio and optical indicates that the circumnuclear structure seen in the continuum colour map represents direct emission from hot stars in the ring, i.e. a starburst.

The colours of the blue circumnuclear blobs in a 2'' aperture are given in Table 3. The V-I colours are remarkably uniform and close to normal old stellar population colours. Again, the real colours without the underlying structure must be bluer.

Mrk 3

Mrk 3 is an S0 type galaxy (Jenkins 1981; Wagner 1987) with a Seyfert 2 nucleus (Koski 1978) at a redshift of 0.0134; 1'' corresponds to 410 pc on the sky. A B band image is shown in Plate 11. The X-ray spectrum of Mrk 3 (Awaki et al. 1990) shows a heavy absorbing column (tex2html_wrap_inline3065 cm tex2html_wrap_inline3067) and a strong Fe K line. This can be explained by absorption in and scattering off an obscuring torus. Awaki et al. suggest Mrk 3 being an intermediate case between less obscured (e.g. Mrk 348) and completely absorbed (e.g. NGC 1068) Seyfert 2s.

The H I mass is tex2html_wrap_inline3069 tex2html_wrap3141  (Pedlar et al. 1984), large for ellipticals. This may be due to interaction with UGC 03422, located 6' NW along PA = 155tex2html_wrap3143,  although there is no evidence for this in direct optical images (Wagner 1987). At 5 GHz, Mrk 3 exhibits a pair of highly collimated jets perpendicular to the major axis of the galaxy, 2'' in extent along PA = 84tex2html_wrap3145  (Kukula et al. 1993). Embedded in the jets are compact components, the western end terminating in a bright lobe containing a hotspot. Mrk 3 has a dust-dominated steep spectrum in the NIR (Edelson & Malkan 1986), high optical polarisation (tex2html_wrap_inline3081 for lines and continuum at PA = 130tex2html_wrap3147; Schmidt & Miller 1985) and shows broad lines in polarised light (Miller & Goodrich 1990).

Recent HST imaging by Capetti et al. (1995b) revealed a striking spiral shape of the ionized gas emission, with a large number of resolved knots in a fainter extended region. The brightest part of the spiral is confined into a cone of tex2html_wrap_inline308540tex2html_wrap3149  full opening angle, and its morphology is similar to the radio (Kukula et al. 1993). The optical emission lines are blue asymmetric, indicating absorption by dust surrounding the NLR clouds. The nuclear emission thus seems to be heavily absorbed in the line of sight, while the ionizing flux is intrinsically not strongly collimated.

The B-I image of the central tex2html_wrap_inline3091 (tex2html_wrap_inline3093) region of Mrk 3 is shown in Plate 12. The central region is redder than the surrounding bulge at PA tex2html_wrap_inline3095100tex2html_wrap3151  with extent tex2html_wrap_inline309911''. Within this red elongation, there is an arclike blue extension, with two main blue maxima, roughly elongated at PA = 95tex2html_wrap3153.  The W maximum is bluer at tex2html_wrap_inline31051.3'' from the nucleus, while the E maximum is at 0.8'' distance from the nucleus. In the E, the elongation curves towards larger PA, fading away at tex2html_wrap_inline31114.6'' from nucleus. The double structure is roughly aligned with the radio axis (85tex2html_wrap3155; Ulvestad & Wilson 1984). The opening angle of the ``cone'' agrees well with that measured from the emission lines (Capetti et al. 1995b), and the curvature of the blue continuum is similar (S-shaped) to that seen in the emission lines (Capetti et al. 1995b) and in the radio (Kukula et al.\ 1993). Most likely, the blue structure is scattered light from the nucleus following the biconical structure in Mrk 3. Outside of the blue emission, the colour is typical of galactic bulges, displaying no sign of star forming regions in its host galaxy, rendering the possibility of jet-induced star formation unlikely.

Pogge & De Robertis (1993) presented a ``UV/Red'' colour map of Mrk 3, which revealed two distinct structures: a pair of UV (blue) knots on either side of the nucleus along PA = 85tex2html_wrap3157,  separated by 1.5'', and a diffuse blue component with a distinct bi-conical morphology extending out to 5'' on either side of the nucleus along PA = 114tex2html_wrap3159.  Our B-I map is in good agreement with their map, with the PA's agreeing within the errors. We discuss the origin of these blue maxima in Sect. 6.2.

The V-R map of Mrk 3 (Plate 13), which resembles an [OIII]/Htex2html_wrap_inline3129 excitation map, shows a spiral structure in the outer regions with a radius tex2html_wrap_inline31314'' from the nucleus, in agreement with Capetti et al. (1995b) results. Interestingly, there is also a red double structure in the nuclear region, which corresponds well to the blue maxima in the B-I map (Plate 12). The colours of the maxima in a 2'' aperture are given in Table 3. The BVRI colours are redder than for the NGC 1068 elongation, but still in agreement with scattering, being contaminated by the underlying red stellar population.

Mrk  78

Mrk 78 is a Seyfert 2 galaxy at redshift 0.038 (1'' equals 1.15 kpc). Plate 14 shows its B band image. At 2 cm, Mrk 78 shows a triple structure, with a bright core and fainter components to the E and W, parallel to the NLR axis (Pedlar et al. 1989). There is no detailed morphological correlation, however, between the radio and the ENLR (Unger et al. 1987), with the axes misaligned by tex2html_wrap_inline316525tex2html_wrap3201  (Capetti et al. 1994). This misalignment can be caused by the ENLR being in a different plane from the rest of the galaxy, or by different covering factor E and W of the nucleus (Pedlar et al. 1989).

The [OIII] emission is biconical with opening angle tex2html_wrap_inline316940tex2html_wrap3203  extended along the radio axis at PA = 67tex2html_wrap3205  (tex2html_wrap_inline3175 in extent; Capetti et al. 1994). There is also a resolved obscuring zone close to the center, elongated E-W, with tex2html_wrap_inline31770.8'' extent and displaced from the center of the cone by tex2html_wrap_inline31810.2''. The lack of far-UV emission, comparison with the FIR emission, and photon budget arguments indicate a strongly anisotropic nuclear source. Capetti et al. suggest that obscuring dust (both on pc and kpc scales) prevents UV radiation to escape along our line of sight, while the stellar continuum remains largely unobscured.

We present our B-I map in Plate 15. It shows the nucleus as redder than the bulge of the galaxy. The reddest region close to the nucleus is slightly extended (tex2html_wrap_inline3187) along PAtex2html_wrap_inline3189330tex2html_wrap3207.  This extension is almost perpendicular to the E-W [OIII] morphology (Capetti et al.\ 1994), and may show the location of the obscuring dust in this galaxy. There is also an arc-like blue region forming a broken ring from E through N to SW at a distance tex2html_wrap_inline31933'' from the nucleus. Again, although this could be a star forming region around the nucleus, or show a gradient in the stellar population, there is no direct evidence for this. A more appealing possibility is that it represents a scattering mirror which is less efficient or further away from the nucleus than in the cases of NGC 1068, Mrk 3 and Mrk 573 (see Sect. 6.2). The colours of the blue ring are redder in V-I, but bluer in B-V than for the NGC 1068 elongation (Table 3).

Mrk  348

Mrk 348 is an almost face-on S0/a-sb type galaxy (Huchra 1977) with a Seyfert 2 nuclear spectrum (Koski 1978). The B band image is shown in Plate 16. The velocity field of the neutral hydrogen and tidal plumes indicate an extensive tidal perturbation by a companion galaxy, NGC 266 (Simkin et al. 1987). Its small reddening (tex2html_wrap_inline3211 both for lines and continuum) is mainly produced in our Galaxy. At a redshift of Mrk 348 (0.014), 1'' corresponds to 420 pc projected distance on the sky.

Spectropolarimetry has revealed the presence of broad wings to the permitted lines in Mrk 348 (Miller & Goodrich 1990). The radio structure is linear on a scale of 0.2'' (Neff & de Bruyn 1983; Ulvestad & Wilson 1984). There is also an extended distribution of ionized gas in the nuclear region (Bergeron & Durret 1987). The tex2html_wrap_inline3217 luminosity of Mrk 348 is a factor of 30 brighter (Warwick et al. 1989) and the Fe Ktex2html_wrap_inline3219 line an order of magnitude fainter (Koyama et al. 1989) than in NGC 1068. Probably Mrk 348 is seen directly through much less column density than in the case of NGC 1068.

We present our B-I map of the central tex2html_wrap_inline3223 (tex2html_wrap_inline3225) of Mrk 348 in Plate 17. It shows the nucleus as redder than the near-nuclear region. At larger distances, the host galaxy appears again as red. Between these, there is an intermediate blue ring-like region, tex2html_wrap_inline32273'' in diameter elongated at PA tex2html_wrap_inline323115tex2html_wrap3235.  Again, although the map of Mrk 348 is less clear than for the other galaxies, this blue structure may be related to scattering of the nuclear light, with less efficient ``mirrors'' than in the other galaxies studied.

Mrk  573

Mrk 573 is a Seyfert 2 galaxy at a redshift of 0.017 (1'' corresponds to 510 pc projected distance on the sky). Plate 18 shows its B band morphology. The extinction towards the nucleus, tex2html_wrap_inline3241, is probably mostly intrinsic to the galaxy. It has a triple radio source at 6 cm with two lobes of similar luminosity at distance tex2html_wrap_inline32431.5'' from the nucleus along the radio axis at PA = 125tex2html_wrap3313  (Ulvestad & Wilson 1984). Narrow band [OIII] and Htex2html_wrap_inline3249 images show an elongated emission line nebulosity extending about tex2html_wrap_inline3251 along the radio axis, and two oppositely directed cones of ionising radiation (Haniff et al. 1991; Tsvetanov & Walsh 1992). The Htex2html_wrap_inline3253 peak coincides with the continuum peak, but the [OIII] maximum is shifted 4'' along the radio axis to SE (Whittle et al. 1988; Tsvetanov & Walsh 1992). Perpendicular to the line image elongation there is a low-ionisation valley through nucleus at PA tex2html_wrap_inline325740tex2html_wrap3315  (Tsvetanov & Walsh 1992).

Kinney et al. (1991) compared the number of ionising photons seen by the ENLR gas to the number of ionising photons seen in Earth for a sample of Seyfert 2 galaxies. For Mrk 573, they found a ratio tex2html_wrap_inline32611. However, corrected for extinction and biconical covering factor, the ratio becomes tex2html_wrap_inline3263 5 (Tsvetanov & Walsh 1992), providing strong evidence for anisotropy of the radiation field.

Pogge & De Robertis (1993) presented a ``UV/Red'' map of Mrk 573, where they find a biconical structure oriented at PA = 124tex2html_wrap3317  with an opening angle tex2html_wrap_inline326760tex2html_wrap3319.  They also find a curved feature 3.5'' SE of the nucleus and a similar feature 4.5'' NW. Both are coincident with [OIII] emission line features, but outside the radio continuum lobes. Recently, Pogge & De Robertis (1995) obtained narrow band images in five emission lines and a continuum band. In addition to the features mentioned above, which they found to be elongated perpendicular to the radio axis and rotated following the spiral arms rotation, they found two further arc-like structures at 1.7'' SE and 2.2'' NW of the nucleus at PA 124tex2html_wrap3321, bracketing the radio lobes. Based on the morphology and ionisation gradient, Pogge & De Robertis (1995) suggest that the inner pair of features is reminiscent of edge-brightened bow shocks where the emission line gas is interacting with the radio plasma from the AGN, while the outer pair may represent episodic ejection of plasma from the nucleus.

We present our B-I map of the central tex2html_wrap_inline3283 (tex2html_wrap_inline3285) of Mrk 573 in Plate 19. It shows two blue maxima on either side of the nucleus with a separation of tex2html_wrap_inline32877'' at PA tex2html_wrap_inline3291135tex2html_wrap3323  and a red ``bridge'' perpendicular to them crossing the nucleus.  The double structure is roughly parallel to the radio axis (PA = 124tex2html_wrap3325; Ulvestad & Wilson 1984) but the blue component is more extended than the radio continuum emission. There is also a tightly wound spiral arm structure tracing an elliptical ring with major axis tex2html_wrap_inline3297 aligned at PA tex2html_wrap_inline329910tex2html_wrap3327.  Most likely, the blue maxima define the geometry of the radiation cones, with the red bridge indicating the large scale obscuring material across the nucleus. While the blue maxima could alternatively be understood as arising from star formation triggered by the radio jet, there is no sign of young blue stars in the continuum images. Furthermore, the ENLR line ratios indicate an AGN-type ionising spectrum. Note, however, that the 10 tex2html_wrap_inline3303m flux of Mrk 573 is relatively larger than in typical Seyfert 2s, suggesting that a starburst component may contribute in the nucleus (Maiolino et al. 1995). There is a good general correspondence between the maps of , Pogge & De Robertis (1993, 1995) and our B-I map (Plate 19); except that we can resolve the blue maxima, which are more diffuse and situated further away from the nucleus than in NGC 1068 and Mrk 3. For discussion of the blue maxima, see Sect. 6.2. The colours of the blue maxima in a 3'' aperture are given in Table 3. They are bluer in B-V and V-R than those of the NGC 1068 elongation, and in good agreement with arising from scattering.

NGC 1667

Except for its supernova 1986N, NGC 1667 has not been studied in detail so far. It is a relatively isolated type Sc galaxy with a Seyfert 2 nucleus at z = 0.0153; 1'' corresponds to 470 pc on the sky. A B band image of NGC 1667 is shown in Plate 20. Evidence for star formation activity in its host galaxy comes from starburst-like FIR colours, slightly extended MIR emission, and optical morphology and emission line ratios (see references in Heckman et al. 1989). The radio emission is diffuse and single-component (Ulvestad & Wilson 1989), indicating that most of the radio emission in NGC 1667 does not arise from the AGN.

We present our large scale B-I map (tex2html_wrap_inline3337) of NGC 1667 in Plate 21. The image delineates the spiral structure of this galaxy, the individual H II regions in the spiral arms, and the geometry of dusty material, as in the case of NGC 3227 (Plate 5). Note especially the red ``bubble'' extending up to 9'' N of the nucleus. Plate 22 shows the B-I map of the central tex2html_wrap_inline3343 (tex2html_wrap_inline3345) nuclear region. In addition to the innermost spiral arm, there is a red double structure across the nucleus with a separation tex2html_wrap_inline33473'' at PA tex2html_wrap_inline3351 45tex2html_wrap3355.  The colours of these red maxima are given in Table 3. Although this red structure could be related to the material obscuring a Seyfert 1 nucleus, we defer further discussion, but encourage finding direct evidence of anisotropy in this little studied Seyfert 2.

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