Linear radio structures in selected Seyfert galaxies

Transcription

1 Mon. Not. R. Astron. Soc. 404, (2010) doi: /j x Linear radio structures in selected Seyfert galaxies E. Xanthopoulos, 1,2 A. H. C. Thean, 3 A. Pedlar 3 anda.m.s.richards 3 1 University of California Davis, Department of Physics, Davis, CA 95616, USA 2 IGPP/Lawrence Livermore National Laboratory, Livermore, CA 94550, USA 3 University of Manchester, Jodrell Bank Centre for Astrophysics Alan-Turing Building, Oxford Road, Manchester M13 9PL Accepted 2010 January 25. Received 2010 January 21; in original form 2009 April 6 1 INTRODUCTION Over the last decade, a lot of studies have been done in order to understand the physical relation between the radio-emitting relativistic plasma and the ionized gas in the narrow-line region (NLR). The close relation has been noted by de Bruyn & Wilson (1978) and Wilson & Heckman (1985). Also, the close association of the radio jets with the NLR has been studied with increasing detail, initially using simple analytical models (Pedlar, Unger & Dyson 1985; Taylor et al. 1989), and then progressing to highly sophisticated hydro-code numerical simulations (e.g. Steffen et al. 1997). High-resolution radio observations have revealed double and triple radio sources in many Seyfert nuclei, suggesting that the collimated ejection of material is occurring in these objects in the same way as the radio jets in quasars and radio galaxies. But, very few Seyfert galaxies have been found to have radio structures that can be described as jets (Kukula et al. 1993; Ghosh et al. 1994; Oosterloo et al. 2000; Whittle & Wilson 2004; Rosario et al. 2005; Rosario 2007). With such objects we can determine with high precision the degree of bending in the jet and the true position angle (PA) of the inner jet. It is this PA which is likely to be related to the central engine, rather than the larger scale structures which can be bent by interactions with the galactic environment (e.g. NGC 4051; Christopoulou et al. 1997). Several Seyfert galaxies show optical emission lines from a wedge-shaped region of ionized gas which is roughly aligned with the radio structure (Wilson & Tsvetanov 1994). It is generally assumed that these are due to cones of ionizing radiation exanthop@igpp.ucllnl.org ABSTRACT High-resolution Multi-Element Radio Linked Interferometer Network 5-GHz observations of seven Seyfert galaxies, selected as the ones previously showing evidence of collimated ejection, have been compared with high-resolution Hubble Space Telescope data. A radio and optical/near-ultraviolet emission correlation is apparent in all the sources. The radio maps reveal rich structures in the entire sample. NGC 2639 and TXFS have multipleknot parsec-scale extended structures, Mrk 1034NED02, Mrk 1210, NGC 4922NED02 and NGC 5506 reveal one-sided jets, while IC 1481 exhibits jet features. Interaction between these very small (mostly mas) sub-kpc jets with dense material gives rise to symptoms of a disrupted medium, signatures of which are present in all the sources. Key words: methods: observational galaxies: jets galaxies: Seyfert. from the active galactic nuclei (AGN) which are centred on the radio axis. The angular resolution of the Multi-Element Radio Linked Interferometer Network (MERLIN) at 5 GHz is equivalent to that of the Hubble Space Telescope (HST), making these radio images ideal for comparison with the structure of the NLR and extended NLR (ENLR) and perfect cases to study in detail the individual small-scale jets as revealed in, for example, Capetti et al. (1995), Falcke, Wilson & Simpson (1998) and Capetti et al. (1999). The fact that Falcke et al. (2001) always saw some obscuring material on scales of several tens to hundreds of parsecs in the Seyfert galaxies they observed with HST could indicate that the torus on mas (i.e. sub-pc) scale is related to large-scale dust lanes and is not an isolated nuclear feature. MERLIN observations could reveal the orientation and geometry of the nuclear disc in relation to the dust lanes seen with HST. All the known water megamasers are contained in galaxies which have some level of activity, and interferometric studies show that the maser emission often originates within about 1 pc or less of the nucleus (Claussen & Lo 1986; Haschick et al. 1990). Hence, the megamaser phenomenon may be related to dense molecular gas in the nuclei of such galaxies. Braatz et al. (1998) find a strong correlation between the 5-GHz power and 22-GHz maser brightness and suggest that this confirms the location of the masers in a disc which is also implicated in collimated radio jets. Both the nuclear and surrounding regions of AGN are highly complex and host a wealth of physical processes. Gas in different phases (atomic, molecular and ionized) is observed in these regions. This gas can carry the signatures of the effect of the AGN on its surrounding medium. For example, the interaction of a radio jet with the (rich) gaseous medium can cause the jets to be disrupted, smothered, deflected or destroyed (e.g. Whittle & Wilson 2004), C 2010 The Authors. Journal compilation C 2010 RAS

2 Linear radio structures 1967 Table 1. General characteristics of the Seyfert galaxies. Object z AGN type Hubble type Linear scale References (NED) (kpc arcsec 1 ) (for AGN type) Mrk 1034NED Sy1 Sab Mazzarella & Balzano (1986) Mrk Sy2 Sa Hewitt & Burbidge (1991) NGC Sy1.9/LINER (R)SA(r)a:/Sb Braatz et al. (1994); Wilson & Colbert (1995) NGC 4922NED Sy2/LINER S Andreasian & Khachikian (1986); Alonso-Herrero et al. (1999) NGC NLSy1 Sa pec sp Nagar et al. (2002) TXFS LINER S/S0? Bennert et al. (2004) IC LINER Sb pec Huchra & Burg (1992) and this in turn can provide a mechanism that can create shocks, destroy line-emitting clouds, drive outflows with high velocities (e.g. Fosbury et al. 1998; Batcheldor et al. 2007) or even trigger star formation (e.g. van Breugel & Dey 1993; Croft et al. 2006). The purpose of the present study is to investigate the variety of collimated ejection in low-luminosity AGN and increase the small number of multiple component jets known (e.g. Cecil et al. 2000; Momjian et al. 2003; Mundell et al. 2003). We present new MER- LIN maps for Mrk 1034NED02, NGC 2639, NGC 4922NED02, NGC 5506, IC 1481, and rereduce MERLIN 5-GHz data for Mrk 1210 (Middelberg et al. 2004) and TXFS (Taylor et al. 2004). We intend to determine the radio and optical/near-ultraviolet (near-uv) alignment of the collimated ejection on mas (sub-pc) scales, by comparing the MERLIN radio maps with HST optical observations, and examine the interaction (and its effects) of the linear radio structures with both the near-nuclear and the surrounding medium. In Section 2, we present the MERLIN observations and reductions and introduce the HST data. In Section 3, we describe the radio maps and the optical/near-uv images for all the galaxies. The paper ends with the discussion and conclusions. Throughout this paper, we use H 0 = 65 km s 1 Mpc 1 unless otherwise specified. 2 OBSERVATIONS AND REDUCTIONS 2.1 Radio data MERLIN observations at 5 GHz of the seven Seyfert galaxies were performed between 1998 and The general characteristics of the objects in our sample are shown in Table 1. In column 1, we give the most common name of the galaxy. Columns 2 4 present the redshift (z), the Seyfert/LINER type and the Hubble type, respectively, for each galaxy. In column 5, one can find the linear scale for each Table 2. Journal of the observations. galaxy, while in the last column we provide the reference(s) for the Seyfert/LINER type. Four of the seven Seyfert galaxies (Mrk 1034, NGC 2639, 4922 and 5506) were selected as the strongest radio Seyfert galaxies detected in the extended 12-μm survey (Rush, Malkan & Spinoglio 1993) in which the Very Large Array (VLA) 8.4-GHz maps (Thean et al. 2000) show evidence of extended structures consistent with collimated ejection. The rest, Mrk 1210, TXFS , IC 1481 (including again NGC 2639 and 5506), are Seyfert galaxies in which water masers have been previously detected. A journal of the observations is presented in Table 2. The observations were made with six telescopes and all four polarizations centred on MHz. The data were recorded with a bandwidth of 16 MHz separated into 16 1-MHz channels. Observations of each target were interleaved with those of a carefully chosen nearby calibrator, shown in column 5 of Table 2, that was used for phase calibrations purposes. The cycle time between each target and calibrator is given in column 6. In columns 2 and 3, we have included the J2000 radio coordinates of the sources measured from our MERLIN maps using the AIPS task JMFIT, a two-dimensional elliptical Gaussian fitting program. The positional accuracy for these MERLIN observations is better than 20 mas for most of the target sources and better than 30 mas for the low declination sources in the sample. Column 4 shows the date of the observations. The data were edited, corrected for elevation-dependent effects, non-closing errors and bandpass response, and were flux-calibrated using the standard MERLIN analysis programs. The bandpass and non-closing corrections were determined from the strong point source OQ208. The flux calibration was determined using 3C286. Further processing of the data was carried out with the National Radio Astronomy Observatory AIPS package. For the two objects (NGC 2639 and TXFS ) for which we had more than one run of observations, the data for the target and the phase calibrator were combined using the AIPS task DBCON. Then, the MERLIN Object Right ascension Declination Date of obs. Phase cal. Cycle time (target+cal) (h m s, J2000) (, J2000) (min) Mrk 1034NED December min + 2min Mrk March min min NGC December min + 2min NGC December min + 2min NGC 4922NED December min + 2min NGC December min + 2min TXFS February min + 2min TXFS March min + 2min IC December min + 2min

3 1968 E. Xanthopoulos et al. Table 3. MERLIN radio properties of the galaxies. Object Flux int Flux peak Beam (FWHM) PA Linear Extent Type (mjy) (mjy) ( arcsec) ( ) ( arcsec) Mrk 1034NED ± ± ± One-jet Mrk ± ± ± One-jet NGC ± ± ± Two-lobes and two-jets NGC 4922NED ± ± ± One-jet NGC ± ± ± One-jet TXFS ± ± ± One-jet and hint of second jet IC ± ± ± One-jet and hint of second jet pipeline automated data calibration and imaging procedure was used for all the data sets. The fully calibrated data on the target were then mapped using AIPS and different weighting functions. An integrated and a peak flux density was measured for the radio emission in each source using again the AIPS task JMFIT. Flux density errors were derived by adding, in quadrature, the conservative 5 per cent flux scale error, the rms noise in the final image and the error in the Gaussian fitting (JMFIT). From the radio images, we also extract the size of the jets, the total linear extent of each source and the PA of the radio structures, using the AIPS task TVDIST. These measurements are presented in Table 3. In column 1, we give the name of the object, columns 2 and 3 present the integrated and peak flux densities of the radio emission and the respective errors, the size and PA of the fitted beam are seen in columns 4 and 5. Finally in columns 6 and 7 one can find the total linear extent of the radio structure and the type of the radio features found (jets, lobes, etc.). 2.2 HST data HST archive data were available for all the galaxies in the paper. These are described for each individual galaxy below. After aligning the individual HST images with the radio MERLIN maps, the radio data were overlaid on the optical data. In this way, we can study in detail the radio and optical/uv emission correlation in these galaxies. It is unfortunate that although we have such high angular resolution for the MERLIN (0.04 arcsec ) and HST images [0.027 and arcsec for Advanced Camera for Surveys (ACS), and 0.10 arcsec for Wide Field Planetary Camera 2 (WFPC2) and arcsec for Faint Object Camera (FOC)] the registration of the two images cannot be better than 0.5 arcsec, due to absolute astrometric uncertainties for HST data. This problem can be mitigated in cases where there is a bright compact core in the HST image which is reasonably assumed to be associated with the compact inverted spectrum radio core. This is actually the case for all our galaxies. We therefore register the MERLIN and HST images of our galaxies by shifting the peak of the optical/uv bright core to the centre of the radio nucleus. Using this registration, contours of the MERLIN C-band image are overlaid on the HST images Mrk 1034 HST images of Mrk 1034NED02 were obtained in 2006 with the Wide Field Channel (WFC) of the ACS camera as part of the HST proposal on an ACS survey of a complete sample of luminous infrared galaxies in the local universe. The images were taken through the F435W, Johnson B filter (effective wavelength 4297 Å / 1038 Å) with a 1275-s exposure, and through the F814W, broad I filter (effective wavelength 8333 Å /2511 Å) with a 730-s exposure. The WFC has a arcsec 2 field of view and a plate scale of arcsec pixel 1. Basic two-dimensional image reductions (overscan, bias, dark subtraction and flat-fielding) as well as cosmic ray rejections from the CR-SPLIT=2 data were performed with the CALACS pipeline processing. The observations used the dither patterns and so further processing by PyDrizzle corrected for geometric distortion of the ACS camera and combined the dithered images into one final reduced and calibrated image. Using IRAF,we removed any remaining cosmic ray events from the data and rotated the image to the cardinal orientation [north (N) is up and east (E) to the left] by means of the keyword ORIENTAT in the data header. At the redshift z = of the galaxy, 1 pixel corresponds to 35.1 pc for H 0 = 65 km s 1 Mpc 1. The images, calibrated in counts in e, were multiplied by the PHOTFLAM keyword from the image header and divided by the exposure time and so were converted to erg cm 2 s 1 Å 1. B and I magnitudes were measured using the zero point, PHOTZPT. A B I image of the host galaxy of Mrk 1034NED02 was created by dividing the flux calibrated and converted to magnitudes B (F435W) by the I (F814W) band image (with no rescaling). The images were first sky subtracted and registered and then calibrated as mentioned above Mrk 1210 and NGC 2639 HST images of Mrk 1210 and of NGC 2639 were obtained in 2002 with the high-resolution channel (HRC) of the ACS camera as part of the HST proposal 9379 on near-uv imaging of Seyfert galaxies. Each image was taken through the F330W (HRC U) filter (effective wavelength 3354 Å /588 Å) with a 1200-s exposure. The HRC has a29 25 arcsec 2 field of view and a plate scale of arcsec pixel 1. Basic two-dimensional image reductions and further image processing, as were described for the Mrk 1034NED02 data, were also followed here. At the redshift z = of Mrk 1210, 1 pixel corresponds to 7.99 pc, while at the redshift of for NGC pixel corresponds to 6.62 pc (both for H 0 = 65 km s 1 Mpc 1 ). The UV observation through the F330W filter is the optimal configuration to detect faint star-forming regions around the nuclei NGC 4922 The double system NGC 4922 was observed with the HST in 1995 using the WFPC2 as part of the HST proposal 5479 on subarcsecond structure in nearby AGN (Malkan, Gorjian & Tam 1998). The 500-s exposure was taken through the broad-band filter F606W (effective wavelength 5997 Å /1502 Å) which includes both the standard WFPC2 V and R bands. NGC 4922A = NGC 4922NED01, the

4 south largest galaxy, is centred on the planetary camera (PC) CCD of WFPC2, which has a plate scale of arcsec pixel 1 and a field of view of arcsec 2. NGC 4922B = NGC 4922NED02, the galaxy to the N, which was detected by our radio observations, falls on the wide field chip. Each wide-field CCD has a plate scale of 0.1 arcsec pixel 1 and a field of view of arcmin 2.Same initial and further reduction steps as described before were also followed here. At the redshift of for NGC 4922NED02, 1 pixel corresponds to 50.5 pc for H 0 = 65 km s 1 Mpc NGC 5506 The HST image was obtained in 1995 with the FOC, in its FOC/96 configuration, as part of the HST proposal 5144 on the Study of optical emission associated with radio jets and hot spots. The image was taken through the F501N ([O III] interference) filter (effective wavelength 5010 Å /74 Å) with a s exposure. The FOC/96 has a arcsec 2 field of view and a plate scale of arcsec pixel 1. Routine calibrations were applied to the FOC image at the Space Telescope Institute while further typical image processing steps, as described above, were performed by us. At the redshift z = of the galaxy, 1 pixel corresponds to 1.92 pc for H 0 = 65 km s 1 Mpc TXFS TXFS was observed with the PC plate scale of arcsec pixel 1 and a field of view of arcsec 2 on board the HST as part of the HST proposal 7278 on The connection between the obscuring torus and masing disc in H 2 O Megamasers. It was observed in three filters: F814W (red continuum WFPC2 I, effective wavelength 8203 Å /1758 Å), F547M (green continuum Strömgren y wider, effective wavelength 5446 Å /486.6 Å) and F673N (redshifted Hα + [NII] λλ6548, 6583, effective wavelength 6732 Å /47.2 Å) with total integration times of 120, 320 and 1200 s, respectively; all exposures being split in two or three integrations to allow cosmic ray rejection. The images were processed through the standard WFPC2 pipeline data reduction at the Space Telescope Science Institute while further data reduction was done in IRAF and included cosmic ray rejection, rotation to the cardinal orientation and flux calibration. For the continuum filters, a constant background was determined in an emission-free region of the PC (to represent sky brightness) and subtracted from the image. The galaxy continuum near the Hα + [N II] line was determined by combining the red and green continuum images, scaled to the filter width of F673N and weighted by the relative offset of their mean wavelengths from the redshifted Hα + [N II] emission. This continuum was then subtracted from the on-band image to obtain a continuum-free image of Hα + [N II] which will show the pure emission. No shifts were applied between the images because there were all taken within one orbit and at the same position of the PC chip. At the redshift z = of TXFS , 1 pixel corresponds to pc for H 0 = 65 km s 1 Mpc IC 1481 IC 1481 was observed with the WFPC2 as part of the same HST proposal as TXFS It was observed in three filters: F814W (red continuum WFPC2 I, effective wavelength 8203 Å /1758 Linear radio structures 1969 Å), F547M (green continuum Strömgren y wider, effective wavelength 5446 Å /486.6 Å) and FR680N (Linear Ramp filter , W, redshifted Hα + [N II] λλ6548, 6583) with total integration times of 160, 320 and 900 s, respectively, all exposures being split in two or three integrations to allow cosmic-ray rejection. The galaxy falls on the wide-field chip (WF2) and so we have a plate scale of 0.1 arcsec pixel 1 and a field of view of arcmin 2. We followed the same steps, as described in TXFS , in order to determine the galaxy continuum and obtain the continuumfree Hα + [N II] image of IC 1481 (although here we applied a shift between the images in order to register them). At the redshift z = of IC 1481, 1 pixel corresponds to 44.3 pc for H 0 = 65 km s 1 Mpc 1. 3 MAPS AND RESULTS 3.1 Mrk 1034NED02 The host galaxy (HST/ACS WFC B I band image) of Mrk 1034NED02 is presented in Fig. 1. Part of the bright nucleus and the prominent spiral arms are clearly seen. The blue core (B I = 1.30 mag) is arcsec 2 (or pc) in size, measured from both the B- andi-band data. The inset image, on the top-right corner, shows the 5-GHz MERLIN map of Mrk 1034NED02 overlaid on the HST/ACS WFC I band image of the central part of the host galaxy. All the radio emission from the 5-GHz MERLIN observations is contained within the optical core, which is not a compact component but rather has an elongated kidney-bean shaped structure. At the resolution of arcsec, we are able to see for the first time radio structure from the core of Mrk 1034NED02 with the 5-GHz MERLIN observations. The one-sided jet, at a PA of 89 from N to E, is arcsec or 103 pc in extent. We measure an integrated flux density for the source of 4.02 ± 0.37 mjy while the peak flux density is 2.05 ± 0.18 mjy. The total radio linear extent of the source is arcsec or 153 pc. As expected, the high-resolution observations has resolved all of the extended diffuse emission and as a result we measure lower values for the integrated flux compared to the previous radio observations (Maslowski & Kellermann 1988; Sopp & Alexander 1992; Condon et al. 1996; Thean et al. 2000). The ACS/WFC HST (0.049 arcsec resolution) data were able to resolve the optical nucleus of Mrk 1034NED02 and the finer details are revealed in the I-band image of the central part of the galaxy (Fig. 2, bottom panel). The kidney-bean morphology is a mini structure of three components [ mini Fanaroff Riley type II (FR II) type morphology], a core, which is pointed in the same side and direction as the radio jet, and two compact features on opposite sides of the core. The radio jet coincides with the central I component and SW feature which points downwards. The outer optical contours become again pointed along the PA of the radio jet. The nuclear optical emission of Mrk 1034NED02 has also an intimate relationship with the more extended radio emission that was revealed by Thean et al. (2000) in their VLA 8.4-GHz observations. The extended wings on opposite sides of the core in the Thean et al. (2000) data have both the appearance and extent ( 2.1 arcsec atapaof65 from the N) of the inner optical nuclear structure of the galaxy as seen in both Figs 1 and 2, including the curved feature that is observed in the central optical emission at the end of the radio jet (top panel of Fig. 2). It may well agree with Wang, Wiita & Hooda (2000) who describe a curved interaction area where the jet hits the cloud, and squeezes a weak stream of plasma downwards.

5 1970 E. Xanthopoulos et al. Figure 1. HST/ACS WFC B I image of the host galaxy of Mrk 1034NED02 (see the text for details). The relative intensity of the grey-scale image, which corresponds to the to range of B/I magnitude ratios, is shown by the bar on the right-hand side of the figure. The spiral structure of the galaxy is seen prominently. In the top-right corner, presented as an inset figure, is the 5-GHz MERLIN map of Mrk 1034NED02 (resolution arcsec) overlaid on the grey-scale HST/ACS WFC (F814W) I-band image (resolution arcsec) of the central part of the same galaxy. Contour levels are at 2.5e-4 (1,2,4, 8) Jy beam 1. The peak flux is mjy beam 1 and the rms noise level is 98 μjy beam 1. The inner core optical emission is curved towards the S just below the tip of the W radio jet, and is almost emerging from the end of the radio jet extension. 3.2 Mrk 1210 The tightly wound spiral structure of the host galaxy of Mrk 1210 is prominent in our HST ACS/HRC near-uv (F330W) image (Fig. 3). The face-on spiral morphology of the Sa type galaxy (Malkan et al. 1998) on a first inspection seems that of a normal galaxy with spiral arms following a ring of star-forming regions. However, when we zoom in the bright nucleus, the inner contours are off centred and not circular. They appear elongated in the SE NW direction and pointed towards the E.Muñoz Marín et al. (2007) characterize the near-uv emission of this galaxy as a ring traced by star-forming regions. They also hint at a bright double nucleus which may corroborate the appearance of the elongated inner contours, although the elongation can be a result of the presence of the nuclear radio jet(s). The double nucleus nature may also be supported by the results of the Very Long Baseline Array (VLBA) 6-cm observations (Middelberg et al. 2004) which resolve the brightest source, that is the core, into an arc of four components. The inset image in the lower-right corner of Fig. 3 presents the 5-GHz MERLIN map of Mrk 1210 overlaid on the HST image. Our MERLIN 5-GHz map agrees with the same map presented by Middelberg et al. (2004). All the MERLIN 5-GHz emission is contained within the inner few nuclear contours of the near- UV emission as seen in the contour map of the host galaxy. The radio emission consists of two radio components, most possibly the bright core and a jet emerging from the nucleus and pointing to the SE. This may be further supported by European VLBI Network (EVN) and VLBA 18-cm images (Middelberg et al. 2004) that detect a bright compact object resolved in five features all contained in our MERLIN bright core and another component (marginal detection), at a distance of 117 mas to the SE from the bright compact source, which is spatially coincident with our MERLIN SE extended component and which they consider a continuation of the radio ejecta. Based on physical arguments, Middelberg et al. (2004) propose a system composed of a core and an outflow to explain the radio structure in Mrk This seems to agree with optical and near-infrared (NIR) spectroscopy findings of Mrk 1210 by Mazzalay & Rodríguez-Ardila (2007) that advocate the presence of nuclear outflow in this galaxy instead of a hidden broad-line region. Although Middelberg et al. (2004) are unable to identify the

6 Figure 2. Top panel: 5-GHz MERLIN map of Mrk 1034NED02 (resolution arcsec) overlaid on the grey-scale HST/ACS WFC (F814W) I- band image (resolution arcsec) of the central part of the same galaxy. Contour levels are at 2.5e-4 (1,2,4,8)Jybeam 1. The peak flux is mjy beam 1 and the rms noise level is 98 μjy beam 1. The bar on the top shows the relative intensity of the grey-scale image. Bottom panel: HST ACS/WFC (F814W) I-band (resolution arcsec) contour map of the core of Mrk 1034NED02. Contour levels are at 1.590e-20 (2,4,8,16, 20, 25, 27, 28, 29, 30, 33) erg cm 2 s 1 Å 1. The peak flux is 8.609e-18 erg cm 2 s 1 Å 1 and the rms noise level is 5.3e-21 erg cm 2 s 1 Å 1. The core is resolved into an extended structure with multiple features and shows an intimate connection with the radio emission. core in any of the components in their brightest source, since all of them have steep spectra, they also rule out that the SE component, as seen in our MERLIN map, can host the nucleus of Mrk 1210 as it lacks a flat spectrum that would indicate the nucleus. It might be the case that the AGN lies somewhere between the components of their brightest source but the nucleus is not detected because it is free free absorbed at 6 cm (e.g. NGC 3079, Kondratko, Greenhill & Moran 2005; NGC 7674, Middelberg et al. 2004). Linear radio structures 1971 The extent of the jet structure in our MERLIN 5 GHz is arcsec or 49.4 pc at a PA of 125 from N to E. The PA is perpendicular to the optical polarization vector (PA = 29 ) measured by Tran (1995), as is expected in Seyfert 2 galaxies. A two-dimensional elliptical Gaussian fitting with AIPS measures an integrated radio flux for Mrk 1210 of ± 1.95 mjy (peak flux ± 1.16) mjy, in agreement with Middelberg et al. (2004) from the same 5-GHz radio data, but much lower than previous single-dish 5-GHz radio observations of this source (Griffith et al. 1995). The total radio linear extent of Mrk 1210 along the orientation of the radio jet axis is arcsec or 69 pc. The UV emission follows intimately the radio emission and it is pointed SE following the direction of the jet emanating from the core (Fig. 3, inset). Both the orientation and the elongation of the UV and radio emission are also in perfect agreement with the [O III] emission and the slight elongated to SE Hα emission visible in a point spread function subtracted Hα+[NII] image, both data presented by Falcke et al. (1998). Falcke et al. (1998) find that the [O III]/Hα ratio has an unusually high ratio of 5 for the central peak. This corresponds to the nuclear line emission that they find to be compact and <40 pc in extent, and hence corresponds to the core of our observations. This ratio is 2 for the faint extended emission to the SE and that corresponds to the possible jet emission seen in our images. 3.3 NGC 2639 MERLIN observations at 5 GHz of NGC 2639 are shown for the fist time (Fig. 4, inset image, top-left corner). The radio map unveils a compact core and two symmetric extended structures on opposite sides of the core, rich with hotspots and detailed structure. The two-sided radio structure consists of symmetric S-shaped jets, with a number of apparently discrete components, emerging from the compact core. Two small jets, directed towards SE (the left jet) and NW (the right jet), begin from the 0.2 arcsec in extent core. The first hotspots are evident very near the nucleus in each jet on opposite sides of the core. Then, at a core separation of 0.27 arcsec each jet bends by around 90, the left towards the N and the right one towards the S orientation and for about 0.2 arcsec. Following, both jets return to the initial orientation and are almost parallel to their previous SE and NW jets, respectively, each side radio emission creating an S-shape morphology. Some diffuse emission is seen further out enveloping the radio lobes on both sides of the radio structure. The right jet structure features further out two larger hotspots while the jet on the left has a more disturbed and diffused emission. Each radio lobe is 0.7 arcsec or 172 pc. The total extent of the MERLIN radio structure of NGC 2639 is 1.6 arcsec or 392 pc at a PA of 112. The size and orientation agree well with what is found from the same frequency VLA observations of Ulvestad & Wilson (1989). We measure an integrated flux of ± 4.85 mjy which when we compare with previous radio flux measurements, at the same and higher frequencies (Hummel et al. 1982; Ulvestad & Wilson 1984, 1989; Wilson et al. 1998; Roy et al. 2000; Thean et al. 2000; Ho & Ulvestad 2001; Leipski et al. 2006), adds to previous indications of the variability of the radio source. The spectrum of the nuclear source is flat between 5 and 15 GHz (Wilson et al. 1998), and so such a comparison is possible. The triple structure that we see in the MERLIN map is the same as the typical structure of an FR II type galaxy. The near-uv HST/ACS image of the host galaxy of NGC 2639 is presented in Fig. 4. This configuration is optimal to detect faint young and middle-aged star-forming regions around these nuclei,

7 1972 E. Xanthopoulos et al. Figure 3. HST ACS/HRC (F330W) (resolution arcsec) contour map of the host galaxy of Mrk Contour levels are at 2.033e-23 (1,2,4,8,16, 32, 256) erg cm 2 s 1 Å 1. The peak flux is 1.186e-20 erg cm 2 s 1 Å 1 and the rms noise level is 5.8e-24 erg cm 2 s 1 Å 1. N is up and E to the left. In the bottom-right corner, inset figure, the 5 GHz MERLIN contour map of Mrk 1210 is overlaid on the HST ACS/HRC (F330W) grey-scale image of the central structure of Mrk Contour levels are at 9.023e-4 (1,2,4,8,16,24)Jybeam 1. The peak flux is mjy beam 1 and the rms noise level is 281 μjy beam 1. The Clean beam ( arcsec 2 at PA=22. 98) is plotted in the lower-left corner. The sidebar shows the relative intensity of the grey-scale image. The central near-uv structure follows closely the extended radio emission. and separate their light from the underlying bulge emission. The inclined spiral structure is bisected by a dust lane crossing the galaxy in the SE NW direction. Large-scale dust obscuration and narrow dust lanes passing through the nucleus have also been observed by Falcke et al. (2001) with HST in Hα and continuum filters. The PA of this dust lane is almost identical to the PA of the small radio jets initially emerging from the nucleus. The inner near-uv region is more disturbed and seems to be more elongated along the radio structure. The near-uv emission axis coincides with the PA of both the small-scale radio structure and the outer larger-scale radio structure (after the jets return to their initial orientation following the bending of the jets). The core and lobes of the radio emission seem to fit perfectly within the central UV emission (seen in the smaller picture in the top-left corner of Fig. 4), that is within a region of 1.6 arcsec or 392 pc. More extended UV emission is enveloping the radio structure in a swirling type morphology. The near-uv image shows many clusters in the outer region, several kpc away from the centre. The UV image does not show a bright nucleus for NGC However, such a nucleus is seen in Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and Paα line images of NGC 2639 (Böker et al. 1999). The UV morphology is hampered by strong dust extinction.

8 Linear radio structures 1973 Figure 4. HST ACS/HRC (F330W) (resolution arcsec) contour map of NGC Contour levels are at 2.878e-23 (1,2,4,6)ergcm 2 s 1 Å 1. The peak flux is 2.858e-22 erg cm 2 s 1 Å 1 and the rms noise level is 6.7e-24 erg cm 2 s 1 Å 1. N is up and E to the left. In the top-left corner, inset figure, the 5-GHz MERLIN map of NGC 2639 is overlaid on the HST ACS/HRC (F330W) grey-scale image of the central structure of NGC Contour levels are at 1.297e-4 (1, 2, 4, 8, 16, 32, 64, 128, 256, 512) Jy beam 1. The peak flux is mjy beam 1 and the rms noise level is 48 μjy beam 1.The clean beam ( arcsec 2 at PA = ) is seen in the lower-left corner. The bar on the top shows the relative intensity of the grey-scale image. The very symmetric and rich radio structure on opposite sides of the core is evident. 3.4 NGC 4922NED02 The northern component NGC 4922NED02 (=NGC 4922B), of the galaxy pair NGC 4922 (Fig. 5), was observed at 5 GHz with MERLIN. This is the first radio map of the Seyfert nucleus at this frequency (seen in the top-right corner of Fig. 5). The MERLIN map shows a compact component, the nucleus, which is elongated in the SW NE direction following the direction of the interaction of the two galaxies in the system, and a jet of arcsec or 59 pc in extent, emerging from the nucleus and directed towards the NE. We measure a total flux of 5.39 ± 0.58 mjy for the radio emission, a total linear size of arcsec or 89 pc and a PA of 39 from NtoE. The elongation of the nucleus and the radio jet structure coincide with the elongated optical nuclear structure of NGC 4922B revealed in the HST WFPC2 F606W image of the galaxy pair NGC 4922 (Fig. 5). The optical nucleus seems also to be extended to the S of the core. The star formation appears more pronounced to the S slightly displaced from the nucleus. The optical emission follows a curved morphology just below the nucleus. The optical emission of NGC 4922B to the N ends in a double-fork morphology with tails of emission beginning from the edges of the fork and enveloping

9 1974 E. Xanthopoulos et al. Figure 5. HST WFPC2 (F606W) image of the binary system NGC N is up and E to the left. The galaxy to the S is NGC 4922A, while the galaxy to the N is NGC 4922B. The MERLIN observations were centred on NGC 4922B, the Seyfert nucleus. Contour levels are at 9.636e-20 (1,1.5,2,2.5,4,8,16,32) erg cm 2 s 1 Å 1. The peak flux is 8.035e-18 erg cm 2 s 1 Å 1 and the rms noise level is 4.7e-21 erg cm 2 s 1 Å 1. The very disturbed morphology of NGC 4922B is clearly seen. In the top-right corner, inset figure, the 5-GHz MERLIN map centred on NGC 4922B (=PGC /FIRST J ) is overlaid on the HST WFPC2 (F606W) grey-scale image of the central structure of NGC 4922B. Contour levels are at 6.0e-4 (1,2,4,6)Jybeam 1.The peak flux is mjy beam 1 and the rms noise level is 126 μjy beam 1. The clean beam ( arcsec 2 at PA = ) is plotted in the lower-left corner. The sidebar shows the relative intensity of the grey-scale image. The radio map reveals an NE jet structure.

10 the galaxy pair as diffuse emission (not shown here). One fork end is orientated in the direction of the interacting pair while the other fork end follows the direction of the inner extended contours and curved morphology orientation. 3.5 NGC 5506 Our MERLIN 5-GHz observations (Fig. 6 inset) reveal the compact core which is slightly elongated towards the NE SW direction, in agreement with the findings of Unger et al. (1986). This elongated structure is also aligned with both the galactic major axis (Wilson et al. 1976) and the PA of the optical continuum polarization (Martin et al. 1983). Such alignment between the radio axis and optical Linear radio structures 1975 continuum polarization is normally only found in Seyfert 1 galaxies (Antonucci 1983). The more outer contours of the core are pointed towards the SW where Ulvestad, Wilson & Sramek (1981) find a jet extension. A bubble-like jet of 370 mas or 51 pc is emanating from the nucleus and is directed NW. The edge of the structure is curved and seems to be bending towards the W. We also see probably the hotspot of the radio jet appearing very near the nucleus. The PA of the axis of this radio jet emission is 155. The 5-GHz observations resolve the loop identified by Wehrle & Morris (1987) and Colbert et al. (1996a). This is just traceable to the NW of the core in the Thean et al. (2000) 8.4-GHz VLA map, that also shows a lot of diffuse emission around the core. Colbert et al. (1996a) note that the diffuse, bubble-like radio structures extend out of the disc of the Figure 6. HST FOC/96 (F501N) (resolution arcsec) contour map of the host galaxy of NGC Contour levels are at 2.201e-19 ( 1, 1, 2, 4, 8, 16) erg cm 2 s 1 Å 1. The peak flux is 6.894e-18 erg cm 2 s 1 Å 1 and the rms noise level is 1.7e-19 erg cm 2 s 1 Å 1. N is up and E to the left. Both the more disturbed central emission as well as the fan-shaped extended structure are clearly seen. In the top-right corner, the 5-GHz MERLIN map of NGC 5506 is overlaid on the aligned HST FOC/96 (F501N) grey-scale image of the central structure of NGC Contour levels are at 2.185e-3 (1,2,4,6,10,16, 20, 24) Jy beam 1. The peak flux is mjy beam 1 and the rms noise level is 546 μjy beam 1. The clean beam ( arcsec 2 at PA=19. 60) is plotted in the lower-left corner. The bar on the top shows the relative intensity of the grey-scale image. The bubble-like emission extending to the NW is seen. The optical emission follows closely the radio jet emission.

11 1976 E. Xanthopoulos et al. host galaxy to radii of approximately 30 arcsec from the nucleus in PA of 140. Wehrle & Morris (1987) attribute the loop to either a bubble of hot plasma rising from the nucleus or a magnetically dominated coronal arch. We measure a total linear extent for the radio emission along the PA of the radio jet of arcsec or pc. The integrated flux density is ± 5.16 mjy and the peak flux of ± 2.66 mjy. This flux is lower than the 160 ± 8 mjy flux density found by Ulvestad et al. (1981) from GHz VLA observations of NGC This is expected since the higher resolution MERLIN observations have resolved most of the diffuse emission outside the core. Ulvestad et al. (1981) measure a spectral index of 0.56 ± 0.05 (20-6 cm) when combined with the results of de Bruyn & Wilson (1976). A core flux of ± 4.39 mjy is calculated from our 5-GHz MERLIN observations. This is also much less than that calculated by Ulvestad et al. (1981) (114 ± 6 mjy) for the unresolved radio core. Our measurements further show that there is significant emission on scales larger than the MERLIN 5 GHz beam size, as is evident in the VLA image of Colbert et al. (1996a). Recent VLBI 18- and 6-cm observations of NGC 5506 (Middelberg et al. 2004) resolve the nuclear radio emission into three compact, non-collinear core components within the central 5 pc and a diffuse region of 8 pc extent to the N. All three components and the diffuse emission to the N (80 40 mas) are contained within the compact core in our MERLIN 5-GHz map and also within the single beam of mas of the MERLIN 5-GHz data. Optically, NGC 5506 is a highly inclined system, Sa type, with dust disc and dust lanes passing close to the core (bisected nucleus). It has a bright nucleus, filaments and wisps (Malkan et al. 1998). In Fig. 6, we present the HST/FOC (F501N) image of NGC The optical narrow-line emission displays a beautiful fan-line triangular morphology ( 2.43 arcsec or 332 pc in extent) that begins at the nucleus and becomes more diffuse as it spreads towards the N. The emission just to the N of the nucleus appears very disturbed. The FOC emission has the same appearance as the F606W WFPC2 image (Malkan et al. 1998) that also shows a very bright nucleus in the apex of a fan-shaped emission extending towards the N. Malkan et al. (1998) note that the S side is the nearer side and so the jet is projected against the farthest side of the galaxy. It is clear that what we see in the HST optical images is the ionization cone on the farthest side of the galaxy where the jet is projected. Most possibly heavy dust obscuration to the S side of the core in combination with projection effects makes the opposite cone and opposite jet invisible from our view. This is supported by the results of an HST V H colour map of NGC 5506 (Martini et al. 2003), which shows a loosely wound spiral with dust lanes present to the S of the nucleus and so suggests a very dusty spiral. The colour map reveals the blue fan-shaped structure to the N of the bright nucleus with the same morphology, extent and PA as that seen in the V images. Further support comes also from NIR spectroscopy (8 13 μm) by Roche et al. (2007), who find that in NGC 5506 the depth of the silicate absorption increases from N to S across the nucleus, suggestive of a dusty structure on scales of tens of parsecs. When the MERLIN radio contours are overlaid on the optical HST/FOC (F501N) image, as can be seen in the top-right corner of Fig. 6, an intimate correspondence between the radio and optical emission of the core, the near nuclear region and the bubble jet to the NW is unveiled. Combined EVN and MERLIN 6-cm data (Middelberg et al. 2004) yield very faint diffuse structure N of the core, and this emission follows the direction of the bubble jet resolved in our 5-GHz map as well as the extended optical emission seen in our FOC image. The diffuse structure is interesting because it suggests a physical connection between the AGN and the radio emission on kpc scales observed with the VLA C-configuration by Colbert et al. (1996a). This diffuse northern emission could be plasma that has flowed through the hotspots that is either deflected by the shock or rising buoyantly in the pressure gradient in the interstellar medium (ISM). If initially jets in Seyfert galaxies are relativistic and then disrupted by the ISM, we might expect significant faint, diffuse radio emission which is actually present in NGC 5506 (brighter than 2 mjy beam 1 ) (see Middelberg et al and references therein). The axis of the optical and radio emission coincides with the minor-axis outflows from the nucleus found by Colbert et al. (1996a,b), in both the radio (VLA 4.9 GHz observations) and optical (continuum and Hα narrow-line images and long-slit spectra) regimes. Diffuse radio emission is found out to several kpc from the nucleus in the direction of the minor axis, and double-peaked emission-line profiles are found in minor-axis spectra from regions 500 pc above and below the disc (Wilson, Baldwin & Ulvestad 1985; Maiolino et al. 1994). These features imply the presence of a minor-axis wind which is blowing a shell of material northwards (and perhaps southwards) from the nucleus. 3.6 TXFS The HST/WFPC2 continuum image of the host galaxy of TXFS (Fig. 7) shows an elongated, along PA 55, and highly inclined (i = 70 ) galaxy. The galaxy has a butterfly nucleus, a sign of dust obscuration, a dust lane crossing the nucleus that presumably represents its normal ISM. In the far-ir, the galaxy is associated with the IRAS source F which has a 60- μm flux, four orders of magnitudes stronger than the far-ir extension of the normal radio power-law, indicating the presence of dust (Koekemoer et al. 1995). A higher reddening of the NW side of the galaxy compared to the SE indicates that the NW side is the nearer side of the galaxy disc. The 5-GHz MERLIN data for TXFS were first presented in Taylor et al. (2004). The reanalysed MERLIN 5-GHz map is seen in the top-right corner of Fig. 7 overlaid on the HST WFPC2 (F637N) Hα+[N II] pure line image. The two radio maps are almost identical (with the exception of the NW extended structure that is more pronounced in the Taylor et al map). The radio emission is composed of a compact core ( arcsec 2 in size) and an extended structure along the NW SE direction. This linear structure extends on both sides of the core, most possibly pinning the presence of bidirectional jets emerging from the nucleus. Detailed structure is revealed in the SE jet which consists of multiple knots and a bright possible end hotspot. The linear extent of this jet is arcsec or 271 pc at a PA of 146. The linear structure follows the same direction as the 8.4-GHz (Falcke et al. 2000) and the 22-GHz emission (Taylor et al. 2002). The more outer contours of the compact core appear to be pointed in the same direction as the pc-scale jets hinted at the highest resolution 22-GHz observations (Taylor et al. 2002) along PA The total linear extent that we measure from the MERLIN 5-GHz observations is arcsec or 410 pc along PA 144. The integrated flux density is ± 1.09 mjy which is lower than what was measured by Taylor et al. (2002, 32.2 ± 0.98 mjy at 4.86 GHz) for TXFS This value is also lower than the integrated flux density of the same 5-GHz MERLIN data of Taylor et al. (2004, 31.1 ± 0.93 mjy). When we measure the integrated flux density in our data using a window that contains the total radio structure of TXFS and the task IMSTAT, we find a value of ± 1.44 mjy, which

12 Linear radio structures 1977 Figure 7. The HST WFPC2 continuum map of the host galaxy of TXFS Contour levels are set at 5.548e-20 (1,2,4,8)ergcm 2 s 1 Å 1.The continuum peak flux is 5e-17 erg cm 2 s 1 Å 1 and the rms noise level is 1.8e-20 erg cm 2 s 1 Å 1. N is up and E to the left. The core of the galaxy can be described as a butterfly emission. In the top-right corner, the 5-GHz MERLIN contour map of TXFS is overlaid on the HST WFPC2 (F673N) narrow-band grey-scale image of the Hα emission of TXFS Contour levels are at 1.750e-4 (1, 2, 4, 8, 16, 32, 64) Jy beam 1. The peak flux is mjy beam 1 and the rms noise level is 65 μjy beam 1. The clean beam ( arcsec 2 at PA=9. 87) is plotted in the lower-left corner. The radio emission coincides with the Hα emission at the core while it seems to be deflected at the end hotspot. However, on the larger scale, the SE radio jet PA coincides with that of the Hα emission (see the Hα extended emission in the lower-left corner of the inset figure). Both the radio and optical narrow-line emissions are perpendicular to the major axis of the host galaxy. is much closer to the Taylor et al. (2004) findings. However, the peak flux of our MERLIN map (14.25 mjy) is slightly higher than the Taylor et al. (2004) peak flux of the compact core (13.6 mjy). They also find that a little less than half of the total emission, 13.6 mjy, is in the compact core with a size <45 mas, and this is exactly what we measure from our rereduced data. We believe that any difference that we find in the fluxes between the two identical maps is caused by different weighting of the same data during the mapping process that affects the proportion of flux on different baseline lengths. The Hα+[N II] emission reveals a very bright red core and a SE NW extended emission, roughly along PA 40 ± 5, in agreement with the radio emission orientation. The jet-like feature is perpendicular to the nuclear dust lane and the galaxy major axis. There appears to be a knot-to-knot and feature-to-feature strong correlation between the optical and radio emission, especially along the SE jet structure. Even the diffuse radio emission just below the core coincides with the more gaseous structure that we observe in the optical emission in the form of a plume arcsec in extent. The emission extends further towards the SW, with a broad wiggly structure near the nucleus and a blob arcsec away from the nucleus and towards the SW. This large blob of emission does not coincide with the end hotspot of the radio jet ( arcsec 2 ), as we would expect, but appears to be somehow deflected to the N of the radio emission. Most possibly, the presence of some strong and dense material or some other event taking place at this point must

13 1978 E. Xanthopoulos et al. have caused the radio emission to derail from the axis of alignment. However, as we see at the larger and more outer scale 1 kpc away, the SE radio jet emission continues the intimate relationship with the optical emission (Fig. 7, inset image). The radio and optical correlation is also evident in the opposite NW side of the core. In the radio map we see radio emission arcsec at PA 42.This emission most possibly hints at the presence of the counter jet. It agrees well with the diffuse emission to the NW along a PA of 36 resolved by Taylor et al. (2004) in VLBA 1.4-GHz observations, as well as with VLA and MERLIN orientations on somewhat larger scales. The nucleus is not very bright in Hα+[N II], presumably because of obscuration by the dust lane. The symmetric structure and tight collimation between the radio and Hα+[N II] emission suggest that the radio continuum is produced by jets oriented at a large angle to the line of sight propagating perpendicular to the accretion disc. Falcke et al. (2000) suggest that this alignment reflects an interaction between the radio jet and the ISM. They find that the axes of the nuclear dust disc, the radio emission and the optical line emission in this galaxy apparently define the axis of the AGN. The nuclear accretion disc, obscuring torus and large-scale molecular gas layer are roughly coplanar. 3.7 IC 1481 Our 5-GHz MERLIN map is the first radio observation of IC 1481 at this frequency. The observations reveal a compact component with ear-type extended structures on both sides of the core (Fig. 8 inset). The jet-like extension towards NW is arcsec (38 pc) atapaof 141, while there is a hint of a similar linear structure pointing SE. We measure a total flux of this compact structure of 1.29 ± 0.15 mjy and a peak flux of 0.99 ± 0.10 mjy beam 1.As expected, the higher resolution MERLIN observations have completely dissolved the more extended and diffuse radio emission. Hence, we find the much lower radio flux density, compared to previous radio observations (Dressel & Condon 1978; Condon, Cotton & Broderick 2002), which is mostly that of the nucleus. The total linear extent of this central radio emission is arcsec or 76 pc. At a larger field of view, there are hints of very compact features located on both sides of the central core component (not shown here). These possible hotspots follow the SE NW diagonal path at the same PA of 140 as the small-scale radio emission (total linear extent from one end hotspot to the other is 1.67 kpc). WeoverlaidtheMERLINradiomapontheHα emission-line image after aligning both observations (inset image in the lowerleft corner of Fig. 8). The Hα+[N II] image reveals a bright-red core and an E W extended optical emission (with an E plume structure) that seems to follow the radio emission. The HST WFPC2 F814W image of the host galaxy of IC 1481 is presented in Fig. 8. It shows an outer ring-like envelope and there is a bright star in the N. When we zoom into the centre, the inner contours are somewhat elongated in the direction of the radio jet-like structure. Falcke et al. (2001) suggested that the optical appearance of the system, which is very irregular, might be a site of an ongoing galaxy merger. The HST WFPC2 F547M image (Fig. 9) shows a more pronounced elongated emission along the direction of the jet extensions and has a closer alignment with the small-scale radio emission. The most characteristic feature of the V image is a fan-like extended structure lying well outside the nuclear radio source and spreading along the SE direction. The large fan-like V complex has an apex close to the end of the eastern radio jet, and a corrugated southern boundary. The eastern side is best described as a large centrally illuminated fan-like gas structure that is penetrated, accelerated and ultimately weakens the radio source. The NW optical emission appears more disturbed. This side can be described as an initially disrupted jet that fills, accelerates and then leaks out of a complex region of ionized gas. In this side, the initial encounter may have been more damaging to the cloud and hence the presence of the more disturbed optical emission. The core of the optical emission shows a bump to the W. Although a careful examination shows that most probably this feature is caused by a foreground star that happened to be in the line of sight of the nucleus of the galaxy, we also leave the small possibility that we might be dealing with a multiple/double nucleus, the result of an on going merger as Falcke et al. (2001) suggested. 4 DISCUSSION AND CONCLUSIONS It is now widely appreciated that Seyferts, in common with radio galaxies, show evidence for collimated ejection and/or radio jets. The conical or biconical morphology of the UV and optical continuum in Seyferts has been found to be aligned with the radio axis (Pogge & de Robertis 1993), and in most of these cases the emission-line gas displays morphological and kinematic signatures of disturbance or acceleration by the nuclear jet. In this area, high-resolution images obtained with MERLIN at 5 GHz have provided the clearest evidence for linear radio jets in Seyfert galaxies to date. Previous high-angular-resolution MERLIN studies have revealed highly collimated radio jets in objects which are poorly resolved in VLA images. For this reason, we observed with MER- LIN, at 5 GHz, seven radio-quiet active galaxies (Mrk 1034NED02, Mrk 1210, NGC 2639, NGC 4922NED02, NGC 5506, TXFS and IC 1481) that were found to show hints of linear structures of multiple components in previous lower resolution radio observations. We then combined the MERLIN radio data with HST archival images and used information from published data in other wavelengths, in order to (i) examine in detail the radio emission in these galaxies, (ii) search for radio-optical/near UV correspondence and (iii) look for signs of jet ISM interaction and symptoms of a disrupted medium. Our study shows the following. (1) Mrk 1034NED02 has an E W elongated core radio structure and a one-sided W jet emerging from the nucleus that is aligned with the nuclear optical emission. The HST data resolve the optical blue core that is elongated in the same E W direction. Kotilainen (1998) suggested the blue elongation might represent scattered light from the nucleus. However, he hinted at the idea that the blue structures might be due to an intrinsically non-stellar continuum, e.g. emission from high-velocity shock waves generated from the interaction of a radio jet with the ENLR gas (e.g. Sutherland 1993). According to published information, Mrk 1034NED02 has all the characteristics of the presence of circumnuclear starbursts present, i.e. strong far-ir emission (Rush et al. 1993), strong CO emission from cool dust (Kandalyan 2003), strong extended mid-ir emission and spectral features from warm dust (Rush et al. 1993) and large NIR light-to-mass ratios (Maiolino et al. 1995). These support the presence of intrinsically blue regions of (jet-induced) star formation present in the nuclear, circumnuclear and more extended regions in Mrk 1034NED02. The present data may suggest that in Mrk 1034NED02 a dense (molecular?) cloud enters the inner jet flow, perhaps brought in by galactic rotation (Kandalyan 2003). The western side jet is

14 Linear radio structures 1979 Figure 8. HST WFPC2 (F814W) (resolution 0.10 arcsec) contour map of the IC 1481 host galaxy. Contour levels are at 9.178e-20 (1, 2, 4, 8, 16, 32, 128) erg cm 2 s 1 Å 1. The peak flux is 5.824e-17 erg cm 2 s 1 Å 1 and the rms noise level is 1.5e-20 erg cm 2 s 1 Å 1. N is up and E to the left. Hints of linear structures are evident on opposite sides of the core. In the lower-left corner, the 5-GHz MERLIN contour map of IC 1481 is overlaid on the HST WFPC2 (FR680N) grey-scale image of the Hα emission of IC Contour levels are at 1.4e-4 (1,2,4,6,7)Jybeam 1. The peak flux is mjy beam 1 and the rms noise level is 65 μjy beam 1. The clean beam ( arcsec 2 at PA=21. 75) is plotted in the lower-left corner. The sidebar shows the relative intensity of the grey-scale image. The Hα emission follows the radio structure of IC 1481 (it is extended in the same direction W E as the radio bumps seen in the radio map). disrupted by this dense obstacle without significantly being distorted or accelerated (this is the point where we see the termination end of the radio jet in our images). The eastern side jet is, however, completely destroyed. After some time, the western jet begins to disrupt, ablate and accelerate the gas, either downstream or laterally as the jet drives a channel into the cloud complex. This is the case of the downward curved optical morphology after the end of the radio jet. (2) The radio emission of Mrk 1210 is composed of two components, most possibly the bright core and a SE jet. Both are contained within the central near-uv emission. The near-uv contours of the host galaxy are elongated in the same direction as the radio structure supporting close morphological radio UV connection. This suggests a possible jet cloud interaction as in NGC 7319 (Xanthopoulos et al. 2004), where jet-induced star formation in a Seyfert galaxy was unveiled for the first time through radio UV

15 1980 E. Xanthopoulos et al. Figure 9. HST WFPC2 (F547M) V-band image (resolution 0.10 arcsec) of the centre of IC The contour map is overlaid on the same grey-scale image in order to accentuate important features in the image. Contour levels are at 3.289e-20 (10, 16, 20, 26, 32, 60, 100) erg cm 2 s 1 Å 1.The peak flux is 3.727e-18 erg cm 2 s 1 Å 1 and the rms noise level is 1.05e- 20 erg cm 2 s 1 Å 1. The sidebar shows the relative intensity of the greyscale image. N is up and E to the left. Note the fan-shape emission to the SE from the nucleus, and the bump to the W of the core. correlations. If this is the case, then (i) where is the counter jet and (ii) is there evidence for the presence of cloud/dense ambient gas that interacts with the SE jet and would be responsible for destroying or inhibiting/suppressing the NW jet? Although our observations show only the SE extension, previous radio observations by Middelberg et al. (2004) may hint at a second extension to the NW, while the overall structure is a triple radio source (however, they note that both the SE and the NW components have steep radio spectra and the location of the nucleus is unclear). Evidence for the presence of cloud/dense ambient gas comes from H 2 emission that is detected up to distance of 250 pc in Mrk 1210 (Martini et al. 2003; Mazzalay & Rodríguez-Ardila 2007), both NE and SW from the centre, predominantly excited by stellar processes. This is supported by the presence of a circumnuclear starburst (Schulz & Henkel 2003), and strong H 2 O megamaser emission coming from a small region around the nucleus (Braatz, Wilson & Henkel 1994), that can be associated with star-forming regions. Furthermore, a total of nearly 300 M of hot molecular gas are measured within the inner 500 pc of Mrk It is interesting that despite the low molecular gas content Mrk 1210 displays one of the highest star formation efficiencies among water maser galaxies. This further supports the fact that the molecular lines in Mrk 1210 are mainly excited by UV heating from stars. Weak Paβ is detected in the 500 pc NE, suggesting that the extended emission is dominated by matter-bounded clouds. The analysis of the emission-line profiles supports nuclear outflow in this object (Middelberg et al. 2004). (3) Two symmetric S-shaped jets with rich morphology on opposite sides, E W direction, of a compact core compose the MERLIN 5-GHz radio structure of NGC The facts that (i) the near-uv emission axis coincides with the PA of both the small-scale radio structure and the outer larger-scale radio structure, (ii) the inner near-uv region is more elongated along the radio structure and (iii) the PA of the dust lane crossing the host galaxy is almost identical to the PA of the small radio jets emerging from the nucleus support correlation between jet emission and star formation. The S-shaped morphology, that we see in NGC 2639, is often explained by precession or wobbling of the central engine (e.g. Nakamura, Uchida & Hirose 2001; Caproni, Mosquera Cuesta & Abraham 2004). However, the fact that the left jet has a more disturbed appearance further out, compared to its counter jet, gives support to external, radio plasma and ambient medium interaction, further away from the nucleus (e.g. Fiedler & Henriksen 1984; Hardee 1982, 2003). In addition, NGC 2639 resembles in many respects the case of the LINER galaxy NGC 4278 (Giroletti, Taylor & Giovannini 2005), in which it is possible that the large bends visible in the images would be real and not amplified by geometrical effects, suggesting a strong interaction of the jets with the surrounding medium. The combination of a strong interaction with a dense medium and a low-power core could well account for the small size of NGC 2639, while relatively low-velocity jets cannot bore through the local ISM and escape. Emission in this galaxy originates in radio jets via the synchrotron process. Similar also to the powerful radio-loud AGN, NGC 2639 contains cold gas traced by CO rotational lines with double-horned line profiles suggesting a rotating disc in the nuclear region (Raluy, Planesas & Colina 1998). The mapped region has a diameter of 40 arcsec, which corresponds to a size 9.8 kpc, and is extended in the direction NW SE, in agreement with the PA of the galaxy (140 ). The distribution of the integrated emission has an elliptical shape with major and minor axes of arcsec 2, in agreement with the 58 inclination angle of the galaxy. The accumulation of clouds of dense molecular gas around the nucleus of this water maser galaxy seems necessary for the water megamaser to be produced (Martin et al. 1989; Krause et al. 1990). The anticorrelation found also between the molecular gas surface density in the inner region of NGC 2639 (Raluy et al. 1998) and the variation rate of the maser intensity can be interpreted in terms of the maser emission being produced or further enhanced by the interaction of the nuclear jets with clouds of matter surrounding the active nucleus at different scales. (4) The radio emission of NGC 4922NED02 is composed of a compact component elongated in the SW NE direction and a NE jet, all contained within the optical core of the host galaxy. NGC 4922B is a known H 2 O maser (Henkel et al. 2005) that belongs to their jet maser sample. These are objects that contain nuclear jets that are orientated close to the disc of the galaxy and the plane of the sky and provide insight into the interaction of nuclear jets with dense warm molecular gas in the central parsecs of galaxies. There is plentiful evidence for dense material in the central parsecs of NGC 4922B. 21-cm line observations (Gavazzi 1987) reveal a complex H I spectrum with a velocity consistent with the velocity derived from the optical emission lines of NGC 4922B, while the measurements of low-lying CO rotational line transitions at millimetre and submillimetre wavelengths, that are often used as tracers of molecular hydrogen (Yao et al. 2003), add more support to dense material in NGC 4922B. The production of the soft X-rays in this galaxy is star formation activity via supernova remnants and/or Population I massive X-ray binaries, plus a large-scale supernova-driven wind (see also Domingue, Sulentic & Durbala 2005). Ground-based, mid-ir data suggest that the northern component of the NGC 4922 pair of galaxies is also responsible for more than 90 per cent of the total IRAS flux (Alonso-Herrero et al. 1999; Gorjian et al. 2004), indicating the presence of warm dust heated by either an active nucleus and/or star formation processes in this Seyfert 2/LINER galaxy. It is possible that in NGC 4922B jet/ism interaction causes the N jet to be halted/smothered (and may be a counterjet to be completely