arXiv:astro-ph/0402142v1 6 Feb 2004 ai ootclcniumwscniee,adasmlrres similar a and th considered, of was ratio continuum the optical when to classes radio t two these found radio-qui into (1989) separated al. into quasars et Kellermann divided categories. conventionally radio-loud and are galaxies Active Introduction 1. edo Send .Middelberg, E. DI ilb netdb adlater) hand by inserted be will Astrophysics (DOI: & Astronomy oinadPoete fNcerRdoCmoet nSeyfer in Components Radio Nuclear of Properties and Motion ff on lota h ioa lgmn fp n p ai stru radio kpc and pc s of not alignment was bimodal the which that scale, also Se VLBI found of the on made sub component observations or flat-spectrum VLBI pc one of the literature the with from interaction compiled by jet relativistic initially pcs efidapoe oini G 64o (0 of 7674 NGC in com motion low-luminosity proper a a is find 7674 we NGC epochs, that suggest NG We symmetry. in configurations of Seyferts the of pr whereas radio-quietness and structures, the linear structure simple, their of study causes to possible 1210 many Mrk the and 2110 NGC 5506, NGC Abstract. 2004 1, January accepted 2002; 1, January Received n,i G 56 efida3 a find we 5506, NGC in and, e words. Key jet the of deflection from or precession jet from result could 1 2 3 4 hssml fSyet,wihsosauiomdistribution uniform a shows which Seyferts, of sample this int compon is radio motion of non-relativistic this motion whether outward discuss the briefly that showing work earlier 5 6 7 8 9 rn eussto requests print e-mail: H¨ugel dem Auf f¨ur Radioastronomie, Max-Planck-Institut aty nttt,Uiest fGoign adee 12, e-mail: Landleven Groningen, of University Institute, Kapteyn utai eecp ainlFclt,P o 6 pig N Epping, 76, e-mail: Box PO Facility, National Telescope Australia eateto srnm,Uiest fMrln,CleeP College e-mail: Maryland, of University Astronomy, of Department onHpisUiest,Dprmn fPyisadAstrono and e-mail: Physics of Department University, Hopkins John e-mail: D-370 11, Universit¨ats-Sternwarte, Geismarlandstrasse edaice ntttdrUiesta on usle 1 Nussallee Universit¨at Bonn, der Geod¨atisches Institut SRN ..Bx2 90A wneo,TeNetherlands The Dwingeloo, e-mail: AA 7990 2, Box P.O. ASTRON, NF rer bevtr,LroE em ,Foec 50125 Florence 5, Fermi E. Largo Observatory, Arcetri INAF, mdeb ry kiham [email protected] tkrichbaum, aroy, emiddelb, [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] 1 erpr V,MRI n LAosrain t1 m mad3 and cm 6 cm, 18 at observations VLBA and MERLIN EVN, report We aais efr,Glxe:atv,Glxe:jets Galaxies: active, Galaxies: Seyfert, Galaxies: .L Roy, L. A. .Middelberg E. : aucitn.h4137 no. manuscript 1 , 7 .M Nagar, M. N. σ pe ii f0 of limit upper aaisSe ihVLBI with Seen Galaxies Colbert, 2 , 9 . 5 .P Krichbaum, P. T. 50 .Witzel A. c . 92 o h opnnssprtdb 3 by separated components the for 3G¨ottingen, Germany 83 isct h e-omto rcs rrslsfo deceler from results or process jet-formation the to rinsic p cl neselrmdu.W obndorsml iha with sample our combined We medium. interstellar scale -pc ± ,D515Bn,Germany Bonn, D-53115 7, hat ult eir 56adMk11 aemlil opnnswt oceraxi clear no with components multiple have 1210 Mrk and 5506 C et 0 h opnn ofiuain nNC77 n G 10are 2110 NGC and 7674 NGC in configurations component The . yitrcinwt a nteitrtla medium. interstellar the in gas with interaction by fmslgmn ewe 0 between misalignment of 9 -32 on Germany Bonn, D-53121 69, . 77A rnne,TeNetherlands The Groningen, AD 9747 trsdslydb ai aaisadqaasi o displa not is quasars and galaxies radio by displayed ctures 07) W Australia SW, fr aais n on htms efr ulihv tle at have nuclei Seyfert most that found and galaxies, yfert 1 y 40NrhCalsSre,Blioe D228 USA 21218, MD Baltimore, Street, Charles North 3400 my, Italy , r,M 04,USA 20742, MD ark, hc fe xii paetspruia oin Radio-q motion. cor superluminal luminous apparent exhibit compact often and which lobes radio large-scale Radio-lou have quasars. jects Lac radio-loud BL work and include galaxies 2000) category radio al. radio-loud objects, et the al. (White of et all Members Miller since. not 1986, ers but al. 2002) et al. Peacock Ivezi´c et (e.g. 1990, most by found been has n .J Fricke, J. K. and nsi efr aaisi o-eaiitco csae.W scales. pc on non-relativistic is galaxies Seyfert in ents e nteseta nie esrda rscrslto.W resolution. arcsec at measured indices spectral the in een c atsmercojc.Cmaigteiae tdi at images the Comparing object. symmetric pact ewe h w eta opnnssprtdb 8 pc 282 by separated components central two the between prmtoso csae n oadsm osrit on constraints some add to and scales pc on motions oper 1 .P Norris, P. R. 6c fteSyetglxe G 7674, NGC galaxies Seyfert the of cm .6 6 3 ◦ .S Wilson, S. A. . c u eut ofimadextend and confirm results Our pc. 8 n 90 and ◦ h rqetmisalignment frequent The . 4 .Falcke, H. to fan of ation e by yed 1 ff ue1,2018 15, June , erent 8 list .J M. J. E. ast e e s ob- d t uiet es - 2 Middelberg et al.: Motion and Properties of Nuclear Radio Components in Seyfert Galaxies Seen with VLBI objects include Seyfert galaxies and radio-quiet quasars; most slowed by interaction with the NLR gas. Were such braking of their radio emission is confined to the sub-kpc scale. to occur, one would expect shocks in the jet and possibly jet Recent Very Long Baseline Interferometry (VLBI) has disruption. The in-situ particle acceleration associated with shown that many Seyferts also contain compact nuclear ra- shocks could lead then to extended regions of radio emis- dio components with brightness temperatures ≫ 107 K despite sion. 3) to measure spectral indices, which provide a useful having radio luminosities over a factor of 108 less than the most diagnostic of emission mechanisms, source compactness, and powerful active galaxies (e.g. Ulvestad et al. 1998; Ulvestad possible foreground free-free absorption. Most spectral index et al. 1999a; Ulvestad et al. 1999b; Roy et al. 2000; Mundell measurements of Seyfert galaxies to date have used the VLA, et al. 2000), and that they harbour compact components mov- ATCA or WSRT, whose relatively large beams can average ing at non-relativistic speeds. In all cases, the central masses together many components in the nucleus. Measurements are inferred to be black holes of millions to billions of solar with parsec-scale resolution in Seyfert galaxies enable one masses, so the central potentials are relativistically deep. Since to separate a possible compact, flat-spectrum component most accretion energy is released close to the hole, jets might from surrounding steep-spectrum, optically-thin synchrotron be expected to be relativistic in all cases. Why then are the ra- emission, but such measurements require VLBI with matched dio sources in Seyferts small, weak and slow compared to those beams at two frequencies and are still relatively few. In in radio-loud objects? contrast, powerful radio sources have been well studied with Various answers to this question have been proposed, in- VLBI and flat-spectrum core components are often seen at the volving either “intrinsic” differences in the central engine or base of a conically-expanding jet, thus indicating the location “extrinsic” differences in the surrounding medium. Intrinsic of the nucleus. Do Seyfert nuclei also show a single, compact, differences that have been suggested include (i) systematically synchrotron self-absorbed core component at the base of a lower black hole masses in Seyferts (Laor 2000), (ii) lower collimated jet? black hole spins (Wilson & Colbert 1995), (iii) a “magnetic switch” that was identifiedby Meier et al. (1997)duringnumer- To meet these goals, we observed using Global VLBI, the ical modeling of jets, (iv) the production in Seyferts of buoyant European VLBI Network (EVN), the Multi-Element Radio plasmons that bubble up through the density gradient of the Linked Interferometer Network (MERLIN) and the Very Long narrow line region (NLR) instead of a collimated relativistic jet Baseline Array (VLBA) the Seyfert galaxies NGC 5506 and (Pedlar et al. 1985, Whittle et al. 1986, Taylor et al. 1989), (v) NGC 7674 at multiple epochs for proper motion measurements a large thermal plasma fraction in the jet in Seyferts (Bicknell of nuclear radio sources, and Mrk 1210 and NGC 2110 for the et al. 1998), or (vi) radiative inefficiency (Falcke & Biermann first epoch. To enlarge the sample size for deriving statistics on 1995). Extrinsic differences generally invoke the rapid decel- the spectral and geometric propertiesof the pc scale structure of eration of initially relativistic jets by collisions in a dense sur- Seyferts, we made a complete literature review of VLBI obser- −1 −1 rounding BLR or ISM (e.g. Norman & Miley 1984). With so vations of Seyfert galaxies. We assume H0 = 75 kms Mpc many possible causes, and so few observational constraints, the and q0 = 0.5 throughout this paper, and calculate spectral in- question remains open why Seyfert galaxies have such low ra- dices using S ∝ να. dio luminosities. One approach to investigate whether the radio quietness of 2. The Sample and Observations Seyferts is due to intrinsic or extrinsic causes is to measure speeds of radio components as close to the nucleus as possible, The galaxies presented here are part of a sample selected for hopefully before interactions with the ISM affect them much. global VLBI observations in 1994 to investigate the dichotomy How close that needs to be is unclear, but signs of jet-NLR between radio-loud and radio-quiet AGN. The sample com- interaction have been seen on scales of tens of pc (Whittle et al. prised 16 Seyfert 2 galaxies from V´eron-Cetty & V´eron (1991) 1986; Wilson & Tsvetanov 1994; Bower et al. 1995; Capetti with 6 cm core flux densities > 70 mJy and δ > −10◦. The lit- et al. 1997; Falcke et al. 1998), so sub-pc scales are desirable. erature was then carefully reviewed for updated spectral clas- Proper motions of Seyfert galaxy radio components have now sifications, some of the objects were discarded and others were been measured with VLBI in a few cases (e.g. Mrk 231 and incorporated based on their new classification and flux density Mrk 348 by Ulvestad et al. 1999b, NGC 1068 by Roy et al. measurements. 2000, and NGC 4151 by Ulvestad et al. 1998, Ulvestad et al. Global VLBI, 1994. The sample was observed in 1994 2002), and the motions were found to be ≤ 0.25 c. In IIIZw2, with global VLBI at 6 cm, using the ten VLBA stations, v < 0.04 c was seen between two barely-resolved components the phased VLA, Effelsberg, Onsala, Medicina and Noto for over eight months, followed by a sudden displacement at v ≥ 36 min in six snapshots over 11 h, yielding a ∼ 30 min integra- 1.25 c over seven months (Brunthaler et al. 2000). tion of each object (Krichbaum et al. 1994, unpublished). The The observations presented here were made for three observations yielded only the detection of point sources in the reasons: 1) to measure proper motions of radio components cases of NGC 7674,NGC 2110 and Mrk 1210,and lacked short on pc scales. 2) to look for extended, low surface-brightness baselines and spectral index information. Only the NGC 5506 radio emission on the scale of the NLR. In the event that slow data are included in this paper. proper motions were measured, the next question would be EVN and MERLIN, 1999/2000. Four of those objects whether components were ejected slowly or whether they (NGC 5506, NGC 7674, NGC 2110 and Mrk 1210) were re- were ejcted at high, perhaps relativistic velocities and then observed in 1999 and 2000 with better sensitivity and better Middelberg et al.: Motion and Properties of Nuclear Radio Components in Seyfert Galaxies Seen with VLBI 3
Source RA (J2000) DEC (J2000) D Type Purpose Position reference NGC 5506 14 13 14.87926 −03 12 27.6514 24.71a) S1n1b) Science Target Roy et al. (1997)1c) 1404-015 14 04 45.8949 −01 30 21.937 Phase calibrator - NGC 7674 23 27 56.712 +08 46 44.14 1162a) S1h2b) Science Target Krichbaum et al. (1994)2c) 2327+096 23 27 33.5808 +09 40 09.460 Phase calibrator Browne et al. (1998) NGC 2110 05 52 11.376 −07 27 22.52 31.13a) S1i3b) Science Target Ulvestad & Wilson (1983)3c) 0541-056 05 41 38.0848 −05 41 49.395 Phase calibrator - Mrk 1210 08 04 05.856 +05 06 49.83 53.94a) S1h4b) Science Target Falcke et al. (1998)4c) 0803+043 08 03 56.4444 +04 21 02.724 Phase calibrator Browne et al. (1998) Table 1. List of observed sources. Column 2 and 3 give the coordinates used in the observations and for correlation, column 4 gives the source distances in Mpc. Column 5 gives the classification according to V´eron-Cetty & V´eron (2001). Remarks: 1a) Keel (1996), 1b) Nagar et al. (2002b), 1c) Coordinates from Roy et al. (1997) observations (unpublished), which based on VLA observations by Ulvestad & Wilson (1984), 2a) Nishiura et al. (2000), 2b) V´eron-Cetty & V´eron (2001), 2c) VLA 6 cm (unpublished), 23 25 24.410 +08 30 12.60 (B1950) precessed to J2000 by NRAO’s scheduling software SCHED, Keel (1996), 3a) Nelson & Whittle (1995), 3b) V´eron-Cetty & V´eron (1998), 3c) 05 49 46.376 −07 28 01.99 (B1950) precessed to J2000 by SCHED, 4a) de Vaucouleurs et al. (1991), 4b) V´eron-Cetty & V´eron (1998), 4c) Based on 6 cm VLA observations from Falcke et al. (1998).
(u, v)-plane coverage using EVN and MERLIN at 18 cm and – NGC 7674 was observed at 18 cm on April 13, 1985 by 6 cm. Unger et al. (1988) with four EVN stations using a band- The observations were made with eight EVN stations width of 28 MHz and dual circular polarization. We re- (Effelsberg, Cambridge 32 m, Jodrell Bank Mk 1 at 18 cm reduced those data from the archive in Bonn following the and Jodrell Bank Mk 2 at 6 cm, Medicina, Noto, Onsala 25 m, same procedures as for the 1999 EVN 18 cm data, yield- Westerbork array, Torun) at 18 cm (1.65GHz) on November ing images with rms = 0.46 mJybeam−1 and a beam size 11, 1999, and at 6 cm (4.99GHz) on February 28, 2000. The of 39.3 × 24.2 mas. Amplitude calibration data were not 18 cm EVN observations were recorded in MkIII mode with a archived, so we used Tsys and gain measurements from the bandwidth of 56 MHz, 1 bit sampling and dual circular polar- immediately preceding experiment (Porcas, priv. comm.). ization, and were correlated in Bonn on the MkIII correlator. – Mrk 1210 was observed on August 6, 1998 with the VLBA The 6 cm EVN observations were recorded in the new MkIV at 18 and 6 cm with on-source times of 160 min and mode with a bandwidthof 32 MHz, 2 bit sampling and dual cir- 140 min, a bandwidth of 32 MHz, 2 bit sampling and dual cular polarization, and were correlated at the Joint Institute for circular polarization. Phase-referencing was made to the VLBI in Europe (JIVE). The MERLIN observations were cor- same calibrator as for the EVN and MERLIN observations. related in real time with a bandwidthof 32 MHz, 2 bit sampling The observations yielded thermal noise limited images and dual circular polarization. As all sources are weak, phase- with resolutions of 18.6 × 14.2 mas and 3.9 × 1.8 mas at referencing was used for initial phase calibration. Detailsofthe 18cm and6 cm. calibrators are given in Table 1. All calibrators lie within 6◦ of the targets. Phase calibrator scans were obtained every 5 min to 10 min at 18 cm and every4 min at 6 cm. Data reduction was done using the Astronomical Image Additional data. Further observationspresented in this pa- Processing System (AIPS). The EVN and VLBA amplitudes per were done in the framework of other projects: were calibrated using Tsys measurements and the MERLIN amplitudes were calibrated using the flux density calibrator – NGC 5506 was observed during two further VLBI experi- DA 193 assuming flux densities of 2.12 Jy at 18 cm and ments, designed for proper motion measurement. 5.15 Jy at 6 cm. Residual delays and phases were determined (i) February 5, 1997: NGC 5506 was observed with the by fringe fitting on the phase reference calibrators and in- ten VLBA stations at 18 cm, 6 cm and 3.6 cm with on- terpolating the solutions onto the targets. If the target was source times of 91 min, 64 min and 43 min, a bandwidth of visible in the resulting phase-referenced image, one to three 32 MHz, 2 bit sampling and right circular polarization. iterations of phase self-calibration and deconvolution using (ii) May 31, 2000: NGC 5506 was observed with the ten CLEAN were performed until a thermal noise limited im- VLBA stations at 18 cm and 6 cm with on-source times of age was achieved. Absolute position information is then pre- 204 min at each wavelength, a bandwidth of 32 MHz, 2 bit served as the self-calibration uses phase-referenced, and there- sampling and left circular polarization. fore position-referenced, model components. If the target was The 1994 and 2000 VLBA observations and the EVN ob- not visible after phase referencing,the data were self-calibrated servations were reduced in AIPS using the same procedure using a point source as the initial model. In this case, absolute for each (see below), including the same weighting, taper position information was lost (see Table 2). and restoring beam size, and the 1997 VLBA observations Absolute source positions were measured from the phase- were reduced using the same methods except for slightly referenced, self-calibrated images, where available; other- different tapering. All 18 cm and 6 cm images are shown in wise, relative source positions were measured from the self- Fig. 1. calibrated images. Positions were always measured by fittinga 4 Middelberg et al.: Motion and Properties of Nuclear Radio Components in Seyfert Galaxies Seen with VLBI two-dimensional parabola to the source and recording the po- rection (i.e. perpendicular to the disk), but could not establish sition of the peak. This was found superior to fitting Gaussians whether this is starburst or AGN driven. At higher resolution because the images of the sources were not always well de- (∼ 0.7′′), a MERLIN18cm imagebyUngeret al. (1986)shows scribed by a Gaussian. Phase-referenced positions are based that NGC 5506 has a compact core with a diffuse halo of a few on the calibrator positions in Table 1. Absolute position un- arcseconds diameter. The compact core is slightly extended and certainties are typically 10−7 times the source-calibrator sep- aligns with the major axis of the host galaxy and with the posi- aration (Lestrade 1991). Our random position uncertainties tion angle of the optical continuum polarization (Martin et al. should be a small fraction of the beamwidth, and proportional 1983). This is contrary to the situation in most Seyfert 2s, in to beamwidth/SNR, where SNR is the signal to noise ratio of which the polarization tends to be perpendicular to the jet axis the source component. (Antonucci & Miller 1985) and strengthens the Sy 1 classifica- Flux densities were measured by integrating over the source tion. region in the image plane. The estimated 1 σ flux density errors A water maser was discovered in NGC 5506 by Braatz comprise a 5 % (MERLIN) or 10 % (other arrays) flux density et al. (1994) during a survey of Seyfert galaxies. Unfortunately, scale calibration uncertainty, ≈ 0.2 mJy additive thermal noise, its weakness, 60 mJy in the broad component, has prevented the exact value depending on the source size and the particular detailed VLBI imaging. image, and 5 % to 10 % uncertainty in the integration over the source region, depending on the size and flux density. The combined estimated errors of the integrated flux densities in 3.1.1. Results Table2 representimagerms noiseplus 11 % (S ν < 20 mJy) and 7%(S ν > 20 mJy) for MERLIN observations and 15 % (S ν < 20mJy) and11% (S ν > 20 mJy) for other VLBI observations. The peak flux density errors are image rms noise plus 5 % for B1 6 cm B1 6 cm 10 B0 10 MERLIN observations and 10 % for other VLBI observations. B0 0 B2 0 B2 Spectral indices were measured by tapering the 6 cm data -10 -10 1994.16 1994.16 global VLBI global VLBI and restoring with the 18 cm beam size. The uncertainty on the 18 cm spectral indices, based on the uncertainties on the flux density 10 measurements is 0.09 (1 σ) for a 20 mJy component. Despite 0 10 the taper, some beam mismatch remained because the 18 cm -10 0 1997.10 -10 and tapered 6 cm arrays were not perfectly scaled arrays; the VLBA resulting magnitude of the effect is difficult to quantify because milliarcsec 10 it depends on the source structure. The largest effect probably 1997.10 0 VLBA comes from the short baselines at 18 cm, since these were not -10 removed to match the 6 cm short-baseline length. This would milliarcsec