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Astronomy Astrophysics A&A 417, 925–944 (2004) Astronomy DOI: 10.1051/0004-6361:20040019 & c ESO 2004 Astrophysics Motion and properties of nuclear radio components in Seyfert galaxies seen with VLBI E. Middelberg1,A.L.Roy1,7,N.M.Nagar2,9, T. P. Krichbaum1, R. P. Norris3,A.S.Wilson4, H. Falcke1,8, E. J. M. Colbert5, A. Witzel1, and K. J. Fricke6 1 Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, 53121 Bonn, Germany e-mail: emiddelb;aroy;tkrichbaum;[email protected] 2 Kapteyn Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands e-mail: [email protected] 3 Australia Telescope National Facility, PO Box 76, Epping, NSW, Australia e-mail: [email protected] 4 Department of Astronomy, University of Maryland, College Park, MD 20742, USA e-mail: [email protected] 5 John Hopkins University, Department of Physics and Astronomy, 3400 North Charles Street, Baltimore, MD 21218, USA e-mail: [email protected] 6 Universit¨ats-Sternwarte, Geismarlandstrasse 11, 37083 G¨ottingen, Germany e-mail: [email protected] 7 Geod¨atisches Institut der Universit¨at Bonn, Nussallee 17, 53115 Bonn, Germany 8 ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands e-mail: [email protected] 9 INAF, Arcetri Observatory, Largo E. Fermi 5, Florence 50125, Italy Received 28 November 2002 / Accepted 14 January 2004 Abstract. We report EVN, MERLIN and VLBA observations at 18 cm, 6 cm and 3.6 cm of the Seyfert galax- ies NGC 7674, NGC 5506, NGC 2110 and Mrk 1210 to study their structure and proper motions on pc scales and to add some constraints on the many possible causes of the radio-quietness of Seyferts. The component configurations in NGC 7674 and NGC 2110 are simple, linear structures, whereas the configurations in NGC 5506 and Mrk 1210 have multiple components with no clear axis of symmetry. We suggest that NGC 7674 is a low-luminosity compact symmetric object. Comparing the im- ages at different epochs, we find a proper motion in NGC 7674 of (0.92±0.07) c between the two central components separated by 282 pc and, in NGC 5506, we find a 3 σ upper limit of 0.50 c for the components separated by 3.8 pc. Our results confirm and extend earlier work showing that the outward motion of radio components in Seyfert galaxies is non-relativistic on pc scales. We briefly discuss whether this non-relativistic motion is intrinsic to the jet-formation process or results from deceleration of an initially relativistic jet by interaction with the pc or sub-pc scale interstellar medium. We combined our sample with a list compiled from the literature of VLBI observations made of Seyfert galaxies, and found that most Seyfert nuclei have at least one flat-spectrum component on the VLBI scale, which was not seen in the spectral indices measured at arcsec resolution. We found also that the bimodal alignment of pc and kpc radio structures displayed by radio galaxies and quasars is not displayed by this sample of Seyferts, which shows a uniform distribution of misalignment between 0◦ and 90◦. The frequent misalignment could result from jet precession or from deflection of the jet by interaction with gas in the interstellar medium. Key words. galaxies: seyfert – galaxies: active – galaxies: jets 1. Introduction workers since. Members of the radio-loud category include BL Lac objects, radio galaxies and radio-loud quasars. Radio- Active galaxies are conventionally divided into radio-quiet loud objects have large-scale radio lobes and compact lumi- and radio-loud categories. Kellermann et al. (1989) found that nous cores which often exhibit apparent superluminal motion. quasars separated into these two classes when the ratio of their Radio-quiet objects include Seyfert galaxies and radio-quiet radio to optical continuum was considered, and a similar re- quasars; most of their radio emission is confined to the sub-kpc sult has been found by most (e.g. Peacock et al. 1986; Miller scale. et al. 1990; Ivezi´c et al. 2002) but not all (White et al. 2000) Recent Very Long Baseline Interferometry (VLBI) has Send offprint requests to: E. Middelberg, shown that many Seyferts also contain compact nuclear ra- e-mail: [email protected] dio components with brightness temperatures 107 K despite Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20040019 926 E. Middelberg et al.: Motion and properties of nuclear radio components in Seyfert galaxies seen with VLBI having radio luminosities over a factor of 108 less than the most emission mechanisms, source compactness, and possible fore- powerful active galaxies (e.g. Ulvestad et al. 1998; Ulvestad ground free-free absorption. Most spectral index measure- et al. 1999a; Ulvestad et al. 1999b; Roy et al. 2000; Mundell ments of Seyfert galaxies to date have used the VLA, ATCA et al. 2000), and that they harbour compact components mov- or WSRT, whose relatively large beams can average together ing at non-relativistic speeds. In all cases, the central masses many components in the nucleus. Measurements with parsec- are inferred to be black holes of millions to billions of solar scale resolution in Seyfert galaxies enable one to separate a masses, so the central potentials are relativistically deep. Since possible compact, flat-spectrum component from surrounding most accretion energy is released close to the hole, jets might steep-spectrum, optically-thin synchrotron emission, but such be expected to be relativistic in all cases. Why then are the ra- measurements require VLBI with matched beams at two fre- dio sources in Seyferts small, weak and slow compared to those quencies and are still relatively few. In contrast, powerful radio in radio-loud objects? sources have been well studied with VLBI and flat-spectrum Various answers to this question have been proposed, in- core components are often seen at the base of a conically- volving either “intrinsic” differences in the central engine or expanding jet, thus indicating the location of the nucleus. Do “extrinsic” differences in the surrounding medium. Intrinsic Seyfert nuclei also show a single, compact, synchrotron self- differences that have been suggested include (i) systematically absorbed core component at the base of a collimated jet? lower black hole masses in Seyferts (Laor 2000), (ii) lower To meet these goals, we observed using Global VLBI, the black hole spins (Wilson & Colbert 1995), (iii) a “magnetic European VLBI Network (EVN), the Multi-Element Radio switch” that was identified by Meier et al. (1997) during nu- Linked Interferometer Network (MERLIN) and the Very Long merical modelling of jets, (iv) the production in Seyferts of Baseline Array (VLBA) the Seyfert galaxies NGC 5506 buoyant plasmons that bubble up through the density gradi- and NGC 7674 at multiple epochs for proper motion measure- ent of the narrow line region (NLR) instead of a collimated ments of nuclear radio sources, and Mrk 1210 and NGC 2110 relativistic jet (Pedlar et al. 1985; Whittle et al. 1986; Taylor for the first epoch. To enlarge the sample size for deriv- et al. 1989), (v) a large thermal plasma fraction in the jet in ing statistics on the spectral and geometric properties of the Seyferts (Bicknell et al. 1998), or (vi) radiative inefficiency pc scale structure of Seyferts, we made a complete literature (Falcke & Biermann 1995). Extrinsic differences generally in- review of VLBI observations of Seyfert galaxies. We assume −1 −1 voke the rapid deceleration of initially relativistic jets by col- H0 = 75 km s Mpc and q0 = 0.5 throughout this paper, and lisions in a dense surrounding BLR or ISM (e.g. Norman & calculate spectral indices using S ∝ να. Miley 1984). With so many possible causes, and so few ob- servational constraints, the question remains open why Seyfert 2. The sample and observations galaxies have such low radio luminosities. One approach to investigate whether the radio quietness of The galaxies presented here are part of a sample selected for Seyferts is due to intrinsic or extrinsic causes is to measure global VLBI observations in 1994 to investigate the dichotomy speeds of radio components as close to the nucleus as possible, between radio-loud and radio-quiet AGN. The sample com- hopefully before interactions with the ISM affect them much. prised 16 Seyfert 2 galaxies from V´eron-Cetty & V´eron (1991) How close that needs to be is unclear, but signs of jet-NLR in- with 6 cm core flux densities >70 mJy and δ>−10◦. The lit- teraction have been seen on scales of tens of pc (Whittle et al. erature was then carefully reviewed for updated spectral clas- 1986; Wilson & Tsvetanov 1994; Bower et al. 1995; Capetti sifications, some of the objects were discarded and others were et al. 1997; Falcke et al. 1998), so sub-pc scales are desir- incorporated based on their new classification and flux density able. Proper motions of Seyfert galaxy radio components have measurements. now been measured with VLBI in a few cases (e.g. Mrk 231 Global VLBI, 1994. The sample was observed in 1994 and Mrk 348 by Ulvestad et al. 1999b, NGC 1068 by Roy et al. with global VLBI at 6 cm, using the ten VLBA stations, 2000, and NGC 4151 by Ulvestad et al. 1998; Ulvestad et al. the phased VLA, Effelsberg, Onsala, Medicina and Noto 2002), and the motions were found to be ≤ 0.25 c. In III Zw 2, for 36 min in six snapshots over 11 h, yielding a ∼30 min in- v<0.04 c was seen between two barely-resolved compo- tegration of each object (Krichbaum et al.
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