Motion and Properties of Nuclear Radio Components in Seyfert Galaxies Seen with VLBI

Astronomy & Astrophysics manuscript no. h4137 June 15, 2018 (DOI: will be inserted by hand later) Motion and Properties of Nuclear Radio Components in Seyfert Galaxies Seen with VLBI E. Middelberg,1 A. L. Roy,1,7 N. M. Nagar,2,9 T. P. Krichbaum,1 R. P. Norris,3 A. S. Wilson,4 H. Falcke,1,8 E. J. M. Colbert,5 A. Witzel1 and K. J. Fricke,6 1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-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 Universitäts-Sternwarte, Geismarlandstrasse 11, D-37083 Göttingen, Germany e-mail: [email protected] 7 Geodätisches Institut der Universität Bonn, Nussallee 17, D-53115 Bonn, Germany 8 ASTRON, P.O. Box 2, 7990 AA Dwingeloo, The Netherlands e-mail: [email protected] 9 INAF, Arcetri Observatory, Largo E. Fermi 5, Florence 50125, Italy Received January 1, 2002; accepted January 1, 2004 Abstract. We report EVN, MERLIN and VLBA observations at 18 cm, 6 cm and 3.6 cm of the Seyfert galaxies 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 images 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 arXiv:astro-ph/0402142v1 6 Feb 2004 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 has been found by most (e.g. Peacock et al. 1986, Miller et al. 1990, Ivezićet al. 2002) but not all (White et al. 2000) work- Active galaxies are conventionally divided into radio-quiet ers since. Members of the radio-loud category include BL Lac and radio-loud categories. Kellermann et al. (1989) found that objects, radio galaxies and radio-loud quasars. Radio-loud ob- quasars separated into these two classes when the ratio of their jects have large-scale radio lobes and compact luminous cores radio to optical continuum was considered, and a similar result which often exhibit apparent superluminal motion. Radio-quiet Send offprint requests to: E. Middelberg 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 Véron-Cetty & Véron (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.

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