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THE RADIO PROPERTIES of SEYFERT NUCLEI 1 Introduction 2

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THE RADIO PROPERTIES of SEYFERT NUCLEI 1 Introduction 2 THE RADIO PROPERTIES OF SEYFERT NUCLEI ANDY THEAN Istituto di Radioastronomia, C.N.R., via P.Gobetti, 101, 40129 Bologna, ITALY E–mail: [email protected] ALAN PEDLAR Jodrell Bank, University of Manchester, Macclesfield, Cheshire, SK11 9DL, U.K. MAREK KUKULA Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, U.K. STEFI BAUM, CHRIS O’DEA STScI, 3700 San Martin Drive, Baltimore, Maryland, 21218, U.S.A. The radio structures produced by Seyfert nuclei are small and difficult to detect using current tech- nology, nevertheless, their strengths, sizes, morphologies, orientations and spectral energy distri- butions provide a rich source of information about the central engines of AGN. We have made a high–resolution radio survey of a large sample of Seyfert galaxies. Here we discuss the effects of instrumental limitations on our results and assess the impact of the Square Kilometer Array (SKA) on future studies of this kind. The sensitivity of the SKA would significantly improve coverage of the radio luminosity versus redshift plane for Seyfert galaxies and this would improve the struc- tural information available about nuclear radio sources. The resolution of the SKA would facilitate spectral index measurements of compact radio components. 1 Introduction Seyfert nuclei are a common class of Active Galactic Nuclei (AGN) usually found in spiral galaxies. Important information about the way they release energy can be obtained from radio observations of their inner parsecs. Radio measurements are free from the effects of dust, which plays a key rˆole in determining the properties of Seyferts at other wavelengths, and allow direct comparisons between obscured and un–obscured objects. Where collimated radio structures are observed they indicate the strength and orientation of nuclear outflows and by studying the effect of these outflows on the surrounding inter–stellar medium we may attempt to parameterise their energies and ages [1–4]. However, the radio structures produced by Seyfert nuclei are rarely larger than a kiloparsec or brighter than tens of millijanskys and therefore, using current technology, the quality of the radio information available for Seyfert galaxies rarely rivals that available for powerful radio galaxies. In this article we present the results of a high–resolution radio survey of Seyfert galaxies from the extended 12 m sample and assess the impact of the Square Kilometer Array (SKA) on future work of this kind. 2 Observations of the extended 12 mAGNsample From an initial sample of 893 galaxies with 12 m flux densities greater than or equal to 0.22 Jy, Rush et al (1993) [5] defined a subsample of 118 active galaxies. Selection was made at 12 m in order to overcome the wavelength–dependent selection effects which have affected previous surveys. It has been proposed that mid–infrared wavelengths carry an approximately constant fraction of the bolometric M.P. van Haarlem (editor) Perspectives on Radio Astronomy – Science with Large Antenna Arrays Netherlands Foundation for Research in Astronomy – 1999 24 Mean VLA detection limit Projected SKA detection limit 23 22 21 logP (W/Hz) 20 19 18 0.001 0.01 0.1 redshift Figure 1: The radio luminosity versus redshift plane sampled by our observations of the extended 12 m Seyferts. Arrows indicate upper–limits on P. Notice the potential improvement offered by the SKA. luminosity for quasars and both types of Seyfert [6] and this claim is supported by the equal redshift distributions of type 1 and type 2 Seyferts in the sample; after the exclusion of 5 radio–loud sources, the redshift distributions of type 1 and type 2 Seyferts are matched at 98.7% confidence using the Kolmogorov–Smirnov test. We have made radio observations of the extended 12 m AGN sample with the Very Large Array (VLA) in A–configuration snapshot mode at 8.4 GHz. This resulted in maps with an average 1– noise ½ ¼¼ level of 53 Jy beam and a resolution of 0.24 . 3 The luminosity–redshift plane In common with all flux–limited surveys, the coverage of the luminosity–redshift plane for our observations of the extended 12 m sample is truncated by the detection limit. The dashed line in ½ Figure 1 shows the variation in the minimum detectable radio power, P (WHz ), with redshift for our VLA observations assuming a mean 3– detection limit of 0.16 mJy (the detection limit for individual sources varies according to map quality). The projected minimum detectable radio power of the SKA at 20 cm is shown by the dot-dashed line (assuming a detection limit of 0.0005 mJy with 15 minute integrations). Since the SKA would be around two orders of magnitude more sensitive than the VLA in snapshot mode, it would improve coverage of the luminosity–redshift plane significantly and allow more radio components to be detected; roughly twice as many under the conservative assumption that the luminosity–redshift plane is filled uniformly. Further improvements in detection rate would be expected at 20 cm because at this wavelength the sources themselves are usually brighter than at higher frequencies. 3.1 The core radio luminosities of the two Seyfert types For the extended 12 m sample, we have found that the compact radio components found in Seyfert type 1 sources have an indistinguishable 8.4 GHz luminosity distribution to that of Seyfert type 2 sources [10]. This result supports the standard Seyfert unification model, according to which the radio–generating engines in the two Seyfert types are identical. High–resolution, low–frequency radio 200 10000 1000 100 Size (pc) 10 Seyfert 1s Seyfert 2s VLA field-of-view Limit VLA uv-under-sampling limit VLA resolution limit SKA resolution limit (20 cm) 0.001 0.01 0.1 redshift Figure 2: The linear size versus redshift plane sampled by our observations of the extended 12 m sample. Size upper–limits on unresolved sources are denoted by arrows. The sensitivity of the SKA would significantly improve radio size measurements; at present the sizes of some sources are probably under–estimated because weak secondary components are undetectable. surveys of Seyferts are needed to compare the spectral indices of their compact radio components. At present, MERLIN is the most suitable instrument for mapping Seyferts at frequencies lower than 8.4 GHz with sufficient resolution to minimise the contribution of the host galaxy, but many Seyferts are not bright enough for such observations. Routine high–resolution spectral index measurements with the SKA would allow the spectral ages of Seyferts to be deduced, as with radio galaxies [7]. Such observations might also allow the effects of free–free absorption on sub–parsec scales to be systematically investigated [8]; models by Krolik & Lepp (1989) [9] predict a low frequency cut–off below 10 GHz due to free–free absorption by electrons in a putative torus surrounding an AGN. 4 The size–redshift plane Figure 2 shows how the portion of the linear size versus redshift plane sampled by our observations is truncated by the field–of–view limit (solid line), the uv–under–sampling limit for individual components (dashed line) and the resolution limit (dotted line). The dot–dashed line shows the 0.1¼¼ resolution of the SKA at 20 cm. The range of resolvable source sizes is not a strong function of redshift; around ninety percent of the sources lie in the redshift range 0.003 to 0.070 and of the resolved sources in this range 70% are found in the regions where the effect of redshift on the field–of–view and resolution limits is negligible. The detectable source size is a stronger function of redshift due to the truncation of the luminosity–redshift plane shown in Figure 1. i.e. sources sizes can be underestimated because weak secondary components are undetectable. In addition, the sizes we are able to measure for sources with diffuse, extended components are known to be under–estimates because our observations are insensitive to flux components with angular sizes larger than 3.5¼¼ (the uv-under–sampling limit). The resolution of the SKA will improve coverage of the linear size versus redshift plane, especially at high frequencies. However, the most significant improvement on size measurements is likely to arise from the improved sensitivity of the instrument. 201 4.1 Comparing the radio structures of the two Seyfert types Highly–collimated radio structures are hard to explain using models in which Seyfert activity is caused by nuclear starbursts. The observation of such structures is evidence of a similarity between the central engines of Seyfert galaxies and radio galaxies, and may be indicative of accretion–driven nuclear activity. Almost 50% of each Seyfert type observed in the extended 12 m sample were unresolved by our VLA observations. The SKA would be important in determining whether this approximates the true fraction of unresolved Seyferts, or whether the high fraction of unresolved sources is caused by resolution and sensitivity limitations. Using faint, highly–collimated radio structures as a diagnostic of accretion–driven activity would be useful method of assessing the frequency of weak accretion–driven activity in starburst galaxies, Low Ionisation Nuclear Emission line Region galaxies (LINERs) and “normal” galaxies. We have found that the compact radio sources found in Seyfert type 1 and type 2 sources have indistinguishable size distributions [10]. This result supports the standard Seyfert unification model, according to which the intrinsic lengths of the radio structures produced by the two Seyfert types have the same distribution. However, the size comparison is insensitive to systematic size differences less than around a factor of 5, and therefore a larger sample is needed to improve sensitivity to the effects of orientation; the unified scheme predicts that the apparent sizes of Seyfert 1s should be on average 2 to 3 times smaller than those of Seyfert 2s because their axes lie closer to the line–of–sight.
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