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Radio Continuum Morphology of Southern Seyfert Galaxies?

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Radio Continuum Morphology of Southern Seyfert Galaxies? ASTRONOMY & ASTROPHYSICS JUNE II 1999, PAGE 457 SUPPLEMENT SERIES Astron. Astrophys. Suppl. Ser. 137, 457–471 (1999) Radio continuum morphology of southern Seyfert galaxies? R. Morganti1, Z.I. Tsvetanov2, J. Gallimore3,??, and M.G. Allen4 1 Istituto di Radioastronomia, CNR, Via Gobetti 101, 40129 Bologna, Italy e-mail: [email protected] 2 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, U.S.A. 3 NRAO, 520 Edgemont Rd., Charlottesville, VA 22903, U.S.A. 4 Mount Stromlo and Siding Spring Observatories, Private Bag, Weston Creek Post Office, ACT 2611, Australia Received October 19, 1998; accepted April 16, 1999 Abstract. We present radio observations for 29 southern 1. Introduction Seyfert galaxies selected from a volume limited sample with cz < 3600 km s−1, and declination δ<0◦. Objects Radio emission from Seyfert galaxies traces energetic, me- with declination −30◦ <δ<0◦ were observed with the chanical processes occurring in the active nucleus. The Very Large Array (VLA) at 6 cm (4.9 GHz) and objects radio continuum spectra of Seyferts are almost invariably with δ<−30◦ were observed with the Australia Telescope steep, consistent with optically-thin synchrotron emission. Compact Array (ATCA) at 3.5 cm (8.6 GHz). Both the In those sources where the radio continuum has been re- VLA and the ATCA observations have a resolution of solved, the morphology is commonly linear, interpreted to ∼ 100. These new observations cover more than 50% of trace a stream of ejected plasma, or a jet, originating from the southern sample with all but two of the 29 objects de- the central engine. For example, most well-resolved Seyfert tected above our limit of 0.15 mJy. Combining these data radio sources are double or triple sources straddling the with data available from the literature gives almost 85% active nucleus. The radio emission of the nearest and coverage of the southern sample and further expands the brightest Seyferts resolves into tightly collimated struc- radio observations of a distance limited sample by 22%. tures originating from the active nucleus, consistent with Collecting additional sources from the literature, con- a jet interpretation (e.g., NGC 1068: Gallimore et al. 1996; sisting of known Seyferts with cz < 4600 km s−1, we find Muxlow et al. 1996; Wilson & Ulvestad 1983; NGC 4151: that 38% of the sources are unresolved at arcsecond res- Pedlar et al. 1993; Mrk 3: Kukula et al. 1993). There is olution, and 34% have linear radio structure. Our results evidence that Seyfert radio jets impact the dense gas in expand upon and are consistent with earlier studies. We the near-nucleus environment and affect the gas distribu- confirm a correlation between the size of the radio struc- tion, morphology, and ionization of the narrow-line region ture and the radio luminosity. A comparison between (NLR). The orientation of the radio jet is also the only Seyfert types 1 and 2 finds that type 2s tend to have a straightforward measure of the symmetry axis of the ac- larger linear size. There is no statistically significant dif- tive nucleus, useful for testing orientation-based unifying ference in radio power between types 1 and 2, although schemes for AGNs. all the most powerful objects appear to be Seyfert 2’s. Early studies searching for differences between No significant difference has been found in the spectral Seyfert 1’s and 2’s (de Bruyn & Wilson 1978; Meurs & indices. Wilson; Ulvestad & Wilson 1984a; Ulvestad & Wilson 1984b) concluded that Seyfert 2’s have stronger and Key words: radio continuum: galaxies — galaxies: Seyfert larger radio sources than Seyfert 1’s, however, these studies were biased by optical selection: weaker Seyfert 2 galaxies were omitted from the samples. There was found to be little or no difference between Seyfert 1 and 2 radio sources in follow-up studies which properly considered relatively unbiased, volume-limited samples (Edelson Send offprint requests to: R. Morganti 1987; Ulvestad & Wilson 1989; Giuricin et al. 1990). ? Based on observations done with the Australia Telescope More specifically, there is no statistically significant Compact Array (ATCA) and the Very Large Array (VLA). difference in the distribution of radio luminosity, and only ?? Jansky Fellow. a marginal difference in the distribution of radio source 458 R. Morganti et al.: Radio continuum morphology of southern Seyfert galaxies sizes, with Seyfert 2’s tending to be slightly larger than Array (ATCA)2 aperture synthesis telescopes. Observing Seyfert 1’s (although only at < 90% significance, Wilson configurations and frequencies were chosen to match the 1991). In contrast, unifying schemes predict that the radio ∼ 100 resolution of the optical narrow-band observations. jets in Seyfert 2’s should appear larger in projection, We discuss the radio properties and statistics for the since the unifying model orients the collimating disk more 29 surveyed sources, and a more detailed comparison of nearly edge-on in narrow-line AGNs. It is not simple to the radio and optical properties will be presented in a reconcile the statistics of radio sources with the unified forthcoming paper. scheme hypothesis. While there is no tendency for the radio sources to orient in any preferred direction with respect to the plane 2. Observations of the host galaxy (Ulvestad & Wilson 1984a), the radio sources are commonly elongated in the same direction as 2.1. Sample the NLR (Haniff et al. 1988; Pogge 1989; Wilson et al. 1988), i.e. the inner part of the optical emission regions The sample includes 51 well classified (from their optical (extending up to few kpc). In some Seyfert 2s the NLR spectra) Seyfert galaxies south of declination 0◦ and with has a conical or bi-conical shape with the nucleus at the redshift cz < 3600 km s−1. More details about selection apex (e.g. Pogge 1989, 1997). Wilson & Tsvetanov (1994) and optical observations of this sample are presented in showed that the radio axis is invariably co-aligned with Tsvetanov et al. (in preparation), Tsvetanov et al. (1998). the ionization cone axis for the 11 ionization cones known The 29 sources listed in Table 1 were observed with at that time. either the VLA or ATCA, depending on their declina- The alignment of the radio jet and NLR raises a ques- tion. One source, NGC 3393, was observed with both tion of energetics — does the radio jet ionize and heat arrays. The array configurations and frequencies were cho- the NLR significantly compared to ionizing radiation from sen to obtain a resolution of ∼ 100 in order to match the AGN? In several well-studied cases, there is a detailed the resolution of the optical data. In particular, the VLA morphological association between the NLR and the radio was used in its B and BNA configurations, providing a emission (e.g. Whittle et al. 1988; Whittle 1989; Capetti resolution of ∼ 100 at 6 cm (4.9 GHz). To obtain similar et al. 1996; Gallimore et al. 1996). Currently, the numbers resolution with the ATCA required an observing frequency of such well-studied cases is too small to address questions of 3.5 cm (8.6 GHz) with the 6 km configuration. Owing to of interaction and energetics in a statistically complete missing short baselines in either array, our data are more and meaningful sense. sensitive to compact emission from the circumnuclear re- To this end, Tsvetanov et al. (in preparation) have gion but less sensitive to extended, diffuse emission. In recently assembled a volume-limited sample (cz < addition, faint, diffuse emission from active spirals tends 3600 km s−1) of well-classified Seyfert galaxies. The to have very steeply falling radio spectra with increasing main advantage of this sample is that all of the sources frequency; at observing wavelengths of 3.5 and 6 cm any have been extensively observed in the optical with diffuse emission will probably have dropped below our sur- narrow-band imaging done with the ESO NTT, 3.6 m face brightness detection limit. and 2.2 m telescopes. This survey provides high-quality Many of those sources lying north of δ ∼−30◦ have emission-line maps (in [O III] λ5007 and Hα+[N II]) been observed in previous studies. The data available in with a typical resolution of ∼ 100. The sample includes the literature will be included in our discussion in Sect. 4. objects with and without known extended emission line Thoughout the paper we adopt a Hubble constant −1 −1 regions, providing detailed information about the extent, H0 =75kms Mpc . morphology, and degree of ionization (obtained from the [O III] λ5007/Hα+[N II]ratio) of the emission-line regions. It is therefore important to obtain for such 2.2. VLA observations a sample detailed radio images in order to perform a detailed comparison of the radio and optical morphology VLA snapshot observations were obtained for eight of the on arcsecond (hundred-pc) scales. survey Seyferts in the declination range −30◦ <δ<0◦. Here we present a radio imaging survey of the The VLA observations are summarized in Table 1. The Tsvetanov et al. sample, made using the Very Large observations were carried out using the standard 6 cm Array (VLA)1 and the Australia Telescope Compact (4.9 GHz) continuum mode, that is, with two 50 MHz- wide channels at bandwidth-separated frequencies (4.835 & 4.885 GHz). Three sources were observed using the 1 The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under contract with National 2 Operated by the CSIRO Australia Telescope National Science Foundation. Facility. R. Morganti et al.: Radio continuum morphology of southern Seyfert galaxies 459 Table 1. VLA observations (6 cm) Object α2000 δ2000 Resolution rms Peak Array arcsec arcsec deg mJy beam−1 mJy beam−1 NGC 1097 02 46 19.1 −30 16 28 2.1 1.3 −59 0.051 3.78 BNA NGC 1320 03 24 48.7 −03 02 33 2.5 1.4 −38 0.081 1.98 B NGC 3393 10 48 24.0 −25 09 40 2.2 1.3 −3 0.049 15.80 BNA NGC 3660 11 23 32.2 −08 39 30 2.1 1.4 −1 0.049 0.49 B NGC 4968 13 07 06.0 −23 40 43 2.3 1.4 −4 0.046 13.06 BNA NGC 5427 14 03 25.9 −06 01 50 2.1 1.4 −17 0.048 2.49 B NGC 7172 22 02 02.1 −31 52 12 3.6 1.0 36 0.053 3.62 BNA NGC 7314 22 35 46.0 −26 03 02 3.0 1.2 31 0.034 2.74 BNA B-array (8 Apr.
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