3 Results

Table 3 summarizes our measurements for the 29 observed Seyferts. The listed radio morphologies follow the convention of Ulvestad & Wilson (1989). The angular and linear size of the radio emission, measured for those Seyferts with well-resolved radio structure, were measured to the outermost significant surface brightness contour; upper limits are given for unresolved Seyferts. The radio maps of the resolved objects are presented in Figs. 1 to 4 for the objects observed with the VLA and in Figs. 5 to 15 for the objects observed with ATCA. To emphasize ring or shell structures that can be lost on contour maps, we present all maps as contours on top of greyscale.

  

Figure 1: NGC 1097 (uniform weighting) Contours: 1.0 mJy beam-1 $\times$-0.16, -0.22, 0.16, 0.22, 0.31, 0.44, 0.62, 0.97, 1.21, 1.70, 2.38, 3.33

  

Figure 2: NGC 3393. Contours: 1.0 mJy beam-1 $\times$-0.20, -0.36, 0.20, 0.36, 0.66, 1.20, 2.19, 4.02, 7.36, 13.5

  

Figure 3: NGC 7172. Contours: 1.0 mJy beam-1 $\times$-0.16, -0.25, 0.16, 0.25, 0.38, 0.57, 0.87, 1.32, 2.00, 3.04

  

Figure 4: NGC 7172. Contours: 1.0 mJy beam-1 $\times$-0.16, -0.25, 0.16, 0.25, 0.38, 0.58, 0.89, 1.38, 2.12, 3.26

  

Table 3: The radio parameters from VLA & ATCA observations

Most of the resolved sources bear a linear radio structure as is commonly observed in Seyfert nuclei. In NGC 1097 and NGC 1365 we resolve a central, unresolved radio source, apparently related to the AGN, and a ring associated with star forming regions (compare with Hummel et al. 1987 and Sandqvist et al. 1995, respectively). NGC 7582 displays diffuse radio structure. Only two objects of the observed (IC 5201 and NGC 7590) were undetected; both are part of the group observed at 3 cm with ATCA.

Previous observations for 9 of the 29 objects have been reported in the literature by various authors. Where possible we have used these data in combination with our new observations to estimate a radio spectral index, $\alpha$, defined as $S\propto \nu^{\alpha}$. We caution that there are inestimable uncertainties in the spectral indices for extended sources owing to differences in the (u,v) coverage between our observations and those in the literature. Nevertheless, the indices are accurate for unresolved sources and otherwise provide a sense of the spectral shape for the extended sources.

In the next section we discuss each source individually and also summarize relevant radio and optical observations available in the literature. Where possible, the flux at 20 cm (1.4 GHz), and the Parkes-Tidbinbilla interferometer (PTI) flux measurements at 13 cm (2.3 GHz) taken from literature, are listed. The PTI observations have an effective resolution of $\sim 0$$.\!\!^{\prime\prime}$1, much higher that the data presented in this paper. The main information we can derive from PTI observations is whether a compact radio structure (<0$.\!\!^{\prime\prime}$1) is present. Where possible we compute spectral indices between 20 and 6 cm ($\alpha_{6}^{20}$) and between 6 and 3 cm ($\alpha_{3}^{6}$), and compare with previously reported values.

Our snapshot observations are insensitive to structures larger than 15-18 arcsec, or around 4 kpc at the redshift limit of this survey. The observations therefore resolve out diffuse emission from galaxy disks, but we remain sensitive to radio emission from compact star-forming regions. An interesting result of this survey is that star-forming rings are not common. Only NGC 1097 and NGC 1365 display starburst rings, similar to the one observed in NGC 1068, for example, even though our observations would have detected and resolved such rings out to the redshift limit of the survey. Otherwise the radio structures resolved by this survey resemble the classic linear radio structures commonly observed in Seyfert nuclei, and we assume that these structures are jets powered by the AGN. Of course, we are unable to distinguish jets and compact nuclear starbursts in unresolved sources, corresponding to linear scales  pc at the redshift limit of the survey.

3.1 Notes on individual sources

TOL 0109-383 (NGC 424): This object has been observed by Ulvestad & Wilson (1989) at 20 and 6 cm. With $\sim1$$^{\prime\prime}$ resolution they found a slightly extended source with flux of 22.3 mJy at 20 cm, and 14.9 mJy at 6 cm. In our data we also find a structure that is slightly extended to the east (Fig. 5), although the very elongated beam of our observations makes this very uncertain. Ulvestad & Wilson (1989) found that the dominant component has a flat spectrum with a spectral index between 20 and 6 cm of $\alpha^6_{20} = -0.17$. Our 3 cm measurement shows that the spectrum remains flat between 6 and 3 cm, $\alpha^3_6=-0.21$.

NGC 1097: Originally classified as a LINER by the optical emission-line spectrum, the recent appearance of broad Balmer-line emission and a featureless blue continuum implies that it has a Seyfert 1 nucleus (Storchi-Bergmann et al. 1993). The radio structure (Fig. 1) comprises an unresolved point source and the well-known star-forming ring. A detailed radio study of this object was performed by Hummel et al. (1987) and shows an overall steep spectrum ($\alpha$ between -0.6 and -0.8) and an inverted spectrum ($\alpha = 1.0$) for the nuclear component. The flux of the nucleus measured from our data is very similar to that found by Hummel et al. (1987). An upper limit to the core flux ($S_{\rm 13\ cm}<5$ mJy) has been obtained from PTI observations by Sadler et al. (1995, hereafter S95).

NGC 1365: This is a well known southern barred galaxy. It was observed in the radio continuum (20, 6 and 2 cm) by Sandqvist et al. (1995, hereafter SJL95). As in SJL95, our map (Fig. 6) shows a ring of emission with angular dimensions $8\hbox{$^{\prime\prime}$}\times 20\hbox{$^{\prime\prime}$}$. This ring-like emission is similar to that found in NGC 1097. SJL95 also identify the existence of a jet-like structure originating from the nucleus and about $5\hbox{$^{\prime\prime}$}$ long in position angle (PA) $125^\circ$ (i.e., aligned with the minor axis of the galaxy). We observe a similar structure in our map. The detailed study of the spectral data (SJL95) indicate that both the jet and the nucleus have a steep spectral index. This jet-like feature appears to be aligned with the axis of the ionized gas. NGC 1365 has also been observed with the PTI at 13 cm ($\sim$$0.1\hbox{$^{\prime\prime}$}$ resolution) by Roy et al. (1994) (hereafter R94) and S95 in which a 4 mJy component was detected. This object has been extensively studied in H I by Jörsäter & van Moorsel (1995).

NGC 1320 (Mrk 607): This object is unresolved in our VLA observations. PTI observations place an upper limit of S < 4 mJy on the compact flux density at 13 cm (R94).

NGC 1386: Ulvestad & Wilson (1984b) observed this object at 20 and 6 cm. In their observations, NGC 1386 is barely resolved with an extension to the southwest (${\rm PA}=-125^\circ$), and it is unresolved in our 3.5 cm map. Ulvestad & Wilson (1984b) give a flux density of 13 mJy and 23.0 mJy at 6 and 20 cm, respectively, and a spectral index of $\alpha^6_{20} = -0.47$. The spectral index between 6 and 3cm is very flat, $\alpha^3_6 = -0.05$. PTI observations find a flux density of 4 mJy at 13 cm (R94, S95).

NGC 1566: A possible faint blob of radio emission is detected 3$^{\prime\prime}$ north of central peak (Fig. 7). Observations with the PTI at 13 cm (R94 and S95) give a 5 mJy flux.

ESO 362-G18: This object is not resolved by our observations. PTI observations give an upper limit of S < 4 mJy on the flux density at 13 cm (R94). ESO $362-{\rm G}18$ has been studied in [O III] $\lambda$5007 and H$\alpha$ by Mulchaey et al. (1996).

  

Figure 5: TOL 0109-383. Contours: -1.0, -0.6, 0.6, 1, 1.7, 3, 5, 9 mJy beam-1

  

Figure 6: NGC 1365. Contours: -0.6, -0.4, 0.4, 0.6, 0.9, 1.5, 2.3, 3.6 mJy beam-1

  

Figure 7: NGC 1566. Contours: -0.9, -0.6, 0.6, 0.9, 1.4, 2.2, 3.4, 5.3 mJy beam-1

  

Figure 8: NGC 3281. Contours: -0.9, -0.5, 0.5, 0.9, 1.9, 3.7, 7.2, 14.2 mJy beam-1

  

Figure 9: NGC 4507. Contours: -0.7, -0.5, 0.5, 0.7 1, 1.4, 2, 3 mJy beam-1

  

Figure 10: ESO 137-G34 Contours: -0.5, -0.4, 0.4, 0.5, 0.7, 1, 1.3, 1.8 mJy beam-1

  

Figure 11: ESO138-G01 Contours: -0.7, -0.4, 0.4, 0.7, 1.1, 1.6, 2.5, 3.9 mJy beam-1

  

Figure 12: NGC 6221 Contours: -0.6, -0.4, 0.4, 0.6, 0.9, 1.3, 1.8, 2.7 mJy beam-1

  

Figure 13: NGC 6300 Contours: -0.7, -0.5, 0.5, 0.7, 0.9, 1.1, 1.4, 1.8 mJy beam-1

  

Figure 14: IC 5063 Contours: -1.8, -0.6, 0.6, 1.8, 5.5, 16.3, 48.4, 144.1 mJy beam-1

  

Figure 15: NGC 7582 Contours: -0.6, -0.4 m 0.4, 0.6, 0.9, 1.5, 2.3, 3.6, 5.6 mJy beam-1

NGC 3281: This object is possibly barely resolved (along the north-south direction) in our data (Fig. 8), and was not resolved by Ulvestad & Wilson (1989). Ulvestad & Wilson (1989) find a flux of 61.2 mJy at 20 cm and 26.7 mJy at 6 cm which gives a steep spectral index of $\alpha^{6}_{20} =-0.65$. Our observations show that the spectral index remains steep out to 3 cm, ($\alpha^3_6 = -0.83$).

NGC 3393: This object was observed with both the VLA and ATCA. The two radio maps are very similar and show a double structure. Here we show only the VLA map (Fig. 2), which has a slightly better beam shape. From the two frequencies we derive a spectral index of $\alpha^3_6 =-0.71$ for the NE side and -0.93 for the SW side. The NLR is elongated along a similar position angle as the radio - PA$_{\rm NLR} \approx$ PA$_{\rm radio} = 45^\circ$ - with a close correspondence to the radio morphology. This is very clear in the HST image (Pogge 1997) which shows a spectacular S-shape morphology. A core of 16 mJy was detected with the PTI at 13 cm (R94).

NGC 3660: This object is unresolved in our observations. The source was previously detected by the Effelsberg 100 m telescope (Kollatschny et al. 1983). The total flux measured in our observation (0.5 mJy) is much smaller than that reported by Kollatschny et al. (11 mJy). The reason for this discrepancy is likely to be the presence of other two bright unrelated sources in the field. These sources could not be separated from NGC 3660 by the 100 m telescope but they are in our VLA data.

NGC 3783: The object is unresolved in our 8 GHz observations and was also reported unresolved by Ulvestad & Wilson (1984b) who give a flux density of 13 mJy at 6 cm. A spectral index of $\alpha^3_6 = -0.55$ is derived from Ulvestad & Wilson (1984b) and our flux measurements. An upper limit (S < 5 mJy) was obtained from the PTI observations at 13 cm (R94).

NGC 4507: In our 3 cm data, this object is slightly extended along PA $\sim 10^\circ$ (Fig. 9). This face-on galaxy shows an high excitation NLR with a fairly symmetric edge-brightened bicone shape.

TOL 1238-364 (IC 3639): In our 3 cm map the source is unresolved. TOL 1238-364 object was observed at 20 and 6 cm by Ulvestad & Wilson (1989). At 20 cm the source appears to have a diffuse emission around a relatively strong core. At 6 cm only the core of the source was detected. Ulvestad & Wilson (1989) find a 6 cm flux of 13.6 mJy which combined with our data gives a spectral index of $\alpha^3_6 = -0.53$.A core of 13 mJy was detected with the PTI at 13 cm (R94).

NGC 4968: This object is unresolved in our observations. A core of 10 mJy has been detected with the PTI at 13 cm (R94).

MCG-6-30-15: This is an unresolved and barely detected object. Ulvestad & Wilson (1984b) reported a flux density of 1 mJy at 6 cm and 1.7 at 20 cm, giving a spectral index of $\alpha^{20}_6 = -0.44$. The flux density from our observations give an inverted spectral index at high frequencies $\alpha^3_6 = 0.24$. The upper limit to the flux with the PTI is S < 4 mJy at 13 cm (R94).

ESO 137-G34: This is a low-surface brightness spiral classified as Seyfert 2. The radio morphology (Fig. 10) consists of three knots aligned along PA$_{\rm radio} \sim -40^\circ$. The ionized gas has an S-shape morphology within a larger scale bi-cone. The radio emission is coincident with the inner (linear) part of the line-emitting gas.

ESO 138-G01: This is a Seyfert 2 nucleus residing in early type host galaxy. The radio emission (Fig. 11) is possibly elongated in PA$_{\rm radio} \sim 35^\circ$. The NLR has a jet-like morphology elongated PA$_{\rm NLR}\sim -40^\circ$ The radio emission is also misaligned by $\sim 50^\circ$ with respect to the major axis of the galaxy.

NGC 6221: We resolve a radio triple (see Fig. 12) aligned along PA$_{\rm radio} \sim 40^\circ$. The upper limit to the core flux with the PTI at 13 cm is S < 2 mJy (S94).

NGC 6300: The radio morphology (Fig. 13) of this object is slightly elongated.

IC 5063 (PKS 2048-57): This is an early-type galaxy hosting a Seyfert 2 (Colina et al. 1991). Its radio luminosity is nearly 100 times larger than typical values for nearby Seyferts. Our ATCA radio map (Fig. 14, see also Morganti et al. 1998) resolves a linear radio structure comprising three compact features aligned along PA$_{\rm radio} = 305^\circ$. The spectral index is steep: $\alpha^{20}_6 = -1.1$ (Danziger et al. 1981). The optical line-emitting gas traces an X-shape (Morganti et al. 1998), and the radio emission coincides with the inner part of the line emitting region. Recent H I observations with ATCA and VLBI reveal a well defined H I disk aligned with a morphologically similar system of dust lanes (Morganti et al. 1998; Oosterloo et al. 1998). PTI observations measure a flux of 140 mJy at 13 cm and 10 mJy at 3 cm Slee et al. (1994).

NGC 7172: This object has a linear radio structure (Figs. 3 and 4) which is elongated roughly east-west (PA$_{\rm radio} = 90^\circ$). A 13 cm core flux of 3 mJy has been detected with the PTI (R94).

IC 5201: This source is undetected in our observations. An upper limit S < 5 mJy has been obtained from PTI observations at 13 cm (S94).

NGC 7496: This source is unresolved by our observations. A core flux of 7 mJy has been detected with the PTI at 13 cm (R94).

NGC 7582: Ulvestad & Wilson (1984b) detected a weak core surrounded by an extended emission. They measure 69 mJy at 6 cm and 166 mJy at 20 cm, giving a spectral index of -0.73. We find (Fig. 15) that the spectral index remains steep out to 3 cm with $\alpha^3_6 = -0.72$. For comparison, the NLR is a good example of an edge-brightened, wide angle bi-cone with a cone axis projected along PA$_{\rm e.l.} = 55^\circ$. In addition, the host galaxy has a prominent bar along PA$_{\rm bar} = 155^\circ$. An upper limit to the flux density of S < 6 mJy has been set by the PTI observations (S95, R94).

NGC 7590: This object is not detected in our data; the upper limit to the flux density from PTI is S < 3 mJy at 13 cm (R94).