4. Extended emission or milli halos around CSSs
The sources whose overall separation of the components are less than the IPS cut-off size scintillate as a single source. A comparison of the sizes of the scintillating components with the VLBI-scale structure should enable us to infer the existence of milli-halos on the scale of hundreds of milliarcsec which may have been resolved out in the high-frequency VLBI observations, which are usually between 1.4 and 5 GHz. The extended emission with steeper spectrum may also be more prominent at low frequencies.
In Fig. 4, we plot the sizes of the scintillating components inferred from the IPS observations against the largest angular size estimated from the VLBI images for all objects with an overall angular separation less than about 400 mas. There are 21 objects which satisfy this criterion, and their properties can be found in Table 1 and Table 2.
The sizes of the scintillating components estimated from the IPS observations tend to be larger than the interferometric measurements, except for the source 0428+205. Here, the IPS size is significantly smaller than the LAS, which is defined by the weak extended emission towards the north-west. The peak brightness of this component is about a factor of 37 weaker than the dominant component seen in the VLBI image at 18 cm. The size of the dominant component, whose peak brightness in the VLBI image is 1476 mJy/beam (Dallacasa et al. 1995) and which is likely to contribute to most of the scintillating power, is similar to the IPS size.
Since the uncertainty in the IPS size could be as large as about 50 mas, we identify only those sources whose difference between the IPS and VLBI sizes to be at least about 3 times 50 mas to be strong candidates for milli halos. These include the sources 1127-145, 1341+144, 1354-174, 1518+047 and 1543+005. Those whose difference lies between 2 and 3 times are 1442+101, 2247+140 and 2353+154, and these are possible candidates for millihalos. These 8 sources are described briefly below. The IPS sizes of the remaining 12 sources with LAS 400 mas, namely 0019-000, 0320+053, 0742+103, 0752+342, 0941-080, 1117+146, 1245-197, 1345+125, 2008-068, 2128+048, 2147+145 and 2210+016, are consistent with the VLBI sizes.
There appears to be evidence of extended emission around some of the CSSs on scales few times larger than the known size of the sources. We refer to these structures as milli-halos. These low-frequency halos are expected to have steeper radio spectra and could be contributing less to the total flux density at high frequencies; and could also have been resolved out in the interferometric maps. There is often evidence of missing flux density in the VLBI images, but evidence of these halos from the integrated spectra is difficult to establish because the low-frequency integrated spectra as well as the spectra of individual components are not well-determined. One possible scenario for such structures could be earlier periods of activity which is presently fading out due to radiative and other losses. Another possibility is that the milli-halo is composed of relativistic particles which have diffused out from the jet or the nucleus. Such milli-halos with sizes of the order of about a kpc have been seen in a few nearby AGN from high-resolution and high sensitivity observations (Silver et al. 1998; Carilli et al. 1998). The sizes of extended emission detected in CSSs by IPS observations are in the range of about 250 pc to 4 kpc. These could be similar to the milli-halos seen in the nearby AGNs.
1127-145: This source has a core-jet type of structure at 2.32 GHz with an overall angular size of about 63 mas (Fey et al. 1996). At higher resolution the dominant component is resolved into a double-lobed source with an angular separation between the peaks of emission of about 3.7 mas (Romney et al. 1984; Wehrle et al. 1992; Bondi et al. 1996; Fey et al. 1996). The LAS of the source estimated from the IPS observations is about 350 mas, significantly larger than the known VLBI-scale structure.
1341+144: The source 1341+144 is a possible double with an angular separation between the peaks of emission of about 60 mas (Cotton et al. 1989). The LAS inferred from the IPS observations is about 375 mas.
1354-174: It is a highly asymmetric source where the flux density ratio of the outer components is about 57, and the overall angular size is about 32 mas (Frey et al. 1997). The IPS size of the source is 188 mas.
1442+101: We estimate the size of the scintillating component from IPS observations to be about 150 mas. The VLBI map by Gurvits et al. (1994) shows a core and extension resembling a jet, with the largest separation of the components being 15 mas. The VLBI map at 18 cm by Dallacasa et al. (1995) shows a core and jet-like extension with the core in the northern end, contrary to the Gurvits et al. (1994) image. If the core is towards the south, the possible jet has a very steep spectrum. The LAS of the source from the VLBI image is about 24 mas, while the size estimated from the IPS observations is about 150 mas.
1518+047: Phillips & Mutel (1981) find the source to be double-lobed at 18 cm with an overall LAS of 135 mas. Both the lobes are resolved into two components at 5GHz by Mutel et al. (1985). A recent image by Dallacasa et al. (1998) also shows the double-lobed structure. The IPS size of the source is 325 mas.
1543+005: It is a possible triple source with an overall angular size of 7 mas (Stanghellini et al. 1999). The size of the scintillating component is about 325 mas.
2247+140: MERLIN observations by Spencer et al. (1989) show that it is a single source with an angular size of 200 mas. VLA observation by van Breugel et al. (1984) show that it is resolved into a possible double-lobed source. The IPS size of the source is about 325 mas.
2353+154: Barthel & Miley (1988) found this source to have a single component with an angular size of 50 mas. The IPS size of the source is about 158 mas.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 19100