5 GHz EVN observations of GPS radio sources

1. 5 GHz EVN observations of GPS radio sources Liu Xiang C. Renolds & R. Strom Urumqi Observatory, NAOCAS

2. Properties of GPS sources • Giga-Hz Peaked Spectrum --- GPS • GPS radio sources make up a significant fraction of the bright radio source sample(~10%), but they are not well understood. • The GPS sources are powerful (P_{1.4GHz} >10^{25}W/Hz), compact (<1kpc), • have convex radio spectra that peak between about 0.3-10~GHz (observer's frame). • Only about 12% of GPS sources show extended radio emission (> 1kpc), and it is diffuse and very faint. Most GPS sources appear to be truly compact and isolated (O'Dea1998).

4. Given the fact that GPSs are small radio sources, within AGN, it is quite likely that their low frequency radio emission will be absorbed due to either Synchrotron Self Absorption (SSA) and/or Free-Free absorption (FFA), giving rise to a peaked (GPS) radio spectrum. GPS sources may be the best objects for studying absorption and scattering in AGNs, for example, the HI absorption (Vermeulen et al. 2003), and free-free absorption (Morganti et al. 2004).

5. SSA+FFA Yang, Liu & Shen 2005

6. CSO properties • Compact Symmetric Objects (CSOs) make up a class of radio sources with distinctive radio morphology. • They are powerful and compact sources with overall size < 1kpc, dominated by lobe/jet emission on • both sides of the central engine, and are thought to be relatively • free of beaming effects (Wilkinson et al. 1994). • Their small size is • most likely due to their youth (<10^4 years) and not due to a • dense confining medium (Owsianik & Conway 1998). • A unification scenario assumes that • CSOs evolve into Medium-size Symmetric Objects • (MSOs, 1-15 kpc), • which, in turn, evolve into Large Symmetric Objects • (LSOs, > 15kpc), i.e. large FRII radio sources (Fanti et al. 1995, • Readhead et al. 1996).

8. Proper motion and ages Liu Xiang et al. 2000

9. CSOs are of particular interest in the study of the physics and • evolution of active galaxies as introduced in our previous papers • (Liu Xiang et al. 2002, 2005, hereafter paper I, II). A couple of CSOs • have been found to be very young radio sources with ages of several • centuries (Fanti 2000, Gugliucci et al. 2005). How were they • triggered? One suggestion is that CSO host galaxies have been merging • or interacting, but it is still an open question. It is possible that • a CSO study might find a difference between their host galaxies and • other galaxies if we have a large CSO sample. Secondly, how does a • young radio source evolve? Can we measure intrinsic • hotspot-advance-speeds of young compact radio sources? For compact • flat spectrum sources, which are often core-jet, their jets may be • close to the line of sight and therefore Doppler boosted. For CSOs, • the viewing angle is usually large, so the intrinsic two-sided • jet/hotspot velocity can be readily determined. A large CSO sample would • permit a statistical estimate of the speeds of hotspots in CSOs.

10. FFA and scattering in CSO Beaming, SSA and FFA: It shows that two models containing the same beaming parameters but having quite different absorption mechanisms can fit two lobes equally well. In one model, only FFA (free-free absorption) is needed. In the other model, both the SSA (synchrotron self-absorption) in two lobes and an extra FFA toward the southwest lobe are required. Our analysis cannot unambiguously distinguish between two models. Although the FFA process is invoked in both models, the geometry of the absorbing gas could be quite different. ---Xie, Jiang & Shen 2005 Scattering in CSO Liu & Yang 2003

11. Search CSOs from GPS sample • Why search for CSOs in a sample of GPS sources? A detailed explanation is given in paper II. In brief, • assuming that most of the compact radio sources are intrinsic • CSOs, by selection they will often show a one-sided core-jet due • to Doppler boosting. Compact two-sided jets may be detectable by • searching in a large flat-spectrum sample, as was done by Taylor • & Peck (2003). In a flux-limited complete sample PR+CJ1 (200 • sources with S_5GHz > 0.7 Jy and Dec >35, Xu et al. • 1995), 10 out of 14 CSOs detected • are GPS sources. It is therefore efficient to search for CSOs in • GPS samples (see paper I, II). • GPS sources share some common • properties with CSOs as they are both confined within 1 kpc and • are probably young radio sources (Murgia 2003).

12. GPS samples • There are a few tens of sources in bright GPS source samples, and a • couple of these sources have never (or poorly) been imaged with • VLBI. From the lists of bright GPS source samples (de Vries et • al. 1997, Stanghellini et al. 1998), we have tried to image the GPS • sources with VLBI. The first and second observation runs were made at • 1.6, 2.3 and 8.4~GHz with the EVN (European VLBI Network) for 22 • sources. Multi-frequency VLBI images were obtained and 5 CSOs and 9 • CSO candidates have been found (paper I, II). • In this follow up observation, we aim at imaging the GPS sources at 5~GHz to • confirm the CSO candidates, to find new CSOs from the bright GPS samples, and • to measure the polarization from GPS sources. Our search for CSOs from the GPS • samples is complementary to those by the COINS group (Taylor \& Peck 2003) • which looks for CSOs based on samples consisting mainly of flat spectrum radio • sources.

13. Polarization • GPS radio sources show very low polarization (about 0.2% at • 5~GHz, O'Dea 1998). The low integrated polarization could be due • to large Faraday depths around the radio source. It is found that • some GPS galaxies are unpolarized while some GPS quasars show • relatively higher fractional polarization (Dallacasa 2004). This • may mean that the galaxies are depolarized more than the quasars. • Higher frequency observations are required to determine the • differences between GPS galaxies and quasars in polarization. The • fractional polarization of Compact Steep Spectrum (CSS) sources • tends to show depolarization (typically the degree of polarization • is 1%-3% at 5~GHz, going up to 6%-7% at 8.4~GHz), suggesting • that large Faraday depths are possible (O'Dea 1998). It is often • difficult to measure reliable Rotation Measures (RMs) in GPS • sources because the polarization is so low that it is often • difficult to detect polarization at multiple wavelengths. • Observations at frequencies above the spectral peak are needed to • determine if GPS sources generally have high RMs.

14. AGN and the EVN

15. VLBI observations • EVN observations at • 1.6, 2.3/8.4 GHz, plus MERLIN observed in 1999, 2000, pulished in 2002, A&A, Liu Xiang et al. • 2.3/8.4 GHz, observed in 2002, • pulished in 2005, A&A, Liu Xiang et al. • 5 GHz, observed in 2004, wth new 6cm receiver in Ur!, • accepted in 2005, A&A, Liu Xiang et al.

16. 5 GHz results

19. DA193 • This radio loud quasar is extremely compact, 4mas, is only resolved with VLBI, and is often used as a VLBI calibrator. The total intensity map at 5~GHz clearly shows a core and a jet. • The core is very compact with about 70% of the total flux density, • the jet component is also compact with about 30% of the • total flux density. The jet position angle is -48^{\circ} in the north-west • direction, roughly consistent with -53^{\circ} in a VSOP map at 5~GHz (Scott • et al. 2004). The distance between the jet component and the core is 2.1 mas in • our 5~GHz VLBI observation, which is greater than the 0.7 mas in the previous • VSOP image. If the component detected here is the same as that detected in the • VSOP image, then the increase over about 5.5 years corresponds to an apparent • velocity of 2.4+/-0.6c (we use h=0.75 here). It has previously been suggested that the jet has a superluminal motion of about 2.2+/-0.7h^{-1}c (Lister & Marscher 1998).

20. The polarization image shows a weak feature, with integrated fractional polarization of 1% and chi= 149^\circ. This fraction at 5~GHz is less than that of about 1.5% at 43~GHz (Lister & Marscher 1998), the polarization angle is nearly perpendicular to that at 43~GHz. The result may support the idea that the fractional polarization in GPS sources decreases as the frequency decreases (O'Dea 1998). The WSRT data of DA193 in the VLBI observation has been used to calibrate the polarization anglein the VLBI images. The polarization fraction measured in the WSRT data is 1.3% for DA193.

21. PKS 0554-026 • The 5~GHz VLBI image shows a compact core, with a jet or diffuse • emission within 20~pc . The total flux seen • in the Westerbork local interferometry data has been recovered in • this image. The total flux density has changed from 290 mJy • (observed 1980, Wright & Otrupcek, 1990; Parkes Catalogue) to 171 • mJy in this observation, indicating this source is variable and • probably a core-jet source. The structure at 5~GHz is similar to • that found at 2.3/8.4~GHz (paper II) in general. There is no clear • detection of polarized flux.

22. PKS 0914+11 • The 5~GHz VLBI image shows a symmetric double source, • with a tail associated with the eastern component `C'. • From the displacement between the two components detected in this • 5~GHz VLBI image, it is clear that they correspond to components • `A' and `C' in paper II, meaning that components `B' and `D' from • paper II are undetected here. We have tentatively registered the • single component at 8.4~GHz (paper II) as component `A' in the • 5~GHz image by consideration of its spectrum. Such an • identification means that the spectral index of both components A • and C is steeper between 8.4 and 5~GHz than between 5 and 2.3~GHz • which is typical of the components in • CSOs above the turnover frequency. Component `C' has a rather • steep spectral index of 1.95 between 2.3 and 5~GHz and so could • reasonably be expected to be undetectable at 8.4~GHz. • From the steep spectra of the components and its symmetrical • structure, we classify the source as a CSO. The total flux density • has changed from 140 mJy (observed 1979, Parkes Catalogue 1990) to • 110 mJy in this observation. The total flux density at 5~GHz is • recovered in the VLBI image. No polarization is detected in the • 5~GHz VLBI observation.

23. B3 1133+432 • This is an empty field in the optical. The 5~GHz VLBI image shows a double source. The double • structure exhibits two opposite edge-brightened hotspot/lobes with • spectral indices of 1.08 and 1.35 between 2.3 and 5~GHz for the • northern and southern lobe respectively, also see. We confirm that the source is a CSO. • Orienti et al (2004) imaged the source with the VLBA at 5~GHz, and • it is similar to in general. • 95% of the Effelsberg flux density has been recovered in our 5~GHz image. • No polarization is detected for this source.

24. 1333+589 • It is an empty field in the optical. The GPS source peaks at • 4.9~GHz. The 5~GHz VLBI image shows a double structure. The northern component is more extended • in PA 26^{\circ} than was seen at 2.3/8.4~GHz (paper I) or in • a 5~GHz image by Xu et al. (1995, snapshot observation with MKII • recording in 1.8 MHz bandwidth). The southern component was • measured to have 325 mJy by Xu et al. (1995), but only 175 mJy is • seen in our image, so it has decreased by 46% in 13 years; while • the northern component has increased 46%from 324 mJy to 472 mJy. • There seems to be a new jet-like extension to the northern • component, which may be responsible for the increasing flux • density of the northern component in the past 13 years. The • distance between the two components is 12.8 mas, the same as in Xu • et al. (1995). The northern component shows an inverted spectrum • if it is considered as a single component, and like the southern component it has an edge-brightened hotspot/lobe and steep inverted spectrum. We • classify the source as a CSO. The Effelsberg flux density has been • recovered in the VLBI image. No polarization is detected in the • 5~GHz VLBI observation.

25. 1404+286 (OQ +208) • It shows a similar structure to previous • observations at 5~GHz (e.g. Wang et al. 2003). This is a CSO as • confirmed in paper I. No polarization is detected in the 5~GHz • VLBI observation. It is previously reported that its linear • polarization is less than 0.2% at 5~GHz (Stanghellini et al. • 1998). The source has been found to be a Compton-thick AGN from • the X-ray (Guainazzi et al. 2004) and radio (Yang & Liu 2005), • indicating a dense confining medium in the source. The medium may • have depolarized the radio source.

26. 1433-040 • The source is identified as a galaxy with $m_{r}$=22.3 (Stanghellini et al.\ • 1993). It appeared in the O'Dea (1996) and early GPS sample lists (Spoelstra • 1985). This is the first VLBI image for the source (Fig.~\ref{1433-040_ip}); it • shows a core-jet appearance. It can be fitted with two Gaussian components: a • central compact one and a weaker but broader one to the north-east • ($PA$=13.9$^{\circ}$). It is likely that the more compact component is the • core. • The spectrum of the source is inverted at about 1~GHz with a • spectral index $\alpha \sim -0.18$. The total flux density varies • from 200 mJy (observed 1980, Parkes Catalogue 1990) to 246 mJy • (this observation). 88\% of the total flux density has been • detected in the 5~GHz VLBI image. The core-jet classification is • supported by the positive detection of polarization in this • source. Integrated polarization of 3.6\% is detected in the 5~GHz • VLBI observation, with an EVPA of 143$^{\circ}$ • (Fig.~\ref{1433-040_p}). The polarization seems largely to come • from the jet component because the polarized emission is located • closer to the jet position. We obtained a fractional polarization • of 3.8\% with an EVPA of 134$^{\circ}$ in the WSRT data for this • source. • The first vlbi image and polarization measure for this source.

27. 1509+054 • This object has been classified as a Seyfert-1 galaxy with a • redshift of 0.084 (Chavushyan et al. 2001). The 5~GHz VLBI image • shows an asymmetric double, which is • similar to the 8.4~GHz image (paper I). The eastern component is • more compact than the western one, and the latter can be resolved • into three subcomponents. The VCS2 image • at 8.4~GHz is similar but missing component `C' (Fomalont et al. • 2003). The spectral indices of the eastern and western component • are -1.8 and 0.67 respectively between 5 and 8.4~GHz. From the • compactness and steep rising spectrum, the eastern component is • likely to be the core of the source. The total flux density of the • source has increased from 526 mJy (Dallacasa et al. 2000) to 688 • mJy in this observation, suggesting it is variable and may be a • core-jet source. From its triple-like structure we still retain • the source as a CSO candidate for future classification. Weak • polarization is possibly detected at the central component • in the 5~GHz VLBI image -- we use it as an upper limit, although the EVPA • differs from that of the WSRT result. The source may be associated • with the X-ray source 1WGA J1511.6+0518.

28. 1518+046 • The 5~GHz VLBI image shows a classical double, with two • hotspot/lobes. The identification of these components as hotspots/lobes is • supported by the spectra, as given in paper I. The total flux density is stable • from 1.03 Jy (Parkes Catalogue 1990) to 1.06 Jy in this observation. We • conclude that this is a CSO or MSO (for its size is nearly 1~kpc). We tried to • measure the lobe expansion speed by comparing our map to an early 5~GHz map • (observed in April 1983, Mutel \& Hodges 1985). We measure an expansion of • 2.2+/-1.1 mas in 21.5 years, or a hotspot/lobe proper motion of 0.9+/-0.5c • (we use h=0.75 here). Although it is only a 2sigma detection of motion, it • may suggest that this MSO has higher hotspot/lobe proper motion that is typical • of CSOs. This is important because we have previously had little knowledge • about MSO hotspot velocity. Possible polarization is detected at the hotspots • in the 5~GHz image, we use it as an upper limit although the EVPA differs from that of the WSRT result.

29. 1751+278 • The 5~GHz VLBI image exhibits asymmetric double structure. The • northern component is about 10 times brighter than the southern • one. We derive a spectral index of 0.59 • for the strong one, and 0.84 for the weaker one, between 1.6 and • 5~GHz. Both have steep spectra, suggesting that the source is a • double-lobe source rather than a core-jet source as we discussed • in paper I. We think that component `D' in paper I may not be a • genuine component. The total flux density is stable within 7% • from 280 mJy (Griffith et al. 1990) to 260 mJy in this • observation. The total flux density at 5~GHz is recovered in the • VLBI image. Possible polarization is detected at the strong • component in the 5~GHz image, which we use as an upper limit.

30. 1824+271 • The 5~GHz VLBI image shows a symmetric • double structure. The spectral indices between 5~GHz and 8.4~GHz • (paper II) are steeper than 1 for both components, and are 1.0 and • 0.8 between 2.3~GHz and 5~GHz for components A and B respectively, indicating the source is a • double-lobe source. We confirm that the source is a CSO. The total • flux density has slightly changed from 98 mJy (Griffith et al. • 1990) to 111 mJy in this observation. All of the total flux • density has been detected in the VLBI image. No polarization is • detected in the 5~GHz VLBI and WSRT observations.

31. TXS 2121-014 • The 5~GHz VLBI image exhibits double lobes and a weak core-like • component between the lobes. It is • difficult to estimate the spectral index of the `core' since it is • not separated from the western lobe at 2.3~GHz (paper II). The • spectral indices between 2.3~GHz and 5~GHz are 1.1 and 0.8 for the • eastern and western lobe respectively, we confirm that the source is a CSO. The • total flux density seems stable from 320 mJy (Parkes Catalogue • 1990) to 327 mJy in this observation. No polarization is detected • in the 5~GHz VLBI observation, and the WSRT value is similarly low.

32. PKS 2322-040 • With a typical GPS spectrum peaked at 1.4 GHz, the 5~GHz VLBI • image exhibits double lobe/hotspots and a weak jet-like emission • to the northern lobe. It is similar to • that at 2.3~GHz (paper II) where a sign of the jet is also seen • but is not well resolved because of the lower resolution at • 2.3~GHz. The total flux density seems stable to within 10\%, from • 500 mJy (Parkes Catalogue 1990) to 548 mJy in this observation. • The spectral indices of components `A' and `B' are 0.55 and 1.48 • respectively between 2.3 and 5~GHz. From the steep spectra and symmetric • edge-brightened lobe/hotspots, we classify the source as a CSO. • No polarization is detected in the 5~GHz VLBI and WSRT observations.

33. 2323+790 • This is a galaxy. The NVSS image exhibits • double circular components which are separated by • +/-2arcmin. • It is not clear if they are separate sources. The WENSS map • indicates the source has multiple components. The 5~GHz VLBI image • shows a bright component and two jet-like components, but the • source looks like a double (Fig.~\ref{2323+790_i}). It is the • first VLBI image for the source and we cannot give a proper • classification. This is a S5 source, the total flux density seems • stable at 5~GHz from 448 mJy (S5 data, observed in 1978, K\"uhr et • al. 1981) to 438 mJy in this observation. We retain this source as • a CSO candidate for further classification. 94% of the total flux • density has been detected in the VLBI image. No polarization is • detected in either the 5~GHz VLBI or the WSRT observation.

34. Discussion • CSOs are defined as compact radio sources with relatively steep • spectrum double lobes on the opposite sides of a core. • Ideally a core component with a spectrum flatter than the lobes must be identified before sources can be confirmed as CSOs. • For some CSOs • with a jet axis very close to the plane of sky the core may be so • weak as to be undetectable, yet a CSO identification can still be • secured if there are symmetric edge-brightened lobes (Taylor & • Peck 2003). • For example, of the CSOs 0914+114, 1133+432, 1333+589,1518+046, 1824+271, 2121-014 and 2322-040 in this paper, only the source 2121-014 may show core emission.

35. Discussion • In our sample, the optical counterparts of the 14 sources comprise • 2 quasars and 10 galaxies and 2 empty fields. • 1 quasar (DA193) and 1 galaxy (1433-040) show linear polarizations >1%, and 1 quasar (1518+046) and 2 galaxies (1509+054 and 1751+278) show • possible polarizations of <0.5%, and 9 galaxies show no • polarization, perhaps supporting the idea that quasars may have • relatively higher fractional polarization than galaxies, although • the statistics of this sample are rather small. • The relatively • high polarizations in the core-jet sources may be the result of • less depolarization in these sources. Further study of a large • sample is required.

36. Discussion • These results confirm that the GPS sources often show very low or no • polarization. They may have been largely depolarized, because their convex • spectra indicate the GPS sources may live in more dense environments than • non-GPS sources. For example, the low polarization of DA193 may be partly due • to Faraday depolarization, since very high (>4700radm^{-2}) rest-frame • rotation measures have been measured for this source (Lister & Marscher 1998). • And the Compton-thick medium in OQ208 (Guainazzi et al. 2004) may have led to • undetectable polarization in OQ208. • No correction for the effect of Faraday rotation on \chi has been applied • in the images presented here, so the inferred magnetic field vectors are not • necessarily perpendicular to the electric vectors in the images.

37. Summary and Conclusions • We have obtained 5~GHz total intensity VLBI images for 14 GPS sources. • The parameters of source structure and spectra have been derived. Two core-jet sources 1433-040 and DA193 show integrated fractional • polarizations of 3.8% and 1.3% respectively. Three show possible very weak • polarizations <0.5% and the other nine sources which show no • polarization prove that the GPS sources are generally unpolarized or have very • low polarization. • Three sources 1133+432, 1824+271 and 2121-014 have been • confirmed as CSOs, and four new CSOs have been classified by the • 5~GHz images and the spectral indices, they are 0914+114, • 1333+589, 1518+046, and 2322-040. • Four sources remain CSO candidates, of which 1509+054, 1751+278 and • 2323+790 showing triples or doubles are likely CSOs; 0554-026 is very compact • and probably a core-jet source. However, high resolution VLBI observations are • required for proper classification. • In addition, we estimate that the jet of quasar DA193 has a • superluminal motion of 2.4+/-0.6c in the past 5.5 years; and the • hotspots in quasar 1518+046 show a motion of • 0.9+/-0.5c in past 22 years.

38. Thank you !!