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Astron. Astrophys. 341, 44-57 (1999)

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4. Results and discussion

4.1. The radio light curve of the central source

We have measured the flux density of the central source at 1.4 GHz from the WSRT data by fitting a point-source model directly to the interferometer visibilities. This yields a flux density of [FORMULA] mJy. We have tabulated and plotted several other 1.4-GHz flux density measurements from the literature in Table 2 and Fig. 7. The measurements from Parma et al. (1986), S95 and the FIRST survey have been made using the VLA in its B-configuration, and are therefore sensitive to the same source structures. Fanti et al. (1987) used the VLA in its A-configuration and therefore their observations have been done with a [FORMULA] times higher resolution. However, since the central source is unresolved by them, their flux density measurement should be comparable to the lower resolution ones. We have assumed a 2% gain calibration error in all VLA observations. The value from Colla et al. (1975a) is strictly an upper limit since this measurement has been done using the Nancay radio telescope with a beamsize of [FORMULA] in R.A. and [FORMULA] in Dec. (Colla et al. 1975b) and therefore also includes a fraction of the extended emission.

We find that, after a continuous rise between 1974 and 1994, the core flux density at 1.4 GHz has decreased by [FORMULA] mJy in the three years between the latest VLA observations (FIRST survey) and these observations. Although several GPS sources have been found to be variable, they are usually quasars and not galaxies (e.g. Stanghellini et al. 1998). The (at least) 20 year long continuous rise and subsequent fall in the flux density of B1144+352 is therefore quite a remarkable behaviour. Snellen et al. (1998) present a model for the flux density evolution of GPS sources, in which such a turn-over in flux density is expected. In their model, the radio components are ejected from the nucleus at relativistic velocities and at a large angle to the line of sight. As a result of relativistic beaming, the intensity of the radiation emitted by such a component in the direction of the observer will be very low. Only after the radio component has decelerated sufficiently, it will start to contribute significantly to the observed flux density. While the component is decelerating, it will also expand adiabatically, so that the total emitted flux will drop. The exact behaviour of the observed flux density depends on the relative importance of these two effects during the evolution of the radio source. Snellen et al. (1998) show that during the initial phases of the deceleration the flux density will increase, due to the declining effect of relativistic beaming, while after some time, as a result of adiabatic expansion of the component, the flux density will slowly decrease. According to their model, the eastern lobe visible in the VLBI map of Henstock et al. (1995), which is largely responsible for the observed change in flux density, must be in the expansion phase. This can be tested by monitoring the peak frequency, which should now also be decreasing in time.

[FIGURE] Fig. 7. The 1.4-GHz radio light curve of the core of B1144+352 over the last 25 years. Values and references for the flux densities presented here can be found in Table 2.

4.2. The large scale radio structure

Although we do not detect radio jets to physically connect the central GPS source with the diffuse outer structures, there are several indications that they are truly associated, and that the GPS host galaxy is therefore also the host galaxy of a co-aligned Mpc-sized radio source. In this section, we will look at the evidence for each radio structure separately.

4.2.1. The eastern radio structure

The eastern structure (Fig. 4) resembles an extended FRII-type radio lobe with a leading hotspot and with an overall morphology that points back to the GPS source. The hotspot is also detected in the FIRST survey, with a flux density of [FORMULA] mJy. There is also a second weak ([FORMULA] mJy) radio source in the FIRST survey at R.A. 11h 45m 06:s68, Dec. [FORMULA], which coincides with a local maximum in the radio emission in the WSRT map. The positions of these two sources have been marked with a cross in Fig. 4.

The detection of these sources provides two alternative explanations for the origin of this radio structure. First, it could be an unrelated head-tail radio source with the hotspot actually being the radio core of such a structure. If this were the case then we would expect to find an optical galaxy coinciding with the FIRST radio source. However, there is no object brighter than an R-band magnitude of 20 at this position in the Digitized Sky Survey (DSS). If the host galaxy of this radio source would be in the same group as the GPS host galaxy, its non-detection implies that its absolute magnitude is at least 6 mag. weaker in R-band than that of the GPS host (i.e. [FORMULA]). It is unlikely that such a weak galaxy would harbor an AGN capable of producing the observed radio structure (see, e.g., Owen 1993). The other possibility is that the host galaxy is at a much higher redshift and that the observed position on the sky near the GPS source is just a coincidence. This would imply that the head-tail source is extremely large and powerful. Also, if this radio structure were a head-tail source, one would expect a significant steepening of the radio spectrum towards the end of the tail. We do not observe such a radio spectral behaviour (see Fig. 2a). Therefore, we regard it unlikely that the eastern radio structure is a separate head-tail radio galaxy.

Second, the eastern radio structure could be a somewhat amorphous radio source with, possibly, the second weak radio source detected by FIRST in the middle of this structure as the radio core. Again, the lack of an optical counterpart in the DSS, situated within the boundaries of the eastern radio structure, strongly argues against this scenario.

We can therefore rule out the possibility that the eastern radio structure is a separate and unrelated radio source, leaving only the conclusion that it must be associated with the GPS source. Both the NVSS and the WENSS radio maps show a faint `bridge' connecting the eastern radio structure with the GPS source. The WSRT map does not show this bridge, but that is most likely related to the problem in the data discussed in Sect. 3.2. The detection of this bridge is further evidence that the eastern radio structure is a radio lobe originating from within the GPS host galaxy.

4.2.2. The western radio structure

In the western radio structure, the two northern components (A and B, see Fig. 5) are most likely a separate radio source. The evidence for this is threefold. First, the radio source consisting only of components A and B put together has a morphology which resembles that of a `fat' double-lobed radio source. Second, the observed E-field polarization vectors turn around at the southern edge of component B. Such a signature is normally seen at the outer edge of FRII-type radio sources (e.g. Saikia & Salter 1988). Third, on the DSS a bright ([FORMULA], estimated from the plates) galaxy is situated at a position between the radio components A and B (see Fig. 8). Its B1950.0 coordinates, obtained by fitting a Gaussian to the DSS image, are 11h 44m 31:s25 in R.A. and [FORMULA] in Dec., with an estimated uncertainty of [FORMULA]. In the FIRST survey there is no radio source at the position of the optical galaxy, which implies that the flux density of a possible radio core at 1.4 GHz is [FORMULA] mJy ([FORMULA] limit). Still, on the map showing the spectral index between the WENSS and NVSS surveys (see Fig. 2a) there appears to be a region with a slightly flatter spectrum between components A and B, as compared to the spectrum of those components. This can be expected if a flat spectrum radio component, such as the radio core or a jet, is situated there. We have obtained a spectrum of the optical galaxy with the 2.5-m Isaac Newton Telescope on La Palma on February 24, 1998. We used the IDS spectrograph equipped with a 1k[FORMULA]1k TEK chip and the R158V grating which gives a dispersion of 6.56 Å per pixel. The slitwidth was [FORMULA], equivalent to 2.7 pixels on the CCD, and the central wavelength was 6500 Å. The total integration time was 10 min. The resulting spectrum is presented in Fig. 9. We have not corrected for galactic extinction. We detect H[FORMULA]+[FORMULA]NII [FORMULA] emission with an equivalent width of [FORMULA] Å, strongly suggesting that this galaxy indeed harbours an AGN, albeit not a powerful one. Since there is no other obvious host galaxy candidate in the DSS, it must be the host galaxy of the radio source formed by components A and B. Its redshift is not significantly different from that of the host galaxy of the GPS source ([FORMULA], versus [FORMULA] for the GPS host galaxy; Marcha et al. 1996). The projected distance to the GPS host galaxy is [FORMULA] kpc. Therefore it probably belongs to the same group of galaxies as the GPS host galaxy. From the integrated flux density of components A and B we have calculated a radio power at 1.4 GHz of [FORMULA] W Hz-1. The absolute magnitude of the host in the R-band is [FORMULA]. In a diagram of radio versus optical luminosity (see, e.g., Owen 1993), it is situated in the region occupied by low-luminosity FRI-type sources. Indeed, Owen & White (1991) find that most sources with a `fat double' morphology lie in this region of the diagram. Following the IAU-convention, we will refer to this radio source as B1144+353. Several of its parameters can be found in Table 3.


[TABLE]

Table 3. Properties of the radio source B1144+353.
Notes:
Coordinates are in B1950.0. Flux density and radio power are monochromatic and at 1.4 GHz. [FORMULA] and [FORMULA] are the equipartion magnetic fieldstrength and energy density, respectively, and are calculated using the assumptions made in Miley (1980).


[FIGURE] Fig. 8. Contour plot of our 1.4-GHz WSRT observations of radio components A and B, overlaid on a greyscale plot of the optical field from the DSS. Note the presence of a bright galaxy between the two radio components. Contourlevels are at (0.3, 0.6, 1.2, 2.4, 4.8, 9.6, 19.2) mJy beam-1.

[FIGURE] Fig. 9. Optical spectrum of the bright galaxy situated between components A and B (see Fig. 8). The features we have used to determine the redshift are indicated. There is weak H[FORMULA]+[FORMULA]NII [FORMULA] emission ([FORMULA] Å). The strong absorption feature near 7600 Å is atmospheric.

The southern part of the western source (i.e. radio component C and the southern tail) is either associated with the host galaxy of the GPS source, or is part of the radio source B1144+353, or is a separate radio source. The lack of an association with an optical galaxy on the DSS rules out the last explanation. If it were associated with B1144+353, this radio source would have a highly unconventional morphology: it would be highly asymmetrical and it would have double radio lobes on one side of its nucleus. The bending of the polarization vectors at the southern edge of component B is characteristic of an outward moving shock which compresses the magnetic fields, and normally delimits an FRII-type radio lobe. To explain component C as a part of this radio source would require a rather exotic formation scenario. If it were the result of an earlier period of activity, we would expect to observe a similar structure northward of component A. Also, if the jet of B1144+353 truly reaches as far as component C we would not expect to see the bending of the polarization vectors at the edge of component B. For these reasons we do not believe that component C is physically part of the radio source B1144+353.

The most likely explanation for its existence is therefore that it is related to the GPS host galaxy. There are two further arguments to support this scenario. First, the WSRT observations show that the E-field polarization vectors turn around at the western edge of radio component C, just as they do at the southern edge of radio component B. This suggests that component C is the leading edge of a FRII-type radio lobe. As seen from the western edge of component C, the locus of the polarization vectors is situated in the direction of the GPS source, rather than in the direction of B1144+353. Second, the FIRST radio image of the GPS source (see Fig. 3) shows two jet-like components emanating from the core. The eastern `jet' is clearly pointing towards the large eastern radio lobe, and so it is most likely that the other `jet' points towards another large radio lobe, probably at a similar distance from the nucleus. Since A and B are a separate radio source, component C is the only candidate for such a lobe.

We therefore conclude that component C forms the (former) endpoint of the western jet. FIRST has not detected a hotspot in component C, which results in an upper flux density limit ([FORMULA]) of 0.5 mJy at 1.4 GHz for such a component.

The line connecting the hotspot in the eastern lobe with the maximum in component C does not cross the radio core but passes [FORMULA], or 57 kpc, south of it. This indicates that both jets do not follow a straight path from the core to their endpoint. Another possibility is that the host galaxy has moved towards the north while it was forming the Mpc-scale radio structure. A typical spectral age of a Mpc-sized radio source is [FORMULA] yr (e.g. Mack et al. 1998, Schoenmakers et al. 1998a), so that a velocity of the central galaxy of [FORMULA] km s-1 would be required to explain the possible shift. This is high, but not unconceivable. An argument against this scenario is that the hotspot in the eastern radio lobe, which represents the most recent endpoint of the eastern jet, is situated near the southern edge of the lobe. If the host galaxy had drifted northwards, the current endpoint of the jet would be expected near the northern edge of the lobe. Therefore we believe that a displacement of the host galaxy is not a very likely explanation for its offset from the line connecting the maxima in the outer lobes.

The FIRST survey and VLBI observations (Henstock et al. 1995) show that the pc-to-kpc scale radio axis has a constant position angle of [FORMULA] (measured CCW from the North). However, the WSRT observations show that the position angles of the lines connecting the core with the maxima in the lobes have different position angles (see Fig. 3 and Table 4). On the eastern side there is a change of [FORMULA] in the direction of the radio axis between kpc and Mpc-scales, on the western side the change is [FORMULA]. Since the kpc-scale structure is so well aligned, this indicates that the jets must be bent at a large distance (i.e. farther than 50 kpc) from the core. Alternatively, the radio axis may have changed direction in the course of time. However, even if the former position angle was such that the outflow on the eastern side was pointed directly at the hotspot, the western jet must still have been bent by [FORMULA] to have its endpoint in the maximum of the western lobe. An interesting possibility is that the western jet has been deflected by the bowshock preceeding the expanding southern lobe of B1144+353 (component A). We have however no observational evidence for this, nor the theoretical understanding to judge the feasibility of such a scenario.


[TABLE]

Table 4. Properties of the radio lobes of B1144+352. The values for the western lobe include the southern tail. [FORMULA] is the projected distance between the position of the maximum flux density in the lobe and the GPS source. PA is the position angle of the line connecting the core and the maximum in the lobes, counted CCW from the North. [FORMULA] and [FORMULA] are the flux densities at 1400 and 325 MHz, respectively, and [FORMULA] is the spectral index between these frequencies. [FORMULA] and [FORMULA] are the equipartion magnetic fieldstrength and energy density of the lobes, respectively, and are calculated using the assumptions made in Miley (1980) and the flux densities at 1400 MHz. All values have been calculated assuming that the lobes are associated to the GPS source and therefore have a similar redshift.
Notes:
a) measured from the NVSS map;
b) measured from the WSRT map;
c) measured from the WENSS map;
d) somewhat confused with the source B1144+353.


The orientation of the polarization vectors in components A, B and C is close to what one would expect for such source components. This strongly suggests that there is only marginal Faraday Rotation towards the western radio structure. If the rotation were more than [FORMULA], we would have observed a systematic offset in the polarization angles, and so we believe the rotation must be less than this. This implies that the (absolute) Rotation Measures [FORMULA] rad m-2. In case of [FORMULA] ambiguities in the polarization angle, which we cannot exclude on the basis of our data alone but which are not very likely either, [FORMULA] rad m-2, with n an integer number.

4.2.3. The southern radio tail

The existence of the southern tail of the western radio lobe is enigmatic. Extended radio tails have also been found in the Mpc-sized radio sources NGC 315 and NGC 6251 (e.g. Mack et al. 1997). One, much favoured, scenario is that these tails are material deposited in an earlier phase of the jet-activity, with the jet pointing in a different direction. In the case of B1144+352, there is indeed some evidence for such a change in the jet direction in the history of the radio source, as has been mentioned in the previous section. On the other hand, the lack of a similar tail northwards of the eastern lobe argues against a changing jet axis as the primary cause of the tail in the western lobe.

The tail of B1144+352 resembles the diffuse radio sources sometimes found in rich galaxy clusters (e.g. Röttgering et al. 1994, Feretti & Giovannini 1995, Röttgering et al. 1997). A model for the origin of these radio halo sources is presented by Ensslin et al. (1998). They find that shocks in an intra-cluster medium, caused by the merging of two clusters, can re-accelerate particles efficiently to the energies required to emit radio synchrotron emission. The radio source would then trace the shock front, hence its elongated appearance. A necessary condition is that the low-energy particles that are being accelerated in the shock should be available in the intra-cluster medium. Ensslin et al. suggest that they can be the remnant of a radio source that has already faded away. All radio halo sources known are found in clusters with high ([FORMULA] erg s-1) X-ray luminosities (e.g. Röttgering et al. 1997). However, the X-ray data have shown that B1144+352 is not in such an environment. At most, there is an `intra-group' medium surrounding the central radio source. This is in accordance with the apparently low Rotation Measures towards the western radio structure, since this also implies a low (column) density of thermal electrons along the line of sight.

Is the southern tail perhaps the poor group equivalent of the radio halo sources in rich clusters? The only large radio survey of poor groups of galaxies to date is presented by Burns et al. (1987), who observed 137 poor groups with the WSRT and the VLA. This revealed only one radio source with an extended tail much similar to the tail of B1144+352. However this source (B0153+053) is most likely a head-tail radio galaxy, which Burns et al. found in large numbers in their survey. It is therefore uncertain if there is a type of radio source that exists in groups of galaxies and which is an equivalent of the extended radio halo sources.

4.3. The orientation of the radio source

Giovaninni et al. (1995) have found superluminal motion in the pc-scale radio structure, with an apparent velocity of 2.4[FORMULA]. In the cosmology we assume, this implies that the angle [FORMULA] between the radio axis and the line of sight must be [FORMULA]. Considering the already large linear size of the outer structure, the lack of broad emission lines and non-thermal continuum in the optical spectrum, and the relatively low X-ray luminosity of the central source when compared with broad-line radio galaxies (e.g. Crawford & Fabian 1995), we believe that a value close to the upper limit of [FORMULA] is more appropriate.

If the kpc-scale structure has a similar orientation to the pc-scale structure and if it originated in the core, the armlength asymmetry of the inner structure observed in FIRST can be explained as a pure orientation effect. We further assume that the advance velocity of the outer components is equal on both sides of the nucleus. Since the armlength ratio of the outer components in the FIRST map is 1.684, their advance velocity must be [FORMULA], which is much higher than what is normally found in radio galaxies ([FORMULA], e.g. Alexander & Leahy 1987). This would suggest that the inner lobes are expanding in an extremely low-density environment, much lower than normally found at such distances from the radio core.

Also the flux density ratio of the kpc-scale components on either side of the core, if they are indeed advancing at this velocity, can be predicted under the hypothesis that the two sides are intrinsically equal and that any side-to-side difference is only due to relativistic beaming. Using the formula given in Pearson & Zensus (1987), and assuming that the components are discrete radio sources advancing at an angle [FORMULA] with respect to the line of sight, we find an expected flux density ratio of 7.3. The observed flux density ratio is [FORMULA], which, considering the simplifying assumptions, is not far off. However, velocities of almost [FORMULA] at distances of tens of kpc from the nucleus are not expected in radio sources, and so we believe that the armlength and flux density asymmetries are caused by some other effect, most likely inhomogeneities in the environment.

An alternative hypothesis to explain the large asymmetry in armlength of the inner kpc-scale structure is that of a jet which is ejected alternatively on one side of the nucleus and then on the other (e.g. Rudnick & Edgar 1984). The observation that the radio structure on the eastern side appears to consist of two components, one at 21.6 kpc from the nucleus and the other at 41.2 kpc, while the radio structure on the western side only consists of a single component at 24.5 kpc from the core, appears to support such a scenario. This suggests that the outflow direction of the jet responsible for the formation of the inner structure would have `flipped' at least twice. A consequence of this model is that only one side of the radio source is fuelled by the jet. Since radio hotspots are expected to fade very rapidly once the inflow of jet material has stopped (within a few [FORMULA] yr; e.g. Clarke & Burns 1991), a large asymmetry in flux density is a natural result. Although statistical arguments argue against this scenario (e.g. Ensman & Ulvestad 1984), we cannot rule out the possibility that B1144+352 is a special case.

The projected distance from the hotspot in the eastern lobe to the maximum in component C of the western structure is [FORMULA] kpc. If the Mpc-scale structure also has a similarly oriented radio axis as the pc-scale structure, the true physical size of B1144+352 is [FORMULA] Mpc, which is not an uncommon value for Giant radio sources (e.g. Saripalli et al. 1986).

4.4. Is the nucleus recurrently radio active?

B1144+352 is one of two currently known GPS sources which are also associated with Mpc-sized radio sources. The other case is the source B1245+676 which has outer lobes of 1.9 Mpc projected linear size (de Bruyn et al. in preparation). These two sources can be seen as the extremes of the few known GPS sources with extended radio structures, such as those discussed by Baum et al. (1990) and Stanghellini et al. (1990). In the case of B1144+352, the presence of sharply bound radio structures on Mpc, kpc and pc-scale suggests that the AGN must have gone through several phases of radio activity or, alternatively, that the jets of a continuously operating AGN must have been disrupted (or `smothered'; Baum et al. 1990) several times close to the nucleus. The fact that the core is currently a GPS source, and so either must be very young, or entrapped in a high density environment, adds further weight to the argument that some form of interruption of the jet flow must have occurred. Since superluminal motion has been detected on pc-scales, it is unlikely that the GPS source is confined by a high density medium. Therefore, the most likely scenario for this source is that the nucleus is indeed recurrently radio active. O'Dea (1998) mentions that the large fraction of GPS/CSS galaxies that show evidence for interaction and/or mergers, suggests that such processes must be related with the formation of the radio sources. An apparent close companion is also detected near the host galaxy of B1144+352 (see Sect. 2 and Fig. 1). Perhaps an interaction between the host galaxy and this apparent companion has disturbed the jet flow to the outer lobes, or even halted the jet formation temporarily, giving rise to the subsequent formation of the inner radio structure.

The rise and fall in the 1.4-GHz flux density of the central source over the last 25 years and the presence of the luminous X-ray source suggest that the nucleus is currently active. The distance between the outermost maximum in the eastern inner kpc-structure and the core is [FORMULA] kpc (see Table 1), where [FORMULA] is the angle between the radio axis and the line of sight. This indicates that the nucleus must have been radio active, although not necessarily continuously, for at least the last [FORMULA]) yr. Here, [FORMULA] is the advance velocity of the head of the jet. The presence of the GPS source implies that jet material which is currently expelled from the core does not flow into the outer Mpc-sized lobes anymore, and possible also not into the kpc-sized lobes either. Still, a compact radio hotspot has been detected in the eastern Mpc-sized radio lobe of B1144+352. As has been mentioned in the last section, radio hotspots are expected to fade within a few [FORMULA] yr (Clarke & Burns 1991). Therefore, the detection of the hotspot implies that there must still be jet material arriving there. This means that the disruption of the jet cannot have occurred longer ago than the travel-time of the jet material from the nucleus to this hotspot. The travel-time is [FORMULA] yr, where [FORMULA] is the velocity of the material flowing down the jet and which is most likely close to c. This also gives an maximum age of the inner structure. Given the constraint on the age, we find that the minimal life-time averaged advance velocity of the head of the inner structure, [FORMULA], is [FORMULA]. Depending on how long the radio activity was switched off this value further increases.

If, eventually, the central kpc-scale radio source grows out to a large ([FORMULA] kpc) radio source before the outer lobes have faded, B1144+352 can become a so-called `double-double radio galaxy' (DDRG, Schoenmakers et al. 1998b). DDRGs are radio sources which consist of two unequally sized but well aligned double-lobed radio sources with a coinciding radio core. Six of such objects with inner sources larger than 100 kpc are known now (Schoenmakers et al. 1998b), and they also appear to be the result of recurrent jet-formation activity in the nucleus. Sources such as B1144+352 and B1245+676 are good candidates for the progenitors of these DDRGs.

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Online publication: November 26, 1998
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