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

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

5. Conclusions

We have studied the large-scale radio structure of nearby GPS source B1144+352. We have presented new 1.4-GHz WSRT observations, and we have obtained X-ray data from the ROSAT archive. Our conclusions are the following:

After a steady increase over at least 25 yr, the core flux density at 1.4 GHz has decreased by mJy to mJy (epoch 1997.7) with respect to the measurements in the FIRST survey (epoch 1994.6). This behaviour is remarkable since GPS galaxies in general show only small variations in their flux density, but agrees with the predictions made by a model of the evolutionary scenario of radio components in GPS sources presented by Snellen et al. (1998).
The eastern extended radio structure is a radio lobe with a leading hotspot and can only be associated with the central GPS source.
The southern part of the western structure is a radio lobe with an elongated tail. This lobe is most likely associated with the GPS source. The origin of the southern tail is not clear.
The total projected linear size of the large-scale radio source associated with the GPS source B1144+352 is Mpc, but the detection of superluminal motion in the GPS source suggests that the true (deprojected) physical size may be Mpc.
There are differences of between the pc-to-kpc-scale radio axis and the Mpc-scale radio axis. This means that the Mpc-scale jets must have been bent. It also suggests that the outflow direction of the jet responsible for the pc-to-kpc-scale structure has changed before the formation of the kpc-scale structure.
Since we still see a compact hotspot in the eastern Mpc-scale radio lobe, we can limit the age of the kpc-scale radio source to yr, with the angle of the radio axis with respect to the line of sight. Therefore, the time-averaged advance velocity of the eastern kpc-scale component must be .
The host galaxy of B1144+352 is associated with a somewhat extended X-ray source. The extension appears to have a similar position angle to the central radio source. Assuming that the emission is due to an AGN, with a powerlaw spectrum with photon index 1.8, we find a luminosity of erg s^-1 between 0.1 and 2.4 keV. For a thermal bremsstrahlung model with K and a metal abundance of 0.25 solar, we find a luminosity of erg s^-1 in the same band. Such luminosities are comparable to that of powerful 3CR narrow-line radio galaxies at similar redshift. It strongly suggests that the central part of the host galaxy of B1144+352 does not contain a large amount of cold gas.
In the environment of the GPS source some sort of medium must be present to confine the extended radio structures. Such a medium may reveal itself through the emission of X-rays. Using the lack of a significant detection in our ROSAT data, we have determined a 1 upper limit of erg s^-1 for the keV X-ray luminosity within a 250 kpc radius of the host galaxy of B1144+352. Therefore, the radio source is not in a luminous X-ray cluster. The upper limit of the X-ray luminosity is, however, comparable to that of poor groups of galaxies, so we cannot exclude the presence of an `intra-group' medium. Optical data seem to support the presence of a small group of galaxies around the host galaxy of B1144+352.
The northern part of the western extended radio structure is a separate low-power radio galaxy which we refer to as B1144+353. We have measured the redshift of the host galaxy at , which is not significantly different from that of the GPS host galaxy (). The projected distance to the GPS host galaxy is kpc.

The source B1144+352 is the second GPS source known to be associated with a Mpc-sized radio source. Notwithstanding whether GPS sources are young or situated in dense environments, the existence of the Mpc-sized lobes implies that the jets fuelling the radio source have been able to escape the host galaxy in the past. We conclude that the best explanation for the existence of these two sources is that their jet flow must have been interrupted temporarily, either as a result of a complete halting of the jet formation or as a result of a disruption of the jet flow close to the nucleus. This makes it very likely that they are the progenitors of the DDRGs discussed by Schoenmakers et al. (1998b). In case of the source B1144+352, the observation of superluminal motion on pc-scale indicates that the GPS source it is not confined, or `frustrated', by a high density medium, and that therefore the nucleus is most likely recurrently radio active. The variation in radio peak flux density observed over the last 25 years suggests that a new phase of activity may have started quite recently and that the GPS source which is currently observed is extremely young.

Online publication: November 26, 1998
helpdesk.link@springer.de