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Astron. Astrophys. 332, 395-409 (1998)

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5. The origin of relics

5.1. Radio galaxies

Relics are believed to be remnants of former radio galaxies. The problem that the energy loss time scale is frequently shorter than the time elapsed since the last resupply with energetic electrons from a progenitor radio galaxy is solved in our approach due to the acceleration by the large-scale accretion shocks. Therefore the progenitor radio galaxy needs not to be within the distance from the relic a galaxy can travel with a typical cluster velocity within a cooling time of the electrons. In order to explain the polarization by field compression, it is necessary though that the radio plasma from which the present day radio emission results was injected within the upstream region. The large spatial extent of relics might result from the much lower pressure in the upstream regions, so that the radio lobes could expand over a larger volume than a similar radio galaxy would have occupied in the intra-cluster medium inside the accretion shock.

It is interesting to remember that the minimum magnetic pressure of 1253+275 is near equipartition with the expected thermal pressure at the shock radius (estimated using an extrapolation of the central density profile), whereas minimum pressures of radio lobes of active radio galaxies in the interior of galaxy clusters usually are much lower than the gas pressure at their radii, even if projection effects are taken into account (Feretti et al. 1992). Several effects can explain this latter pressure imbalance, despite the possibility that minimum energies do not describe the physical conditions:

  • a filling factor of the observed radio plasma of a few percent
  • the ubiquitous presence of relativistic protons within the lobes, dominating the energy density of relativistic particles
  • the existence of a significant number of low-energy electrons radiating below the adopted low-frequency cutoff of the minimum energy analysis
  • a significant amount of thermal gas within the lobes.

Under the hypothesis that relics are old lobes, the filling factor and also the low-energy electron population should be similar in both types of sources, and therefore not responsible for differences of cluster relics and lobes in the central regions of clusters. Thermal gas remains to be the explanation, and also relativistic protons, because the possible rich initial proton population of the lobes might be escaped due to the larger age and the higher cosmic ray diffusion coefficient of relics compared to lobes inside clusters.

Giovannini et al. (1985) propose the Coma cluster galaxy IC 3900 to be the origin of 1253+275, which should have moved to another position on the plane of the sky. We point out that the narrow-angle tail (NAT) radio galaxy NGC 4789 is a very attractive possible origin, too. First, a bridge of radio emission connects it with the central ridge of the relic (Giovannini et al. 1991), indicating a geometrical connection. Second, 1253+275 lies on the (projected) line between NGC 4789 and the center of the Coma cluster. NGC 4789 is therefore an ideal candidate for being the source of magnetized plasma injected upstream into the accretion flow. The NAT structure of NGC 4789 might result from the ram-pressure of the infalling matter on the radio jets, enhanced by an ascending movement of the galaxy. Relativistic electrons from the jets should be convected by the flow within the radio bridge to the accretion shock, where they are reaccelerated and radiate. The line-of-sight velocity of NGC 4789 is higher by [FORMULA] than Coma's Hubble velocity (Venturi et al. 1988). This implies, if NGC 4789 is really located upstream above 1253+275, that we see a relic on the back side of the accretion shock sphere. This is sketched in Fig. 2. If NGC 4789 is on a radial ascending orbit, its velocity should be 1480 km/s (assuming [FORMULA]), similar to that of the infalling matter measured in the clusters rest frame [FORMULA] km/s. The radio plasma, which is reaching the shock today, was thus injected into the stream half a Gyr ago, given that its movement is only due to convection by the accretion flow.

5.2. Large-scale fields

A very different nature of the origin of the magnetic fields in the relic is imaginable: from simulations of magnetic field generation and amplification by turbulence in the flows of large-scale structure formation field strengths of the [FORMULA] G level within cosmological filaments are predicted by Biermann et al. (1997). The predicted field strengths at cluster accretion shocks are [FORMULA] G, being in energetic equipartition with the thermal energy density. In the inner regions of clusters, field strengths at the [FORMULA] G level are expected, as Enßlin et al. (1997) argue to be injected from radio galaxies, presuming that their jets also contain energetic protons. If field values of [FORMULA] G are typical for the accretion shock region, it has to be explained what is special at the positions of relic sources. In the case of 1253+275 this should be the presence of NGC 4789 providing preaccelerated electrons injected into the accretion flow, efficiently reaccelerated at the shock front below the galaxy.

It is remarkable that the 1253+275 complex is in the elongation of the Coma cluster, which itself appears to align with Coma/Abell 1367 filament of galaxies as described by Fontanelli (1984). It is possible that the position of the relic traces the working surface of a large-scale stream flowing out of this filament and hitting the sea of cluster gas. The observed radio bridge connecting 1253+275 and the radio halo Coma C (Kim et al. 1989) then might be a deposition of magnetic fields from this stream, illuminated by relativistic electrons accelerated in the shock.

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© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998