Evidence for shock acceleration and intergalactic magnetic fields in a large-scale filament of galaxies ZwC1 2341.1+0000

Abstract

We report the discovery of large-scale diffuse radio emission from what appears to be a large-scale filamentary network of galaxies in the region of cluster ZwCl 2341.1+0000, and stretching over an area of at least 6 h−1 50 Mpc in diameter. Mul- ticolour CCD observations yield photometric redshifts indicating that a significant fraction of the optical galaxies in this region is at a redshift of z=0.3. This is sup- ported by spectroscopic measurements of 4 galaxies in the Sloan Digital Sky Survey (SDSS) at a mean z =0.27. We present VLA images at λ =20 cm (NVSS) and 90 cm, showing the detailed radio structure of the filaments. Comparison with the high resolution FIRST radio survey shows that the diffuse emission is not due to known individual point sources. The diffuse radio-emission has a spectral index α . −0.5, and is most likely synchrotron emission from relativistic charged particles in an inter-galactic magnetic field. Furthermore, this optical/radio structure is detected in X-rays by the ROSAT all-sky survey. It has a 0.1–2.4 keV luminosity of about 1044 erg s−1 and shows an extended highly non-relaxed morphology. These observa- tions suggest that ZwCl 2341.1+0000 is possibly a proto-cluster of galaxies in which we are witnessing the process of structure formation. We show that the energetics of accretion shocks generated in forming large-scale structures are sufficient to pro- duce enough high energy cosmic-ray (CR) electrons required to explain the observed radio emission, provided a magnetic field of strength B & 0.3µG is present there. The latter is only a lower limit and the actual magnetic field is likely to be higher depending on the morphology of the emitting region. Finally, we show results from Preprint submitted to Elsevier Science 1 February 2008a numerical simulation of large-scale structure formation including acceleration of CR electrons at cosmological shocks and magnetic field evolution. Our results are in accord with the observed radio synchrotron and X-ray thermal bremsstrahlung fluxes. Thus we conclude that the reported radio detection is the first evidence of cosmic-ray particle acceleration taking place at cosmic shocks in a magnetized inter-galactic medium over scales of & 5 h−1 50 Mpc.