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‾¹₅ₒ Mpc in diameter.
Multicolour CCD observations yield photometric redshifts indicating that a significant fraction of the optical galaxies in this
region is at a redshift of z 5 = 0.3. This is supported by spectroscopic measurements of 4 galaxies in the Sloan Digitized Sky
Survey (SDSS) survey 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 sˉ¹ and
shows an extended highly non-relaxed morphology. These observations 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 produce enough high energy cosmic-ray (CR)
electrons required to explain the observed radio emission, provided a magnetic field of strength B ≳ 0.3 µG 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 a 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‾¹₅ₒ 21 Mpc.
