Wide-field LOFAR imaging of the field around the double-double radio
  galaxy B1834+620: A fresh view on a restarted AGN and doubeltjes
  The existence of double-double radio galaxies (DDRGs) is evidence for
recurrent jet activity in AGN, as expected from standard accretion models. A
detailed study of these rare sources provides new perspectives for
investigating the AGN duty cycle, AGN-galaxy feedback, and accretion
mechanisms. Large catalogues of radio sources provide statistical information
about the evolution of the radio-loud AGN population out to high redshifts.
Using wide-field imaging with the LOFAR telescope, we study both a well-known
DDRG as well as a large number of radio sources in the field of view. We
present a high resolution image of the DDRG B1834+620 obtained at 144 MHz using
LOFAR commissioning data. Our image covers about 100 square degrees and
contains over 1000 sources. The four components of the DDRG B1834+620 have been
resolved for the first time at 144 MHz. Inner lobes were found to point towards
the direction of the outer lobes, unlike standard FR~II sources. Polarized
emission was detected in the northern outer lobe. The high spatial resolution
allows the identification of a large number of small double-lobed radio
sources; roughly 10% of all sources in the field are doubles with a separation
smaller than 1 arcmin. The spectral fit of the four components is consistent
with a scenario in which the outer lobes are still active or the jets recently
switched off, while emission of the inner lobes is the result of a mix-up of
new and old jet activity. From the presence of the newly extended features in
the inner lobes of the DDRG, we can infer that the mechanism responsible for
their formation is the bow shock that is driven by the newly launched jet. We
find that the density of the small doubles exceeds the density of FR-II sources
with similar properties at 1.4 GHz, but this difference becomes smaller for low
flux densities.
