Galaxy
clusters are the largest gravitationally bound objects
in the Universe and form through accretion of gas and
by mergers with other clusters and galaxy groups. They
are unique laboratories to study some of the most
fundamental questions in astrophysics, related to the
physics of particle acceleration and cosmic rays, the
growth of large-scale structure, and the nature of
dark matter. Massive clusters, particularly merging
clusters, are also powerful cosmic telescopes, capable
of magnifying distant galaxies, thus providing a probe
of the early Universe.
Radio observations of galaxy clusters have revealed the existence of large megaparsec-size diffuse synchrotron emitting sources: so-called radio halos and relics, depending on their location in the cluster center or outskirts. The synchrotron radiation implies the presence of cosmic rays and magnetic fields in the intracluster medium (ICM). With their enormous extent, these sources trace some of the largest particle accelerators in the Universe.
It is
thought that halos and relics follow shocks waves and
turbulence which are created when two or more galaxy
clusters collide and merge. At these shocks, particles
are accelerated to relativistic energies and in the
presence of a magnetic field these relativistic
particles (cosmic rays) emit synchrotron radiation
observable with radio telescopes. The physics of the
acceleration process and origin of cluster-wide
magnetic fields are still poorly understood.
The acceleration mechanism can be studied via radio observations, in particular by mapping out the shape of the radio spectrum via observations at multiple frequencies. The properties of the magnetic field can be determined by obtaining polarization measurements. X-ray observations also play an important role, as they allows us to determine the shock Mach numbers and temperature and density of the ICM.
Over the
past few years, our focus has been on utilizing LOFAR
for two main purposes: (i) constructing a large sample
of clusters to investigate the occurrence rates of
halos and relics, and (ii) conducting detailed studies
of selected clusters and their surroundings.
The diffuse radio emission from merging
clusters, as well as other radio sources, is typically
very faint, making their study difficult. The
brightness of this emission generally increases
towards lower frequencies, and low-frequency
observations are therefore preferred. However,
high-quality low-frequency images are difficult to
make due to the presence of the ionosphere, which
blurs the radio images.
In our group we are working on developing a
calibration and imaging schemes that corrects for the
blurring effects of the ionosphere, allowing detailed
studies cluster and other sources at frequencies below
200 MHz using . LOFAR is new revolutionary radio
telescope operating at the lowest frequencies
accessible from the ground. During the last year, we
have focused on the development of (i) extraction and
re-calibration of sources from the LoTSS survey, (ii)
sub-arcsecond widefield imaging with LOFAR's European
baselines, and (iii) pushing LOFAR to the lowest
possible frequencies that can be observed from the
ground, the decameter band.
In our group, we are developing calibration and
imaging schemes to correct for the blurring effects of
the ionosphere, enabling detailed studies of clusters
and other sources at frequencies below 200 MHz using LOFAR.
LOFAR is a revolutionary radio telescope
operating at the lowest frequencies accessible from
the ground. Over the past year, our focus has been on
(i) extracting and recalibrating sources from the LoTSS survey,
(ii) achieving sub-arcsecond widefield imaging with
LOFAR's European baselines, and (iii) extending
LOFAR's observations to the lowest possible
frequencies observable from the ground, in the
decameter band.