1,308 research outputs found
Multiple density discontinuities in the merging galaxy cluster CIZA J2242.8+5301
CIZA J2242.8+5301, a merging galaxy cluster at z=0.19, hosts a double-relic
system and a faint radio halo. Radio observations at frequencies ranging from a
few MHz to several GHz have shown that the radio spectral index at the outer
edge of the N relic corresponds to a shock of Mach number 4.6+/-1.1, under the
assumptions of diffusive shock acceleration of thermal particles in the test
particle regime. Here, we present results from new Chandra observations of the
cluster. The Chandra surface brightness profile across the N relic only hints
to a surface brightness discontinuity (<2-sigma detection). Nevertheless, our
reanalysis of archival Suzaku data indicates a temperature discontinuity across
the relic that is consistent with a Mach number of 2.5+/-0.5, in agreement with
previously published results. This confirms that the Mach number at the shock
traced by the N relic is much weaker than predicted from the radio. Puzzlingly,
in the Chandra data we also identify additional inner small density
discontinuities both on and off the merger axis. Temperature measurements on
both sides of the discontinuities do not allow us to undoubtedly determine
their nature, although a shock front interpretation seems more likely. We
speculate that if the inner density discontinuities are indeed shock fronts,
then they are the consequence of violent relaxation of the dark matter cores of
the clusters involved in the merger.Comment: 11 pages, 11 figures. Accepted for publication in MNRA
Carbon radio recombination lines from gigahertz to megahertz frequencies towards Orion A
Context. The combined use of carbon radio recombination lines (CRRLs) and the
158 m-[CII] line is a powerful tool for the study of the energetics and
physical conditions (e.g., temperature and density) of photodissociation
regions (PDRs). However, there are few observational studies that exploit this
synergy. Aims. Here we explore the relation between CRRLs and the 158
m-[CII] line in light of new observations and models. Methods. We present
new and existing observations of CRRLs in the frequency range 0.15--230 GHz
with ALMA, VLA, the GBT, Effelsberg 100m, and LOFAR towards Orion~A (M42). We
complement these observations with SOFIA observations of the 158 m-[CII]
line. We studied two PDRs: the foreground atomic gas, known as the Veil, and
the dense PDR between the HII region and the background molecular cloud.
Results. In the Veil we are able to determine the gas temperature and electron
density, which we use to measure the ionization parameter and the photoelectric
heating efficiency. In the dense PDR, we are able to identify a layered PDR
structure at the surface of the molecular cloud to the south of the Trapezium
cluster. There we find that the radio lines trace the colder portion of the
ionized carbon layer, the C/C/CO interface. By modeling the emission of
the ~m-[CII] line and CRRLs as arising from a PDR we derive a thermal
pressure K cm and a radiation field
close to the Trapezium. Conclusions. This work provides
additional observational support for the use of CRRLs and the 158 m-[CII]
line as complementary tools to study dense and diffuse PDRs, and highlights the
usefulness of CRRLs as probes of the C/C/CO interface.Comment: 18 pages, 16 figures, accepted for publication in A&
Deep LOFAR 150 MHz imaging of the Bo\"otes field: Unveiling the faint low-frequency sky
We have conducted a deep survey (with a central rms of )
with the LOw Frequency ARray (LOFAR) at 120-168 MHz of the Bo\"otes field, with
an angular resolution of , and obtained a sample of
10091 radio sources ( limit) over an area of .
The astrometry and flux scale accuracy of our source catalog is investigated.
The resolution bias, incompleteness and other systematic effects that could
affect our source counts are discussed and accounted for. The derived 150 MHz
source counts present a flattening below sub-mJy flux densities, that is in
agreement with previous results from high- and low- frequency surveys. This
flattening has been argued to be due to an increasing contribution of
star-forming galaxies and faint active galactic nuclei. Additionally, we use
our observations to evaluate the contribution of cosmic variance to the scatter
in source counts measurements. The latter is achieved by dividing our Bo\"otes
mosaic into 10 non-overlapping circular sectors, each one with an approximate
area of The counts in each sector are computed in the
same way as done for the entire mosaic. By comparing the induced scatter with
that of counts obtained from depth observations scaled to 150MHz, we find that
the scatter due to cosmic variance is larger than the Poissonian
errors of the source counts, and it may explain the dispersion from previously
reported depth source counts at flux densities . This work
demonstrates the feasibility of achieving deep radio imaging at low-frequencies
with LOFAR.Comment: A\&A in press. 15 pages, 16 figure
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