1,232 research outputs found
The long-wavelength view of GG Tau A: rocks in the ring world
We present the first detection of GG Tau A at centimetre wavelengths, made with the Arcminute Microkelvin Imager Large Array at a frequency of 16 GHz (λ = 1.8 cm). The source is detected at >6 σrms with an integrated flux density of S16GHz = 249 ± 45 µJy. We use these new centimetre-wave data, in conjunction with additional measurements compiled from the literature, to investigate the long-wavelength tail of the dust emission from this unusual protoplanetary system. We use an MCMC-based method to determine maximum likelihood parameters for a simple parametric spectral model and consider the opacity and mass of the dust contributing to the microwave emission. We derive a dust mass of Md ~ 0.1 Msun, constrain the dimensions of the emitting region and find that the opacity index at λ > 7 mm is less than unity, implying a contribution to the dust population from grains exceeding ~4 cm in size. We suggest that this indicates coagulation within the GG Tau A system has proceeded to the point where dust grains have grown to the size of small rocks with dimensions of a few centimetres. Considering the relatively young age of the GG Tau association in combination with the low derived disc mass, we suggest that this system may provide a useful test case for rapid core accretion planet formation models
Multiscale Analysis of the Gradient of Linear Polarisation
We propose a new multiscale method to calculate the amplitude of the gradient
of the linear polarisation vector using a wavelet-based formalism. We
demonstrate this method using a field of the Canadian Galactic Plane Survey
(CGPS) and show that the filamentary structure typically seen in gradients of
linear polarisation maps depends strongly on the instrumental resolution. Our
analysis reveals that different networks of filaments are present on different
angular scales. The wavelet formalism allows us to calculate the power spectrum
of the fluctuations seen in gradients of linear polarisation maps and to
determine the scaling behaviour of this quantity. The power spectrum is found
to follow a power law with gamma ~ 2.1. We identify a small drop in power
between scales of 80 < l < 300 arcmin, which corresponds well to the overlap in
the u-v plane between the Effelsberg 100-m telescope and the DRAO 26-m
telescope data. We suggest that this drop is due to undersampling present in
the 26-m telescope data. In addition, the wavelet coefficient distributions
show higher skewness on smaller scales than at larger scales. The spatial
distribution of the outliers in the tails of these distributions creates a
coherent subset of filaments correlated across multiple scales, which trace the
sharpest changes in the polarisation vector P within the field. We suggest that
these structures may be associated with highly compressive shocks in the
medium. The power spectrum of the field excluding these outliers shows a
steeper power law with gamma ~ 2.5.Comment: 12 pages, 12 figure
Diffuse radio emission in MACS J0025.41222: the effect of a major merger on bulk separation of ICM components
Mergers of galaxy clusters are among the most energetic events in the
Universe. These events have significant impact on the intra-cluster medium,
depositing vast amounts of energy - often in the form of shocks - as well as
heavily influencing the properties of the constituent galaxy population. Many
clusters have been shown to host large-scale diffuse radio emission, known
variously as radio haloes and relics. These sources arise as a result of
electron (re-)acceleration in cluster-scale magnetic fields, although the
processes by which this occurs are still poorly understood. We present new,
deep radio observations of the high-redshift galaxy cluster MACS
J0025.41222, taken with the GMRT at 325 MHz, as well as new analysis of all
archival X-ray observations. We aim to investigate the potential of
diffuse radio emission and categorise the radio population of this cluster,
which has only been covered previously by shallow radio surveys. We produce
low-resolution maps of MACS J0025.41222 through a combination of uv-tapering
and subtracting the compact source population. Radial surface brightness and
mass profiles are derived from the data. We also derive a 2D map of
the ICM temperature. For the first time, two sources of diffuse radio emission
are detected in MACS J0025.41222, on linear scales of several hundred kpc.
Given the redshift of the cluster and the assumed cosmology, these sources
appear to be consistent with established trends in power scaling relations for
radio relics. The X-ray temperature map presents evidence of an asymmetric
temperature profile and tentative identification of a temperature jump
associated with one relic. We classify the pair of diffuse radio sources in
this cluster as a pair of radio relics, given their consistency with scaling
relations, location toward the cluster outskirts, and the available X-ray data.Comment: 20 pages, 15 figures, accepted for publication in A&
Early Science with the Karoo Array Telescope: a Mini-Halo Candidate in Galaxy Cluster Abell 3667
Abell 3667 is among the most well-studied galaxy clusters in the Southern
Hemisphere. It is known to host two giant radio relics and a head-tail radio
galaxy as the brightest cluster galaxy. Recent work has suggested the
additional presence of a bridge of diffuse synchrotron emission connecting the
North-Western radio relic with the cluster centre. In this work, we present
full-polarization observations of Abell 3667 conducted with the Karoo Array
Telescope at 1.33 and 1.82 GHz. Our results show both radio relics as well as
the brightest cluster galaxy. We use ancillary higher-resolution data to
subtract the emission from this galaxy, revealing a localised excess, which we
tentatively identify as a radio mini-halo. This mini-halo candidate has an
integrated flux density of mJy beam at 1.37 GHz,
corresponding to a radio power of P
W Hz, consistent with established trends in mini-halo power scaling.Comment: 17 pages, 10 figures, accepted MNRA
An upper limit on the strength of the extragalactic magnetic field from ultra-high-energy cosmic-ray anisotropy
If ultra-high-energy cosmic rays originate from extragalactic sources, the
offsets of their arrival directions from these sources imply an upper limit on
the strength of the extragalactic magnetic field. The Pierre Auger
Collaboration has recently reported that anisotropy in the arrival directions
of cosmic rays is correlated with several types of extragalactic objects. If
these cosmic rays originate from these objects, they imply a limit on the
extragalactic magnetic field strength of B < 0.7-2.2 x 10^-9 (lambda_B / 1
Mpc)^-1/2 G for coherence lengths lambda_B < 100 Mpc and B < 0.7-2.2 x 10^-10 G
at larger scales. This is comparable to existing upper limits at lambda_B = 1
Mpc, and improves on them by a factor 4-12 at larger scales. The principal
source of uncertainty in our results is the unknown cosmic-ray composition.Comment: 9 pages, 3 figures, accepted by Ap
Unravelling the origin of large-scale magnetic fields in galaxy clusters and beyond through Faraday Rotation Measures with the SKA
We investigate the possibility for the SKA to detect and study the magnetic
fields in galaxy clusters and in the less dense environments surrounding them
using Faraday Rotation Measures. To this end, we produce 3-dimensional magnetic
field models for galaxy clusters of different masses and in different stages of
their evolution, and derive mock rotation measure observations of background
radiogalaxies. According to our results, already in phase I, we will be able to
infer the magnetic field properties in galaxy clusters as a function of the
cluster mass, down to solar-masses. Moreover, using cosmological
simulations to model the gas density, we have computed the expected rotation
measure through shock-fronts that occur in the intra-cluster medium during
cluster mergers. The enhancement in the rotation measure due to the density
jump will permit to constraint the magnetic field strength and structure after
the shock passage. SKA observations of polarised sources located behind galaxy
clusters will answer several questions about the magnetic field strength and
structure in galaxy clusters, and its evolution with cosmic time.Comment: 9 pages, 4 Figures, to appear as part of 'Cosmic Magnetism' in
Proceedings 'Advancing Astrophysics with the SKA (AASKA14)', PoS(AASKA14
Investigating the source of Planck-detected AME: high resolution observations at 15 GHz
The Planck 28.5 GHz maps were searched for potential Anomalous Microwave
Emission (AME) regions on the scale of or smaller, and several
new regions of interest were selected. Ancillary data at both lower and higher
frequencies were used to construct spectral energy distributions (SEDs), which
seem to confirm an excess consistent with spinning dust models. Here we present
higher resolution observations of two of these new regions with the Arcminute
Microkelvin Imager Small Array (AMI SA) between 14 and 18 GHz to test for the
presence of a compact (10 arcmin or smaller) component. For
AME-G107.1+5.2, dominated by the {\sc Hii} region S140, we find evidence for
the characteristic rising spectrum associated with the either the spinning dust
mechanism for AME or an ultra/hyper-compact \textsc{Hii} region across the AMI
frequency band, however for AME-G173.6+2.8 we find no evidence for AME on
scales of arcmin.Comment: 13 pages, 8 figures, 4 tables. Submitted to Advances in Astronomy AME
Special Issu
Tentative Evidence for Relativistic Electrons Generated by the Jet of the Young Sun-like Star DG Tau
Synchrotron emission has recently been detected in the jet of a massive
protostar, providing further evidence that certain jet formation
characteristics for young stars are similar to those found for highly
relativistic jets from AGN. We present data at 325 and 610 MHz taken with the
GMRT of the young, low-mass star DG Tau, an analog of the Sun soon after its
birth. This is the first investigation of a low-mass YSO at at such low
frequencies. We detect emission with a synchrotron spectral index in the
proximity of the DG Tau jet and interpret this emission as a prominent bow
shock associated with this outflow. This result provides tentative evidence for
the acceleration of particles to relativistic energies due to the shock impact
of this otherwise very low-power jet against the ambient medium. We calculate
the equipartition magnetic field strength (0.11 mG) and particle energy
(4x10^40 erg), which are the minimum requirements to account for the
synchrotron emission of the DG Tau bow shock. These results suggest the
possibility of low energy cosmic rays being generated by young Sun-like stars.Comment: 19 pages, 2 figures, accepted for publication in ApJ Letter
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