131 research outputs found
Transfer learning for radio galaxy classification
In the context of radio galaxy classification, most state-of-the-art neural
network algorithms have been focused on single survey data. The question of
whether these trained algorithms have cross-survey identification ability or
can be adapted to develop classification networks for future surveys is still
unclear. One possible solution to address this issue is transfer learning,
which re-uses elements of existing machine learning models for different
applications. Here we present radio galaxy classification based on a 13-layer
Deep Convolutional Neural Network (DCNN) using transfer learning methods
between different radio surveys. We find that our machine learning models
trained from a random initialization achieve accuracies comparable to those
found elsewhere in the literature. When using transfer learning methods, we
find that inheriting model weights pre-trained on FIRST images can boost model
performance when re-training on lower resolution NVSS data, but that inheriting
pre-trained model weights from NVSS and re-training on FIRST data impairs the
performance of the classifier. We consider the implication of these results in
the context of future radio surveys planned for next-generation radio
telescopes such as ASKAP, MeerKAT, and SKA1-MID
Testing protoplanetary disc dispersal with radio emission
We consider continuum free-free radio emission from the upper atmosphere of
protoplanetary discs as a probe of the ionized luminosity impinging upon the
disc. Making use of previously computed hydrodynamic models of disc
photoevaporation within the framework of EUV and X-ray irradiation, we use
radiative transfer post-processing techniques to predict the expected free-free
emission from protoplanetary discs. In general, the free-free luminosity scales
roughly linearly with ionizing luminosity in both EUV and X-ray driven
scenarios, where the emission dominates over the dust tail of the disc and is
partial optically thin at cm wavelengths. We perform a test observation of GM
Aur at 14-18 Ghz and detect an excess of radio emission above the dust tail to
a very high level of confidence. The observed flux density and spectral index
are consistent with free-free emission from the ionized disc in either the EUV
or X-ray driven scenario. Finally, we suggest a possible route to testing the
EUV and X-ray driven dispersal model of protoplanetary discs, by combining
observed free-free flux densities with measurements of mass-accretion rates. On
the point of disc dispersal one would expect to find a M_dot^2 scaling with
free-free flux in the case of EUV driven disc dispersal or a M_dot scaling in
the case of X-ray driven disc dispersal.Comment: Accepted MNRAS, 12 pages, 11 figures, (pdf generation fixed
Limits on the validity of the thin-layer model of the ionosphere for radio interferometric calibration
For a ground-based radio interferometer observing at low frequencies, the
ionosphere causes propagation delays and refraction of cosmic radio waves which
result in phase errors in the received signal. These phase errors can be
corrected using a calibration method that assumes a two-dimensional phase
screen at a fixed altitude above the surface of the Earth, known as the
thin-layer model. Here we investigate the validity of the thin-layer model and
provide a simple equation with which users can check when this approximation
can be applied to observations for varying time of day, zenith angle,
interferometer latitude, baseline length, ionospheric electron content and
observing frequency.Comment: 8 pages, 10 figures, accepted MNRA
Rare Galaxy Classes Identified In Foundation Model Representations
We identify rare and visually distinctive galaxy populations by searching for
structure within the learned representations of pretrained models. We show that
these representations arrange galaxies by appearance in patterns beyond those
needed to predict the pretraining labels. We design a clustering approach to
isolate specific local patterns, revealing groups of galaxies with rare and
scientifically-interesting morphologies.Comment: Accepted at Machine Learning and the Physical Sciences Workshop,
NeurIPS 202
Combining astrophysical datasets with CRUMB
At present, the field of astronomical machine learning lacks widely-used
benchmarking datasets; most research employs custom-made datasets which are
often not publicly released, making comparisons between models difficult. In
this paper we present CRUMB, a publicly-available image dataset of
Fanaroff-Riley galaxies constructed from four "parent" datasets extant in the
literature. In addition to providing the largest image dataset of these
galaxies, CRUMB uses a two-tier labelling system: a "basic" label for
classification and a "complete" label which provides the original class labels
used in the four parent datasets, allowing for disagreements in an image's
class between different datasets to be preserved and selective access to
sources from any desired combination of the parent datasets.Comment: Accepted in Machine Learning and the Physical Sciences Workshop at
NeurIPS 2023; 6 pages, 1 figure, 1 tabl
Anomalous Microwave Emission from Star Forming Regions
The evidence for microwave emission from spinning dust grains has been strengthened considerably by its detection in a number of discrete astrophysical objects associated with star formation. These detections, in combination with statistical constraints on its presence on large angular scales in the diffuse ISM, have provided strong observational confirmation of an emission mechanism still referred to as anomalous. This emission has a peaked spectrum with a maximum in the microwave band; the present review discusses the continuum radio emission mechanisms which can contribute to this region of the electromagnetic spectrum, collects published results on the prevalence of anomalous microwave emission in a variety of star formation regions, presents the overall conclusions that may be drawn from the detections so far, and discusses the prospects for future research on the anomalous microwave emission attributed to spinning dust within star forming regions
Sub-arcsecond high sensitivity measurements of the DG~Tau jet with e-MERLIN
We present very high spatial resolution deep radio continuum observations at
5 GHz (6 cm) made with e-MERLIN of the young stars DG Tau A and B. Assuming it
is launched very close (~=1 au) from the star, our results suggest that the DG
Tau A outflow initially starts as a poorly focused wind and undergoes
significant collimation further along the jet (~=50 au). We derive jet
parameters for DG Tau A and find an initial jet opening angle of 86 degrees
within 2 au of the source, a mass-loss rate of 1.5x10^-8 solar masses/yr for
the ionised component of the jet, and the total ejection/accretion ratio to
range from 0.06-0.3. These results are in line with predictions from MHD
jet-launching theories.Comment: Accepted MNRAS Letter
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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