1,579 research outputs found
SCORPIO-II: Spectral indices of weak Galactic radio sources
In the next few years the classification of radio sources observed by the
large surveys will be a challenging problem, and spectral index is a powerful
tool for addressing it. Here we present an algorithm to estimate the spectral
index of sources from multiwavelength radio images. We have applied our
algorithm to SCORPIO (Umana et al. 2015), a Galactic Plane survey centred
around 2.1 GHz carried out with ATCA, and found we can measure reliable
spectral indices only for sources stronger than 40 times the rms noise. Above a
threshold of 1 mJy, the source density in SCORPIO is 20 percent greater than in
a typical extra-galactic field, like ATLAS (Norris et al. 2006), because of the
presence of Galactic sources. Among this excess population, 16 sources per
square degree have a spectral index of about zero, suggesting optically thin
thermal emission such as Hii regions and planetary nebulae, while 12 per square
degree present a rising spectrum, suggesting optically thick thermal emission
such as stars and UCHii regions.Comment: 12 pages, 11 figures, accepted by MNRA
Automated cross-identifying radio to infrared surveys using the LRPY algorithm: A case study
Cross-identifying complex radio sources with optical or infra red (IR) counterparts in surveys such as the Australia Telescope Large Area Survey (ATLAS) has traditionally been performed manually. However, with new surveys from the Australian Square Kilometre Array Pathfinder detecting many tens of millions of radio sources, such an approach is no longer feasible. This paper presents new software (LRPY - Likelihood Ratio in PYTHON) to automate the process of cross-identifying radio sources with catalogues at other wavelengths. LRPY implements the likelihood ratio (LR) technique with a modification to account for two galaxies contributing to a sole measured radio component. We demonstrate LRPY by applying it to ATLAS DR3 and a Spitzer-based multiwavelength fusion catalogue, identifying 3848 matched sources via our LR-based selection criteria. A subset of 1987 sources have flux density values for all IRAC bands which allow us to use criteria to distinguish between active galactic nuclei (AGNs) and star-forming galaxies (SFG). We find that 936 radio sources (˜47 per cent) meet both of the Lacy and Stern AGN selection criteria. Of the matched sources, 295 have spectroscopic redshifts and we examine the radio to IR flux ratio versus redshift, proposing an AGN selection criterion below the Elvis radio-loud AGN limit for this dataset. Taking the union of all three AGNs selection criteria we identify 956 as AGNs (˜48 per cent). From this dataset, we find a decreasing fraction of AGNs with lower radio flux densities consistent with other results in the literature
Microwave observations of spinning dust emission in NGC6946
We report new cm-wave measurements at five frequencies between 15 and 18GHz
of the continuum emission from the reportedly anomalous "region 4" of the
nearby galaxy NGC6946. We find that the emission in this frequency range is
significantly in excess of that measured at 8.5GHz, but has a spectrum from
15-18GHz consistent with optically thin free-free emission from a compact HII
region. In combination with previously published data we fit four emission
models containing different continuum components using the Bayesian spectrum
analysis package radiospec. These fits show that, in combination with data at
other frequencies, a model with a spinning dust component is slightly preferred
to those that possess better-established emission mechanisms.Comment: submitted MNRA
A blind detection of a large, complex, Sunyaev--Zel'dovich structure
We present an interesting Sunyaev-Zel'dovich (SZ) detection in the first of
the Arcminute Microkelvin Imager (AMI) 'blind', degree-square fields to have
been observed down to our target sensitivity of 100{\mu}Jy/beam. In follow-up
deep pointed observations the SZ effect is detected with a maximum peak
decrement greater than 8 \times the thermal noise. No corresponding emission is
visible in the ROSAT all-sky X-ray survey and no cluster is evident in the
Palomar all-sky optical survey. Compared with existing SZ images of distant
clusters, the extent is large (\approx 10') and complex; our analysis favours a
model containing two clusters rather than a single cluster. Our Bayesian
analysis is currently limited to modelling each cluster with an ellipsoidal or
spherical beta-model, which do not do justice to this decrement. Fitting an
ellipsoid to the deeper candidate we find the following. (a) Assuming that the
Evrard et al. (2002) approximation to Press & Schechter (1974) correctly gives
the number density of clusters as a function of mass and redshift, then, in the
search area, the formal Bayesian probability ratio of the AMI detection of this
cluster is 7.9 \times 10^4:1; alternatively assuming Jenkins et al. (2001) as
the true prior, the formal Bayesian probability ratio of detection is 2.1
\times 10^5:1. (b) The cluster mass is MT,200 = 5.5+1.2\times 10^14h-1M\odot.
(c) Abandoning a physical model with num- -1.3 70 ber density prior and instead
simply modelling the SZ decrement using a phenomenological {\beta}-model of
temperature decrement as a function of angular distance, we find a central SZ
temperature decrement of -295+36 {\mu}K - this allows for CMB primary
anisotropies, receiver -15 noise and radio sources. We are unsure if the
cluster system we observe is a merging system or two separate clusters.Comment: accepted MNRAS. 12 pages, 9 figure
The POlarised GLEAM Survey (POGS) II: Results from an All-Sky Rotation Measure Synthesis Survey at Long Wavelengths
The low-frequency linearly-polarised radio source population is largely
unexplored. However, a renaissance in low-frequency polarimetry has been
enabled by pathfinder and precursor instruments for the Square Kilometre Array.
In this second paper from the POlarised GaLactic and Extragalactic All-Sky
Murchison Widefield Array (MWA) Survey -- the POlarised GLEAM Survey, or POGS
-- we present the results from our all-sky MWA Phase I Faraday Rotation Measure
survey. Our survey covers nearly the entire Southern sky in the Declination
range to at a resolution between around three and
seven arcminutes (depending on Declination) using data in the frequency range
169231 MHz. We have performed two targeted searches: the first covering
25,489 square degrees of sky, searching for extragalactic polarised sources;
the second covering the entire sky South of Declination ,
searching for known pulsars. We detect a total of 517 sources with 200 MHz
linearly-polarised flux densities between 9.9 mJy and 1.7 Jy, of which 33 are
known radio pulsars. All sources in our catalogues have Faraday rotation
measures in the range rad m to rad m. The
Faraday rotation measures are broadly consistent with results from
higher-frequency surveys, but with typically more than an order of magnitude
improvement in the precision, highlighting the power of low-frequency
polarisation surveys to accurately study Galactic and extragalactic magnetic
fields. We discuss the properties of our extragalactic and known-pulsar source
population, how the sky distribution relates to Galactic features, and identify
a handful of new pulsar candidates among our nominally extragalactic source
population.Comment: Replacement of previous version. Only change is minor updates to
catalogues (see ancillary files) which now contain bib code of finalised
manuscript (published in PASA). Manuscript has 31 pages, 10 figures, 5
tables. Four Appendices are included in the ancillary material, showing
further Figures, continuum spectra for a handful of selected sources, and RM
spectra for all 517 source
The POlarised GLEAM Survey (POGS) II: Results from an all-sky rotation measure synthesis survey at long wavelengths
The low-frequency linearly polarised radio source population is largely unexplored. However, a renaissance in low-frequency polarimetry has been enabled by pathfinder and precursor instruments for the Square Kilometre Array. In this second paper from the POlarised GaLactic and Extragalactic All-Sky MWA Survey-the POlarised GLEAM Survey, or POGS-we present the results from our all-sky MWA Phase I Faraday Rotation Measure survey. Our survey covers nearly the entire Southern sky in the Declination range to at a resolution between around three and seven arcminutes (depending on Declination) using data in the frequency range 169-231 MHz. We have performed two targeted searches: the first covering 25 489 square degrees of sky, searching for extragalactic polarised sources; the second covering the entire sky South of Declination, searching for known pulsars. We detect a total of 517 sources with 200 MHz linearly polarised flux densities between 9.9 mJy and 1.7 Jy, of which 33 are known radio pulsars. All sources in our catalogues have Faraday rotation measures in the range to rad m-2. The Faraday rotation measures are broadly consistent with results from higher-frequency surveys, but with typically more than an order of magnitude improvement in the precision, highlighting the power of low-frequency polarisation surveys to accurately study Galactic and extragalactic magnetic fields. We discuss the properties of our extragalactic and known-pulsar source population, how the sky distribution relates to Galactic features, and identify a handful of new pulsar candidates among our nominally extragalactic source population
Study of the Galactic radio sources in the SCORPIO survey resolved by ATCA at 2.1 GHz
We present a catalogue of a large sample of extended radio sources in the
SCORPIO field, observed and resolved by the Australia Telescope Compact Array.
SCORPIO, a pathfinder project for addressing the early operations of the
Australia SKA Pathfinder, is a survey of ~5 square degrees between 1.4 and 3.1
GHz, centered at l=343.5{\deg}, b=0.75{\deg} and with an angular resolution of
about 10 arcsec. It is aimed at understanding the scientific and technical
challenges to be faced by future Galactic surveys. With a mean sensitivity
around 100 Jy/beam and the possibility to recover angular scales at least
up to 4 arcmin, we extracted 99 extended sources, 35 of them detected for the
first time. Among the 64 known sources 55 had at least a tentative
classification in literature. Studying the radio morphology and comparing the
radio emission with infrared we propose as candidates 6 new H II regions, 2 new
planetary nebulae, 2 new luminous blue variable or Wolf--Rayet stars and 3 new
supernova remnants. This study provides an overview of the potentiality of
future radio surveys in terms of Galactic source extraction and
characterization and a discussion on the difficulty to reduce and analyze
interferometric data on the Galactic plane
High resolution AMI Large Array imaging of spinning dust sources: spatially correlated 8 micron emission and evidence of a stellar wind in L675
We present 25 arcsecond resolution radio images of five Lynds Dark Nebulae
(L675, L944, L1103, L1111 & L1246) at 16 GHz made with the Arcminute
Microkelvin Imager (AMI) Large Array. These objects were previously observed
with the AMI Small Array to have an excess of emission at microwave frequencies
relative to lower frequency radio data. In L675 we find a flat spectrum compact
radio counterpart to the 850 micron emission seen with SCUBA and suggest that
it is cm-wave emission from a previously unknown deeply embedded young
protostar. In the case of L1246 the cm-wave emission is spatially correlated
with 8 micron emission seen with Spitzer. Since the MIR emission is present
only in Spitzer band 4 we suggest that it arises from a population of PAH
molecules, which also give rise to the cm-wave emission through spinning dust
emission.Comment: accepted MNRA
Gridded and direct Epoch of Reionisation bispectrum estimates using the Murchison Widefield Array
We apply two methods to estimate the 21~cm bispectrum from data taken within
the Epoch of Reionisation (EoR) project of the Murchison Widefield Array (MWA).
Using data acquired with the Phase II compact array allows a direct bispectrum
estimate to be undertaken on the multiple redundantly-spaced triangles of
antenna tiles, as well as an estimate based on data gridded to the -plane.
The direct and gridded bispectrum estimators are applied to 21 hours of
high-band (167--197~MHz; =6.2--7.5) data from the 2016 and 2017 observing
seasons. Analytic predictions for the bispectrum bias and variance for point
source foregrounds are derived. We compare the output of these approaches, the
foreground contribution to the signal, and future prospects for measuring the
bispectra with redundant and non-redundant arrays. We find that some triangle
configurations yield bispectrum estimates that are consistent with the expected
noise level after 10 hours, while equilateral configurations are strongly
foreground-dominated. Careful choice of triangle configurations may be made to
reduce foreground bias that hinders power spectrum estimators, and the 21~cm
bispectrum may be accessible in less time than the 21~cm power spectrum for
some wave modes, with detections in hundreds of hours.Comment: 19 pages, 10 figures, accepted for publication in PAS
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