Shea D Brown | CSIRO (original) (raw)

Papers by Shea D Brown

We report the a posteriori confirmation of two large-scale filaments along the sight-line of the ... more We report the a posteriori confirmation of two large-scale filaments along the sight-line of the blazar 1ES 1553+113, which correspond to Warm-Hot Intergalactic Medium (WHIM; ??) absorption features in the X-ray and far ultraviolet. ? reported 5 low signal-to-noise absorption features along the sight line to the z ≥ 0.4 blazar 1ES 1553+113. These CV and CVI Kα absorption lines correspond to potential WHIM systems at z X = 0.312, z X = 0.237, and a cluster of lines with an average redshift of < z X >= 0.133. The two highest redshift systems also have corresponding BLA lines detected in the FUV. Unlike the detections of ? and ?, where there was an obvious low redshift intervening structure (the Sculpor Wall at z ≈ 0.03) 1 , the redshifts of ? are at distances where spectroscopic surveys are incomplete, leaving the intervening cosmic-web relatively unexplored. In order to explore the large-scale structure at the relatively high redshift of these absorption features, we utilized the machine-learning generated WISE-SuperCOSMOS photometric redshift catalog of ?. The catalog, which consists of >20 million galaxies, was constructed by jointly analyzing the mid infrared WISE catalog (?) with cross-matched sources in the optical SuperCOSMOS catalog (?). The photometric redshifts were calibrated using an artificial neural-network (ANN) using Galaxy and Mass Assembly (GAMA) spectroscopic data (?), and produce photo-zs with an average accuracy of σ z = 0.033. The catalog redshifts rapidly lose completeness beyond z ≈ 0.3. We separated the catalog into redshift bins of δz = 0.04, which corresponds roughly to the 1σ z = 0.033 uncertainty of the photometric redshifts. We then created all-sky maps of the galaxy distribution (number of galaxies per pixel) using the Healpix scheme (?) with N side =256 (θ pix ∼0.229 arcmin), and the Galactic plane and other contaminated regions masked out (?). In order to highlight any large-scale structure that might be evident in the images, we convolved the map with a FWHM=2.5 • Gaussian kernel. Figure 1 shows the WISE-SuperCOSMOS maps in the 25 • x 25 • field surrounding 1ES 1553+113, plotted over the eight bins that cover the complete redshift range of the survey. We note that the 2.5 • Gaussian smoothing corresponds to 11-16 Mpc at the highest redshifts, and highlights only the largest-scale filaments and super-clusters (e.g., ?). Due to the varying completeness we've normalized the maps in each bin. Using the pixel value of these normalized maps at the location of 1ES 1553+113, the largest intervening structure is at 0.21 < z < 0.25, followed by 0.29 < z < 0.33.

We set limits on the presence of the synchrotron cosmic web through the crosscorrelation of the 2... more We set limits on the presence of the synchrotron cosmic web through the crosscorrelation of the 2.3 GHz S-PASS survey with a model of the local cosmic web derived from constrained magnetohydrodynamic (MHD) simulations. The MHD simulation assumes cosmologically seeded magnetic fields amplified during large-scale structure formation, and a population of relativistic electrons/positrons from proton-proton collisions within the intergalactic medium. We set a model-dependent 3σ upper limit on the synchrotron surface brightness of 0.16 mJy arcmin −2 at 2.3 GHz in filaments. Extrapolating from magnetic field maps created from the simulation, we infer an upper limit (density-weighted) magnetic field of 0.03 (0.13) µG in filaments at the current epoch, and a limit on the primordial magnetic field (PMF) of B P M F <1.0 nG.

Publications of the Astronomical Society of Australia, 2013

Significant new opportunities for astrophysics and cosmology have been identified at low radio fr... more Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the Southern Hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21 cm emission from the epoch of reionisation in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.

Abstract We have performed a stacking experiment to detect faint, diffuse non-thermal radio emiss... more Abstract We have performed a stacking experiment to detect faint, diffuse non-thermal radio emission in X-ray luminous (> 10 44 erg s-1) galaxy clusters in the southern sky with z< 0.2. The increased sensitivity of this experiment over existing pointed observations allows us to test models of cluster radio halo bimodality and models of relativistic particle acceleration.

Description/Abstract We present results from the first cross-correlation search for the synchrotr... more Description/Abstract We present results from the first cross-correlation search for the synchrotron component of the diffuse intergalactic medium (IGM) in filamentary large scale structure (LSS). We used the low resolution (36') Bonn survey at 21cm, with the infrared 2MASS catalog as a tracer of the LSS. Synchrotron emission likely results from LSS formation shocks and feedback from AGN and galactic winds [2].

Abstract EMU is a wide-field radio continuum survey planned for the new Australian Square Kilomet... more Abstract EMU is a wide-field radio continuum survey planned for the new Australian Square Kilometre Array Pathfinder (ASKAP) telescope. The primary goal of EMU is to make a deep (rms~ 10 μJy/beam) radio continuum survey of the entire Southern sky at 1.3 GHz, extending as far North as+ 30 declination, with a resolution of 10 arcsec.

ABSTRACT We derive the best characterization to date of the properties of radio quiescent massive... more ABSTRACT We derive the best characterization to date of the properties of radio quiescent massive galaxy clusters through a statistical analysis of their average synchrotron emissivity. We stacked 105 radio images of clusters from the 843 MHz Sydney University Molonglo Sky Survey, all with LX> 10 44 erg s–1 and redshifts z< 0.2, after removing point-source contamination and rescaling to a common physical size.

ABSTRACT We combine for the first time all available information about the spectral shape and mor... more ABSTRACT We combine for the first time all available information about the spectral shape and morphology of the radio halo of the Coma cluster with the recent γ-ray upper limits obtained by the Fermi-Large Area Telescope (LAT) and with the magnetic field strength derived from Faraday rotation measures. We explore the possibility that the radio emission is due to synchrotron emission of secondary electrons.

ABSTRACT We present a detailed study of the Faraday depth structure of four bright (> 1 Jy), stro... more ABSTRACT We present a detailed study of the Faraday depth structure of four bright (> 1 Jy), strongly polarized, unresolved radio-loud quasars. The Australia Telescope Compact Array (ATCA) was used to observe these sources with 2 GHz of instantaneous bandwidth from 1.1 to 3.1 GHz.

Abstract: With magnetic fields below one microgauss, the minimum pressures of synchrotron plasmas... more Abstract: With magnetic fields below one microgauss, the minimum pressures of synchrotron plasmas start to approach those in the thermal gas in cluster outskirts and the more diffuse WHIM in large-scale structure filaments. We summarize some of our techniques to find the corresponding low surface brightness radio sources and what we have uncovered.

Abstract: We have detected a radio bridge of unpolarized synchrotron emission connecting the NW r... more Abstract: We have detected a radio bridge of unpolarized synchrotron emission connecting the NW relic of the galaxy cluster Abell 3667 to its central regions. We used data at 2.3 GHz from the S-band Polarization All Sky Survey (S-PASS) and at 3.3 GHz from a follow up observation, both conducted with the Parkes Radio Telescope.

Abstract: Significant new opportunities for astrophysics and cosmology have been identified at lo... more Abstract: Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the Southern Hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency.

Abstract The Square Kilometre Array and its pathfinders ASKAP and MeerKAT will produce prodigious... more Abstract The Square Kilometre Array and its pathfinders ASKAP and MeerKAT will produce prodigious amounts of data that necessitate automated source finding. The performance of automated source finders can be improved by pre-processing a dataset.

Abstract Large-scale nonthermal (synchrotron) emission in galaxy clusters may go undetected by ra... more Abstract Large-scale nonthermal (synchrotron) emission in galaxy clusters may go undetected by radio interferometers. We present results from a study of X-ray clusters at 1.4 GHz with the 100-m Green Bank Telescope (GBT). After subtraction of point sources and diffuse emission using pre-existing interferometer data (NVSS), we find a significant excess of diffuse emission in four of seven Abell clusters observed.

Abstract Shocks and turbulence generated during large-scale structure formation are predicted to ... more Abstract Shocks and turbulence generated during large-scale structure formation are predicted to produce large-scale, low surface-brightness synchrotron emission. On the largest scales, this emission is globally correlated with the thermal baryon distribution, and constitutes the 'synchrotron cosmic-web'. I present the observational prospects and challenges for detecting this faint emission with upcoming SKA pathfinders.

The Astrophysical Journal, Jan 1, 2007

We detect a dip of 20-45% in the surface brightness and number counts of NVSS sources smoothed to... more We detect a dip of 20-45% in the surface brightness and number counts of NVSS sources smoothed to a few degrees at the location of the WMAP cold spot. The dip has structure on scales of ∼ 1 • to 10 • . Together with independent all-sky wavelet analyses, our results suggest that the dip in extragalactic brightness and number counts and the WMAP cold spot are physically related, i.e., that the coincidence is neither a statistical anomaly nor a WMAP foreground correction problem. If the cold spot does originate from structures at modest redshifts, as we suggest, then there is no remaining need for non-Gaussian processes at the last scattering surface of the CMB to explain the cold spot. The late integrated Sachs-Wolfe effect, already seen statistically for NVSS source counts, can now be seen to operate on a single region. To create the magnitude and angular size of the WMAP cold spot requires a ∼ 140 Mpc radius completely empty void at z≤1 along this line of sight. This is far outside the current expectations of the concordance cosmology, and adds to the anomalies seen in the CMB.

The Astronomical …, Jan 1, 2007

We present the first installment of HI sources extracted from the Arecibo Legacy Fast ALFA (ALFAL... more We present the first installment of HI sources extracted from the Arecibo Legacy Fast ALFA (ALFALFA) extragalactic survey, initiated in 2005. Sources have been extracted from 3-D spectral data cubes exploiting a matched filtering technique and then examined interactively to yield global HI parameters. A total of 730 HI detections are catalogued within the solid angle 11 h 44 m < R.A.(J2000) < 14 h 00 m and +12 • < Dec.(J2000) < +16 • , and redshift range −1600 km s −1 < cz < 18000 km s −1 . In comparison, the HI Parkes All-Sky Survey (HIPASS) detected 40 HI signals in the same region. Optical counterparts are assigned via examination of digital optical imaging databases. ALFALFA HI detections are reported for three distinct classes of signals: (a) detections, typically with S/N > 6.5; (b) high velocity clouds in the Milky Way or its periphery; and (c) signals of lower S/N (to ∼ 4.5) which coincide spatially with an optical object of known similar redshift. Although this region of the sky has been heavily surveyed by previous targeted observations based on optical flux-or size-limited samples, 69% of the extracted sources are newly reported HI detections. The resultant positional accuracy of HI sources is dependent on S/N: it averages 24 ′′ (20 ′′ median) for all sources with S/N > 6.5 and is of order ∼17 ′′ (14 ′′ median) for signals with S/N > 12. The median redshift of the sample is ∼7000 km s −1 and its distribution reflects the known local large scale structure including the Virgo cluster and the void behind it, the A1367-Coma supercluster at cz ∼7000 km s −1 and a third more distant overdensity at cz ∼13000 km s −1 . Distance uncertainties in and around the Virgo cluster perturb the derived HI mass distribution. Specifically, an apparent deficiency of the lowest HI mass objects can be attributed, at least in part, to the incorrect assignment of some foreground objects to the cluster distance. Several extended HI features are found in the vicinity of the Virgo cluster. A small percentage (6%) of HI detections have no identifiable optical counterpart, more than half of which are high velocity clouds in the Milky Way vicinity; the remaining 17 objects do not appear connected to or associated with any known galaxy. Based on these initial results, ALFALFA is expected to fulfill, and even exceed, its predicted performance objectives in terms of the number and quality of HI detections.

The Astronomical Journal, Jan 1, 2009

We present a study of the spectral, polarimetric, morphological and environmental properties of t... more We present a study of the spectral, polarimetric, morphological and environmental properties of the diffuse radio source 0809+39 using observations taken with the Westerbork Synthesis Radio Telescope, the Very Large Array, and archival optical and X-ray data. The source has two distinct diffuse, steep-spectrum components, one in the north that is highly polarized, and a linear southern component undetected in polarization. We discuss several plausible origins for each component, and conclude that the northern bright polarized component is most likely a radio relic associated with a poor z∼0.2 cluster of galaxies, with a radio/X-ray luminosity ratio two orders of magnitude above typical values. The southern component is aligned with a more extended filament of galaxies ∼5 Mpc long at z∼0.04. Deep optical and X-ray follow-ups are still needed in order to confirm and understand the physical origins of the synchrotron emission. Whatever the details of these origins, 0809+39 highlights the utility of synchrotron radiation for illuminating the diffuse components of low density environments unrelated to rich clusters.

Arxiv preprint arXiv: …, Jan 1, 2009

We report the a posteriori confirmation of two large-scale filaments along the sight-line of the ... more We report the a posteriori confirmation of two large-scale filaments along the sight-line of the blazar 1ES 1553+113, which correspond to Warm-Hot Intergalactic Medium (WHIM; ??) absorption features in the X-ray and far ultraviolet. ? reported 5 low signal-to-noise absorption features along the sight line to the z ≥ 0.4 blazar 1ES 1553+113. These CV and CVI Kα absorption lines correspond to potential WHIM systems at z X = 0.312, z X = 0.237, and a cluster of lines with an average redshift of < z X >= 0.133. The two highest redshift systems also have corresponding BLA lines detected in the FUV. Unlike the detections of ? and ?, where there was an obvious low redshift intervening structure (the Sculpor Wall at z ≈ 0.03) 1 , the redshifts of ? are at distances where spectroscopic surveys are incomplete, leaving the intervening cosmic-web relatively unexplored. In order to explore the large-scale structure at the relatively high redshift of these absorption features, we utilized the machine-learning generated WISE-SuperCOSMOS photometric redshift catalog of ?. The catalog, which consists of >20 million galaxies, was constructed by jointly analyzing the mid infrared WISE catalog (?) with cross-matched sources in the optical SuperCOSMOS catalog (?). The photometric redshifts were calibrated using an artificial neural-network (ANN) using Galaxy and Mass Assembly (GAMA) spectroscopic data (?), and produce photo-zs with an average accuracy of σ z = 0.033. The catalog redshifts rapidly lose completeness beyond z ≈ 0.3. We separated the catalog into redshift bins of δz = 0.04, which corresponds roughly to the 1σ z = 0.033 uncertainty of the photometric redshifts. We then created all-sky maps of the galaxy distribution (number of galaxies per pixel) using the Healpix scheme (?) with N side =256 (θ pix ∼0.229 arcmin), and the Galactic plane and other contaminated regions masked out (?). In order to highlight any large-scale structure that might be evident in the images, we convolved the map with a FWHM=2.5 • Gaussian kernel. Figure 1 shows the WISE-SuperCOSMOS maps in the 25 • x 25 • field surrounding 1ES 1553+113, plotted over the eight bins that cover the complete redshift range of the survey. We note that the 2.5 • Gaussian smoothing corresponds to 11-16 Mpc at the highest redshifts, and highlights only the largest-scale filaments and super-clusters (e.g., ?). Due to the varying completeness we've normalized the maps in each bin. Using the pixel value of these normalized maps at the location of 1ES 1553+113, the largest intervening structure is at 0.21 < z < 0.25, followed by 0.29 < z < 0.33.

We set limits on the presence of the synchrotron cosmic web through the crosscorrelation of the 2... more We set limits on the presence of the synchrotron cosmic web through the crosscorrelation of the 2.3 GHz S-PASS survey with a model of the local cosmic web derived from constrained magnetohydrodynamic (MHD) simulations. The MHD simulation assumes cosmologically seeded magnetic fields amplified during large-scale structure formation, and a population of relativistic electrons/positrons from proton-proton collisions within the intergalactic medium. We set a model-dependent 3σ upper limit on the synchrotron surface brightness of 0.16 mJy arcmin −2 at 2.3 GHz in filaments. Extrapolating from magnetic field maps created from the simulation, we infer an upper limit (density-weighted) magnetic field of 0.03 (0.13) µG in filaments at the current epoch, and a limit on the primordial magnetic field (PMF) of B P M F <1.0 nG.

Publications of the Astronomical Society of Australia, 2013

Significant new opportunities for astrophysics and cosmology have been identified at low radio fr... more Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the Southern Hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21 cm emission from the epoch of reionisation in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.

Abstract We have performed a stacking experiment to detect faint, diffuse non-thermal radio emiss... more Abstract We have performed a stacking experiment to detect faint, diffuse non-thermal radio emission in X-ray luminous (> 10 44 erg s-1) galaxy clusters in the southern sky with z< 0.2. The increased sensitivity of this experiment over existing pointed observations allows us to test models of cluster radio halo bimodality and models of relativistic particle acceleration.

Description/Abstract We present results from the first cross-correlation search for the synchrotr... more Description/Abstract We present results from the first cross-correlation search for the synchrotron component of the diffuse intergalactic medium (IGM) in filamentary large scale structure (LSS). We used the low resolution (36') Bonn survey at 21cm, with the infrared 2MASS catalog as a tracer of the LSS. Synchrotron emission likely results from LSS formation shocks and feedback from AGN and galactic winds [2].

Abstract EMU is a wide-field radio continuum survey planned for the new Australian Square Kilomet... more Abstract EMU is a wide-field radio continuum survey planned for the new Australian Square Kilometre Array Pathfinder (ASKAP) telescope. The primary goal of EMU is to make a deep (rms~ 10 μJy/beam) radio continuum survey of the entire Southern sky at 1.3 GHz, extending as far North as+ 30 declination, with a resolution of 10 arcsec.

ABSTRACT We derive the best characterization to date of the properties of radio quiescent massive... more ABSTRACT We derive the best characterization to date of the properties of radio quiescent massive galaxy clusters through a statistical analysis of their average synchrotron emissivity. We stacked 105 radio images of clusters from the 843 MHz Sydney University Molonglo Sky Survey, all with LX> 10 44 erg s–1 and redshifts z< 0.2, after removing point-source contamination and rescaling to a common physical size.

ABSTRACT We combine for the first time all available information about the spectral shape and mor... more ABSTRACT We combine for the first time all available information about the spectral shape and morphology of the radio halo of the Coma cluster with the recent γ-ray upper limits obtained by the Fermi-Large Area Telescope (LAT) and with the magnetic field strength derived from Faraday rotation measures. We explore the possibility that the radio emission is due to synchrotron emission of secondary electrons.

ABSTRACT We present a detailed study of the Faraday depth structure of four bright (> 1 Jy), stro... more ABSTRACT We present a detailed study of the Faraday depth structure of four bright (> 1 Jy), strongly polarized, unresolved radio-loud quasars. The Australia Telescope Compact Array (ATCA) was used to observe these sources with 2 GHz of instantaneous bandwidth from 1.1 to 3.1 GHz.

Abstract: With magnetic fields below one microgauss, the minimum pressures of synchrotron plasmas... more Abstract: With magnetic fields below one microgauss, the minimum pressures of synchrotron plasmas start to approach those in the thermal gas in cluster outskirts and the more diffuse WHIM in large-scale structure filaments. We summarize some of our techniques to find the corresponding low surface brightness radio sources and what we have uncovered.

Abstract: We have detected a radio bridge of unpolarized synchrotron emission connecting the NW r... more Abstract: We have detected a radio bridge of unpolarized synchrotron emission connecting the NW relic of the galaxy cluster Abell 3667 to its central regions. We used data at 2.3 GHz from the S-band Polarization All Sky Survey (S-PASS) and at 3.3 GHz from a follow up observation, both conducted with the Parkes Radio Telescope.

Abstract: Significant new opportunities for astrophysics and cosmology have been identified at lo... more Abstract: Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the Southern Hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency.

Abstract The Square Kilometre Array and its pathfinders ASKAP and MeerKAT will produce prodigious... more Abstract The Square Kilometre Array and its pathfinders ASKAP and MeerKAT will produce prodigious amounts of data that necessitate automated source finding. The performance of automated source finders can be improved by pre-processing a dataset.

Abstract Large-scale nonthermal (synchrotron) emission in galaxy clusters may go undetected by ra... more Abstract Large-scale nonthermal (synchrotron) emission in galaxy clusters may go undetected by radio interferometers. We present results from a study of X-ray clusters at 1.4 GHz with the 100-m Green Bank Telescope (GBT). After subtraction of point sources and diffuse emission using pre-existing interferometer data (NVSS), we find a significant excess of diffuse emission in four of seven Abell clusters observed.

Abstract Shocks and turbulence generated during large-scale structure formation are predicted to ... more Abstract Shocks and turbulence generated during large-scale structure formation are predicted to produce large-scale, low surface-brightness synchrotron emission. On the largest scales, this emission is globally correlated with the thermal baryon distribution, and constitutes the 'synchrotron cosmic-web'. I present the observational prospects and challenges for detecting this faint emission with upcoming SKA pathfinders.

The Astrophysical Journal, Jan 1, 2007

We detect a dip of 20-45% in the surface brightness and number counts of NVSS sources smoothed to... more We detect a dip of 20-45% in the surface brightness and number counts of NVSS sources smoothed to a few degrees at the location of the WMAP cold spot. The dip has structure on scales of ∼ 1 • to 10 • . Together with independent all-sky wavelet analyses, our results suggest that the dip in extragalactic brightness and number counts and the WMAP cold spot are physically related, i.e., that the coincidence is neither a statistical anomaly nor a WMAP foreground correction problem. If the cold spot does originate from structures at modest redshifts, as we suggest, then there is no remaining need for non-Gaussian processes at the last scattering surface of the CMB to explain the cold spot. The late integrated Sachs-Wolfe effect, already seen statistically for NVSS source counts, can now be seen to operate on a single region. To create the magnitude and angular size of the WMAP cold spot requires a ∼ 140 Mpc radius completely empty void at z≤1 along this line of sight. This is far outside the current expectations of the concordance cosmology, and adds to the anomalies seen in the CMB.

The Astronomical …, Jan 1, 2007

We present the first installment of HI sources extracted from the Arecibo Legacy Fast ALFA (ALFAL... more We present the first installment of HI sources extracted from the Arecibo Legacy Fast ALFA (ALFALFA) extragalactic survey, initiated in 2005. Sources have been extracted from 3-D spectral data cubes exploiting a matched filtering technique and then examined interactively to yield global HI parameters. A total of 730 HI detections are catalogued within the solid angle 11 h 44 m < R.A.(J2000) < 14 h 00 m and +12 • < Dec.(J2000) < +16 • , and redshift range −1600 km s −1 < cz < 18000 km s −1 . In comparison, the HI Parkes All-Sky Survey (HIPASS) detected 40 HI signals in the same region. Optical counterparts are assigned via examination of digital optical imaging databases. ALFALFA HI detections are reported for three distinct classes of signals: (a) detections, typically with S/N > 6.5; (b) high velocity clouds in the Milky Way or its periphery; and (c) signals of lower S/N (to ∼ 4.5) which coincide spatially with an optical object of known similar redshift. Although this region of the sky has been heavily surveyed by previous targeted observations based on optical flux-or size-limited samples, 69% of the extracted sources are newly reported HI detections. The resultant positional accuracy of HI sources is dependent on S/N: it averages 24 ′′ (20 ′′ median) for all sources with S/N > 6.5 and is of order ∼17 ′′ (14 ′′ median) for signals with S/N > 12. The median redshift of the sample is ∼7000 km s −1 and its distribution reflects the known local large scale structure including the Virgo cluster and the void behind it, the A1367-Coma supercluster at cz ∼7000 km s −1 and a third more distant overdensity at cz ∼13000 km s −1 . Distance uncertainties in and around the Virgo cluster perturb the derived HI mass distribution. Specifically, an apparent deficiency of the lowest HI mass objects can be attributed, at least in part, to the incorrect assignment of some foreground objects to the cluster distance. Several extended HI features are found in the vicinity of the Virgo cluster. A small percentage (6%) of HI detections have no identifiable optical counterpart, more than half of which are high velocity clouds in the Milky Way vicinity; the remaining 17 objects do not appear connected to or associated with any known galaxy. Based on these initial results, ALFALFA is expected to fulfill, and even exceed, its predicted performance objectives in terms of the number and quality of HI detections.

The Astronomical Journal, Jan 1, 2009

We present a study of the spectral, polarimetric, morphological and environmental properties of t... more We present a study of the spectral, polarimetric, morphological and environmental properties of the diffuse radio source 0809+39 using observations taken with the Westerbork Synthesis Radio Telescope, the Very Large Array, and archival optical and X-ray data. The source has two distinct diffuse, steep-spectrum components, one in the north that is highly polarized, and a linear southern component undetected in polarization. We discuss several plausible origins for each component, and conclude that the northern bright polarized component is most likely a radio relic associated with a poor z∼0.2 cluster of galaxies, with a radio/X-ray luminosity ratio two orders of magnitude above typical values. The southern component is aligned with a more extended filament of galaxies ∼5 Mpc long at z∼0.04. Deep optical and X-ray follow-ups are still needed in order to confirm and understand the physical origins of the synchrotron emission. Whatever the details of these origins, 0809+39 highlights the utility of synchrotron radiation for illuminating the diffuse components of low density environments unrelated to rich clusters.

Arxiv preprint arXiv: …, Jan 1, 2009

Advances in radio spectro-polarimetry offer the possibility to disentangle complex regions where ... more Advances in radio spectro-polarimetry offer the possibility to disentangle complex regions where relativistic and thermal plasmas mix in the interstellar and intergalactic media. Recent work has shown that apparently simple Faraday Rotation Measure (RM) spectra can be generated by complex sources. This is true even when the distribution of RMs in the complex source greatly exceeds the errors associated with a single component fit to the peak of the Faraday spectrum. We present a convolutional neural network (CNN) that can differentiate between simple Faraday thin spectra and those that contain multiple or Faraday thick sources. We demonstrate that this CNN, trained for the upcoming Polarisation Sky Survey of the Universe's Magnetism (POS-SUM) early science observations, can identify two component sources 99% of the time, provided that the sources are separated in Faraday depth by >10% of the FWHM of the Faraday Point Spread Function, the polarized flux ratio of the sources is >0.1, and that the Signal-to-Noise radio (S/N) of the primary component is >5. With this S/N cutoff , the false positive rate (simple sources mis-classified as complex) is <0.3%. Work is ongoing to include Faraday thick sources in the training and testing of the CNN.