E. B. Fomalont

National Radio Astronomy Observatory, Charlottesville, Virginia, United States

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Publications (132)357.74 Total impact

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    07/2015; 808(1). DOI:10.1088/2041-8205/808/1/L3
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    ABSTRACT: A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ~15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from September to late November 2014, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ~350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.
    04/2015; 808(1). DOI:10.1088/2041-8205/808/1/L1
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    ABSTRACT: A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ~15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from September to late November 2014, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ~350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.
    The Astrophysical Journal Letters 04/2015; · 5.60 Impact Factor
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    ABSTRACT: We present Atacama Large Millimeter/submillimeter Array (ALMA) observations from the 2014 Long Baseline Campaign in dust continuum and spectral line emission from the HL Tau region. The continuum images at wavelengths of 2.9, 1.3, and 0.87 mm have unprecedented angular resolutions of 0.075 arcseconds (10 AU) to 0.025 arcseconds (3.5 AU), revealing an astonishing level of detail in the circumstellar disk surrounding the young solar analogue HL Tau, with a pattern of bright and dark rings observed at all wavelengths. By fitting ellipses to the most distinct rings, we measure precise values for the disk inclination (46.72pm0.05 degrees) and position angle (+138.02pm0.07 degrees). We obtain a high-fidelity image of the 1.0 mm spectral index ($\alpha$), which ranges from $\alpha\sim2.0$ in the optically-thick central peak and two brightest rings, increasing to 2.3-3.0 in the dark rings. The dark rings are not devoid of emission, we estimate a grain emissivity index of 0.8 for the innermost dark ring and lower for subsequent dark rings, consistent with some degree of grain growth and evolution. Additional clues that the rings arise from planet formation include an increase in their central offsets with radius and the presence of numerous orbital resonances. At a resolution of 35 AU, we resolve the molecular component of the disk in HCO+ (1-0) which exhibits a pattern over LSR velocities from 2-12 km/s consistent with Keplerian motion around a ~1.3 solar mass star, although complicated by absorption at low blue-shifted velocities. We also serendipitously detect and resolve the nearby protostars XZ Tau (A/B) and LkHa358 at 2.9 mm.
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    ABSTRACT: We present Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm continuum images of the asteroid 3 Juno obtained with an angular resolution of 0.042 arcseconds (60 km at 1.97 AU). The data were obtained over a single 4.4 hr interval, which covers 60% of the 7.2 hr rotation period, approximately centered on local transit. A sequence of ten consecutive images reveals continuous changes in the asteroid's profile and apparent shape, in good agreement with the sky projection of the three-dimensional model of the Database of Asteroid Models from Inversion Techniques. We measure a geometric mean diameter of 259pm4 km, in good agreement with past estimates from a variety of techniques and wavelengths. Due to the viewing angle and inclination of the rotational pole, the southern hemisphere dominates all of the images. The median peak brightness temperature is 215pm13 K, while the median over the whole surface is 197pm15 K. With the unprecedented resolution of ALMA, we find that the brightness temperature varies across the surface with higher values correlated to the subsolar point and afternoon areas, and lower values beyond the evening terminator. The dominance of the subsolar point is accentuated in the final four images, suggesting a reduction in the thermal inertia of the regolith at the corresponding longitudes, which are possibly correlated to the location of the putative large impact crater. These results demonstrate ALMA's potential to resolve thermal emission from the surface of main belt asteroids, and to measure accurately their position, geometric shape, rotational period, and soil characteristics.
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    ABSTRACT: We present initial results of very high resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations of the $z$=3.042 gravitationally lensed galaxy HATLAS J090311.6+003906 (SDP.81). These observations were carried out using an extended configuration as part of Science Verification for the 2014 ALMA Long Baseline Campaign, with baselines of up to 15 km. We present continuum imaging at 151, 236 and 290 GHz, at angular resolutions as fine as 23 milliarcseconds (mas; corresponding to an un-magnified spatial scale of 180 pc at z=3.042). The ALMA images clearly show two main gravitational arc components with emission tracing a radius of 1.5". We also present imaging of CO(10-9), CO(8-7), CO(5-4) and H2O line emission. The CO data has an angular resolution of 170 mas and the emission is found to broadly trace the gravitational arc structures. We detect H2O line emission but only using the shortest baselines. The ALMA continuum and spectral line fluxes are consistent with previous Plateau de Bure Interferometer and Submillimeter Array observations despite the increase in angular resolution. Finally, we detect weak unresolved continuum emission at all three observed frequencies from a position that is spatially coincident with the centre of the foreground lensing galaxy.
    03/2015; 808(1). DOI:10.1088/2041-8205/808/1/L4
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    ABSTRACT: For ALMA to produce high quality images of astronomical objects with sub-arcsecond resolution at frequencies above 85 GHz, the radio signals must be combined from up to 66 antennas spread over 15 km with a maximum path length delay difference of about 0.025 mm. This accuracy requires precise antenna structures, stable electronics, compensation for many temporal changes in the system and the measurement of the path-changing water vapour emission in the line of sight. The final stage of path length calibration is provided by frequent observations of relatively strong, point-like distant radio sources, quasars, that lie within a few degrees of the astronomical object. The ALMA Quasar Catalogue was implemented to provide a database that contains the essential parameters for hundreds of quasars and their brightness variations at several frequencies as a function of time. This paper describes the filling of the catalogue and the use of these quasar test signals to provide the path length accuracy needed for the imaging of radio sources.
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    ABSTRACT: In the past decade, our understanding of galaxy evolution has been revolutionized by the discovery that luminous, dusty starburst galaxies were 1,000 times more abundant in the early Universe than at present. It has, however, been difficult to measure the complete redshift distribution of these objects, especially at the highest redshifts (z > 4). Here we report a redshift survey at a wavelength of three millimetres, targeting carbon monoxide line emission from the star-forming molecular gas in the direction of extraordinarily bright millimetre-wave-selected sources. High-resolution imaging demonstrates that these sources are strongly gravitationally lensed by foreground galaxies. We detect spectral lines in 23 out of 26 sources and multiple lines in 12 of those 23 sources, from which we obtain robust, unambiguous redshifts. At least 10 of the sources are found to lie at z > 4, indicating that the fraction of dusty starburst galaxies at high redshifts is greater than previously thought. Models of lens geometries in the sample indicate that the background objects are ultra-luminous infrared galaxies, powered by extreme bursts of star formation.
    Nature 03/2013; 495(7441). DOI:10.1038/nature12001 · 42.35 Impact Factor
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    ABSTRACT: In the past decade, our understanding of galaxy evolution has been revolutionized by the discovery that luminous, dusty, starburst galaxies were 1,000 times more abundant in the early Universe than at present. It has, however, been difficult to measure the complete redshift 2 distribution of these objects, especially at the highest redshifts (z > 4). Here we report a redshift survey at a wavelength of three millimeters, targeting carbon monoxide line emission from the star-forming molecular gas in the direction of extraordinarily bright millimetrewave-selected sources. High-resolution imaging demonstrates that these sources are strongly gravitationally lensed by foreground galaxies. We detect spectral lines in 23 out of 26 sources and multiple lines in 12 of those 23 sources, from which we obtain robust, unambiguous redshifts. At least 10 of the sources are found to lie at z > 4, indicating that the fraction of dusty starburst galaxies at high redshifts is greater than previously thought. Models of lens geometries in the sample indicate that the background objects are ultra-luminous infrared galaxies, powered by extreme bursts of star formation.
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    ABSTRACT: We present Atacama Large Millimeter/submillimeter Array (ALMA) 860 micrometer imaging of four high-redshift (z=2.8-5.7) dusty sources that were detected using the South Pole Telescope (SPT) at 1.4 mm and are not seen in existing radio to far-infrared catalogs. At 1.5 arcsec resolution, the ALMA data reveal multiple images of each submillimeter source, separated by 1-3 arcsec, consistent with strong lensing by intervening galaxies visible in near-IR imaging of these sources. We describe a gravitational lens modeling procedure that operates on the measured visibilities and incorporates self-calibration-like antenna phase corrections as part of the model optimization, which we use to interpret the source structure. Lens models indicate that SPT0346-52, located at z=5.7, is one of the most luminous and intensely star-forming sources in the universe with a lensing corrected FIR luminosity of 3.7 X 10^13 L_sun and star formation surface density of 4200 M_sun yr^-1 kpc^-2. We find magnification factors of 5 to 22, with lens Einstein radii of 1.1-2.0 arcsec and Einstein enclosed masses of 1.6-7.2x10^11 M_sun. These observations confirm the lensing origin of these objects, allow us to measure the their intrinsic sizes and luminosities, and demonstrate the important role that ALMA will play in the interpretation of lensed submillimeter sources.
    The Astrophysical Journal 03/2013; 767(2). DOI:10.1088/0004-637X/767/2/132 · 6.28 Impact Factor
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    ABSTRACT: Deep radio observations at 1.4 GHz for the Extended Chandra Deep Field South were performed in 2007 June through September and presented in a first data release. The survey was made using six separate pointings of the Very Large Array with over 40 hr of observation per pointing. In the current paper, we improve on the data reduction to produce a second data release (DR2) mosaic image. This DR2 image covers an area of about a third of a square degree, reaches a best rms sensitivity of 6 μJy, and has a typical sensitivity of 7.4 μJy per 2.''8 by 1.''6 beam. We also present a more comprehensive catalog, including sources down to peak flux densities of five or more times the local rms noise along with information on source sizes and relevant pointing data. We discuss in some detail the consideration of whether sources are resolved under the complication of a radio image created as a mosaic of separate pointings each suffering some degree of bandwidth smearing, and the accurate evaluation of the flux densities of such sources. Finally, the radio morphologies and optical/near-IR counterpart identifications are used to identify 17 likely multiple-component sources and arrive at a catalog of 883 radio sources, which is roughly double the number of sources contained in the first data release.
    The Astrophysical Journal Supplement Series 03/2013; 205(2):13. DOI:10.1088/0067-0049/205/2/13 · 14.14 Impact Factor
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    ABSTRACT: The Russian Academy of Sciences and Federal Space Agency, together with the participation of many international organizations, worked toward the launch of the RadioAstron orbiting space observatory with its onboard 10-m reflector radio telescope from the Baikonur cosmodrome on July 18, 2011. Together with some of the largest ground-based radio telescopes and a set of stations for tracking, collecting, and reducing the data obtained, this space radio telescope forms a multi-antenna ground-space radio interferometer with extremely long baselines, making it possible for the first time to study various objects in the Universe with angular resolutions a million times better than is possible with the human eye. The project is targeted at systematic studies of compact radio-emitting sources and their dynamics. Objects to be studied include supermassive black holes, accretion disks, and relativistic jets in active galactic nuclei, stellar-mass black holes, neutron stars and hypothetical quark stars, regions of formation of stars and planetary systems in our and other galaxies, interplanetary and interstellar plasma, and the gravitational field of the Earth. The results of ground-based and inflight tests of the space radio telescope carried out in both autonomous and ground-space interferometric regimes are reported. The derived characteristics are in agreement with the main requirements of the project. The astrophysical science program has begun.
    Astronomy Reports 03/2013; 57(3):153-194. DOI:10.1134/S1063772913030025 · 0.80 Impact Factor
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    ABSTRACT: The next-generation space VLBI mission, VSOP-2, is expected to provide unprecedented spatial resolutions at 8.4, 22, and 43 GHz. In this report, phase referencing with VSOP-2 is examined in detail based on a simulation tool called ARIS. The criterion for successful phase referencing was to keep the phase errors below one radian. Simulations with ARIS reveal that phase referencing achieves good performance at 8.4 GHz, even under poor tropospheric conditions. At 22 and 43 GHz, it is recommended to conduct phase referencing observations under good or typical tropospheric conditions. The satellite is required to have an attitude-switching capability with a one-minute or shorter cycle, and an orbit determination accuracy
  • R. Dodson · E. Fomalont · K. Wiik · Tuorla Obs
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    ABSTRACT: In February 1997, the Japanese radio astronomy satellite HALCA was launched to provide the space-bourne element for the VLBI Space Observatory Programme (VSOP) mission. A significant fraction of the mission time was to be dedicated to the VSOP Survey of bright compact Active Galactic Nuclei (AGN) at 5 GHz, which was lead by ISAS. The VSOP Survey Sources are an unbiased dataset of 294 targets, of which 82 % were successfully observed. These are now undergoing statistical analysis to tease out the characteristics of typical AGN sources. We present here the summary of the imaging and conclusions we have reached.
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    L. Petrov · Y. Y. Kovalev · E. B. Fomalont · and D. Gordon
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    ABSTRACT: This paper presents accurate absolute positions from a 24 GHz Very Long Baseline Array (VLBA) search for compact extragalactic sources in an area where the density of known calibrators with precise coordinates is low. The goals were to identify additional sources suitable for use as phase calibrators for galactic sources, determine their precise positions, and produce radio images. In order to achieve these goals, we developed a new software package, PIMA, for determining group delays from wide-band data with much lower detection limits. With the use of PIMA, we have detected 327 sources out of 487 targets observed in three 24 hr VLBA experiments. Among the 327 detected objects, 176 are within 10° of the Galactic plane. This VGaPS catalog of source positions, plots of correlated flux density versus projected baseline length, contour plots, as well as weighted CLEAN images, and calibrated visibility data are available on the Web in FITS format. Approximately one-half of objects from the 24 GHz catalog were observed at dual-band 8.6 GHz and 2.3 GHz experiments. Position differences at 24 GHz versus 8.6/2.3 GHz for all but two objects on average are strictly within reported uncertainties. We found that for two objects with complex structures, positions at different frequencies correspond to different components of a source.
    The Astronomical Journal 06/2011; 142(2):35. DOI:10.1088/0004-6256/142/2/35 · 4.05 Impact Factor
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    L. Petrov · Y. Y. Kovalev · E. B. Fomalont · D. Gordon
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    ABSTRACT: We have carried our a series of Very Long Baseline Array (VLBA) astrometric observation of the Cassini spacecraft, which is currently orbiting Saturn, to determine the position of Saturn's center of mass in the International Celestial Reference Frame (ICRF). Our positional accuracies are typically 0.2-0.3 milli-arcseconds, corresponding to about 2 km at the distance of Saturn. The goal of the project is to improve the Saturn ephemeris. This paper reports the results of our first eight epochs of observing, from October 2006 through April 2009. A new planetary ephemeris (DE 422) has been fit to these data, plus two earlier VLBA observations of Cassini by others and a Cassini-based VLBA gravitational deflection experiment by Fomalont et al. in February 2009. Post-fit residuals for DE 422 have a mean offset less than 0.2 mas in both coordinates, but not all of our phase reference sources have ICRF position with this accuracy yet. Future observations will improve reference source positions, and will continue to follow Saturn through more than 1/4 of its orbital period. (The Cassini mission is currently funded to operate until 2017.) We are grateful to Larry Teitelbaum for support of this project through the Advanced Tracking and Observational Techniques of JPL's Interplanetary Network Directorate, and to John Benson and the VLBA operations team at NRAO for their excellent support of these observations. The VLBA is a facility of the National Radio Astronomy Observatory, which is operated by Associated Universities, Inc., under a cooperative agreement with the National Science Foundation. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
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    ABSTRACT: The planetary ephemeris is an essential tool for interplanetary spacecraft navigation, studies of solar system dynamics (including, for example, barycenter corrections for pulsar timing ephemeredes), the prediction of occultations, and tests of general relativity. We are carrying out a series of astrometric VLBI observations of the Cassini spacecraft currently in orbit around Saturn, using the Very Long Baseline Array (VLBA). These observations provide positions for the center of mass of Saturn in the International Celestial Reference Frame (ICRF) with accuracies ~0.3 milli-arcsecond (1.5 nrad), or about 2 km at the average distance of Saturn. This paper reports results from eight observing epochs between 2006 October and 2009 April. These data are combined with two VLBA observations by other investigators in 2004 and a Cassini-based gravitational deflection measurement by Fomalont et al. in 2009 to constrain a new ephemeris (DE 422). The DE 422 post-fit residuals for Saturn with respect to the VLBA data are generally 0.2 mas, but additional observations are needed to improve the positions of all of our phase reference sources to this level. Over time we expect to be able to improve the accuracy of all three coordinates in the Saturn ephemeris (latitude, longitude, and range) by a factor of at least three. This will represent a significant improvement not just in the Saturn ephemeris but also in the link between the inner and outer solar system ephemeredes and in the link to the inertial ICRF. Comment: Accepted for publication in the Astronomical Journal
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    Edward Fomalont · Sergei Kopeikin
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    ABSTRACT: Since VLBI techniques give microarcsecond position accuracy of celestial objects, tests of GR using radio sources as probes of a gravitational field have been made. We present the results from two recent tests using the VLBA: In 2005, the measurement of the classical solar deflection; and in 2002, the measurement of the retarded gravitational deflection associated with Jupiter. The deflection experiment measured γ to an accuracy of 3 × 10 −4 ; the Jupiter experiment measured the retarded term to 20% accuracy. The controversy over the interpretation of the retarded term is summarized.
    Proceedings of the International Astronomical Union 01/2010; 248. DOI:10.1017/S1743921308019613
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Publication Stats

2k Citations
357.74 Total Impact Points

Institutions

  • 1979–2015
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States
  • 2013
    • University of Chicago
      • Kavli Institute for Cosmological Physics
      Chicago, Illinois, United States
  • 2005–2009
    • University of California, Santa Cruz
      • Department of Astronomy and Astrophysics
      Santa Cruz, California, United States
  • 2002–2009
    • University of Missouri
      • Department of Physics and Astronomy
      Columbia, Missouri, United States
  • 2000
    • National Astronomical Observatory of Japan
      • Astronomy Data Center
      Edo, Tōkyō, Japan
  • 1997
    • The Commonwealth Scientific and Industrial Research Organisation
      • Australia Telescope National Facility
      Canberra, Australian Capital Territory, Australia
  • 1984
    • University of Groningen
      Groningen, Groningen, Netherlands
  • 1982
    • Leiden University
      • Leiden Observartory
      Leyden, South Holland, Netherlands
  • 1976–1982
    • Queen's University
      Kingston, Ontario, Canada
  • 1978
    • Queens University of Charlotte
      New York, United States
  • 1973–1978
    • Max Planck Institute for Radio Astronomy
      Bonn, North Rhine-Westphalia, Germany
  • 1974
    • National Astronomy and Ionosphere Center
      Arecibo, Arecibo, Puerto Rico