arXiv subjects

E. B. Fomalont

Publications and source records attributed to E. B. Fomalont.

At least 37 records · Page 2Linked to original sources

arXiv2008

The VLA 1.4GHz Survey of the Extended Chandra Deep Field South: First Data Release

We have observed the Extended Chandra Deep Field South (E-CDF-S) using a mosaic of six deep Very Large Array (VLA) pointings at 1.4GHz. In this paper, we present the survey strategy, description of the observations, and the first data release. The observations were performed during June through September of 2007 and included from 15 to 17 ``classic'' VLA antennas and 6 to 11 that had been retrofitted for the Expanded VLA (EVLA). The first data release consists of a 34.1' x 34.1' image and the attendant source catalog. The image achieves an rms sensitivity of 6.4 uJy per 2.8" x 1.6" beam in its deepest regions, with a typical sensitivity of 8 uJy. The catalog is conservative in that it only lists sources with peak flux densities greater than seven times the local rms noise, yet it still contains 464 sources. Nineteen of these are complex sources consisting of multiple components. Cross matching of the catalog to prior surveys of the E-CDF-S confirms the linearity of the flux density calibration, albeit with a slight possible offset (a few percent) in scale. Improvements to the data reduction and source catalog are ongoing, and we intend to produce a second data release in January 2009.

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arXiv2007

The VSOP 5 GHz Active Galactic Nucleus Survey: V. Imaging Results for the Remaining 140 sources

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. Approximately twenty-five percent of the mission time was dedicated to the VSOP Survey of bright compact Active Galactic Nuclei (AGN) at 5 GHz. This paper, the fifth in the series, presents images and models for the remaining 140 sources not included in Paper III, which contained 102 sources. For most sources, the plots of the uv-coverage, the visibility amplitude versus uv-distance, and the high resolution image are presented. Model fit parameters to the major radio components are determined, and the brightness temperature of the core component for each source is calculated. The brightness temperature distributions for all of the sources in the VSOP AGN survey are discussed.

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arXiv2007

The micro-Jy Radio Source Population: the VLA-CDFS View

We analyse the 267 radio sources from our deep (flux limit of 42 microJy at the field center at 1.4 GHz) Chandra Deep Field South 1.4 and 5 GHz VLA survey. The radio population is studied by using a wealth of multi-wavelength information, including morphology and spectral types, in the radio, optical, and X-ray bands. The availability of redshifts for ~ 70% of our sources allows us to derive reliable luminosity estimates for the majority of the objects. Contrary to some previous results, we find that star-forming galaxies make up only a minority (~ 1/3) of sub-mJy sources, the bulk of which are faint radio galaxies, mostly of the Fanaroff-Riley I type.

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arXiv2006

The Fifth VLBA Calibrator Survey: VCS5

This paper presents the fifth part of the Very Long Baseline Array (VLBA) Calibrator Survey (VCS), containing 569 sources not observed previously with very long baseline interferometry in geodetic or absolute astrometry programs. This campaign has two goals: (i) to observe additional sources which, together with previous survey results, form a complete sample, (ii) to find new strong sources suitable as phase calibrators. This VCS extension was based on three 24-hour VLBA observing sessions in 2005. It detected almost all extragalactic flat-spectrum sources with correlated flux density greater than 200 mJy at 8.6 GHz above declination -30 degrees which were not observed previously. Source positions with milliarcsecond accuracy were derived from astrometric analysis of ionosphere-free combinations of group delays determined from the 2.3 GHz and 8.6 GHz frequency bands. The VCS5 catalog of source positions, plots of correlated flux density versus projected baseline length, contour plots and FITS files of naturally weighted CLEAN images, as well as calibrated visibility function files are available on the Web at http://vlbi.gsfc.nasa.gov/vcs5

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arXiv2006

The Radio/Optical Catalog of the SSA13 Field

We present a 1.4-GHz catalog of 810 radio sources (560 sources in the complete sample) with 1.8" resolution found within a 17' radius in the SSA13 field (RA=13h12m,DEC=42d38'). The radio image from the VLA has an rms noise level of 4.82 microJy/beam at the field center, and Subaru optical images in r-band (6300A) and z-band (9200A) have a three-sigma detection magnitude of 26.1 and 24.9, respectively. 88% of the radio sources are identified with an optical counterpart, and there is significantly more reddening for objects fainter than 24-mag. The radio and optical parameters are tabulated, and source morphologies are displayed by radio contours overlaying optical false-color images. The radio structures show a wealth of complexity and these are classified into a small number of categories. About one-third of the radio sources are larger than 1.2" and their orientation is often similar to that of the associated galaxy or binary-galaxy system. The density of sources in the SSA13 field above 75 microJy is 0.40 per square arcmin, with a slope of -2.43 in the differential counts. The radio spectral index may steepen for sources below 75 microJy. We estimate that at most 40% of the microJansky radio sources are dominated by AGN processes.

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arXiv2005

Pulsar Astrometry at the Microarcsecond Level

Determination of pulsar parallaxes and proper motions addresses fundamental astrophysical questions. We have recently finished a VLBI astrometry project to determine the proper motions and parallaxes of 27 pulsars, thereby doubling the total number of pulsar parallaxes. Here we summarise our astrometric technique and present the discovery of a pulsar moving in excess of 1000 km/s. As an example of the application of high precision pulsar astrometry we also infer the identification of 2 pulsars originating from a disrupted binary in the Cygnus Superbubble.

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arXiv2005

Getting Its Kicks: A VLBA Parallax for the Hyperfast Pulsar B1508+55

The highest velocity neutron stars establish stringent constraints on natal kicks, asymmetries in supernova core collapse, and the evolution of close binary systems. Here we present the first results of a long-term pulsar astrometry program using the VLBA. We measure a proper motion and parallax for the pulsar B1508+55, leading to model-independent estimates of its distance (2.37+0.23-0.20 kpc) and transverse velocity (1083+103-90 km/s), the highest velocity directly measured for a neutron star. We trace the pulsar back from its present Galactic latitude of 52.3 degrees to a birth site in the Galactic plane near the Cyg OB associations, and find that it will inevitably escape the Galaxy. Binary disruption alone is insufficient to impart the required birth velocity, and a natal kick is indicated. A composite scenario including a large kick along with binary disruption can plausibly account for the high velocity.

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arXiv2005

VLBA Imaging Polarimetry of Active Galactic Nuclei - An Automated Approach

We present full polarization Very Long Baseline Array (VLBA) observations at 5 GHz and 15 GHz of 24 compact active galactic nuclei (AGN). These sources were observed as part of a pilot project to demonstrate the feasibility of conducting a large VLBI survey to further our understanding of the physical properties and temporal evolution of AGN jets. The sample is drawn from the Cosmic Lens All-Sky Survey (CLASS) where it overlaps with the Sloan Digital Sky Survey at declinations north of 15 degrees. There are 2100 CLASS sources brighter than 50 mJy at 8.4 GHz, of which we have chosen 24 for this pilot study. All 24 sources were detected and imaged at 5 GHz with a typical dynamic range of 500:1, and 21 of 24 sources were detected and imaged at 15 GHz. Linear polarization was detected in 8 sources at both 5 and 15 GHz, allowing for the creation of Faraday rotation measure (RM) images. The core RMs for the sample were found to have an average absolute value of 390 +/- 100 rad/m^2. We also present the discovery of a new Compact Symmetric Object, J08553+5751. All data were processed automatically using pipelines created or adapted for the survey.

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arXiv2005

High Resolution Studies of Radio Sources in the Hubble Deep and Flanking Fields

Eighteen days of MERLIN data and 42 hours of A-array VLA data at 1.4 GHz have been combined to image a 10-arcmin field centred on the Hubble Deep and Flanking Fields (HDF and HFF). A complete sample of 92 radio sources with 1.4-GHz flux densities above 40 microJy has been imaged using MERLIN+VLA. The images are amongst the most sensitive yet made at 1.4 GHz, with rms noise levels of 3.3 microJy/beam in the 0.2-arcsec images. Virtually all the sources are resolved, with angular sizes in the range 0.2 to 3 arcsec. No additional sources were detected down to 23 microJy in the central 3 arcmin, indicating that sources fainter than 40 microJy are heavily resolved with MERLIN and must have typical angular sizes greater than 0.5 arcsec. Compact radio sources were used to align the optical data to the ICRF, to <50 mas in the HDF. We find a statistical association of very faint (2 microJy and above) radio sources with optically bright HDF galaxies down to about 23 mag. Of the 92 radio sources above 40 microJy, about 85 percent are identified with galaxies brighter than about I = 25 mag; the remaining 15 percent are associated with optically faint systems. We identify several very red, optically faint systems including the the strongest sub-mm source in the HDF, HDF850.1. 72 percent of the radio sources are starburst or AGN-type systems; the remainder are unclassified. The proportion of starburst systems increases with decreasing flux density; below 100 microJy 70 percent of the sources are starburst-type systems in the redshift range 0.3 -- 1.3. Chandra detections are associated with 55 of the 92 radio sources but their X-ray flux densities do not appear to be correlated with the radio flux densities or morphologies.

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arXiv2004

The VSOP 5-GHz AGN Survey: IV. The Angular Size/Brightness Temperature Distribution

The VSOP (VLBI Space Observatory Programme) mission is a Japanese-led project to study radio sources with sub-milliarcsec angular resolution, using an orbiting 8-m telescope on board the satellite HALCA with a global earth-based array of telescopes. A major program is the 5 GHz VSOP Survey Program, which we supplement here with VLBA observations to produce a complete and flux-density limited sample. Using statistical methods of analysis of the observed visibility amplitude versus projected (u,v) spacing, we have determined the angular size and brightness temperature distribution of bright AGN radio cores. On average, the cores have a diameter (full-width, half-power) of 0.20 mas which contains about 20% of the total source emission, and (14+/-6)% of the cores are <0.04 mas in size. About (20+/-5)% of the radio cores have a source frame brightness temperature 10^{13}K, and (3+/-2)% have 10^{14}K. A model of the high brightness temperature tail suggests that the radio cores have a brightness temperatures approx 10^{12}K, and are beamed toward the observer with an average bulk motion of v/c=0.993 +/- 0.004.

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arXiv2004

The VSOP 5 GHz AGN Survey: III. Imaging Results for the First 102 Sources

The VSOP mission is a Japanese-led project to study radio sources with sub-milliarcsec resolution using an orbiting 8 m telescope, HALCA, along with global arrays of Earth-based telescopes. Approximately 25% of the observing time is devoted to a survey of compact AGN which are stronger than 1 Jy at 5 GHz-the VSOP AGN Survey. This paper, the third in the series, presents the results from the analysis of the first 102 Survey sources. We present high resolution images and plots of visibility amplitude versus projected baseline length. In addition, model-fit parameters to the primary radio components are listed, and from these the angular size and brightness temperature for the radio cores are calculated. For those sources for which we were able to determine the source frame core brightness temperature, a significant fraction (53 out of 98) have a source frame core brightness temperature in excess of 10^12 K. The maximum source frame core brightness temperature we observed was 1.2 X 10^13 K. Explaining a brightness temperature this high requires an extreme amount of relativistic Doppler beaming. Since the maximum brightness temperature one is able to determine using only ground-based arrays is of the order of 10^12 K, our results confirm the necessity of using space VLBI to explore the extremely high brightness temperature regime.

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arXiv2004

The VSOP 5 GHz AGN Survey II. Data Calibration and Imaging

The VSOP mission is a Japanese-led project to study radio sources with sub-milliarcsecond angular resolution using an orbiting 8-m telescope, HALCA and global arrays of Earth-based telescopes. Approximately 25% of the observing time has been devoted to a survey of compact AGN at 5 GHz which are stronger than 1 Jy -- the VSOP AGN Survey. This paper, the second in a series, describes the data calibration, source detection, self-calibration, imaging and modeling, and gives examples illustrating the problems specific to space VLBI. The VSOP Survey web-site which contains all results and calibrated data is described.

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arXiv2004

From Reference Frames to Relativistic Experiments: Absolute and Relative Radio Astrometry

Reference systems and frames are crucial for high precision absolute astrometric work, and their foundations must be well-defined. The current frame, the International Celestial Reference Frame, will be discussed: its history, the use of the group delay as the measured quantity, the positional accuracy of 0.3 milliarcsec, and possible future improvements. On the other hand, for the determination of the motion of celestial objects, accuracies approaching 0.01 milliarcsec can be obtained by measuring the differential position between the target object and nearby stationary sources. This astrometric technique uses phase referencing, and the current techniques and limitations are discussed, using the results from four experiments. Brief comments are included on the interpretation of the Jupiter gravity deflection experiment of September 2002.

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arXiv2004

The VSOP 5 GHz Continuum Survey, recent results

In February 1997 the Japanese radio astronomy satellite Halca was launched to provide the space-bourne element for the VSOP mission. Approximately twenty-five percent of the mission time has been dedicated to the VSOP Survey, a 5 GHz survey of bright, compact AGN. Both the final, calibrated, high resolution images and plots of visibility amplitude versus uv distance for the first 102 of the sources have been prepared and has been submitted. Papers on the methods and the models from fitting the cumulative {\em uv} amplitudes will also be submitted. The analysis of the second half is well underway.

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arXiv2004

The Evolution of the Ultraluminous Infrared Galaxy Population from Redshift 0 to 1.5

We use redshift observations of two deep 1.4GHz fields to probe the evolution of the bright end of the radio galaxy luminosity function to z=1.5. We show that the number of galaxies with radio power that would correspond locally to an ultraluminous infrared galaxy (ULIG) evolves rapidly over this redshift range. The optical spectra and X-ray luminosities are used to refine this result by separating the sources with clear active galactic nucleus (AGN) signatures from those that may be dominated by star formation. Both populations show extremely rapid evolution over this redshift range. We find that the number of sources with ULIG radio power and no clear AGN signatures evolves as (1+z)^(7).

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arXiv2003

The Measurement of the Light Deflection from Jupiter: Experimental Results

We have determined the relativistic light deflection of the quasar J0842+1835 as Jupiter passed within 3.7' on 2002 September 8, by measuring the time delay using the VLBA and Effelsberg radio telescopes at 8.4 GHz. At closest approach, General Relativity (GR) predicts a radial (static) deflection of 1190 microarcsec, and tangential (retarded) deflection in the direction of Jupiter's motion of 51 microarcsec. Our experiment achieved an rms position error of <10 microarcsec, and measured this retarded deflection to be 0.98 +/- 0.19 (rms error) times that predicted by GR. The increase positional accuracy for this VLBI phase referencing experiment was achieved by using two calibrator sources. Comments on the interpretation of this experiment are given.

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arXiv2002

The Properties of Microjansky Radio Sources in the HDF-N, SSA13, and SSA22 Fields

(abridged) We present multiwavelength observations for a large sample of microjansky radio sources detected in ultradeep 1.4GHz maps centered on the Hubble Deep Field-North (HDF-N) and the Hawaii Survey Fields SSA13 and SSA22. Our spectroscopic redshifts for 169 radio sources reveal a flat median redshift distribution, and these sources are hosted by similarly luminous optical L* galaxies, regardless of redshift. This is a serious concern for radio estimates of the local star formation rate density, as a substantial fraction of the ultraviolet luminosity density is generated by sub-L* galaxies at low redshifts. From our submillimeter measurements for 278 radio sources, we find error-weighted mean 850micron fluxes of 1.72$\pm$0.09 mJy for the total sample, 2.37$\pm$0.13 mJy for the optically-faint (I>23.5) subsample, and 1.04$\pm$0.13 mJy for the optically-bright (I<23.5) subsample. We significantly (>3σ) detect in the submillimeter 50 of the radio sources, 38 with I>23.5. Spectroscopic redshifts for three of the I<23.5 submillimeter-detected radio sources are in the range z=1.0-3.4, and all show AGN signatures. Using only the submillimeter mapped regions we find that 69\pm9% of the submillimeter-detected radio population are at I>23.5. We also find that 66\pm7% of the S850>5 mJy (>4σ) sources are radio-identified. We find that millimetric redshift estimates at low redshifts are best made with a FIR template intermediate between a Milky Way type galaxy and a starburst galaxy, and at high redshifts with an Arp220 template.

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arXiv2002

The Micro-Jansky Sky at 8.4 GHz

We present the results from two radio integrations at 8.4 GHz using the VLA. One of the fields, at 13h,+43d (SA13 field), has an rms noise level of 1.49 microJy and is the deepest radio image yet made. Thirty-four sources in a complete sample were detected above 7.5 microJy and 25 are optically identified to a limit of I=25.8, using our deep HST and ground-based images. The radio sources are usually located within 0.5" (typically 5 kpc) of a galaxy nucleus, and generally have a diameter less than 2.5". The second field at 17h, +50d (Hercules Field) has an rms noise of 35 microJy and contains 10 sources. We have also analyzed a complete flux density-limited sample at 8.4 GHz of 89 sources from five deep radio surveys, including the Hubble deep field. Half of all the optical counterparts are with galaxies brighter than I=23 mag, but 20% are fainter than I=25.5 mag. We confirm the tendency for the micro-Jansky radio sources to prefer multi-galaxy systems. The distribution of the radio spectral index between 1.4 and 8.4 GHz peaks at alpha = -0.75~ with a median value of -0.6. The average spectral index becomes steeper (lower values) for sources below 35 microJy, and for sources identified with optical counterparts fainter than I=25.5 mag. The differential radio count between 7.5 and 1000 microJy has a slope of -2.11 +/-0.13 and a surface density of 0.64 sources per square-arcmin with flux density greater than $7.5 microJy.

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