We used the Karl G. Jansky Very Large Array to image one primary beam area at 3 GHz with 8" FWHM resolution and 1.0 mu Jy beam super(-1) rms noise near the pointing center. The P(D) distribution from ...the central 10 arcmin of this confusion-limited image constrains the count of discrete sources in the 1 < S( mu Jy) < 10 range. At this level, the brightness-weighted differential count S super(2)n(S) is converging rapidly, as predicted by evolutionary models in which the faintest radio sources are star-forming galaxies; and asymptotically =96% of the background originating in galaxies has been resolved into discrete sources. About 63% of the radio background is produced by active galactic nuclei (AGNs), and the remaining 37% comes from star-forming galaxies that obey the far-infrared (FIR)/radio correlation and account for most of the FIR background at lambda asymptotically = 160 mu m. Our new data confirm that radio sources powered by AGNs and star formation evolve at about the same rate, a result consistent with AGN feedback and the rough correlation of black hole and stellar masses. The confusion at centimeter wavelengths is low enough that neither the planned Square Kilometre Array nor its pathfinder ASKAP EMU survey should be confusion limited, and the ultimate source detection limit imposed by "natural" confusion is < or =, slant0.01 mu Jy at nu = 1.4 GHz. If discrete sources dominate the bright extragalactic background reported by ARCADE 2 at 3.3 GHz, they cannot be located in or near galaxies and most are < or =, slant0.03 mu Jy at 1.4 GHz.
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 (10 AU) to 0 025 (3.5 AU), revealing an astonishing level of detail in the circumstellar disk surrounding the young solar analog 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 ( ) and position angle We obtain a high-fidelity image of the 1.0 mm spectral index ( ), which ranges from 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, and 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−1 consistent with Keplerian motion around a ∼1.3 star, although complicated by absorption at low blueshifted velocities. We also serendipitously detect and resolve the nearby protostars XZ Tau (A/B) and LkH 358 at 2.9 mm.
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.
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.
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 super(+) sub(-) super(0) sub(0) super(.) sub(.) super(2) sub(2) super(3) sub(0) kpc) and transverse velocity (1083 super(+) sub(-) super(1) sub(9) super(0) sub(0) super(3) km s super(-1)), the highest velocity directly measured for a neutron star. We trace the pulsar back from its present Galactic latitude of 52.3 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.
The Fourth VLBA Calibrator Survey: VCS4 Petrov, L; Kovalev, Y. Y; Fomalont, E. B ...
The Astronomical journal,
03/2006, Letnik:
131, Številka:
3
Journal Article
Recenzirano
Odprti dostop
This paper presents the fourth extension to the VLBA Calibrator Survey, containing 258 new sources not previously observed with very long baseline interferometry (VLBI). This survey, based on three ...24 hr Very Long Baseline Array observing sessions, fills remaining areas on the sky above declination -40° where the calibrator density is less than one source within a 4° radius disk in any given direction. The share of these areas was reduced from 4.6% to 1.9%. Source positions were derived from astrometric analysis of group delays determined at 2.3 and 8.6 GHz frequency bands using the Calc/Solve software package. The VCS4 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 online and can be found using the search keyword 'VCS4.'
Astrometry can bring powerful constraints to bear on a variety of scientific questions about neutron stars, including their origins, astrophysics, evolution, and environments. Using phase-referenced ...observations at the Very Long Baseline Array (VLBA), in conjunction with pulsar gating and in-beam calibration, we have measured the parallaxes and proper motions for 14 pulsars. The smallest measured parallax in our sample is 0.13 - 0.02 mas for PSR B1541+09, which has a most probable distance of 7.2+1.3 -1.1 kpc. We detail our methods, including initial VLA surveys to select candidates and find in-beam calibrators, VLBA phase-referencing, pulsar gating, calibration, and data reduction. The use of the bootstrap method to estimate astrometric uncertainties in the presence of unmodeled systematic errors is also described. Based on our new model-independent estimates for distance and transverse velocity, we investigate the kinematics and birth sites of the pulsars and revisit models of the Galactic electron density distribution. We find that young pulsars are moving away from the Galactic plane, as expected, and that age estimates from kinematics and pulsar spindown are generally in agreement, with certain notable exceptions. Given its present trajectory, the pulsar B2045 - 16 was plausibly born in the open cluster NGC 6604. For several high-latitude pulsars, the NE2001 electron density model underestimates the parallax distances by a factor of 2, while in others the estimates agree with or are larger than the parallax distances, suggesting that the interstellar medium is irregular on relevant length scales. The VLBA astrometric results for the recycled pulsar J1713+0747 are consistent with two independent estimates from pulse timing, enabling a consistency check between the different reference frames.
The Fifth VLBA Calibrator Survey: VCS5 Kovalev, Y. Y; Petrov, L; Fomalont, E. B ...
The Astronomical journal,
04/2007, Letnik:
133, Številka:
4
Journal Article
Recenzirano
Odprti dostop
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: (1) to observe additional sources that, together with previous survey results, form a complete sample, and (2) to find new strong sources suitable as phase calibrators. This VCS extension was based on three 24 hr 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° that 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 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 World Wide Web.
We report 20 and 6 cm VLA deep observations of the CDF-S including the Extended CDF-S. We discuss the radio properties of 266 cataloged radio sources, of which 198 are above a 20 cm completeness ...level reaching down to 43 muJy at the center of the field. Survey observations made at 6 cm over a more limited region cover the original CDF-S to a comparable level of sensitivity as the 20 cm observations. Of 266 cataloged radio sources, 52 have X-ray counterparts in the CDF-S and a further 37 have counterparts in the E-CDF-S area not covered by the 1 Ms exposure. Using a wide range of material, we have found optical or infrared counterparts for 254 radio sources, of which 186 have either spectroscopic or photometric redshifts. Three radio sources have no apparent counterpart at any other wavelength. Measurements of the 20 cm radio flux density at the position of each CDF-S X- ray source detected a further 30 radio sources above a conservative 3 capital sigma detection limit. X-ray and submillimeter observations have been traditionally used as a measure of AGN and star formation activity, respectively. These new observations probe the faint end of both the star formation and radio galaxy/AGN population, as well as the connection between the formation and evolution of stars and SMBHs. Both of the corresponding gravitational and nuclear fusion driven energy sources can lead to radio synchrotron emission. AGN and radio galaxies dominate at high flux densities. Although emission from star formation becomes more prominent at the microjansky levels reached by deep radio surveys, even for the weakest sources, we still find an apparent significant contribution from low-luminosity AGN as well as from star formation.
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