The Sixth VLBA Calibrator Survey: VCS6 Petrov, L; Kovalev, Y. Y; Fomalont, E. B ...
The Astronomical journal,
08/2008, Letnik:
136, Številka:
2
Journal Article
Radio source counts constrain galaxy populations and evolution, as well as the global star formation history. However, there is considerable disagreement among the published 1.4-GHz source counts ...below 100 μJy. Here, we present a statistical method for estimating the μJy and even sub-μJy source count using new deep wide-band 3-GHz data in the Lockman Hole from the Karl G. Jansky Very Large Array. We analysed the confusion amplitude distribution P(D), which provides a fresh approach in the form of a more robust model, with a comprehensive error analysis. We tested this method on a large-scale simulation, incorporating clustering and finite source sizes. We discuss in detail our statistical methods for fitting using Markov chain Monte Carlo, handling correlations, and systematic errors from the use of wide-band radio interferometric data. We demonstrated that the source count can be constrained down to 50 nJy, a factor of 20 below the rms confusion. We found the differential source count near 10 μJy to have a slope of -1.7, decreasing to about -1.4 at fainter flux densities. At 3 GHz, the rms confusion in an 8-arcsec full width at half-maximum beam is ~ 1.2 μJy beam^sup -1^, and a radio background temperature ~14 mK. Our counts are broadly consistent with published evolutionary models. With these results, we were also able to constrain the peak of the Euclidean normalized differential source count of any possible new radio populations that would contribute to the cosmic radio background down to 50 nJy. PUBLICATION ABSTRACT
We present a detailed analysis of 256 radio sources from our deep (flux density limit of 42 mJy 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 multiwavelength information in the radio, optical, and X-ray bands. The availability of redshifts for ~80% of the sources in our complete sample allows us to derive reliable luminosity estimates for the majority of the objects. X-ray data, including upper limits, for all our sources turn out to be a key factor in establishing the nature of faint radio sources. Due to the faint optical levels probed by this study, we have uncovered a population of distant active galactic nuclei (AGNs) systematically missing from many previous studies of sub-millijansky radio source identifications. We find that, while the well-known flattening of the radio number counts below 1 mJy is mostly due to star-forming galaxies, these sources and AGNs make up an approximately equal fraction of the sub-millijansky sky, contrary to some previous results. The AGNs include radio galaxies, mostly of the low-power, Fanaroff-Riley I type, and a significant radio-quiet component, which amounts to approximately one fifth of the total sample. The ratio of radio-to-optical luminosity depends more on radio luminosity, rather than being due to optical absorption.
18 days of MERLIN data and 42 h of A-array VLA data at 1.4 GHz have been combined to image a 10-arcmin field centred on the Hubble Deep Field (HDF). This area also includes the Hubble Flanking Fields ...(HFF). A complete sample of 92 radio sources with S1.4 > 40 μJy was detected using the VLA data alone and then imaged with the MERLIN+VLA combination. The combined images offer (i) higher angular resolution (synthesized beams of diameter 0.2–0.5 arcsec), (ii) improved astrometric accuracy, and (iii) improved sensitivity compared with VLA-only data. The images are amongst the most sensitive yet made at 1.4 GHz, with rms noise levels of 3.3 μJy beam−1 in the 0.2-arcsec images. Virtually all the sources are resolved, with angular sizes in the range 0.2 to 3 arcsec. The central 3-arcmin square was imaged separately to search for sources down to 27 μJy. No additional sources were detected, indicating that sources fainter than 40 μJy are heavily resolved with MERLIN and must have typical angular sizes > 0.5 arcsec. Radio sources associated with compact galaxies have been used to align the HDF, the HFF and a larger CFHT optical field to the radio-based International Celestial Reference Frame. The HST optical fields have been registered to <50 mas in the HDF itself, and to ≤150 mas in the outer parts of the HFF. We find a statistical association of very faint (≥2 μJy) radio sources with optically bright HDF galaxies down to ∼23 mag. Of the 92 radio sources above 40 μJy, ∼85 per cent are identified with galaxies brighter than I= 25 mag; the remaining 15 per cent are associated with optically faint systems close to or beyond the HFF (or even the HDF) limit. The high astrometric accuracy and the ability of radio waves to penetrate obscuring dust have led to the correct identification of several very red, optically faint systems, including the the strongest submillimetre source in the HDF, HDF 850.1. On the basis of their radio structures and spectra, 72 per cent (66 sources) can be classified as starburst or active galactic nucleus-type systems; the remainder are unclassified. The proportion of starburst systems increases with decreasing flux density; below 100 μJy > 70 per cent of the sources are starburst-type systems associated with major disc galaxies 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. The most recent submillimetre results on the HDF and HFF do not provide any unambiguous identifications with these latest radio data, except for HDF 850.1, but suggest at least three strong candidates.
We present astrometric results for compact extragalactic objects observed with the Very Long Baseline Array at radio frequencies of 24 and 43 GHz. Data were obtained from ten 24 hr observing sessions ...made over a five-year period. These observations were motivated by the need to extend the International Celestial Reference Frame (ICRF) to higher radio frequencies to enable improved deep space navigation after 2016 and to improve state-of-the-art astrometry. Source coordinates for 268 sources were estimated at 24 GHz and for 131 sources at 43 GHz. The median formal uncertainties of right ascension and declination at 24 GHz are 0.08 and 0.15 mas, respectively. Median formal uncertainties at 43 GHz are 0.20 and 0.35 mas, respectively. Weighted root-mean-square differences between the 24 and 43 GHz positions and astrometric positions based on simultaneous 2.3 and 8.4 GHz Very Long Baseline Interferometry observations, such as the ICRF, are less than about 0.3 mas in both coordinates. With observations over five years we have achieved a precision at 24 GHz approaching that of the ICRF but unaccounted systematic errors limit the overall accuracy of the catalogs.
The Radio/Optical Catalog of the SSA 13 Field Fomalont, E. B; Kellermann, K. I; Cowie, L. L ...
The Astrophysical journal. Supplement series,
12/2006, Letnik:
167, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We present a 1.4 GHz catalog of 810 radio sources (560 sources in the complete sample) found in the SSA 13 field (R.A. = 13 super(h)12 super(m), decl. = 4238'). The 1.4 GHz radio image was obtained ...from a 91 hr VLA integration with an rms noise level of 4.82 kJy beam super(-1) at the field center. Optical images in the R band (6300 A) and z band (9200 A) with 3 s detection magnitudes of 26.1 and 24.9, respectively, were obtained from three observing nights on the 8 m Subaru Telescope. We find that 88% c 2% of the radio sources are identified with an optical counterpart. There is significantly more reddening for the optical counterparts that are fainter than 24 mag, probably caused by the somewhat larger redshifts of these faint galaxies. The radio and optical parameters are tabulated, and source morphologies are displayed by radio contours overlaid on optical false-color images. The brightness distributions show a wealth of complexity and are classified into a small number of categories. About one-third of the radio sources are larger than l."2, and their orientation is often similar to that of the associated galaxy or binary galaxy system. Radio emission is sometimes located outside of the nuclear regions of the galaxy. The density of sources in the SSA 13 field above 75 kJy is 0.40 arcmin super(-2), with a slope of -2.43 in the differential counts. This source density is somewhat higher than that in the Hubble Deep Field North. The radio spectral index may steepen for sources below 75 kJy and is consistent with the difference in the slope of the source counts observed between 1.4 and 8.4 GHz. We estimate that at most 40% of the microjansky radio sources are dominated by AGN processes, while the remainder are mostly the consequence of star formation and associated supernova activity.
The Second VLBA Calibrator Survey: VCS2 Fomalont, E. B; Petrov, L; MacMillan, D. S ...
The Astronomical journal,
11/2003, Letnik:
126, Številka:
5
Journal Article
We have measured the submilliarcsecond structure of 274 extragalactic sources at 24 and 43 GHz in order to assess their astrometric suitability for use in a high-frequency celestial reference frame ...(CRF). Ten sessions of observations with the Very Long Baseline Array have been conducted over the course of ~5 years, with a total of 1339 images produced for the 274 sources. There are several quantities that can be used to characterize the impact of intrinsic source structure on astrometric observations including the source flux density, the flux density variability, the source structure index, the source compactness, and the compactness variability. A detailed analysis of these imaging quantities shows that (1) our selection of compact sources from 8.4 GHz catalogs yielded sources with flux densities, averaged over the sessions in which each source was observed, of about 1 Jy at both 24 and 43 GHz, (2) on average the source flux densities at 24 GHz varied by 20%-25% relative to their mean values, with variations in the session-to-session flux density scale being less than 10%, (3) sources were found to be more compact with less intrinsic structure at higher frequencies, and (4) variations of the core radio emission relative to the total flux density of the source are less than 8% on average at 24 GHz. We conclude that the reduction in the effects due to source structure gained by observing at higher frequencies will result in an improved CRF and a pool of high-quality fiducial reference points for use in spacecraft navigation over the next decade.
The VLBA Calibrator Survey—VCS1 Beasley, A. J; Gordon, D; Peck, A. B ...
The Astrophysical journal. Supplement series,
07/2002, Letnik:
141, Številka:
1
Journal Article
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