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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

FLASH pilot survey: detections of associated 21 cm H i absorption in GAMA galaxies at 0.42 < z < 1.00

ABSTRACT We present the results of a search for associated 21 cm H i absorption at redshift 0.42 < z < 1.00 in radio-loud galaxies from three Galaxy And Mass Assembly (GAMA) survey fields. These observations were carried out as part of a pilot survey for the Australian SKA Pathfinder (ASKAP) First Large Absorption Survey in H i (FLASH). From a sample of 326 radio sources with 855.5 MHz peak flux density above 10 mJy, we detected two associated H i absorption systems, in SDSS J090331+010847 at z = 0.522 and SDSS J113622+004852 at z = 0.563. Both galaxies are massive (stellar mass $\gt 10^{11}\, \mathrm{M}_\odot$) and have optical spectra characteristic of luminous red galaxies, though spectral energy distribution fitting implies that SDSS J113622+004852 contains a dust-obscured starburst with star formation rate ∼69 M⊙ yr−1. The H i absorption lines have a high optical depth, with τpk of 1.77 ± 0.16 for SDSS J090331+010847 (the highest value for any z > 0.1 associated system found to date) and 0.14 ± 0.01 for SDSS J113622+004852. In the redshift range probed by our ASKAP observations, the detection rate for associated H i absorption lines (with τpk > 0.1 and at least 3σ significance) is $2.9_{-2.6}^{+9.7}$ per cent. Although the current sample is small, this rate is consistent with a trend seen in other studies for a lower detection rate of associated 21 cm H i absorption systems at higher redshift. We also searched for OH absorption lines at 0.67 < z < 1.34, but no detection was made in the 145 radio sources searched.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2010

VLBI2020: From Reality to Vision

The individual apparent motions of distant radio sources are believed to be caused by the effect of intrinsic structure variations of the active galactic nuclei (AGN). However, some cosmological models of the expanded Universe predict that systematic astrometric proper motions of distant quasars do not vanish as the radial distance from the observer to the quasar grows. These systematic effects can even increase with the distance, making it possible to measure them with high-precision astrometric techniques like VLBI. The Galactocentric acceleration of the Solar System barycenter may cause a secular aberration drift with a magnitude of 4 uas/yr. The Solar System motion relative to the cosmic microwave background produces an additional dipole effect, proportional to red shift. We analyzed geodetic VLBI data spanning from 1979 until 2009 to estimate the vector spherical harmonics in the expansion of the vector field of the proper motion of 687 radio sources. The dipole and quadrupole vector spherical harmonics were estimated with an accuracy of 1-5 as/yr. We have shown that over the next decade the geodetic VLBI may approach the level of accuracy on which the cosmological models of the Universe could be tested. Hence, it is important to organize a dedicated observational program to increase the number of measured proper motions to 3000.

Titov, Oleg↗
NASA NTRS2012

VLBI: A Fascinating Technique for Geodesy and Astrometry

Since the 1970s Very Long Baseline Interferometry (VLBI) has proven to be a primary space-geodetic technique by determining precise coordinates on the Earth, by monitoring the variable Earth rotation and orientation with highest precision, and by deriving many other parameters of the Earth system. VLBI provides an important linkage to astronomy through, for instance, the determination of very precise coordinates of extragalactic radio sources. Additionally, it contributes to determining parameters of relativistic and cosmological models. After a short review of the history of geodetic VLBI and a summary of recent results, this paper describes future perspectives of this fascinating technique. The International VLBI Service for Geodesy and Astrometry (IVS), as a service of the International Association of Geodesy (IAG) and the International Astronomical Union (IAU), is well on its way to fully defining a next generation VLBI system, called VLBI2010. The goals of the new system are to achieve on scales up to the size of the Earth an accuracy of 1 mm in position and of 0.1 mm/year in velocity. Continuous observations shall be carried out 24 h per day 7 days per week in the future with initial results to be delivered within 24 h after taking the data. Special sessions, e.g. for monitoring the Earth rotation parameters, will provide the results in near real-time. These goals require a completely new technical and conceptual design of VLBI measurements. Based on extensive simulation studies, strategies have been developed by the IVS to significantly improve its product accuracy through the use of a network of small (approx 12 m) fast-slewing antennas. A new method for generating high precision delay measurements as well as improved methods for handling biases related to radio source structure, system electronics, and deformations of the antenna structures has been developed. Furthermore, as of January 2012, the construction of ten new VLBI2010 sites has been funded, with good prospects for one dozen more antennas, which will improve the geographical distribution of geodetic VLBI sites on Earth and provide an important step toward a global VLBI2010 network. Within this paper, the Global Geodetic Observing System (GGOS) of the IAG will also be introduced and the contribution of VLBI to GGOS will be described.

Geodesy↗
DOE OSTI2022

Cross-correlation of Dark Energy Survey Year 3 lensing data with ACT and Planck thermal Sunyaev-Zel’dovich effect observations. I. Measurements, systematics tests, and feedback model constraints

Here we present a tomographic measurement of the cross-correlation between thermal Sunyaev-Zel’dovich (TSZ) maps from Planck and the Atacama Cosmology Telescope and weak galaxy lensing shears measured during the first three years of observations of the Dark Energy Survey. This correlation is sensitive to the thermal energy in baryons over a wide redshift range and is therefore a powerful probe of astrophysical feedback. We detect the correlation at a statistical significance of 21σ, the highest significance to date. We examine the TSZ maps for potential contaminants, including cosmic infrared background and radio sources, finding that cosmic infrared background has a substantial impact on our measurements and must be taken into account in our analysis. We use the cross-correlation measurements to test different feedback models. In particular, we model the TSZ using several different pressure profile models calibrated against hydrodynamical simulations. Our analysis marginalizes over redshift uncertainties, shear calibration biases, and intrinsic alignment effects. We also marginalize over Ω m and σ 8 using Planck or DES priors. We find that the data prefer the model with a low amplitude of the pressure profile at small scales, compatible with a scenario with strong active galactic nuclei feedback and ejection of gas from the inner part of the halos. When using a more flexible model for the shear profile, constraints are weaker, and the data cannot discriminate between different baryonic prescriptions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

CHANDRA and XMM-NEWTON observations of the bimodal PLANCK SZ -detected clustered CKG345.40-39.34 (A3716) with high and low entropy subcluster cores

Here, we present results from Chandra, XMM-Newton, and ROSAT observations of the Planck SZ-detected cluster A3716 (PLCKG345.40-39.34-G345). We show that G345 is, in fact, two subclusters separated on the sky by 400 kpc. We measure the subclusters' gas temperatures (~2–3 keV), total (~1–2 × 10 14 ${{M}_{\odot }}$) and gas (~1–2 × 10 13 ${{M}_{\odot }}$) masses, gas mass fraction within r500, entropy profiles, and X-ray luminosities (~10 43 erg s -1 ). Using the gas density and temperature profiles for both subclusters, we show that there is good (0.8σ) agreement between the expected Sunyaev–Zel'dovich signal predicted from the X-ray data and that measured from the Planck mission, and better agreement within 0.6σ when we re-computed the Planck value assuming a two component cluster model, with relative amplitudes fixed based on the X-ray data. Dynamical analysis shows that the two galaxy subclusters are very likely ($\gt 97\%$ probability) gravitationally bound, and in the most likely scenario, the subclusters will undergo core passage in 500 ± 200 Myr. The northern subcluster is centrally peaked and has a low entropy core, while the southern subcluster has a high central entropy. Finally, the high central entropy in the southern subcluster can be explained either by the mergers of several groups, as suggested by the presence of five giant ellipticals or by active galactic nucleus energy injection, as suggested by the presence of a strong radio source in one of its massive elliptical galaxies, or by a combination of both processes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

THE LOW-FREQUENCY RADIO CATALOG OF FLAT-SPECTRUM SOURCES

A well known property of the γ-ray sources detected by Cos-B in the 1970s, by the Compton Gamma-Ray Observatory in the 1990s, and recently by the Fermi observations is the presence of radio counterparts, particularly for those associated with extragalactic objects. This observational evidence is the basis of the radio-γ-ray connection established for the class of active galactic nuclei known as blazars. In particular, the main spectral property of the radio counterparts associated with γ-ray blazars is that they show a flat spectrum in the GHz frequency range. Our recent analysis dedicated to search blazar-like candidates as potential counterparts for the unidentified γ-ray sources allowed us to extend the radio-γ-ray connection in the MHz regime. We also showed that blazars below 1 GHz maintain flat radio spectra. Thus, on the basis of these new results, we assembled a low-frequency radio catalog of flat-spectrum sources built by combining the radio observations of the Westerbork Northern Sky Survey and of the Westerbork in the southern hemisphere catalog with those of the NRAO Very Large Array Sky survey (NVSS). This could be used in the future to search for new, unknown blazar-like counterparts of γ-ray sources. First, we found NVSS counterparts of Westerbork Synthesis Radio Telescope radio sources, and then we selected flat-spectrum radio sources according to a new spectral criterion, specifically defined for radio observations performed below 1 GHz. We also described the main properties of the catalog listing 28,358 radio sources and their logN-logS distributions. Finally, a comparison with the Green Bank 6 cm radio source catalog was performed to investigate the spectral shape of the low-frequency flat-spectrum radio sources at higher frequencies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

VLBI ASTROMETRY OF PSR J2222-0137: A PULSAR DISTANCE MEASURED TO 0.4% ACCURACY

The binary pulsar J2222-0137 is an enigmatic system containing a partially recycled millisecond pulsar and a companion of unknown nature. While the low eccentricity of the system favors a white dwarf companion, an unusual double neutron star system is also a possibility, and optical observations will be able to distinguish between these possibilities. In order to allow the absolute luminosity (or upper limit) of the companion object to be properly calibrated, we undertook astrometric observations with the Very Long Baseline Array to constrain the system distance via a measurement of annual geometric parallax. With these observations, we measure the parallax of the PSR J2222-0137 system to be 3.742{sup +0.013}{sub -0.016} mas, yielding a distance of 267.3{sup +1.2}{sub -0.9} pc, and measure the transverse velocity to be 57.1{sup +0.3}{sub -0.2} km s{sup -1}. Fixing these parameters in the pulsar timing model made it possible to obtain a measurement of Shapiro delay and hence the system inclination, which shows that the system is nearly edge-on (sin i = 0.9985 {+-} 0.0005). Furthermore, we were able to detect the orbital motion of PSR J2222-0137 in our very long baseline interferometry (VLBI) observations and measure the longitude of ascending node {Omega}. The VLBI astrometry yields the most accurate distance obtained for a radio pulsar to date, and is furthermore the most accurate parallax for any radio source obtained at ''low'' radio frequencies (below {approx}5 GHz, where the ionosphere dominates the error budget). Using the astrometric results, we show that the companion to PSR J2222-0137 will be easily detectable in deep optical observations if it is a white dwarf. Finally, we discuss the implications of this measurement for future ultra-high-precision astrometry, in particular in support of pulsar timing arrays.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1977

An experiment protocol for a search for radio signals of extraterrestrial intelligent origin in the presence of man-made radio frequency sources

Some aspects of signal extraction in a microwave search for evidence of extraterrestrial intelligence are examined. Parametric relations are summarized which are applicable to a microwave search of constrained duration that employs FFT spectrum-analyzer receivers, with sensitivity enhancement by spectrum accumulation and detection by a threshold criterion. Three types of natural and man-made false alarms are identified, the probability of false alarm in a single data channel is computed, and the implications of false alarms for a constant-beamwidth sky survey are considered. It is shown that the key to an efficient search is the prompt and unambiguous elimination of false alarms. An experimental protocol is suggested which eliminates spurious signals primarily through procedural techniques involving antenna repointing, delayed repeated observations, and storage of particular historical parameters for suspect signals.

Edelson, R. E.↗
NASA NTRS1978

Infrared observations of the galactic center. I - Nature of the compact sources

Photometry from 1.25 to 12 micrometers and spectrophotometry from 8 to 13 micrometers of the compact sources found in the galactic-center region are reported. In addition, revised 10 and new 20 micrometers maps with 2''.3 resolution are given. The nature of the compact sources is discussed. Some are best identified as stars or star clusters; the brightest source at 2 micrometers is probably a supergiant, and the infrared source near the nonthermal radio source is probably a stellar cluster with density greater than 1 million solar masses/cu pc. Other sources emit most of their luminosity at wavelengths of 10 micrometers and greater; this emission is probably from heated dust. One of the sources is observationally similar to extremely red OH/infrared stars. Other sources have luminosities and linear sizes similar to those of compact H II regions; emission from optically thin silicate dust is seen in these.

Becklin, E. E.↗
NASA NTRS1984

A comparison of the properties of highly polarized QSOs versus low-polarization QSOs

An optical linear-polarization survey testing the relationships between polarization and other properties of QSOs is presented. Polarimetric observations of 239 QSOs have been made. Data from the literature are used to analyze the relationships between polarization and radio, optical, and X-ray properties. The highly polarized QSOs are generally compact radio sources, are associated with radio properties such as low-frequency variability and superluminal motion, exhibit large-amplitude rapid photometric variability, have steep nonthermal optical continua, and may show excess X-ray emission. No relationship is seen between polarization and redshift, optical luminosity, or the equivalent width of emission lines. The results are discussed in the context of both isotropic and anisotropic (beaming) models. While the associations between the optical and radio properties of highly polarized QSOs provide strong motivation for the anisotropic model, the lack of associations with redshift, optical luminosity, and emission-line strength is inconsistent with this model. It is concluded that the highly polarized QSOs are probably fundamentally different in their physical properties from low-polarization QSOs.

Moore, R. L.↗
NASA NTRS1985

QUASAT: An orbiting very long baseline interferometer program using large space antenna systems

QUASAT, which stands for QUASAR SATELLITE, is the name given to a new mission being studied by NASA. The QUASAT mission concept involves a free flying Earth orbiting large radio telescope, which will observe astronomical radio sources simultaneously with ground radio telescopes. The primary goal of QUASAT is to provide a system capable of collecting radio frequency data which will lead to a better understanding of extremely high energy events taking place in a variety of celestial objects including quasars, galactic nuclei, interstellar masers, radio stars and pulsars. QUASAT's unique scientific contribution will be the increased resolution in the emission brightness profile maps of the celestial objects.

Jordan, J. F.↗
NASA NTRS1990

Testing for lightning as a source of radio bursts observed on the nightside of Venus

In certain previous studies of radio burst events recorded by the Pioneer Venus Orbiting Electric Field Detector (OEFD), data were sorted for statistical purposes according to occurrence at filter band frequencies smaller than or greater than typical values of the ambient electron gyrofrequency. The expectation in making this distinction was that the lowest frequency signals, at 100 Hz, were candidates for propagation through the ionosphere to the spacecraft in the whistler mode, and that the higher frequency signals, if of subionospheric origin, would require some different ionospheric penetration mechanism. On the basis of certain assumptions about the homogeneity and horizontal stratification of the Venusian nightside ionosphere, methods were developed for case-by-case testing of the hypothesis that any particular burst event originated in subionospheric lightning. The tests, which are capable of refinement, allow prediction of the resonance cone angle, refractive index, wave dispersion, and wave polarization. The tests have been applied to data from 11 periods along 7 orbits, and are believed to represent an improved way of categorizing OEFD burst data for purposes of investigating source/propagation mechanisms. Four of the five burst events that were not found consistent with the lightning hypothesis involved receptions at multiple OEFD filter band frequencies.

Sonwalkar, Vikas S.↗
NASA NTRS1990

The evolution of weak radio galaxies at radio and optical wavelengths

The statistical properties of radio sources are reviewed. The linear sizes of gE radio galaxies increase mildly with radio power, and decrease drastically with redshift. Combined with the redshift distribution, this causes angular sizes to decrease continuously towards fainter fluxes. Nonetheless, the fraction of large sources is not negligible at any flux level, requiring a revision of the resolution correction to the source counts. The 1.4 GHz counts are now well established (within 5 percent) from 10,575 sources in 24 different surveys. This makes it possible to study fied-to-field anisotropies. None are found in the wide area surveys for S(1.4) greater than 10 mJy, and only marginal anisotropies in some small-area surveys. These are compared to the redshift distribution and the angular correlation function of radio sources.

Windhorst, Rogier↗
NASA NTRS1992

DSS-13 26-meter antenna upgraded radiometer system

The Deep Space Station (DSS)-13 26-m antenna radiometer system was upgraded with an IBM-compatible computer-controlled configuration with improved supporting hardware and software. Software was generated to analyze results and correct for antenna mispointing, tropospheric loss, and other observing errors. This total power radiometer configuration provides a prototype for the new DSS-13 34-m antenna. The radiometer system is described in terms of the theory, instrumentation hardware, computer configuration, and operational features and performance. The system is used to obtain antenna efficiency and pointing model data and is useful for radio source calibrations required for radio astronomy. Some recent results are given.

Stelzried, C. T.↗
NASA NTRS1981

The 2f/p/ circumterrestrial radio radiation as seen from ISEE 3

A narrowband highly spin-modulated radio source has been observed with the radio astronomy experiment on ISEE 3 from a position 0.01 AU upstream of the earth. The source is interpreted as radio line emission generated at twice the local plasma frequency in the vicinity of the earth's bow shock by energetic particles. The bandwidth of the 2f(p) emission varies from less than 3 kHz to more than 20 kHz and is probably broadened by solar wind density variations across the source region. The 2f(p) source is visible about 50% of the time, and the true position of its centroid is usually located within 30 earth radii of the subsolar point, but at times is observed at a distance of up to 60 earth radii. But most of these variations might result from changes in the orientation of the polarization vector in a partially polarized source or from refraction across an electron density front.

Hoang, S.↗
DOE OSTI2016

Constraints on feedback in the local universe: The relation between star formation and AGN activity in early-type galaxies

We address the relation between star formation and active galactic nucleus (AGN) activity in a sample of 231 nearby (0.0002 < z < 0.0358) early-type galaxies by carrying out a multi-wavelength study using archival observations in the UV, IR, and radio. Our results indicate that early-type galaxies in the current epoch are rarely powerful AGNs, with P<10{sup 22} WHz{sup −1} for a majority of the galaxies. Only massive galaxies are capable of hosting powerful radio sources while less massive galaxies are hosts to lower radio power sources. Evidence of ongoing star formation is seen in approximately 7% of the sample. The star formation rate (SFR) of these galaxies is less than 0.1 M{sub ⊙} yr{sup −1}. They also tend to be radio faint (P<10{sup 22} WHz{sup −1}). There is a nearly equal fraction of star-forming galaxies in radio faint (P<10{sup 22} WHz{sup −1}) and radio bright galaxies (P⩾10{sup 22} WHz{sup −1}) suggesting that both star formation and radio mode feedback are constrained to be very low in our sample. We notice that our galaxy sample and the Brightest Cluster Galaxies follow similar trends in radio power versus SFR. This may be produced if both radio power and SFR are related to stellar mass.

79 ASTRONOMY AND ASTROPHYSICS↗