Thousands Of Radio Sources Research Articles

Radio Astrometry at Different Frequencies

The very long baseline interferometry technique allows us to determine the positions of thousands of radio sources using the absolute astrometry approach. I have investigated the impacts of a selection of observing frequencies in a range from 2 to 43 GHz in single-band, dual-band, and quad-band observing modes on astrometric results. I processed seven data sets in a range of 72,000 to 6.9 million observations, estimated source positions, and compared them. I found that source positions derived from dual-band, quad-band, and 23.6 GHz single-band data agree at a level below 0.2 mas. Comparison of independent data sets allowed me to assess the error levels of individual catalogs: 0.05–0.07 mas per position component. Further comparison showed that individual catalogs have systematic errors at the same level. The positions from 23.6 GHz single-band data show systematic errors related to the residual ionosphere contribution. Analysis of source position differences revealed systematic errors along jet directions at a level of 0.09 mas. Network-related systematic errors affect all the data, regardless of frequency. Comparison of position estimates allowed me to derive the stochastic error model that closes the error budget. Based on the collected evidence, I have made a conclusion that the development of frequency-dependent reference frames of the entire sky is not warranted. In most cases dual-band, quad-band, and single-band data at a frequency of 22 GHz and higher can be used interchangeably, which allows us to exploit the strength of a specific frequency setup for given objects. Mixing observations at different frequencies causes errors not exceeding 0.07 mas.

MUSER and IPS telescopes for solar and space weather observations

This paper describes the recently built solar radio spectroscopy-imaging facilities, including near future developments and upgrades, as well as the IPS (Interplanetary Scintillation) telescopes under construction in China for solar and heliospheric studies. MUSER (Mingantu spectral radioheliograph), renamed from CSRH (Chinese spectral radioheliograph) after the construction, covers 400 MHz-15 GHz frequency range which was established during 2009–2016 in Mingantu Observing Station at Zhengxiangbaiqi, Inner Mongolia of China, under the National Major Scientific Research Facility Program of China. At moment MUSER is composed of two arrays with MUSER-I covering 400 MHz-2.0 GHz with 40 4.5 m mesh antennas and MUSER-II covering 2–15 GHz with 60 2 m dish antennas. MUSER will be extended to have its third array MUSER-L covering 30–400 MHz frequency regime with 224 LPDAs (log-periodic dipole antennas) under the Meridian-II Project, which is a National Science Infrastructure Project of China, to be constructed in the current two years. MUSER will provide solar radio images monitoring the solar eruptions from solar surface into interplanetary space. An IPS telescope array is going to be built in the current two years under the same National Science Infrastructure Project of China, with the main IPS telescope of three 140 m × 40 m cylinder antennas located in MUSER site and two 30 m antennas in two nearby counties each about 200 km away. The working frequency will be at 327 MHz and 654 MHz with dual linear polarizations. The cylinder antennas will have a sky zenith angle of 60 degrees and be able to observe thousands of radio sources. The IPS telescope will provide important information about solar wind and solar eruptions from the Sun to the Earth environment. The MUSER and IPS telescopes at Mingantu Observing Station, National Space Science Center of Chinese Academy of Sciences will play important role in solar and space weather studies.

A search for missing radio sources at z ≳ 4 using Lyman dropouts

ABSTRACT Using the Lyman Dropout technique, we identify 148 candidate radio sources at z ≳ 4–7 from the 887.5 MHz Australian Square Kilometre Array Pathfinder (ASKAP) observations of the GAMA23 field. About 112 radio sources are currently known beyond redshift z ∼ 4. However, simulations predict that hundreds of thousands of radio sources exist in that redshift range, many of which are probably in existing radio catalogues, but do not have measured redshifts, either because their optical emission is too faint or because of the lack of techniques that can identify candidate high-redshift radio sources (HzRSs). Our study addresses these issues using the Lyman Dropout search technique. This newly built sample probes radio luminosities that are 1–2 orders of magnitude fainter than known radio-active galactic nuclei (AGN) at similar redshifts, thanks to ASKAP’s sensitivity. We investigate the physical origin of radio emission in our sample using a set of diagnostics: (i) radio luminosity at 1.4 GHz, (ii) 1.4 GHz to 3.4 μm flux density ratio, (iii) Far-IR detection, (iv) WISE colour, and (v) SED modelling. The radio/IR analysis has shown that the majority of radio emission in the faint and bright end of our sample’s 887.5 MHz flux density distribution originates from AGN activity. Furthermore, ∼10 per cent of our sample are found to have a 250 μm detection, suggesting a composite system. This suggests that some high-z radio-AGNs are hosted by SB galaxies in contrast to low-z radio-AGNs, which are usually hosted by quiescent elliptical galaxies.

The LOFAR LBA Sky Survey

Context. The LOw Frequency ARray (LOFAR) is the only radio telescope that is presently capable of high-sensitivity, high-resolution (i.e. < 1 mJy beam−1 and < 15″) observations at ultra-low frequencies (< 100 MHz). To utilise these capabilities, the LOFAR Surveys Key Science Project is undertaking a large survey to cover the entire northern sky with Low Band Antenna (LBA) observations. Aims. The LOFAR LBA Sky Survey (LoLSS) aims to cover the entire northern sky with 3170 pointings in the frequency range between 42 − 66 MHz, at a resolution of 15″ and at a sensitivity of 1 mJy beam−1 (1σ). In this work, we outline the survey strategy, the observational status, and the calibration techniques. We also briefly describe several of our scientific motivations and present the preliminary public data release. Methods. The preliminary images were produced using a fully automated pipeline aimed at correcting all direction-independent effects in the data. Whilst the direction-dependent effects, such as those from the ionosphere, have not yet been corrected, the images presented in this work are still ten times more sensitive than previous available surveys at these low frequencies. Results. The preliminary data release covers 740 deg2 around the HETDEX spring field region at an angular resolution of 47″ with a median noise level of 5 mJy beam−1. The images and the catalogue of 25 247 sources have been publicly released. We demonstrate that the system is capable of reaching a root mean square (rms) noise of 1 mJy beam−1 and an angular resolution of 15″ once direction-dependent effects are accounted for. Conclusions. LoLSS will provide the ultra-low-frequency information for hundreds of thousands of radio sources, providing critical spectral information and producing a unique data set that can be used for a wide range of science topics, such as the search for high redshift galaxies and quasars, the study of the magnetosphere of exoplanets, and the detection of the oldest populations of cosmic-rays in galaxies, clusters of galaxies, as well as those produced by active galactic nuclei.

Extragalactic Astrophysics With Next-Generation CMB Experiments

Planck, SPT, and ACT surveys have clearly demonstrated that Cosmic Microwave Background (CMB) experiments, while optimized for cosmological measurements, have made important contributions to the field of extragalactic astrophysics in the last decade. Future CMB experiments have the potential to make even greater contributions. One example is the detection of high-z galaxies with extreme gravitational amplifications. The combination of flux boosting and of stretching of the images has allowed the investigation of the structure of galaxies at z ≃ 3 with the astounding spatial resolution of about 60 pc. Another example is the detection of proto-clusters of dusty galaxies at high z when they may not yet possess the hot intergalactic medium allowing their detection in X-rays or via the Sunyaev-Zeldovich effect. Next generation CMB experiments, like PICO, CORE, CMB-Bharat from space and Simons Observatory and CMB-S4 from the ground, will discover several thousands of strongly lensed galaxies out to z ~ 6 or more and of galaxy proto-clusters caught in the phase when their member galaxies where forming the bulk of their stars. They will also detect tens of thousands of local dusty galaxies and thousands of radio sources at least up to z ≃ 5. Moreover they will measure the polarized emission of thousands of radio sources and of dusty galaxies at mm/sub-mm wavelengths.

Exploring cosmic origins with CORE: Extragalactic sources in cosmic microwave background maps

We discuss the potential of a next generation space-borne Cosmic Microwave Background (CMB) experiment for studies of extragalactic sources. Our analysis has particular bearing on the definition of the future space project, CORE, that has been submitted in response to ESA's call for a Medium-size mission opportunity as the successor of the Planck satellite. Even though the effective telescope size will be somewhat smaller than that of Planck, CORE will have a considerably better angular resolution at its highest frequencies, since, in contrast with Planck, it will be diffraction limited at all frequencies. The improved resolution implies a considerable decrease of the source confusion, i.e. substantially fainter detection limits. In particular, CORE will detect thousands of strongly lensed high-z galaxies distributed over the full sky. The extreme brightness of these galaxies will make it possible to study them, via follow-up observations, in extraordinary detail. Also, the CORE resolution matches the typical sizes of high-z galaxy proto-clusters much better than the Planck resolution, resulting in a much higher detection efficiency; these objects will be caught in an evolutionary phase beyond the reach of surveys in other wavebands. Furthermore, CORE will provide unique information on the evolution of the star formation in virialized groups and clusters of galaxies up to the highest possible redshifts. Finally, thanks to its very high sensitivity, CORE will detect the polarized emission of thousands of radio sources and, for the first time, of dusty galaxies, at mm and sub-mm wavelengths, respectively.

Open our eyes to wider fields in VLBI surveys

AbstractThe observation and imaging of hundreds or thousands of radio sources with the technique of very long baseline interferometry (VLBI) is a computationally intensive task. However, these surveys allow us to conduct statistical investigations of large source samples, and also to discover new phenomena or types of objects. The field of view of these high-resolution VLBI imaging observations is typically a few arcseconds at cm wavelengths. For practical reasons, often a much smaller fraction of the field, the central region is imaged only. With an automated process we imaged the ~1.5-arcsec radius fields around more than 1000 radio sources, and found a variety of extended radio structures. Some of them are yet unknown in the literature.

Quasi Stellar Objects and Active Galactic Nuclei

The role of the Schmidt telescopes in the discovery of the Quasi Stellar Objects and of the Active Galactic Nuclei, and in the understanding of their properties was and continues to be of the greatest importance. Thousands of Radio-Sources have been quickly associated to their optical counterparts thanks to the worldwide availability of the Palomar Observatory Sky Survey plates and charts and more recently of the films of the ESO B Survey. Other thousands of QSOs and AGNs devoid of radio emission are found by the large Schmidts nowaday in operation. This wealth of data give fundamental cosmological knowledge and insight in the physical processes occuring in these objects. I'll concentrate in this Review on two specific topics, namely on the discovery techniques and on the study of the optical variability. To both subjects, the 67/92 cm Schmidt telescope here at Asiago has made significant contributions. The first topic is treated in several excellent papers, such as the one by M. Smith (1978) and the one by P. Veron (1983); the material presented in the second part is largely new. In the following, I'll use rather loosely the terms QSOs and AGNs to designate a variety of objects including Quasars (those QSOs in catalogs of Radio-Sources), high-redshift compact galaxies with emission lines, BL LACset similia.

Optical identifications of radio sources using accurate radio and optical positions.

The first results of a program of optical identification of several thousands of radio sources based on the sole criterion of coincidence of accurate radio and optical positions are presented. Radio positions of ~1 arcsec accuracy for 605 radio sources have already been published by Douglas, Bash, Ghiho, Moseley, and Torrence (1973), who give further details of the overall program. Optical positions a re given for 181 objects near some of these and other radio sources, measured with typical rms accuracy of 0.5 arcsec. Of 145 objects within 10 arcsec of the radio positions measured by Douglas et al., 81 are within 3 arcsec, including 38 spectroscopically confirmed QSOs, 15 additional blue stellar objects, and 15 galaxies. Finding charts are given for 43 objects which are within 3 arcsec of the radio positions (5 arcsec for galaxies and blue stellar objects) and for which finding charts do not exist in the literature. The reliability of these new identifications, based on position coincidence alone, is approximately 95%. A number of previously suggested identifications have been rejected on the basis of the new and more accurate radio and optical positions. (auth)