A catalogue is presented based on the radio positions of 436 extragalactic sources distributed over the entire sky. The positional accuracy of the sources is better than 3 milliarcsec (mas) in both coordinates, with the majority of the sources having errors better than 1mas. This catalogue is based upon a general solution of all applicable dual frequency 2.3 and 8.4GHz Mark-III VLBI data available through the end of 1993 consisting of 1,015,292 pairs of group delay and phase delay rate observations. Details and positions are also given for an additional 124 objects that either need further observation or are currently unsuitable for the definition of a reference frame. The final orientation of the catalogue has been obtained by a rotation of the positions into the system of the International Earth Rotation Service and is consistent with the FK5 J2000.0 optical system, within the limits of the link accuracy.
We survey four nearby irregular galaxies for radio supernova remnants (SNRs) using deep (1{sigma}~20uJy), high-resolution (~20pc) Very Large Array continuum data at 20, 6, and 3.6cm. We identify discrete sources in these galaxies and use radio spectral indices and H{alpha} images to categorize them as SNRs, HII regions, or background radio galaxies. Our classifications are generally in good agreement with the literature. We identify a total of 43 SNR candidates: 23 in NGC 1569, seven in NGC 4214, five in NGC 2366, and eight in NGC 4449. Only one SNR - the well-studied object J1228+441 in NGC 4449 - is more luminous at 20cm than Cas A. By comparing the total thermal flux density in each galaxy with that localized in HII regions, we conclude that a significant fraction must be in a diffuse component or in low-luminosity HII regions.
Information on physical characteristics of astrometric radio sources, such as magnitude and redshift, is of great importance for many astronomical studies. However, data usually used in radio astrometry is often incomplete and outdated. Our purpose is to study the optical characteristics of more than 4000 radio sources observed by the astrometric VLBI technique since 1979. We also studied the effect of the asymmetry in the distribution of the reference radio sources on the correlation matrices between vector spherical harmonics of the first and second degrees. The radio source characteristics were mainly taken from the NASA/IPAC Extragalactic Database (NED). Characteristics of the gravitational lenses were checked with the CfA-Arizona Space Telescope LEns Survey. SIMBAD and HyperLeda databases were also used to clarify the characteristics of some objects. Also we simulated and investigated a list of 4000 radio sources evenly distributed around the celestial sphere. We estimated the correlation matrices between the vector spherical harmonics using the real as well as modelled distribution of the radio sources.
Positions and flux densities at 750 and 1400MHz of 726 radio sources, measured with the 300-foot telescope, are given. Most of the positions are accurate to about +/-30" in each coordinate.
A complete sample of core-dominated radio sources has been studied using the interplanetary-scintillation method. In total, 72 sources were observed, with scintillations detected in 28 of them. The remaining sources have upper limits on their flux densities. Integrated flux densities are estimated for 24 sources. Cut-offs have been observed in the spectra of many sources. The thermal-electron densities have been estimated, assuming that these cut-offs are due to free-free absorption of the synchrotron radio emission.
A complete sample of radio sources has been studied using the interplanetary scintillation method. In total, 32 sources were observed, with scintillations detected in 12 of them. The remaining sources have upper limits for the flux densities of their compact components. Integrated flux densities are estimated for 18 sources.
The results of 0.97, 2.3, 3.9, 7.7, 11.1 and 21.7GHz observations of a complete sample of radio sources obtained on the RATAN-600 radio telescope are presented. The sample is comprised of sources from the 4.85-GHz MGB survey, and contains all sources at declinations 10-12.5 degrees (J2000) with galactic latitudes |b|>15 degrees and flux densities S(4.85)>200mJy. Optical identifications have been obtained for about 86% of the radio sources with flat spectra and 59% of those with steep spectra.
The Abell clusters 2125 and 2645 have different radio source populations, despite being very similar in richness (Abell class 4) and redshift (0.25). The number density of radio sources in Abell 2125 is almost an order of magnitude more than that in Abell 2645, based on observations to the same optical and radio luminosities of the two clusters. About 30% of the radio sources in Abell 2125 shows signs of star formation, with the largest concentration of them in the southwest clump 2Mpc from the cluster center.
We present a VLA C-configuration imaging survey of 14 rich Abell clusters. The observations were undertaken with the intent of characterizing the galaxy orbits within the cluster by combining galaxy redshifts with indicators of orbital shape. We present maps of the observed clusters, detailed maps of resolved sources detected in the survey, comparisons with optical images of the clusters, and tables of source parameters for all detected sources with measured flux values.
Statistically complete samples comprising 33 bright spiral galaxies that are strong radio sources were selected. Sixteen of the galaxies have integrated radio-to-optical flux ratios greater than 10 times the median value for normal spirals. The remainder contain radio cores <=20" in size and stronger than 90mJy at 1415MHz or 60mJy at 2695MHz. High-resolution (2" at 1413MHz or 0.6" at 4885MHz) maps of these galaxies were made with the Very Large Array. The radio sources are usually confined to the central 1kpc of the galaxies, lying within and extending parallel to their stellar disks. There is no evidence for ejection from central components. Most of the sources appear to be coextensive with regions of intense star formation (bolometric luminosities ~10^10-10^11L_{Sun}). The typical radio luminosity, ~10^21^W/Hz/sr at 1413MHz, of the resolved sources can be explained by synchrotron radiation from supernova remnants (SNRs) produced at the rate of ~1/yr. Five of the radio sources are <=1pc in size and probably are not related to star formation or SNRs. The radio and 10-{mu}m flux densities of the extended sources are roughly proportional. The bolometric mass-to luminosity ratios in some active regions are too low to have been maintained for a Hubble time. Nearly all of the strong, extended radio sources are found in galaxies with nearby companions, so most of the episodic bursts of star formation are apparently triggered by galaxy-galaxy interactions.
