We have used the Very Large Array (VLA) to conduct a survey for radio continuum emission in the sample of 52 Seyfert nuclei selected from the optical spectroscopic galaxy catalog of Ho, Filippenko, and Sargent. This Seyfert sample is the most complete and least biased available, and, as such, it will be useful for a variety of statistical analyses. Here we present the observations, measurements, and an atlas of radio maps.The observations were made at 6 cm in the B-array and at 20 cm in the A-array, yielding matched angular resolutions of ∼1 ′′ . We detected 44 objects (85%) at 6 cm and 37 objects (71%) at 20 cm above a 3 σ threshold of 0.12 mJy beam −1 . The sources have a wide range of radio powers (P ≈ 10 18 -10 25 W Hz −1 ), spectral indices (α 20 6 ≈ +0.5 to -1), and linear sizes (L ≈ few tens pc -15 kpc). The morphology of the radio emission is predominantly that of a compact core, either unresolved or slightly resolved, occasionally accompanied by elongated, jet-like features. Linearly-polarized emission was detected at 6 cm in 12 sources, 9 of which were also detected at 20 cm.

We have assembled an 8.4 GHz survey of bright, flat-spectrum ( > À0:5) radio sources with nearly uniform extragalactic (jbj > 10 ) coverage for sources brighter than S 4:8 GHz ¼ 65 mJy. The catalog is assembled from existing observations (especially CLASS and the Wright et al. PMN-CA survey), augmented by reprocessing of archival VLA and ATCA data and by new observations to fill in coverage gaps. We refer to this program as CRATES, the Combined Radio All-Sky Targeted Eight GHz Survey. The resulting catalog provides precise positions, subarcsecond structures, and spectral indices for some 11,000 sources. We describe the morphology and spectral index distribution of the sample and comment on the survey's power to select several classes of interesting sources, especially high-energy blazars. Comparison of CRATES with other high-frequency surveys also provides unique opportunities for identification of high-power radio sources.

Abstract. This paper presents the results of a high resolution radio imaging survey of all known (96) low-luminosity active galactic nuclei (LLAGNs) at D ≤ 19 Mpc. We first report new 2 cm (150 mas resolution using the VLA) and 6 cm (2 mas resolution using the VLBA) radio observations of the previously unobserved nuclei in our samples and then present results on the complete survey. We find that almost half of all LINERs and low-luminosity Seyferts have flat-spectrum radio cores when observed at 150 mas resolution. Higher (2 mas) resolution observations of a flux-limited subsample have provided a 100% (16 of 16) detection rate of pc-scale radio cores, with implied brightness temperatures > ∼ 10 8 K. The five LLAGNs with the highest core radio fluxes also have pc-scale "jets". Compact radio cores are almost exclusively found in massive ellipticals and in type 1 nuclei (i.e. nuclei with broad Hα emission). Only a few "transition" nuclei have compact radio cores; those detected in the radio have optical emission-line diagnostic ratios close to those of LINERs/Seyferts. This indicates that some transition nuclei are truly composite Seyfert/LINER+H II region nuclei, with the radio core power depending on the strength of the former component. The core radio power is correlated with the nuclear optical "broad" Hα luminosity, the nuclear optical "narrow" emission-line luminosity and width, and with the galaxy luminosity. In these correlations LLAGNs fall close to the low-luminosity extrapolations of more powerful AGNs. The scalings suggest that many of the radio-non-detected LLAGNs are simply lower power versions of the radio-detected LLAGNs. The ratio of core radio power to nuclear optical emissionline luminosity increases with increasing bulge luminosity for all LLAGNs. Also, there is evidence that the luminosity of the disk component of the galaxy is correlated with the nuclear emission-line luminosity (but not the core radio power). About half of all LLAGNs with multiple epoch data show significant inter-year radio variability. Investigation of a sample of ∼150 nearby bright galaxies, most of them LLAGNs, shows that the nuclear (≤150 mas size) radio power is strongly correlated with both the black hole mass and the galaxy bulge luminosity; linear regression fits to all ∼150 galaxies give: log P 2 cm = 1.31(±0.16) log M MDO + 8.77 and log P 2 cm = 1.89(±0.21) log L B (bulge) − 0.17. Low accretion rates (≤10 −2 −10 −3 of the Eddington rate) are implied in both advection-and jet-type models. In brief, all evidence points towards the presence of accreting massive black holes in a large fraction, perhaps all, of LLAGNs, with the nuclear radio emission originating in either the accretion inflow onto the massive black hole or from jets launched by this black hole-accretion disk system.

The edge-on starburst galaxy NGC 253 has been imaged with the VLA at wavelengths ranging from 1.3 to 20 cm, with resolutions between 1 and 15 pc. These images reveal a large number of compact radio sources embedded within the di †use radio structure in the inner 200 pc of the galaxy. We have identiÐed approximately 64 individual compact radio sources in the galaxy. Of the strongest 17 sources, for which the Ñux densities are high enough to measure spectral indices with moderately low errors, about half have Ñat spectra and half have steep spectra ; this indicates that perhaps half of the individual radio sources are dominated by thermal radio emission from H II regions. Over an 8 yr timescale, few, if any, of the strongest sources have varied in Ñux density, with limits of 1%È2% yr~1 on the rate of decrease. At the highest resolution of 1 pc, a number of radio sources are beginning to be resolved, and at least 15 di †erent sources can be identiÐed within the innermost 20 pc of the galaxy.Individual radio sources have been explored in more detail. The strongest source, 5.79[39.0 (TH2), assumed to be at the nucleus of the galaxy, has a brightness temperature greater than 20,000 K at 22 GHz and greater than 40,000 K at 15 GHz. It is unresolved at the VLA and may be either an active galactic nucleus or a very compact (nonvariable) supernova remnant. A resolved Ñat-spectrum source, 5.72[40.1 (TH6), is located D20 pc to the southwest. It has an apparent size of 2.4 ] 1.2 pc and appears to be an H II region similar to the inner part of 30 Doradus, containing approximately 105 M _ in stars, as well as D600 of ionized gas. Source counts, the lack of variability, and the lack of new M _ sources imply that the radio supernova rate is no more than 0.3 yr~1, consistent with estimates made in other wave bands.