James S. Ulvestad

CRATES: An All-Sky Survey of Flat-Spectrum Radio Sources

We have assembled an 8.4 GHz survey of bright, flat-spectrum ({alpha} > -0.5) radio sources with nearly uniform extragalactic (|b| > 10{sup o}) coverage for sources brighter than S{sub 4.8 GHz} = 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, sub-arcsecond structures, and spectral indices for some 11,000 sources. We describe the morphology and spectral index distribution of the sample and comment on the surveys 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.

Jet Directions in Seyfert Galaxies

Here we present the study of the relative angle between the accretion disk (or radio jet) and the galaxy disk for a sample of Seyfert galaxies selected from a mostly isotropic property, the 60 μm flux, and warm infrared colors. We used VLA A-array 3.6 cm continuum data and ground-based optical imaging, homogeneously observed and reduced to minimize selection effects. For parts of the analysis we enlarged the sample by including galaxies serendipitously selected from the literature. For each galaxy we have a pair of points (i, δ), which are the inclination of the galaxy relative to the line of sight and the angle between the jet projected into the plane of the sky and the host galaxy major axis, respectively. For some galaxies we also had information about which side of the minor axis is closer to Earth. This data is combined with a statistical technique, developed by us, to determine the distribution of β angles in three dimensions, the angle between the jet and the host galaxy plane axis. We found from an initial analysis of the data of the 60 μm sample, where Seyfert 1 and 2 galaxies were not differentiated, that the observed distribution of i and δ values can be well represented either by a homogeneous sin β distribution in the range 0° ≤ β ≤ 90° or in 0° ≤ β ≤ 65°, but not by an equatorial ring. A more general model, which tested β-distributions in the range β1 ≤ β ≤ β2, for different ranges of β1 and β2 values, required β2 to be larger than 65° and gave preference for β1 smaller than 40°-50°. An important result from our analysis was obtained when we determined whether the jet was projected against the near or the far side of the galaxy and differentiated between Seyfert 1 and Seyfert 2 galaxies, which showed that the model could not represent Seyfert 1 galaxies adequately. We found that the inclusion of viewing angle restrictions for Seyfert 1 galaxies, namely, that a galaxy can be recognized as a Seyfert 1 only if the angle between the jet and the line of sight (||) is smaller than a given angle c and that the galaxy inclination i is smaller than an angle ic, gave rise to statistically acceptable models. This indication that there is a difference in viewing angle to the central engine between Seyfert 1 galaxies and Seyfert 2 galaxies is a direct and independent confirmation of the underlying concepts of the unified model. We discuss possible explanations for the misalignment between the accretion disk and the host galaxy disk: warping of the accretion disk by self-irradiation instability, by the Bardeen-Petterson effect, or by a misaligned gravitational potential of a nuclear star cluster surrounding the black hole, as well as feeding of the accretion disk by a misaligned inflow of gas from minor mergers, capture of individual stars or gas from the nuclear star cluster, and the capture of individual molecular clouds from the host galaxy.

Radio structures of Seyfert galaxies. VII - Extension of a distance-limited sample

The VLA has been used at 6 and 20 cm to observe 27 Seyfert galaxies with recessional velocities less than 4600 km/s that had not been mapped previously. The sample shows weak trends for Seyfert 2 galaxies to have more luminous and larger radio sources than Seyfert 1 galaxies. A 20 cm radio luminosity function is constructed for each Seyfert type and shown to be fairly flat for powers betwen 10 to the 20th and 10 to the 23rd W/Hz. About 10 percent of the galaxies in the present sample may have flat-spectrum components contributing a substantial amount of their total flux density at centimeter wavelengths. 39 refs.

Radio Sources in Low-Luminosity Active Galactic Nuclei. II. Very Long Baseline Interferometry Detections of Compact Radio Cores and Jets in a Sample of LINERs

We have used the VLBA at 5 GHz to observe all galaxies with nuclear radio flux densities above 3.5 mJy found in a VLA survey at 15 GHz of a sample of nearby LINER galaxies. All galaxies were detected revealing high brightness temperature (Tb ∼ 10 8 K) radio sources. Free-free emission is unlikely since it greatly overpredicts the soft X-ray luminosities. We infer the presence of AGN-like, non-thermal radio emission most likely powered by under-fed black holes. Together with our VLA sample we estimate from our observations that at least half of LINER galaxies host genuine AGN. We find no evidence for highly inverted radio cores as predicted in the ADAF model: the (non-simultaneous) spectral indices are on average around α = 0.0. In the two brightest sources we detect some extended emission, which appears to originate in jets in at least one of these galaxies. Together with the spectral indices this suggests that the nuclear emission at centimeter radio waves is largely dominated by emission from radio jets, very similar to the situation in more luminous AGN. The energy released in these jets could be a significant fraction of the energy budget in the accretion flow. Subject headings: galaxies: active — galaxies: jets — galaxies: nuclei — galaxies: structure — galaxies: Seyferts — radio continuum: galaxies

Testing the Unified Model with an Infrared-selected Sample of Seyfert Galaxies

We present a series of statistical tests done to a sample of 29 Seyfert 1 and 59 Seyfert 2 galaxies selected from mostly isotropic properties, their far-infrared fluxes and warm infrared colors. Such selection criteria provide a profound advantage over the criteria used by most investigators in the past, such as ultraviolet excess. These tests were done using ground-based high-resolution Very Large Array A-configuration 3.6 cm radio and optical B and I imaging data. From the relative number of Seyfert 1s and Seyfert 2s, we calculate that the torus half-opening angle is 48°. We show that, as seen in previous papers, there is a lack of edge-on Seyfert 1 galaxies, suggesting that dust and gas along the host galaxy disk probably play an important role in hiding some nuclei from direct view. We find that there is no statistically significant difference in the distribution of host galaxy morphological types and radio luminosities of Seyfert 1s and Seyfert 2s, suggesting that previous results showing the opposite may have been due to selection effects. The average extension of the radio emission of Seyfert 1s is smaller than that of Seyfert 2s by a factor of ~2-3, as predicted by the unified model. A search for galaxies around our Seyferts allows us to put a lower and an upper limit on the possible number of companions around these galaxies of 19% and 28%, respectively, with no significant difference in the number of companion galaxies between Seyfert 1s and Seyfert 2s. We also show that there is no preference for the radio jets to be aligned closer to the host galaxy disk axis in late-type Seyferts, unlike results claimed by previous papers. These results, taken together, provide strong support for a unified model in which type 2 Seyferts contain a torus seen more edge-on than the torus in type 1 Seyferts.

A SUBKILOPARSEC DISK IN MARKARIAN 231

We present imaging with the Very Long Baseline Array of the neutral hydrogen 21 cm absorption-line system seen toward the nuclear regions of Mrk 231 at z☉ = 0.04217, and imaging of the radio continuum emission at 1.4 GHz on scales ranging from a few parsecs to a few hundred parsecs. These data indicate the existence of a subkiloparsec gas disk in Mrk 231, as seen in H I 21 cm absorption and in radio continuum emission. The radio continuum morphology is consistent with a disk of maximum radius of 440 mas (260 h-1 pc), at an inclination angle of 45°, with a major axis oriented east-west. The H I 21 cm absorption shows an east-west gradient in position and velocity of about ±110 km s-1 out to radii of 100 mas (60 h-1 pc). We identify this H I and radio continuum disk as the inner part of the molecular disk seen on a factor of 3 larger scale. The physical conditions for the thermal and nonthermal gas in the subkiloparsec disk of Mrk 231 are similar to those proposed for compact nuclear starburst galaxies and, in particular, to the conditions proposed for the subkiloparsec gas disk in Arp 220. From the neutral hydrogen velocity field we derive a gravitational mass enclosed within a 50 h-1 pc radius of 3 × 108 h-1 M☉, and from the radio continuum emission we derive a massive star formation rate in the disk of 60 M☉ yr-1. We also present a search for H I 21 cm absorption associated with the optical broad absorption line (BAL) systems toward Mrk 231. We do not detect H I 21 cm absorption associated with any of the optical BAL systems. These negative results require that the neutral atomic gas in the BAL clouds be fairly warm (spin temperature Ts > 50 K), unless the Na I abundance is higher than solar, or the dust-to-gas ratio is higher than Galactic, or the observed extinction toward the nucleus of Mrk 231 is not due to the BAL gas.

Statistical Properties of Radio Emission from the Palomar Seyfert Galaxies

We have carried out an analysis of the radio and optical properties of a statistical sample of 45 Seyfert galaxies from the Palomar spectroscopic survey of nearby galaxies. We find that the space density of bright galaxies (-22 mag ≤ M ≤ -18 mag) showing Seyfert activity is (1.25 ± 0.38) × 10-3 Mpc-3, considerably higher than found in other Seyfert samples. Host galaxy types, radio spectra, and radio source sizes are uncorrelated with Seyfert type, as predicted by the unified schemes for active galaxies. Approximately half of the detected galaxies have flat or inverted radio spectra, more than expected based on previous samples. Surprisingly, Seyfert 1 galaxies are found to have somewhat stronger radio sources than Seyfert 2 galaxies at 6 and 20 cm, particularly among the galaxies with the weakest nuclear activity. We suggest that this difference can be accommodated in the unified schemes if a minimum level of Seyfert activity is required for a radio source to emerge from the vicinity of the active nucleus. Below this level, Seyfert radio sources might be suppressed by free-free absorption associated with the nuclear torus or a compact narrow-line region, thus accounting for both the weakness of the radio emission and the preponderance of flat spectra. Alternatively, the flat spectra and weak radio sources might indicate that the weak active nuclei are fed by advection-dominated accretion disks.

The Radio Quiescence of Active Galaxies with High Accretion Rates

We present 6 cm Very Large Array observations of the Greene & Ho sample of 19 low-mass active galaxies with high accretion rates. This is one of the only studies of a uniform sample of narrow-line Seyfert 1 (NLS1) galaxies with such high sensitivity and resolution. Although we detect only one source, the entire sample is very radio quiet down to strong limits. GH 10 was found to have a radio power of 8.5 × 1021 W Hz-1 and a ratio R ≡ f6 cm/f4400 A of 2.8. The 3 σ upper limits for the remaining nondetections correspond to radio powers from 3 × 1020 to 8 × 1021 W Hz-1 and 0.47 < R < 9.9. Stacking all nondetections yields an even stronger upper limit of R ≤ 0.27. An assessment of existing observations in the literature confirms our finding that NLS1s are consistently radio-quiet, with a radio-loud fraction of 0%-6%, which is significantly lower than the 10%-20% observed in the general quasar population. By analogy with stellar mass black holes, we argue that AGNs undergo a state transition at Lbol/LEdd ≈ 0.01. Below this value a radiatively inefficient accretion flow effectively drives an outflow, which disappears when the flow turns into an optically thick, geometrically thin disk, or a radiation pressure-dominated slim disk at still higher Lbol/LEdd.

VLBA imaging of central engines in radio-quiet quasars

We have used the Very Long Baseline Array (VLBA) to image five radio-quiet quasars (RQQs) at milliarcsecond resolution, at frequencies between 1.4 and 5 GHz. These quasars have typical total flux densities of a few millijanskys at gigahertz frequencies and are compact on arcsecond scales. The VLBA images reveal that four of the quasars are dominated by unresolved radio cores, while the fifth has an apparent two-sided jet. Typical core brightness temperatures range from 108 to at least 109 K. The compact radio morphologies and X-ray luminosities of many objects in the RQQ sample seem to indicate classical accretion onto black holes as massive as 109 M☉, with emission physics in many ways similar to their radio-loud counterparts. Therefore, the relatively small amount of radiative energy emerging at radio wavelengths in the RQQs may simply be due to the presence of less powerful radio jets.

LOW-LUMINOSITY ACTIVE GALACTIC NUCLEI AT THE HIGHEST RESOLUTION: JETS OR ACCRETION FLOWS?

Six low-luminosity active galactic nuclei have been imaged at multiple frequencies from 1.7--43 GHz (2.3--15 GHz for three of the galaxies) using the Very Long Baseline Array. In spite of dynamic ranges of about 100 in several frequency bands, all six galaxies remain unresolved, with size limits at 8.4 GHz of 10^3--10^4 times the Schwarzschild radii of the black holes inferred at their galactic centers. The galaxy spectra are roughly flat from 1.7 to 43 GHz, rather than steepening to classical optically thin synchrotron spectra at high frequencies. Although the spectral slopes somewhat resemble predictions for advection-dominated accretion flows, the luminosities are too high for the black hole masses of the galaxies and the slight spectral steepening at high frequencies cannot be explained by standard simple models of such accretion flows. In contrast, compact jets can accommodate the average spectral index, the relatively high radio luminosity, and the unresolved appearance, but only if the jets in all six galaxies are fairly close to our line of sight. This constraint is in agreement with inclination angle predictions for five of the six AGNs based on the dusty torus unification model.