We present sensitive, multifrequency Very Long Baseline Array (VLBA) images of the nuclear radio sources of NGC 1068. At 5 and 8.4 GHz, the radio continuum source S1, argued to mark the location of the hidden active nucleus, resolves into an elongated, $0.8 pc source oriented nearly at right angles to the radio jet axis but more closely aligned to the distribution of the nuclear H 2 O maser spots. S1 is detected at 5 GHz but not at 1.4 GHz, indicating strong free-free absorption below 5 GHz, and it has a flat spectrum between 5 and 8.4 GHz. A 5-8.4 GHz spectral index map reveals an unresolved, inverted spectrum source at the center of the S1 structure that may mark the AGN proper. The average brightness temperature is too low for synchrotron self-absorption to impact the integrated spectrum significantly. In addition, a careful registration with the nuclear H 2 O masers argues that the S1 continuum source arises from the inner regions of the maser disk rather than a radio jet. The emission mechanism may be direct, thermal free-free emission from an X-ray-heated corona or wind arising from the molecular disk. We demonstrate that the hidden active nucleus is sufficiently luminous, to within the current estimates, to provide the requisite heating. The radio jet components C and S2 both show evidence for free-free absorption of a compact, steep-spectrum source. The free-free absorption might arise from a shock cocoon enveloping the compact radio sources. The presence of H 2 O masers specifically at component C supports the interpretation for the presence of a jet-ISM interaction. Component NE remains a steep-spectrum source on VLBA baselines and appears to be a local enhancement of the synchrotron emissivity of the radio jet. The reason for the enhancement is not clear; the region surrounding component NE is virtually devoid of narrow-line region filaments, and so there is no clear evidence for interaction with the surrounding ISM. Component NE might instead arise in an internal shock or perhaps in denser jet plasma that broke away from an earlier interaction with the circumnuclear ISM.

We report resolution, interferometric observations of the 1.3 mm CO J \ 2 ] 1 line in the 0A .5 ] 0A .9 infrared luminous galactic merger NGC 6240. About half of the CO Ñux is concentrated in a rotating but highly turbulent, thick disk structure centered between the two radio and near-infrared nuclei. A number of gas features connect this D500 pc diameter central disk to larger scales. Throughout this region the molecular gas has local velocity widths which exceed 300 km s~1 FWHM and even reach FWZP line widths of 1000 km s~1 in a number of directions. The mass of the central gas concentration constitutes a signiÐcant fraction of the dynamical mass, pc) D(2È4) ] 109 M gas (R ¹ 470 M _ D We conclude that NGC 6240 is in an earlier merging stage than the prototypical ultralu-(0.3È0.7)M dyn . minous galaxy, Arp 220. The interstellar gas in NGC 6240 is in the process of settling between the two progenitor stellar nuclei, is dissipating rapidly, and will likely form a central thin disk. In the next merger stage, NGC 6240 may well experience a major starburst like that observed in Arp 220.

Seyfert galaxies commonly host compact jets spanning 10-100 pc scales, but larger structures are resolved out in long-baseline aperture synthesis surveys. Previous, targeted studies showed that kiloparsec-scale radio structures ( KSRs) may be a common feature of Seyfert and LINER galaxies, and the origin of KSRs may be starbursts or active galactic nuclei (AGNs). We report a new Very Large Array survey of a complete sample of Seyfert and LINER galaxies. Out of all of the surveyed radio-quiet sources, we find that 44% (19 out of 43) show extended radio structures at least 1 kpc in total extent that do not match the morphology of the disk or its associated star-forming regions. The detection rate is a lower limit owing to the combined effects of projection and resolution. The infrared colors of the KSR host galaxies are unremarkable compared to other Seyfert galaxies, and the large-scale outflows orient randomly with respect to the host galaxy axes. The KSR Seyfert galaxies instead stand out by deviating significantly from the far-infrared-radio correlation for star-forming galaxies, with tendency toward radio excess, and they are more likely to have a relatively luminous, compact radio source in the nucleus; these results argue that KSRs are powered by the AGNs rather than starbursts. The high detection rate indicates that Seyfert galaxies generate radio outflows over a significant fraction of their lifetime, which is much longer than the dynamical timescale of an AGNpowered jet but is comparable instead to the buoyancy timescale. The likely explanation is that the KSRs originate from jet plasma that has been decelerated by interaction with the nuclear interstellar medium ( ISM ). Based on a simple ram pressure argument, the kinetic power of the jet on kiloparsec scales is about 3 orders of magnitude weaker than the power of the jet on 10-100 pc scales. This result is consistent with the interaction model, in which case virtually all of the jet power must be lost to the ISM within the inner kiloparsec.

The VLA has been used at 3.6 and 20 cm to image a sample of about 50 early-type Seyfert galaxies with recessional velocities less than 7,000 km s −1 and total visual magnitude less than 14.5. Emission-line ([OIII] and Hα+[NII]) and continuum (green and red) imaging of this sample has been presented in a previous paper. In this paper, we present the radio results, discuss statistical relationships between the radio and other properties and investigate these relationships within the context of unified models of Seyferts. The mean radio luminosities of early-type Seyfert 1's (i.e. Seyfert 1.0's, 1.2's and 1.5's) and Seyfert 2.0's are found to be similar (consistent with the unified scheme) and the radio luminosity is independent of morphological type within this sample. The fraction of resolved radio sources is larger in the Seyfert 2.0's (93%) than in the Seyfert 1's (64%). However, the mean radio extents of Seyfert 2.0's and Seyfert 1's are not significantly different, though this result is limited by the small number of resolved Seyfert 1's.The nuclear radio structures of Seyfert 2.0's in the early-type sample tend to be aligned with the [OIII] and Hα+[NII] structures even though the radio extents are smaller than the [OIII] and Hα+[NII] extents by a factor of ∼ 2 ->5. This alignment, previously known for individual Seyferts with 'linear' radio sources, is here shown to be characteristic of early-type Seyfert galaxies as a -2class. Seyfert 2.0's in the early-type sample also show a significant alignment between the emission-line ([OIII] and Hα+[NII]) axes and the major axis of the host galaxy. These alignments are consistent with a picture in which the ionized gas represents ambient gas predominantly co-planar with the galaxy stellar disk. This ambient gas is ionized by nuclear radiation that may escape preferentially along and around the radio axis, and is compressed in shocks driven by the radio ejecta. We use this alignment to constrain the product of the velocity of the radio ejecta and the period of any large angle precession of the inner accretion disk and jet : V ejecta × P ≥ 2 kpc.An investigation of a larger sample of Seyferts reveals the unexpected result that the Seyfert 1's with the largest radio extent (≥ 1.5 kpc) are all of type Seyfert 1.2. It appears that classification as this type of intermediate Seyfert depends on some factor other than the relative orientation of the nuclear obscuring torus to the line of sight. Among all the other Seyferts, the distribution of radio extent with Seyfert intermediate type is consistent with the expectations of the unified scheme.

We are conducting a large observing program with the Spitzer Space Telescope to determine the mid-to-far infrared spectral energy distributions of a well-defined sample of 87 nearby, 12 µm-selected Seyfert galaxies. In this paper we present the results of IRS low-resolution spectroscopy of a statistically representative subsample of 51 of the galaxies (59%), with an analysis of the continuum shapes and a comparison of the Seyfert types. We find that the spectra clearly divide into groups based on their continuum shapes and spectral features. The largest group (47% of the sample of 51) shows very red continuum suggestive of cool dust and strong emission features attributed to PAHs. Sixteen objects (31%) have a power-law continuum with spectral indices α 5−20 µm =-2.3 --0.9 that flatten to α 20−35 µm =-1.1 -0.0 at ∼20 µm. Clear silicate emission features at 10 and 18 µm are found in two of these objects (Mrk 6 and Mrk 335). A further 16% of the sample show power-law continua with unchanging slopes of α 5−35 µm =-1.7 --1.1. Two objects are dominated by a broad silicate absorption feature. One object in the sample shows an unusual spectrum dominated by emission features, that is unlike any of the other spectra. Some spectral features are clearly related to a starburst contribution to the IR spectrum, while the mechanisms producing observed power-law continuum shapes, attributed to an AGN component, may be dust or non-thermal emission. The infrared spectral types appear to be related to the Seyfert types. Principal component analysis results suggest that the relative contribution of starburst emission may be the dominant cause of variance in the observed spectra. The derived starburst component of each spectrum, however, contributes <40% of the total flux density. We compare the IR emission with the optically thin radio emission associated with the AGN and find that Sy 1's have higher ratios of IR/radio emission than Sy 2's, as predicted by the unified model if the torus is optically thick in the mid-IR. However, smooth-density torus models predict a much larger difference between type 1's and 2's than the factor of 2 difference observed in our sample; the observed factor of ∼2 difference between the type 1's and 2's in their IR/radio ratios above 15 µm requires the standard smooth-density torus models to be optically thin at these wavelengths. However, the resulting low torus opacity requires that the high observed columns detected in X-ray absorption be produced in gas with very low dust to gas ratio (perhaps within the dust sublimation region). On the other hand, our observations may be consistent with clumpy torus models containing a steep radial distribution of optically thick dense clumps. The selection of our sample at 12 µm, where the torus may be optically thick, implies that there may be orientation-dependent biases in the sample, however we do not find that the sample is biased towards Sy 2's with more luminous central engines as would be expected. We find that the Sy 2's typically show stronger starburst c...

Sensitive high angular and linear resolution radio images of the 240-pc radio jet in NGC 4151, imaged at linear resolutions of 0.3 to 2.6 pc using the VLBA and phased VLA at λ21 cm, are presented and reveal for the first time a faint, highly collimated jet (diameter ∼ <1.4 pc) underlying discrete components, seen in lower 1 Royal Society University Research Fellow resolution MERLIN and VLA images, that appear to be shock-like features associated with changes in direction as the jet interacts with small gas clouds within the central ∼100 pc of the galaxy. In addition, λ21-cm spectral line imaging of the neutral hydrogen in the nuclear region reveals the spatial location, distribution and kinematics of the neutral gas detected previously in a lower resolution MER-LIN study. Neutral hydrogen absorption is detected against component C4W (E+F) as predicted by Mundell et al, but the absorption, extending over 3 pc, is spatially and kinematically complex on sub-parsec scales, suggesting the presence of small, dense gas clouds with a wide range of velocities and column densities. The main absorption component matches that detected in the MERLIN study, close to the systemic velocity (998 km s −1 ) of the galaxy, and is consistent with absorption through a clumpy neutral gas layer in the putative obscuring torus, with higher velocity blue-and red-shifted systems with narrow linewidths also detected across E+F. In this region, average column densities are high, lying in the range 2.7 × 10 19 T S < N H < 1.7 × 10 20 T S cm −2 K −1 (T S is the spin temperature), with average radial velocities in the range 920 < V r < 1050 km s −1 . The spatial location and distribution of the absorbing gas across component E+F rules out component E as the location of the AGN (as suggested by Ulvestad et al.) and, in combination with the well-collimated continuum structures seen in component D, suggests that component D (possibly subcomponent D3) is the most likely location for the AGN. We suggest that components C and E are shocks produced in the jet as the plasma encounters, and is deviated by, dense clouds with diameters smaller than ∼1.4 pc.Comparison of the radio jet structure and the distribution and kinematics of ionized gas in the narrow line region (NLR) suggests that shock-excitation by passage of the radio jet is not the dominant excitation mechanism for the NLR. We therefore favour nuclear photoionization to explain the structure of the NLR, although it is interesting to note that a small number of clouds with low velocity and high velocity dispersion are seen to bound the jet, particularly at positions of jet direction changes, suggesting that some NLR clouds are responsible for bending the jet. Alternatively, compression by a cocoon around the radio jet due to pressure stratification in the jet bow shock could explain the bright, compressed optical line-emitting clouds surrounding the cloud-free channel of the radio jet, as modelled by Steffen et al.

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We report new (1995) Very Large Array observations and (1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999) Effelsberg 100m monitoring observations of the 22 GHz H 2 O maser spectrum of the Seyfert 2 galaxy NGC 1068. The sensitive VLA observations provide a registration of the 22 GHz continuum emission and the location of the maser spots with an accuracy of ∼ 5 mas. Within the monitoring data, we find evidence that the nuclear masers vary coherently on time-scales of months to years, much more rapidly than the dynamical time-scale. We argue that the nuclear masers are responding in reverberation to a central power source, presumably the central engine. Between October and November 1997, we detected a simultaneous flare of the blue-shifted and red-shifted satellite maser lines. Reverberation in a rotating disk naturally explains the simultaneous flaring. There is also evidence that near-infrared emission from dust grains associated with the maser disk also responds to the central engine. We present a model in which an X-ray flare results in both the loss of maser signal in 1990 and the peak of the near-infrared lightcurve in 1994. In support of rotating disk geometry for the nuclear masers, we find no evidence for centripetal accelerations of the redshifted nuclear masers; the limits are ±0.006 km s −1 year −1 , implying that the masers are located within 2 • of the kinematic line-of-nodes. We also searched for high velocity maser emission like that observed in NGC 4258. In both VLA and Effelsberg spectra, we detect no high velocity lines between ∼ ±350 km s −1 -±850 km s −1 relative to systemic, arguing that masers only lie outside a radius of ∼ 0.6 pc (1.9 light-years) from the central engine (assuming a distance of 14.4 Mpc). We also consider possible models for the jet masers near radio continuum component C. We favor a shock-precursor model, in which the molecular gas surrounding the jet is heated by X-ray emission from a shock front between the jet and a molecular cloud.