Abstract. We present the completed results of a high resolution radio imaging survey of all (∼200) low-luminosity active galactic nuclei (LLAGNs) and AGNs in the Palomar Spectroscopic Sample of all (∼488) bright northern galaxies. The high incidences of pc-scale radio nuclei, with implied brightness temperatures > ∼ 10 7 K, and sub-parsec jets argue for accreting black holes in > ∼ 50% of all LINERs and low-luminosity Seyferts; there is no evidence against all LLAGNs being mini-AGNs. The detected parsec-scale radio nuclei are preferentially found in massive ellipticals and in type 1 nuclei (i.e. nuclei with broad Hα emission). The radio luminosity function (RLF) of Palomar Sample LLAGNs and AGNs extends three orders of magnitude below, and is continuous with, that of "classical" AGNs. We find marginal evidence for a low-luminosity turnover in the RLF; nevertheless LLAGNs are responsible for a significant fraction of present day mass accretion. Adopting a model of a relativistic jet from Falcke & Biermann, we show that the accretion power output in LLAGNs is dominated by the kinetic power in the observed jets rather than the radiated bolometric luminosity. The Palomar LLAGNs and AGNs follow the same scaling between jet kinetic power and narrow line region (NLR) luminosity as the parsec to kilo-parsec jets in powerful radio galaxies. Eddington ratios l Edd (=L Emitted /L Eddington ) of ≤10 −1 −10 −5 are implied in jet models of the radio emission. We find evidence that, in analogy to Galactic black hole candidates, LINERs are in a "low/hard" state (gas poor nuclei, low Eddington ratio, ability to launch collimated jets) while low-luminosity Seyferts are in a "high" state (gas rich nuclei, higher Eddington ratio, less likely to launch collimated jets). In addition to dominating the radiated bolometric luminosity of the nucleus, the radio jets are energetically more significant than supernovae in the host galaxies, and are potentially able to deposit sufficient energy into the innermost parsecs to significantly slow the gas supply to the accretion disk.

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.

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The X-ray intensity of knot HST-1, 0.85 ′′ from the nucleus of the radio galaxy M87, has increased by more than a factor of 50 during the last 5 years. The optical increase is similar and our more limited radio data indicate a commensurate activity. We give the primary results of our Chandra X-ray Observatory monitoring program and consider some of the implications of this extreme variability in a relativistic jet. We find that the data support a 'modest beaming synchrotron' model as indicated in our earlier papers. Based on this model, the decay of the X-ray lightcurve appears to be dominated by light travel time across the emitting region of HST-1, rather than synchrotron loss timescales.

We report sub-arcsecond X-ray imaging spectroscopy of M87 and the core of the Virgo cluster with the Chandra X-ray Observatory. The X-ray morphology shows structure on arcsecond (∼ 100 pc) to ten arcminute (∼ 50 kpc) scales, the most prominent feature being an "arc" running from the east, across the central region of M87 and off to the southwest. A ridge in the radio map, ending in an "ear"-shaped structure, follows the arc to the east. Depressions in the X-ray surface brightness correspond to the inner radio lobes and there is no evidence of shock-heated gas surrounding them. There are also at least two approximately circular (centered near the nucleus) "edges" in the X-ray brightness distribution, the radii of which are slightly larger than the nuclear distances of the inner radio lobes and intermediate radio ridges, respectively. We speculate that these discontinuities may be spherical pulses or "fronts" driven by the same jet activity as is responsible for the radio structure; such pulses are found in recent numerical simulations. All these results provide good evidence that the nuclear activity affects the intra-cluster medium. We present a temperature map of the intra-cluster medium, and obtain the temperature, pressure and cooling time as a function of nuclear distance for the arcs and the ambient intra-cluster medium. We show that the gas in the arcs is cooler than, and probably over-pressured with respect to, the ambient intra-cluster medium. The metal abundances of the cooler gas in the arc are somewhat enhanced relative to the ambient intra-cluster medium, favoring a "buoyant plume" origin for the X-ray arc, in which ambient gas near the nucleus is entrained by buoyant radio plasma and carried to larger nuclear distances. The gas within the inner "front" (nuclear distance 3.5 kpc 45) is at least a two-temperature plasma, with the cool component at 1 keV. This cool region is concentrated to the north of the nucleus and is strongly correlated with the Hα + [NII] emission-line distribution.

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A survey for megamaser emission from 354 active galaxies has resulted in the detection of 10 H 2 O new sources, making 16 known altogether. The galaxies surveyed include a distance-limited sample (covering Seyferts and LINERs with recession velocities less than 7000 km s~1) and a magnitude-limited sample (covering Seyferts and LINERs withIn order to determine whether the m B ¹ 14.5). H 2 O-detected galaxies are "" typical ÏÏ active galactic nuclei (AGNs) or have special properties that facilitate the production of powerful masers, we have accumulated a database of physical, morphological, and spectroscopic properties of the observed galaxies. The most signiÐcant Ðnding is that megamasers are H 2 O detected only in Seyfert 2 and LINER galaxies, not Seyfert 1Ïs. This lack of detection in Seyfert 1Ïs indicates either that they do not have molecular gas in their nuclei with physical conditions appropriate to produce 1.3 cm masers or that the masers are beamed away from Earth, presumably in the plane H 2 O of the putative molecular torus that hides the Seyfert 1 nucleus in Seyfert 2Ïs. LINERs are detected at a similar rate to Seyfert 2Ïs, which constitutes a strong argument that at least some nuclear LINERs are AGNs rather than starbursts, since starbursts have not been detected as megamasers. We prefer-H 2 O entially detect emission from the nearer galaxies and from those that are apparently brighter at H 2 O mid-and far-infrared and centimeter radio wavelengths. There is also a possible trend for the H 2 Ogalaxies to be more intrinsically luminous in nuclear 6 cm radio emission than the undetected detected ones, though these data are incomplete. We Ðnd evidence that Seyfert 2Ïs with very high (N H [ 1024 cm~2) X-rayÈabsorbing columns of gas are more often detected as maser emitters than Seyfert 2Ïs H 2 O with lower columns. It may be that the probability of detecting maser emission in Seyfert galaxies H 2 O increases with increasing column of cool gas to the nucleus, from Seyfert 1Ïs through narrow-line X-ray galaxies to Seyfert 2Ïs.