Mas Resolution Research Articles

High Sensitivity Array Observations of thez= 4.4 QSO BRI 1335-0417

We present sensitive phase-referenced VLBI results on the radio continuum emission from the z = 4.4 QSO BRI 1335-0417. The observations were carried out at 1.4 GHz using the High Sensitivity Array. Our sensitive VLBI image at 189 × 113 mas (1.25 × 0.75 kpc) resolution shows continuum emission in BRI 1335-0417 with a total flux density of 208 ± 46 μJy, consistent with the flux density measured with the VLA. The size of the source at FWHM is 255 × 138 mas (1.7 × 0.9 kpc), and the derived intrinsic brightness temperature is ∼3.5 × 104 K. No continuum emission is detected at the full VLBI resolution (32 × 7 mas, 211 × 46 pc), with a 4 σ point-source upper limit of 34 μJy beam-1, or an upper limit to the intrinsic brightness temperature of 5.6 × 105 K. The highest angular resolution with at least a 4.5 σ detection of the radio continuum emission is 53 × 27 mas (0.35 × 0.18 kpc). At this resolution the image shows a continuum feature in BRI 1335-0417 with a size of 64 × 35 mas (0.42 × 0.23 kpc) at FWHM and an intrinsic brightness temperature of ∼2 × 105 K. The extent of the observed continuum sources at 1.4 GHz and the derived brightness temperatures show that the radio emission (and thus, presumably, the far-IR emission) in BRI 1335-0417 is powered by a major starburst with a massive star formation rate of order a few thousand M⊙ yr-1. Moreover, the absence of any compact high brightness temperature source suggests that there is no radio-loud AGN in this z = 4.4 QSO.

Limits to the abundance of surface CO2 ice on Titan

In the context of the Cassini/Huygens mission, we performed supporting ground‐based observations to complement the results from the NASA/ESA/ASI space mission to the Saturnian system with particular focus on Titan. On the nights of 18 and 19 December 2004, we conducted adaptive optics observations with VLT/NACO to search for and map the distribution of CO2 ice deposits on the spatially resolved surface of Titan (65 mas resolution). This experiment became possible because (1) solid CO2 possesses two narrow and strong absorption lines at 2012 nm and 2070 nm that fall into the 2.05 μm window of Titan's atmosphere and (2) the width of these bands matches the band pass of the Fabry‐Perot instrument installed in NACO. We do not detect this chemical compound, but we can derive firm limits on the abundance of CO2 ice on the surface of Titan at sub‐Earth longitudes 284°W and 307°W. With a spatial sampling of 65 mas, we conclude that a partial surface coverage of segregated CO2 ice does not exceed 7% or 14% for bright or dark surface regions, respectively.

A new analysis of the nucleus of NGC 1068 with MIDI observations

We present a new analysis of the first mid-infrared N-band long-baseline interferometric observations of an extragalactic source: the nucleus of the Seyfert 2 galaxy NGC 1068, obtained with MIDI (Mid-InfrareD Interferometer), the mid-infrared beamcombiner at the European Southern Observatory (ESO) Very Large Telescope Interferometer (VLTI). The resolution of mas allows us to study the compact central core of the galaxy between 8 and 13 μm. Both visibility measurements and MIDI spectrum are well reproduced by a simple radiative transfer model with two concentric spherical components. The derived angular sizes and temperatures are ~35 and 83 mas, and ~361 K and 226 K for these two components respectively. Other evidence strongly supports such low temperatures. This modeling also provides the variation of optical depth as a function of wavelength for the extended component across the N-band suggesting the presence of amorphous silicate grains. This shows that MIDI has carried out the first direct observations of the distribution of dust around the central engine.

PEGASE: a DARWIN/TPF pathfinder

The space mission PEGASE, proposed to the CNES (Centre National d'Etudes Spa- tiales = French Space Agency) in the framework of its call for scientific proposals : flying missions, is a 2-aperture interferometer, composed by 3 free flying satellites (2 siderostats and 1 beam combiner), allowing baselines from 50 to 500 m in both nulling and visibility modes. With an angular resolution of a few mas and a spectral resolution of several tens in the spectral range 2.5-5 microns, PEGASE has several goals: - science : spectroscopy of hot jupiters (Pegasides) and brown dwarves, exploration of the inner part of protoplanetary disks - technology : validation in real space conditions of formation flying, nulling and visibility inter- ferometry concepts. PEGASE has been studied at a 0-level. In this paper, we summarize the scientific program and associated technological and mission trade-off coming from this 0-level study. We also discuss how PEGASE can be considered as a TPF/DARWIN pathfinder in an international roadmap towards more complex space interferometry missions such as DARWIN/TPF.

Faint Radio Sources in the NOAO Boötes Field: VLBA Imaging and Optical Identifications

As a step toward investigating the parsec-scale properties of faint extragalactic radio sources, the Very Long Baseline Array (VLBA) was used at 5.0 GHz to obtain phase-referenced images of 76 sources in the NOAO Bootes field. These 76 sources were selected from the Faint Images of the Radio Sky at Twenty cm (FIRST) catalog to have peak flux densities above 10 mJy at 5'' resolution and deconvolved major diameters of less than 3'' at 1.4 GHz. Of these faint radio sources, 57 were identified with accretion-powered radio galaxies and quasars brighter than 25.5 mag in the optical I band. On Very Large Array (VLA) scales at 1.4 GHz, a measure of the compactness of the faint sources (the ratio of the peak flux density from FIRST to the integrated flux density from the NRAO VLA Sky Survey catalog) spans the full range of possibilities arising from source-resolution effects. Of the faint radio sources, 30, or 39%, were detected with the VLBA at 5.0 GHz with peak flux densities above 6 σ ~ 2 mJy at 2 mas resolution. The VLBA detections occur through the full range of compactness ratios. The stronger VLBA detections can themselves serve as phase-reference calibrators, boding well for opening up much of the radio sky to VLBA imaging. For the adopted cosmology, the VLBA resolution corresponds to 17 pc or finer. Most VLBA detections are unresolved or slightly resolved, but one is diffuse and five show either double or core-jet structures; the properties of these latter six are discussed in detail. Three VLBA detections are unidentified and fainter than 25.5 mag in the optical I band; their properties are highlighted because they likely mark optically obscured active nuclei at high redshift.

SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light‐Month

We report on 75 mas resolution, near-IR imaging spectroscopy within the central 30 lt-days of the Galactic center, taken with the new adaptive optics-assisted integral-field spectrometer SINFONI on the ESO VLT. To a limiting magnitude of K ~ 16, 9 of 10 stars in the central 04, and 13 of 17 stars out to 07 from the central black hole have spectral properties of B0-B9 main-sequence stars. Based on the 2.1127 μm He I line width, all brighter early-type stars have normal rotation velocities, similar to solar neighborhood stars. We combine the new radial velocities with SHARP/NACO astrometry to derive improved three-dimensional stellar orbits for six of these "S stars" in the central 05. Their orientations in space appear random. Their orbital planes are not co-aligned with those of the two disks of massive young stars 1''-10'' from Sgr A*. We can thus exclude the hypothesis that the S stars as a group inhabit the inner regions of these disks. They also cannot have been located/formed in these disks and then migrated inward within their planes. From the combination of their normal rotation and random orbital orientations, we conclude that the S stars were most likely brought into the central light-month by strong individual scattering events. The updated estimate of distance to the Galactic center from the S2 orbit fit is R0 = 7.62 ± 0.32 kpc, resulting in a central mass value of (3.61 ± 0.32) × 106 M☉. We happened to catch two smaller flaring events from Sgr A* during our spectral observations. The 1.7-2.45 μm spectral energy distributions of these flares are fit by a featureless, "red" power law of spectral index α' = -4 ± 1 (Sν ~ ν). The observed spectral slope is in good agreement with synchrotron models in which the infrared emission comes from accelerated, nonthermal, high-energy electrons in a radiatively inefficient accretion flow in the central R ~ 10RS region.

Diffraction-limited bispectrum speckle interferometry of the nuclear region of the Seyfert galaxy NGC 1068 in theHandK'bands

We present near-infrared bispectrum speckle interferometry studies of the nuclear region of the Seyfert 2 galaxy NGC 1068. A diffraction-limited K � -band image with 74 mas resolution and the first H-band image with 57 mas resolution were reconstructed from speckle interferograms obtained with the SAO 6 m telescope. The resolved structure consists of a compact core and an extended northern and south-eastern component. The compact core is resolved at all position angles and has a north-western, tail-shaped extension as well as a fainter, south-eastern extension. The K � -band FWHM diameter of this compact core is approximately 18 × 39 mas or 1.3 × 2.8 pc (FWHM of a single-component Gaussian fit; fit range 30-80% of the telescope cut-off frequency; the diameter errors are ±4 mas), and the position angle (PA) of the north-western extension is -16 ± 4 ◦ . If 40% of the flux from the compact Kcore is emission from a point source and 60% from a Gaussian intensity distribution, then a slightly larger FWHM of approximately 26 × 58 mas is obtained for the compact Kcomponent. In the H band, the FWHM diameter of the compact core is approximately 18 × 45 mas (±4 mas), and the PA is -18 ± 4 ◦ . The extended northern component (PA ∼ 0 ◦ ) has an elongated structure with a length of about 400 mas or 29 pc. The extended south-eastern component is fainter than the northern component. The K � -a ndH-band fluxes from the resolved compact core were measured to be 350 ± 90 mJy (i.e., K � ∼ 8.2 m ) and 70 ± 20 mJy (H ∼ 10.4 m ), respectively. The PA of -16 ± 4 ◦ of the compact 18 × 39 mas core is very similar to that of the western wall (PA ∼ -15 ◦ ) of the bright region of the ionization cone. This suggests that the H- and K � -band emission from the compact core is both thermal emission and scattered light from dust near the western wall of a low-density, conical cavity or from the innermost region of a parsec-scale dusty torus that is heated by the central source (the dust sublimation radius of NGC 1068 is approximately 0.1-1 pc). The northern extended 400 mas structure lies near the western wall of the ionization cone and coincides with the inner radio jet (PA ∼ 11 ◦ ). The large distance from the core suggests that the K � -band emission of the northern extended component is scattered light from the western cavity region and the radio jet region.

VLBA Astrometry of Compact Radio Sources in theSpitzerFirst-Look Survey

The NRAO Very Long Baseline Array (VLBA) was used at 1.4 GHz to image 20 compact sources in the inner 1° radius of the Spitzer VLA First-Look Survey. Twelve sources were detected with the VLBA, with peak flux densities above 2 mJy at 9 mas resolution. The positions of these VLBA detections have been measured in the International Celestial Reference Frame (ICRF) to 1 σ accuracies of 8 mas east-west and 4 mas north-south. These positions can help anchor ancillary data for the First-Look Survey to the ICRF and are being reported quickly to the community.

High‐Resolution Imaging of Dust Shells by Using Keck Aperture Masking and the IOTA Interferometer

We present first results of an experiment to combine data from Keck aperture masking and the Infrared-Optical Telescope Array to image the circumstellar environments of evolved stars with ~20 mas resolution. The unique combination of excellent Fourier coverage at short baselines and high-quality long-baseline fringe data allows us to determine the location and clumpiness of the innermost hot dust in the envelopes and to measure the diameters of the underlying stars themselves. We find evidence for large-scale inhomogeneities in some dust shells and also significant deviations from uniform brightness for the photospheres of the most evolved M stars. Deviations from spherically symmetric mass loss in the red supergiant NML Cyg could be related to recent evidence for dynamically important magnetic fields and/or stellar rotation. We point out that dust shell asymmetries, like those observed here, can qualitatively explain the difficulty recent workers have had in simultaneously fitting the broadband spectral energy distributions and high-resolution spatial information, without invoking unusual dust properties or multiple distinct shells (from hypothetical superwinds). This paper is the first to combine optical interferometry data from multiple facilities for imaging, and we discuss the challenges and potential for the future of this method, given current calibration and software limitations.

AGNs in LLAGNs: High resolution radio observations of the Palomar sample

We present completed results of a high resolution radio imaging survey of the Palomar Spectroscopic Sample of all (∼470) nearby bright northern galaxies. Almost half the Palomar sample's galaxies have nuclei with emission-lines characteristic of AGN but with L Hα ≤ 10 40 erg s −1 . These are referred to as low-luminosity AGNs or LLAGNs. The power source of such LLAGNs has been long debated. High resolution radio surveys of the sample – with the VLA at 15 GHz (150 mas resolution), and the VLBA at 5 GHz (2 mas resolution) – have now revealed a high incidence of pc-scale radio cores with implied brightness temperatures [gsim ]10 8 K, and sub-parsec scale jets. The results support the presence of accreting black holes in ≥50% of all LLAGNs; 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). These nuclei follow the usual correlations between radio and emission-line gas properties found in more powerful AGNs. The radio luminosity function (RLF) of Palomar Sample LLAGNs extends three orders of magnitude below, and is continuous with, that of ‘classical’ AGNs. We find marginal evidence for a low-power turnover in the RLF; nevertheless LLAGNs are responsible for a significant fraction of accretion in the local universe. Low accretion rates (≤10 −2 −10 −3 of the Eddington rate) are implied in both advection- and jet-type models. Within the context of jet models, the accretion energy output is dominated by the energy in the observed jets rather than the radiated bolometric luminosity. These jets would be able to dump sufficient energy into the innermost parsecs to significantly slow the accretion inflow. Detailed results can be found in Nagar et al. (2002, 2004) and references therein. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Sensitive VLBI Continuum and HiAbsorption Observations of NGC 7674: First Scientific Observations with the Combined Array VLBA, VLA, and Arecibo

We present phase-referenced VLBI observations of the radio continuum emission from, and the neutral hydrogen 21 cm absorption toward, the luminous infrared galaxy NGC 7674. The observations were carried out at 1380 MHz using the VLBA, the phased VLA, and the 305 m Arecibo radio telescope. These observations constitute the first scientific use of the Arecibo telescope in a VLBI observation with the VLBA. The high- and low-resolution radio continuum images reveal several new continuum structures in the nuclear region of this galaxy. At ~100 mas resolution, we distinguish six continuum structures extending over 14 (742 pc), with a total flux density of 138 mJy. Only three of these structures were known previously. All these structures seem to be related to active galactic nucleus (AGN) activity. The overall S-shaped pattern that the radio structures seem to form could be the result of the interstellar medium diverting the outcoming jets from the central AGN. However, we cannot rule out the possibility of a black hole merger that could result in a similar structural pattern. At the full resolution of the array (11 × 5 mas), we detect only two of the six continuum structures. Both are composed of several compact components with brightness temperatures on the order of 107 K. While it is possible that one of these compact structures could host an AGN, they could also be shocklike features formed by the interaction of the jet with compact interstellar clouds in the nuclear region of this galaxy. Complex H I absorption is detected with our VLBI array at both high and low angular resolution. Assuming that the widest H I feature is associated with a rotating H I disk or torus feeding a central AGN, we estimate an enclosed dynamical mass of ~7 × 107 M☉, comparable to the value derived from the hidden broad Hβ emission in this galaxy. The narrower H I lines could represent clumpy neutral hydrogen structures in the H I torus. The detection of H I absorption toward some of the continuum components and its absence toward others suggest an inclined H I disk or torus in the central region of NGC 7674. The overall averaged H I spectrum toward the continuum structures with H I absorption is very consistent with the Arecibo single-dish H I absorption spectrum at 33 resolution.

The AGN content of ultraluminous IR galaxies: High resolution VLA imaging of the IRAS 1 Jy ULIRG sample

This paper presents the results of a high resolution radio imaging survey of 83 of the 118 ultraluminous infrared galaxies (ULIRGs) in the IRAS 1 Jy ULIRG sample. We have observed these ULIRGs at 15 GHz with the Very Large Array (VLA). We find that ~75% of Seyferts (both type 1 and 2) and LINERs have radio nuclei which are compact at our 150 mas resolution. The detection rate of nuclei is significantly lower (32%); the detections among these are preferentially + LINER/Seyfert composite nuclei. Among ULIRGs with multiple optical or near-IR nuclei our observations detected only one (or no) nucleus; in these the radio detection is typically towards the brightest near-IR nucleus. The compactness of the radio sources, the higher detection rates in AGN-type nuclei than nuclei, the preferential detection of nuclei with unresolved point sources in the near-IR, the low soft X-ray to nuclear radio luminosity ratio (arguing against thermal emission powering the radio nuclei), and the lack of correlation between radio power and Hα luminosity, all support an origin of the detected radio nuclei in AGN related activity. This result is especially interesting for LINER ULIRGs for which signatures of AGNs have often been ambiguous in other wavebands. Such a high incidence of AGN would provide, for the first time, a large sample in which to study the interplay between AGN, starbursts, and galaxy mergers.

VLBI Imaging of Luminous Infrared Galaxies: Active Galactic Nucleus Cores in Markarian 231, UGC 5101, and NGC 7469

We report 18 cm VLBI continuum imaging observations at ~5 mas resolution for UGC 5101, NGC 7469, and Mrk 231, all part of a sample of luminous infrared galaxies that were shown to have strong VLBI radio cores in a previous 18 cm VLBI survey. Mrk 231, generally considered to be a dust-enshrouded quasi-stellar object (QSO), shares many characteristics with quasars, including a broad Sy1 optical emission spectrum, optical and radio variability, and a broad absorption line spectrum. NGC 7469 is a classical Sy1 galaxy, and UGC 5101 is a LINER system. The radio morphology of these three systems on VLBI scales is like that of an active galactic nucleus (AGN), with well-defined ridgelines and high-brightness yet spatially resolved components. The structure and flux densities of these VLBI components are not consistent with starburst-generated radio supernovae of the type found in Arp 220. On scales of ~100 pc and perhaps beyond, the radio continuum in all three objects appears to be dominated by an AGN, not a starburst. Radio emission on larger scales may well originate, in part or in total, in a less compact circumnuclear star-forming region. This is in contrast to the situation thought to prevail in many luminous infrared galaxies (LIGs), which involves a very compact and dense nuclear star-forming region. By placing a lower limit on the AGN-related radio emission, it is possible to shed light on the relative luminosities of the AGN and starburst in these objects. Using the far-infrared-radio correlation for starburst-related radio emission, we show that, despite the unambiguous evidence for AGN activity in these systems, the AGN luminosity, although energetically important, may be exceeded by the extended starburst luminosity in all three cases, unless the bolometric luminosity of the entire system has been underestimated. The lower limits on AGN-related radio emission indicate that these AGNs are somewhat overluminous in the radio compared with radio-quiet Palomar-Green (PG) QSOs and thus that the AGN radio luminosity cannot be used to estimate the bolometric luminosity of the AGN. However, comparing near- and mid-IR luminosities of the LIGs to those of PG QSOs, we find evidence that a significant fraction of the bolometric luminosity of the AGN in Mrk 231 and to a lesser extent UGC 5101 may be radiated away from Earth and thus not detected at any wavelength. The VLBI structure observed in Mrk 231 allows additional interpretation. Confirming and extending VLBI imaging by Ulvestad, Wrobel, and Carilli, our continuum image shows a triple structure, with a core and two lobes, causing it to be classied as a compact symmetric object. It has been suggested that these sources are young, ? 106 yr, with the hot spots representing the working surface of a relativistic jet on the ambient medium. If the southern (primary) lobe/hot-spot in Mrk 231 is confined by ram pressure, we estimate a lobe advance speed, va ~ 10-4c, and an age for the jet/compact source, ? < 106 yr. We have also imaged the 1667 MHz OH maser emission in Mrk 231, which is extended on scales of 50-100 mas (40-80 pc) and probably coincides with the inner region of the disk, which is seen in CO emission and H I absorption. Among OH megamaser sources studied at high sensitivity with milliarcsecond resolution, Mrk 231 is unique in the stringent upper limits placed on the flux density of compact OH structures of the type found in Arp 220 and other LIGs. It is possible that the circumnuclear environment of Mrk 231 has been sufficiently disrupted by the emergent QSO that the cool dense clouds necessary for such compact masers no longer exist.

Extrinsic Radio Variability of JVAS/CLASS Gravitational Lenses

We present flux ratio curves of the fold and cusp (i.e., close multiple) images of six Jodrell Bank VLA Astrometric Survey and Cosmic Lens All-Sky Survey (JVAS/CLASS) gravitational lens systems. The data were obtained over a period of 8.5 months in 2001 with the Multi Element Radio-Linked Interferometer Network (MERLIN) at 5 GHz with 50 mas resolution, as part of a MERLIN Key Project. Even though the time delays between the fold and cusp images are small (≲1 day) compared to the timescale of intrinsic source variability, all six lens systems show evidence that suggests the presence of extrinsic variability. In particular, the cusp images of B2045+265—regarded as the strongest case of the violation of the cusp relation (i.e., the sum of the magnifications of the three cusp images add to zero)—show extrinsic variations in their flux ratios up to ~40% peak to peak on timescales of several months. Its low Galactic latitude of b ≈ -10° and a line of sight toward the Cygnus superbubble region suggest that Galactic scintillation is the most likely cause. The cusp images of B1422+231 at b ≈ +69° do not show strong extrinsic variability. Galactic scintillation can therefore cause significant scatter in the cusp and fold relations of some radio lens systems (up to 10% rms), even though these relations remain violated when averaged over a ≲1 yr time baseline.

Faraday rotation in the CSS QSOs 3C 43 and 3C 454

We present observations of the CSS QSOs 3C 43 and 3C 454 made with the VLA, MERLIN and VLBA at 2, 6 and 18 cm to examine the Faraday rotation in these highly bent sources. 3C 43 is weakly polarized at 18 cm at mas resolution but has a large rotation measure near the bend in the jet. This result is interpreted as indicating jet-ISM interaction. 3C 454 is relatively strongly polarized and has high rotation measure along the jet visible in the high resolution 18 cm images. There is no brightening of the jet near the bend to the west indicating that it is not caused by a jet-cloud collision and may simply be a hydrodynamic instability magnified by a small angle to the line of sight. However, there are clear differences in the sign and magnitude of the Faraday rotation measured at long and short wavelengths. This is interpreted as being due to thermal plasma mixed with the jet. Likely detection of circular polarization at 18 cm supports this interpretation.

Unveiling the Launching Region of YSO Jets with AMBER

The interplay between accretion and ejection in the environment of young stellar objects (YSOs) is believed to be a crucial element in the star formation process. Since most of the properties of the models are set up in the first few AUs from the source (below the so-called Alfven surface), to validate and constrain the models observationally we need very high angular resolution. With HST (resolution ~ 0.″1, i.e. about 14 AU in Taurus) we have been able to access the external border of the acceleration region, for jets in the Taurus-Auriga cloud. Here we see an onion-like kinematic structure in the first 200 AU of the flow, and indications for rotation around the symmetry axis for the resolved low/moderate velocity component. We have now planned observations with AMBER on the VLTI to investigate at 1 mas resolution (in J) the core of the central engine, down to 0.1 AU from the source. Here we describe a joint project by several Institutes in the AMBER consortium dedicated to the study of the morphology and detailed kinematics of a few selected targets. On one hand we will use the large UV coverage of the ATs to explore at medium spectral resolution the structure of the flow. On the other hand the large collection area of the UTs combined with the high resolution mode (R = 10000) of AMBER will allow us to search for interesting kinematic features, among which signatures of rotation around the axis, that would constitute an important validation of the proposed models for the jet launching.

EVN observation of three Broad Absorption Line quasars

We present the results of European VLBI Network observations of 3 Broad Absorption Line quasars at 1.6 GHz. They are all compact with linear sizes less than 1 kpc. The flat spectral source, J1556+3517, is unresolved at about 20 mas resolution, indicating that the orientation of the radio jet in this source is probably near the line of sight. J1312+2319 shows asymmetric two-sided structure on a scale of several hundred pc. This structure resembles that of Compact Symmetric Objects (CSO), however it has a bright central component at the present resolution. The jet orientation of J1312+2319 may be far from the line of sight. J0957+2356 is also unresolved, but with a steep spectrum, therefore the jet orientation in this source is unclear. These results are not consistent with the unification of BAL and non-BAL quasars by orientation.

The Sizes of OH (1720 MHz) Supernova Remnant Masers: MERLIN and Very Long Baseline Array Observations of IC 443

MERLIN and Very Long Baseline Array (VLBA) observations of the 1720 MHz maser emission from the OH molecule in the supernova remnant IC 443 are presented. Based on MERLIN data with a resolution of 160 mas, the deconvolved sizes of the maser sources are in the range 90-180 mas (135-270 AU). The 12 mas resolution VLBA images show compact cores with sizes in the range of 15-55 mas. The maser brightness temperatures are (2-34) × 106 K for the MERLIN sources and (5-19) × 108 K for the VLBA cores, in agreement with theory. Unlike the Zeeman Stokes V profiles observed in other OH (1720 MHz) SNR masers, single-handed circular polarization line profiles are observed in IC 443 on all angular scales from 1000 to 10 mas resolution. For one line component, the observed line width is 0.24 ± 0.07 km s-1, compared to an estimated Doppler width of 0.49 km s-1. This discrepancy in line widths can be accounted for if the maser emission arises from an elongated ellipsoidal region of masing gas.

Radio sources in low-luminosity active galactic nuclei

This paper presents the results of a high resolution radio imaging survey of all known (96) low-luminosity active galactic nuclei (LLAGNs) at 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 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+ 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 emission-line 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: (±0.16) log MMDO + 8.77 and (±0.21(bulge. Low accretion rates (≤ 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.

Bispectrum speckle interferometry of the Red Rectangle: Diffraction-limited near-infrared images reconstructed from Keck telescope speckle data

We present new near-infrared (2.1-3.3μm) images of the Red Rectangle with unprecedented diffraction-limited angular resolutions of 46--68 mas; 4 times higher than that of the Hubble space telescope and almost a factor of two improvement over the previous 6 m SAO telecope speckle images presented by Men'shchikov et al. (1998). The new images, which were reconstructed from Keck telescope speckle data using the bispectrum speckle interferometry method, clearly show two bright lobes above and below the optically thick dark lane obscuring the central binary. X-shaped spikes, thought to trace the surface of a biconical flow, change the intensity distribution of the bright lobes, making them appear broadened or with an east-west double-peak in images with the highest resolution. The striking biconical appearance of the Red Rectangle is preserved on scales from 50 mas to 1' and from the visible (red) to at least 10 μm, implying that large grains of at least several microns in size dominate scattering. The new images supplement previous 76 mas resolution speckle reconstructions at shorter wavelengths of 0.6-0.8 μm (Osterbart et al. 1997) and 0.7-2.2 μm (Men'shchikov et al. 1998), allowing a more detailed analysis of the famous bipolar nebula. The intensity distribution of the images is inconsistent with a flat disk geometry frequently used to model the bipolar nebulae. Instead, a geometrically thick torus-like density distribution with bipolar conical cavities is preferred. The extent of the bright lobes indicates that the dense torus has a diameter of 2100 AU, for an assumed distance of 330 pc. This torus may be the outer reaches of a flared thick disk tapering inwards to the central star, however such a density enhancement on the midplane is not strictly required to explain the narrow dark lane obscuring the central stars.