A. Pedlar

Compact protoplanetary disks around the stars of a young binary system

Planet formation is believed to occur in the disks of gas and dust that surround young solar-type stars. Most stars, however, form in multiple systems, where the presence of a close companion could affect the structure of the disk and perhaps interfere with planet formation. It has been difficult to investigate this because of the resolution needed. Here we report interferometric observations (at a wavelength of 7 mm) of the core of the star-forming region L1551. We have achieved a linear resolution of seven astronomical units (less than the diameter of Jupiters orbit). The core of L1551 contains two distinct disks, with a separation of 45 AU; these appear to be associated with a binary system. Both disks are spatially resolved, with semi-major axes of about 10 AU, which is about a factor of ten smaller than disks around isolated stars. The disk masses are of order 0.05 solar masses, which could be enough to form planetary systems like our own.

H 2O and OH Masers as Probes of the Obscuring Torus in NGC 1068

Original article can be found at: http://adsabs.harvard.edu/abs/ Copyright American Astronomical Society. DOI: 10.1086/177187 [Full text of this article is not available in the UHRA]

Kiloparsec-scale radio emission in Seyfert galaxies : evidence for starburst-driven superwinds ?

We report the results of a pilot program to search for kiloparsec-scale radio emission in 13 Seyfert galaxies, using the Westerbork Synthesis Radio Telescope. While it has been known for some time now that Seyfert galaxies generally contain nuclear, subkiloparsec-scale radio sources, this is the first systematic attempt to image larger scale radio emission in Seyfert galaxies. These observations reveal the presence of kiloparsecscale, diffuse, sometimes bubble-like radio emission in 12 of these sources. This large-scale, extra-nuclear radio emission is randomly oriented with respect to the small-scale nuclear radio source axis but tends to align with the minor axis of the host galaxy disk

A Survey of Kiloparsec-Scale Radio Outflows in Radio-Quiet Active Galactic Nuclei

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 AGN-powered 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.

High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample — I. The observations

We present 8.4 GHz VLA A-configuration observations of 87 sources from the mid-infrared-selected AGN sample of Rush et al (1993). These 0.25 arcsec resolution observations allow elongated radio structures tens of parsecs in size to be resolved and enable radio components smaller than 3.5 arcsec to be isolated from diffuse galactic disc emission. When combined with previous data, matched radio observations covering ninety percent of the sample have been collected and these represent the largest sub-arcsecond-resolution radio imaging survey of a homogeneously-selected sample of Seyfert galaxies to date. We use our observations to identify 5 radio-loud AGN in the sample. The nature of the radio emission from Seyfert nuclei will be discussed in subsequent papers.

THE NUCLEAR REGIONS OF THE SEYFERT GALAXY NGC 4151: PARSEC-SCALE H i ABSORPTION AND A REMARKABLE RADIO JET

Sensitive high angular and linear resolution radio images of the 240 pc radio jet in NGC 4151, imaged at linear resolutions of 0.3-2.6 pc using the VLBA and phased VLA at ?21 cm, are presented and reveal for the first time a faint, highly collimated jet (diameter 1.4 pc) underlying discrete components, seen in lower resolution MERLIN and VLA images, that appear to be shocklike features associated with changes in direction as the jet interacts with small gas clouds within the central ~100 pc of the galaxy. In addition, ?21 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 MERLIN 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 subparsec 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 redshifted systems with narrow line widths also detected across E+F. In this region, average column densities are high, lying in the range 2.7 ? 1019TS < NH < 1.7 ? 1020TS cm-2 K-1 (TS is the spin temperature), with average radial velocities in the range 920 < Vr < 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 active galactic nucleus (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 favor 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 modeled by Steffen et al.

Radio lobes and forbidden O III profile substructure in Seyfert galaxies

High spatial and spectral resolution observations are presented for 10 Seyfert galaxies in the H-beta-forbidden O III 5007 A spectral region. In most of the objects, there is evidence for forbidden O III profile substructure, which appears to be most conspicuous close to the location of a radio lobe. The relative intensities of forbidden O III component emission and ambient forbidden O III emission vary greatly from object to object. A control sample shows little evidence for spatially resolved forbidden O III profile substructure. The forbidden O III components usually have high excitation. The component velocities can fall well outside the maximum galactic rotation amplitude and, in some cases, are opposite to the sense of rotation. This strongly suggests that the component gas does not rotate with the rest of the galactic gas but is instead undergoing systematic outflow. 47 references.

Neutral Hydrogen (21 Centimeter) Absorption in Seyfert Galaxies: Evidence for Free-Free Absorption and Subkiloparsec Gaseous Disks

Active galaxies are thought to be both fueled and obscured by neutral gas removed from the host galaxy and funneled into a central accretion disk. We performed a VLA imaging survey of 21 cm absorption in Seyfert and starburst nuclei to study the neutral gas in the near-nuclear environment. With the exception of NGC 4151, the absorbing gas traces 100 pc-scale, rotating disks aligned with the outer galaxy disk. These disks appear to be rich in atomic gas relative to nuclear disks in nonactive spirals. We find no strong evidence for rapid infall or outflow of neutral hydrogen, but our limits on the mass infall rates are compatible with that required to feed a Seyfert nucleus. Among the galaxies surveyed here, neutral hydrogen absorption traces parsec-scale gas only in NGC 4151. Based on the kinematics of the absorption line, the disk symmetry axis appears to align with the radio jet axis rather than the outer galaxy axis. The most surprising result is that we detect no 21 cm absorption toward the central radio sources of the hidden Seyfert 1 nuclei Mrk 3, Mrk 348, and NGC 1068. Moreover, 21 cm absorption is commonly observed toward extended radio jet structure but appears to avoid central, compact radio sources in Seyfert nuclei. To explain these results, we propose that 21 cm absorption toward the nucleus is suppressed by either free-free absorption, excitation effects (i.e., enhanced spin temperature), or rapid motion in the obscuring gas. Ironically, the implications of these effects is that the obscuring disks must be small, typically not larger than a few tens of parsecs.

Deep MERLIN 5 GHz radio imaging of supernova remnants in the M82 starburst

The results of an extremely deep, 8-d long observation of the central kpc of the nearby starburst galaxy M82 using Multi-Element Radio Linked Interferometer Network (MERLIN) at 5 GHz are presented. The 17 mu Jy beam(-1) rms noise level in the naturally weighted image makes it the most sensitive high-resolution radio image of M82 made to date. Over 50 discrete sources are detected, the majority of which are supernova remnants, but with 13 identified as H II regions. Sizes, flux densities and radio brightnesses are given for all of the detected sources, which are all well resolved with a majority showing shell or partial shell structures. Those sources within the sample which are supernova remnants have diameters ranging from 0.3 to 6.7 pc, with a mean size of 2.9 pc. From a comparison with previous MERLIN 5-GHz observations made in 1992 July, which gives a 9.75-yr timeline, it has been possible to measure the expansion velocities of 10 of the more compact sources, eight of which have not been measured before. These derived expansion velocities range between 2200 and 10 500 km s(-1).

Nuclear Gasdynamics in Arp 220: Subkiloparsec-Scale Atomic Hydrogen Disks

We present new, high angular resolution (~022) MERLIN observations of neutral hydrogen (H I) absorption and λ21 cm radio continuum emission across the central ~900 pc of the ultraluminous infrared galaxy Arp 220. Spatially resolved H I absorption is detected against the morphologically complex and extended λ21 cm radio continuum emission, consistent with two counterrotating disks of neutral hydrogen, with a small bridge of gas connecting the two. Column densities across the two nuclei are high, lying in the range 8 × 1019Ts(K) NH 2.4 × 1020Ts(K) cm-2 (Ts is spin temperature) and corresponding to optical extinctions of 0.052Ts(K) AV 0.155Ts(K) mag, with the higher column densities in the eastern nucleus. Velocity gradients are clearly visible across each nucleus, reaching 1010 ± 20 km s-1 kpc-1 in P.A. ~ 55° and 830 ± 20 km s-1 kpc-1 in P.A. ~ 270° for eastern and western nuclei, respectively. These gradients imply dynamical masses MD = 1.1 × 109( i) (E) and 1.7 × 108( i) M☉ (W), assuming that the neutral gas is distributed in two thin circularly rotating disks. We propose a merger model in which the two nuclei represent the galaxy cores that have survived the initial encounter and are now in the final stages of merging, similar to conclusions drawn from previous CO studies. However, we suggest that instead of being coplanar with the main CO disk (in which the eastern nucleus is embedded), the western nucleus lies above it and, as suggested by the bridge of H I connecting the two nuclei, will soon complete its final merger with the main disk. We suggest that the collection of radio supernovae (RSNe) detected in VLBA studies in the more compact western nucleus represents the second burst of star formation associated with this final merger stage and that free-free absorption due to ionized gas in the bulgelike component can account for the observed RSN distribution.