A. L. Argon

Interstellar Hydroxyl Masers in the Galaxy. I. The VLA Survey

Interstellar OH masers are bright signposts for recently formed massive stars, and the maser emission can be used to study the kinematic and physical conditions of dense molecular material surrounding these stars. We present interferometric maps of 91 interstellar OH maser sources in one or both of the ground-state, main-line, 2Π3/2J = 3/2 OH transitions near 18 cm wavelength. The maps comprising this large, uniformly processed, survey have a spectral resolution of 0.14 km s-1 and an angular resolution of ≈15. We measured the absolute positions of the masers to an accuracy of ≈03 in the E-W direction and ≈05 in the N-S direction, except for those sources with declinations below about -30°, and relative positions of isolated OH maser spots within each source and OH transition to an accuracy of ≈001. This survey forms a nearly complete sample of interstellar OH masers that are stronger than 1 Jy in both right- and left-circular polarization in at least one of the ground-state OH transitions.

Toward a New Geometric Distance To the Active Galaxy NGC 4258. III. Final Results and The Hubble Constant

We report a new geometric maser distance estimate to the active galaxy NGC 4258. The data for the new model are maser line-of-sight (LOS) velocities and sky positions from 18 epochs of very long baseline interferometry observations, and LOS accelerations measured from a 10 yr monitoring program of the 22 GHz maser emission of NGC 4258. The new model includes both disk warping and confocal elliptical maser orbits with differential precession. The distance to NGC 4258 is 7.60 ± 0.17 ± 0.15 Mpc, a 3% uncertainty including formal fitting and systematic terms. The resulting Hubble constant, based on the use of the Cepheid variables in NGC 4258 to recalibrate the Cepheid distance scale, is H0 = 72.0 ± 3.0 km s −1 Mpc −1 .

Full-Polarization Observations of OH Masers in Massive Star-forming Regions. I. Data

We present full-polarization VLBA maps of the ground-state, main-line, 2Π3/2, J = 3/2 OH masers in 18 Galactic massive star-forming regions. This is the first large polarization survey of interstellar hydroxyl masers at VLBI resolution. A total of 184 Zeeman pairs are identified, and the corresponding magnetic field strengths are indicated. We also present spectra of the NH3 emission or absorption in these star-forming regions. Analysis of these data will be presented in a companion paper.

Toward a New Geometric Distance to the Active Galaxy NGC 4258. II. Centripetal Accelerations and Investigation of Spiral Structure

We report measurements of centripetal accelerations of maser spectral components of NGC 4258 for 51 epochs spanning 1994 to 2004. This is the second paper of a series, in which the goal is the determination of a new geometric maser distance to NGC 4258, accurate to possibly ~3%. We measure accelerations using a formal analysis method that involves simultaneous decomposition of maser spectra for all epochs into multiple, Gaussian components. Components are coupled between epochs by linear drifts (accelerations) from their centroid velocities at a reference epoch. For high-velocity emission, accelerations lie in the range –0.7 to +0.7 km s−1 yr−1, indicating an origin within 13° of the disk midline (the perpendicular to the line of sight [LOS] to the black hole). Comparison of the projected positions of high-velocity emission in VLBI images with those derived from acceleration data provides evidence that masers trace real gas dynamics. High-velocity emission accelerations do not support a model of trailing shocks associated with spiral arms in the disk. However, we find strengthened evidence for spatial periodicity in high-velocity emission, of wavelength 0.75 mas. This supports suggestions of spiral structure due to density waves in the nuclear accretion disk of an active galaxy. Accelerations of low-velocity (systemic) emission lie in the range 7.7 to 8.9 km s−1 yr−1, consistent with emission originating from a concavity where the thin, warped disk is tangent to the LOS. A trend in accelerations of low-velocity emission, as a function of Doppler velocity, may be associated with disk geometry and orientation or with the presence of spiral structure.

Interstellar Hydroxyl Masers in the Galaxy. II. Zeeman Pairs and the Galactic Magnetic Field

We have identified and classified Zeeman pairs in the survey by Argon, Reid, & Menten of massive star-forming regions with 18 cm (2Π3/2,J = 3/2) OH maser emission. We have found a total of more than 100 Zeeman pairs in more than 50 massive star-forming regions. The magnetic field deduced from the Zeeman splitting has allowed us to assign an overall line-of-sight magnetic field direction to many of the massive star-forming regions. Combining these data with other data sets obtained from OH Zeeman splitting, we have looked for correlations of magnetic field directions between star-forming regions scattered throughout the Galaxy. Our data do not support a uniform, Galactic-scale field direction, nor do we find any strong evidence of magnetic field correlations within spiral arms. However, our data suggest that in the solar neighborhood the magnetic field outside the solar circle is oriented clockwise as viewed from the north Galactic pole, while inside the solar circle it is oriented counterclockwise. This pattern, including the magnetic field reversal near the Sun, is in agreement with results obtained from pulsar rotation measures.

The Active Jet in NGC 4258 and Its Associated Shocks

We present images and spectra of the active jet and anomalous arms on subparsec through kiloparsec scales in the LINER/Seyfert galaxy NGC 4258 (M106). New VLBA and multicon—guration VLA images show that, on 0.3¨300 pc scales, the jet in projection aligns with (1) the spin axis of the underlying accre- tion disk and (2) two radio hot spots 24A S (840 pc) and 49A N (1.7 kpc) from the nucleus. Hubble Space T elescope WFPC2 (N II) j6583 ) Ha images locate interactions between the jet and the interstellar medium (ISM). The most prominent is a pair of emission line arcs whose apices face away from the galaxy nucleus and envelop the leading edges of the radio hot spots. Ground-based (WHT) spectra with 2 resolution con—rm that the gaseous kinematics and excitation of both arcs have the spatio-kinematic Ae structure expected for jet working surfaces with a shock velocity 350 ^ 100 km s~1. The north shock is oblique and may lie in a nuclear ionization cone. The south shock shows a detached, putative Mach disk. Models suggest that the S shock is a bow shock around a jet whose progress toward us through the galaxy ISM has stalled. This is notable because the inferred out—ow axis is misaligned by D65i (in three dimensions) with the spin axis of the accretion disk. Our emission line ratios and pro—les diagnose the physical properties of the shocks, possible Mach disk, and thence the jets. The shocks lie at one end of a swath of kinematically disturbed gas that reaches back to the previously recognized spiral ii anomalous arms, ˇˇ suggesting that they are linked dynamically by precession of the central engine; although claimed elsewhere to be bar shocks, the anomalous arms are probably a fossil record of chang- ing jet activity in NGC 4258. Our results imply that the jet has recently moved a long way out of the plane of the galaxy. A deep Taurus Tunable Filter Ha image shows that discrete strands in the anom- alous arms persist to galactocentric radii of at least 4@ ((8 kpc), indicating an ongoing ISM interaction. Subject headings: galaxies: individual (NGC 4258) ¨ galaxies: ISMgalaxies: jetsshock waves

A Class of Interstellar OH Masers Associated with Protostellar Outflows

Using the Very Large Array, we have detected weak OH maser emission near the Turner-Welch protostellar source in the W3 OH region. Unlike typical interstellar OH masers, which are associated with ultracompact H II regions, our measured positions and proper motions (from very long baseline interferometry) indicate that these OH masers are associated with a bipolar outflow traced by strong H2O masers. These OH masers may be part of a class of interstellar OH masers that are associated with very young stars that have yet to, or may never, create ultracompact H II regions. This class of OH masers appears to form near the edges of very dense material (within which H2O masers form), where total densities drop precipitously and interstellar UV radiation is sufficient to dissociate the H2O molecules. Observations of this class of OH masers may be an important way to probe the distribution of this important molecule in interstellar shocks at arcsecond resolution or better.

The IC 133 water vapor maser in the galaxy M33: A geometric distance

We report on the results of a 14 yr long VLBI study of proper motions in the IC 133 H2O maser source in the galaxy M33. The method of ordered motion parallax was used to model the three-dimensional structure and dynamics of IC 133 and to obtain a distance estimate, 800 ± 180 kpc. Our technique for determining the distance to M33 is independent of calibrations common to other distance indicators, such as Cepheid period-luminosity relations, and therefore provides an important check for previous distance determinations.

The Sub-PC Scale Accretion Disk of NGC 4258

Water megamasers have been found to trace parsec/sub-parsec, circumnuclear accretion disks in several AGN (e.g., Circinus, NGC 1068 & NGC 4258). High-spatial (0.5 mas) and velocity resolution (0.2 km s−1) VLBA imaging of the disks reveals thin, warped ‘pannekoeken (pancake)’-style structures as opposed to thick tori in the inner regions of the central engines (40 000 Rsch). In this contribution, I will describe some current investigations into the dynamical and physical attributes of the water maser disk in NGC 4258, as revealed by VLBA, VLA and Effelsberg monitoring over 8 years.

VLBI observations of the water vapor megamaser in IC10

We present VLBI synthesis observations of the water vapor megamaser in the south eastern region of IC10. We compare our spectrum with earlier single dish spectra and conclude that the maser is highly variable.