James M. Moran

Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre

The cores of most galaxies are thought to harbour supermassive black holes, which power galactic nuclei by converting the gravitational energy of accreting matter into radiation. Sagittarius A* (Sgr A*), the compact source of radio, infrared and X-ray emission at the centre of the Milky Way, is the closest example of this phenomenon, with an estimated black hole mass that is 4,000,000 times that of the Sun. A long-standing astronomical goal is to resolve structures in the innermost accretion flow surrounding Sgr A*, where strong gravitational fields will distort the appearance of radiation emitted near the black hole. Radio observations at wavelengths of 3.5 mm and 7 mm have detected intrinsic structure in Sgr A*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering. Here we report observations at a wavelength of 1.3 mm that set a size of microarcseconds on the intrinsic diameter of Sgr A*. This is less than the expected apparent size of the event horizon of the presumed black hole, suggesting that the bulk of Sgr A* emission may not be centred on the black hole, but arises in the surrounding accretion flow.

The Submillimeter Array

The Submillimeter Array, a collaborative project of the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics, has begun operation on Mauna Kea in Hawaii. The array comprises a total of eight 6 m telescopes, which will cover the frequency range of 180-900 GHz. All eight telescopes have been deployed and are operational. First scientific results utilizing the three receiver bands at 230, 345, and 690 GHz have been obtained and are presented in the accompanying Letters.

A geometric distance to the galaxy NGC4258 from orbital motions in a nuclear gas disk

The accurate measurement of extragalactic distances is a central challenge of modern astronomy, being required for any realistic description of the age, geometry and fate of the Universe. The measurement of relative extragalactic distances has become fairly routine, but estimates of absolute distances are rare. In the vicinity of the Sun, direct geometric techniques for obtaining absolute distances, such as orbital parallax, are feasible, but such techniques have hitherto been difficult to apply to other galaxies. As a result, uncertainties in the expansion rate and age of the Universe are dominated by uncertainties in the absolute calibration of the extragalactic distance ladder. Here we report a geometric distance to the galaxy NGC4258, which we infer from the direct measurement of orbital motions in a disk of gas surrounding the nucleus of this galaxy. The distance so determined—7.2 ± 0.3 Mpc—is the most precise absolute extragalactic distance yet measured, and is likely to play an important role in future distance-scale calibrations.

A warped accretion disk and wide angle outflow in the inner parsec of the Circinus Galaxy

We present the first VLBI maps of H2O maser emission (λ1.3 cm) in the nucleus of the Circinus galaxy, constructed from data obtained with the Australia Telescope Long Baseline Array. The maser emission traces a warped, edge-on accretion disk between radii of 0.11 ± 0.02 and ~0.40 pc, as well as a wide-angle outflow that extends up to ~1 pc from the estimated disk center. The disk rotation is close to Keplerian (v ∝ r-0.5), the maximum detected rotation speed is 260 km s-1, and the inferred central mass is (1.7 ± 0.3) × 106 M☉. The outflowing masers are irregularly distributed above and below the disk, with relative outflow velocities up to ~±160 km s-1, projected along the line of sight. The flow probably originates closer than 0.1 pc to the central engine, possibly in an inward extension of the accretion disk, although there is only weak evidence of rotation in the outward-moving material. We observe that the warp of the disk appears to collimate the outflow and to fix the extent of the ionization cone observed on larger angular scales. This study provides the first direct evidence (i.e., through imaging) of dusty, high-density, molecular material in a nuclear outflow less than 1 pc from the central engine of a Seyfert galaxy, as well as the first graphic evidence that warped accretion disks can channel outflows and illumination patterns in active galactic nuclei. We speculate that the same arrangement, which in some ways obviates the need for a geometrically thick, dusty torus, may apply to other type 2 active galactic nuclei.

Jet-launching structure resolved near the supermassive black hole in m87

Black Hole Close-Up M87 is a giant elliptical galaxy about 55 million light-years away. Accretion of matter onto its central massive black hole is thought to power its relativistic jet. To probe structures on scales similar to that of the black holes event horizon, Doeleman et al. (p. 355, published online 27 September) observed the relativistic jet in M87 at a wavelength of 1.3 mm using the Event Horizon Telescope, a special purpose, very-long-baseline interferometry array consisting of four radio telescopes located in Arizona, California, and Hawaii. The analysis suggests that the accretion disk that powers the jet orbits in the same direction as the spin of the black hole. High-resolution observations of the jet in the galaxy M87 probe structures very close to the galaxy’s central black hole. Approximately 10% of active galactic nuclei exhibit relativistic jets, which are powered by the accretion of matter onto supermassive black holes. Although the measured width profiles of such jets on large scales agree with theories of magnetic collimation, the predicted structure on accretion disk scales at the jet launch point has not been detected. We report radio interferometry observations, at a wavelength of 1.3 millimeters, of the elliptical galaxy M87 that spatially resolve the base of the jet in this source. The derived size of 5.5 ± 0.4 Schwarzschild radii is significantly smaller than the innermost edge of a retrograde accretion disk, suggesting that the M87 jet is powered by an accretion disk in a prograde orbit around a spinning black hole.

The distance to the center of the Galaxy: H2O maser proper motions in sagittarius B2(N)

Onsala Space Observatory The distance to a star forming region can be determined by measuring the proper motions within H 2 0 maser clusters. If the motions of the maser spots are random, the distance can be determined by applying the technique known as statistical parallax. Alternatively, if organized motions are evident in the proper motions, one can model the source to estimate its the distance. Both methods rely on a comparison of the radial component of the motion (in km/s) and the proper motion on the plane of the sky (in milli-arcseconds/year).

An Unambiguous Detection of Faraday Rotation in Sagittarius A

The millimeter/submillimeter wavelength polarization of Sgr A* is known to be variable in both magnitude and position angle on timescales down to a few hours. The unstable polarization has prevented measurements made at different frequencies and different epochs from yielding convincing measurements of Faraday rotation in this source. Here we present observations made with the Submillimeter Array polarimeter at 227 and 343 GHz with sufficient sensitivity to determine the rotation measure at each band without comparing position angles measured at separate epochs. We find the 10-epoch mean rotation measure to be (-5.6 ± 0.7) × 105 rad m-2; the measurements are consistent with a constant value. We conservatively assign a 3 σ upper limit of 2 × 105 rad m-2 to rotation measure changes, which limits accretion rate fluctuations to 25%. This rotation measure detection limits the accretion rate to less than 2 × 10-7 M☉ yr-1 if the magnetic field is near equipartition, ordered, and largely radial, while a lower limit of 2 × 10-9 M☉ yr-1 holds even for a subequipartition, disordered, or toroidal field. The mean intrinsic position angle is 167° ± 7° and we detect variations of 31 deg. These variations must originate in the submillimeter photosphere, rather than arising from rotation measure changes.

Interferometric measurements of variable 340 GHz linear polarization in sagittarius A

Using the Submillimeter Array, we have made the first high angular resolution measurements of the linear polarization of Sagittarius A* at submillimeter wavelengths and the first detection of intraday variability in its linear polarization. We detected linear polarization at 340 GHz (880 μm) at several epochs. At the typical resolution of 14 × 22, the expected contamination from the surrounding (partially polarized) dust emission is negligible. We found that both the polarization fraction and the position angle are variable, with the polarization fraction dropping from 8.5% to 2.3% over 3 days. This is the first significant measurement of variability in the linear polarization fraction in this source. We also found variability in the polarization and total intensity within single nights, although the relationship between the two is not clear from these data. The simultaneous 332 and 342 GHz position angles are the same, setting a 1 σ rotation measure (RM) upper limit of 7 × 105 rad m-2. From position angle variations and comparison of quiescent position angles observed here and at 230 GHz, we infer that the RM is a few times 105 rad m-2, a factor of a few below our direct detection limit. A generalized model of the RM produced in the accretion flow suggests that the accretion rate at small radii must be low, below 10-6-10-7 M☉ yr-1 depending on the radial density and temperature profiles, but in all cases below the gas capture rate inferred from X-ray observations.

A disk of dust and molecular gas around a high-mass protostar

The processes leading to the birth of low-mass stars such as our Sun have been well studied, but the formation of high-mass (over eight times the Suns mass, M[circdot]) stars remains poorly understood. Recent studies suggest that high-mass stars may form through accretion of material from a circumstellar disk, in essentially the same way as low-mass stars form, rather than through the merging of several low-mass stars. There is as yet, however, no conclusive evidence. Here we report the presence of a flattened disk-like structure around a massive 15M[circdot] protostar in the Cepheus A region, based on observations of continuum emission from the dust and line emission from the molecular gas. The disk has a radius of about 330 astronomical units (au) and a mass of 1 to 8 M[circdot]. It is oriented perpendicular to, and spatially coincident with, the central embedded powerful bipolar radio jet, just as is the case with low-mass stars, from which we conclude that high-mass stars can form through accretion.

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 .