R. L. Plambeck

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.

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.

Giant Molecular Clouds in M33. II. High-Resolution Observations

We present 12CO (J = 1 → 0) observations of 45 giant molecular clouds (GMCs) in M33 made with the BIMA array. The observations have a linear resolution of 20 pc, sufficient to measure the sizes of most GMCs in the sample. We place upper limits on the specific angular momentum of the GMCs and find the observed values to be nearly an order of magnitude below the values predicted from simple formation mechanisms. The velocity gradients across neighboring, high-mass GMCs appear preferentially aligned on scales less than 500 pc. If the clouds are rotating, 40% are counterrotating with respect to the galaxy. GMCs require a braking mechanism if they form from the large-scale radial accumulation of gas. These observations suggest that molecular clouds form locally out of atomic gas, with significant braking by magnetic fields to dissipate the angular momentum imparted by galactic shear. The observed GMCs share basic properties with those found in the Galaxy, such as similar masses, sizes, and line widths, as well as a constant surface density of 120 M☉ pc-2. The size-line width relationship follows ΔV ∝ r0.45±0.02, consistent with that found in the Galaxy. The cloud virial masses imply that the CO-to-H2 conversion factor has a value of 2 × 1020 H2 cm-2 (K km s-1)-1 and does not change significantly over the disk of M33, despite a change of 0.8 dex in the metallicity.

A parallactic distance of 389+24-21 parsecs to the orion nebula cluster from very long baseline array observations

We determine the parallax and proper motion of the flaring, non-thermal radio star GMR A, a member of the Orion Nebula Cluster, using Very Long Baseline Array observations. Based on the parallax, we measure a distance of 389 +24/-21 parsecs to the source. Our measurement places the Orion Nebula Cluster considerably closer than the canonical distance of 480 +/- 80 parsecs determined by Genzel et al. (1981). A change of this magnitude in distance lowers the luminosities of the stars in the cluster by a factor of ~ 1.5. We briefly discuss two effects of this change--an increase in the age spread of the pre-main sequence stars and better agreement between the zero-age main-sequence and the temperatures and luminosities of massive stars.

1.3 mm WAVELENGTH VLBI OF SAGITTARIUS A*: DETECTION OF TIME-VARIABLE EMISSION ON EVENT HORIZON SCALES

Sagittarius A*, the ~4 × 10^6 M_⊙ black hole candidate at the Galactic center, can be studied on Schwarzschild radius scales with (sub)millimeter wavelength very long baseline interferometry (VLBI). We report on 1.3 mm wavelength observations of Sgr A* using a VLBI array consisting of the JCMT on Mauna Kea, the Arizona Radio Observatory’s Submillimeter Telescope on Mt. Graham in Arizona, and two telescopes of the CARMA array at Cedar Flat in California. Both Sgr A* and the quasar calibrator 1924−292 were observed over three consecutive nights, and both sources were clearly detected on all baselines. For the first time, we are able to extract 1.3mmVLBI interferometer phase information on Sgr A* through measurement of closure phase on the triangle of baselines. On the third night of observing, the correlated flux density of Sgr A* on all VLBI baselines increased relative to the first two nights, providing strong evidence for time-variable change on scales of a few Schwarzschild radii. These results suggest that future VLBI observations with greater sensitivity and additional baselines will play a valuable role in determining the structure of emission near the event horizon of Sgr A*.

Dynamically driven evolution of the interstellar medium in M51

Massive star formation occurs in giant molecular clouds (GMCs); an understanding of the evolution of GMCs is a prerequisite to develop theories of star formation and galaxy evolution. We report the highest-fidelity observations of the grand-design spiral galaxy M51 in carbon monoxide (CO) emission, revealing the evolution of GMCs vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (giant molecular associations (GMAs)) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H_2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by large-scale galactic dynamics—their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the interarm region and into the next spiral arm passage.

Imaging the HL Tauri Disk at λ = 2.7 Millimeters with the BIMA Array

We have obtained a subarcsecond image of the disk associated with the T Tauri star HL Tau at a wavelength of 2.7 mm using the new high-resolution capability of the BIMA Array. The disk is elongated with a deconvolved Gaussian source size of 10 ± 02 × 05 ± 02, implying a semimajor axis of 70 ± 15 AU for a distance of 140 pc; the minor axis may be unresolved. The position angle of the major axis (125° ± 10°) is orthogonal to the axis of the optical jet. The disk centroid is coincident with the VLA λ = 3.6 cm source position and nearly coincident with recent measurements of the near-infrared emission peak. The λ = 2.7 mm images, along with previous interferometric measurements at λ = 0.87 mm and flux measurements from 10 μm to 1.3 cm, are well fitted by a simple power-law disk model with a shallow radial dependence to the surface density [Σ(r) ∝ r0 to r-1], an outer radius between 90 and 160 AU, and a dust opacity law proportional to ν1.

A turnstile junction waveguide orthomode transducer

An orthomode transducer with a circular waveguide input and two rectangular waveguide outputs is described. The design utilizes a turnstile junction and two E-plane power combiners. A K-band version of this device has been constructed and tested. From 18 to 26 GHz, the input reflection coefficient was less than -19 dB, the cross-polarization was less than -48 dB, and the transmission loss was ~0.15 dB. We estimate the mechanical tolerances that would be required to scale this device to the 200-270-GHz band for use in a dual-polarization radio-astronomy receiver

A Continuously Tunable 65--15-GHz Gunn Oscillator

A phase-locked second harmonic Gunn oscillator, mechanically tunable from 65 to 115 GHz, has been developed for use as a Iocal oscillator (LO) in millimeter radio astronomy. The oscillators output power is greater than 2 mW over most of its operating range, and exceeds 10 mW from 80 to 102 GHz. Its frequency can be electronically tuned approximately +-200 MHz by varying the bias voltage on the Gunn diode it is phase locked by exploiting this bias tuning. The oscillator consists of a commercially available, packaged GaAs Gunn diode which is mounted in a coaxial resonator of adjustable length. Descriptions of the mechanical design and phase-lock circuit are given. Extensive experimental measurements of the tuning range and power output for oscillators with different resonator dimensions also are reported.

Resolved magnetic-field structure and variability near the event horizon of Sagittarius A∗

Magnetic fields near the event horizon Astronomers have long sought to examine a black holes event horizon—the boundary around the black hole within which nothing can escape. Johnson et al. used sophisticated interferometry techniques to combine data from millimeter-wavelength telescopes around the world. They measured polarization just outside the event horizon of Sgr A*, the supermassive black hole at the center of our galaxy, the Milky Way. The polarization is a signature of ordered magnetic fields generated in the accretion disk around the black hole. The results help to explain how black holes accrete gas and launch jets of material into their surroundings. Science, this issue p. 1242 Magnetic fields around the event horizon of a supermassive black hole have been probed. Near a black hole, differential rotation of a magnetized accretion disk is thought to produce an instability that amplifies weak magnetic fields, driving accretion and outflow. These magnetic fields would naturally give rise to the observed synchrotron emission in galaxy cores and to the formation of relativistic jets, but no observations to date have been able to resolve the expected horizon-scale magnetic-field structure. We report interferometric observations at 1.3-millimeter wavelength that spatially resolve the linearly polarized emission from the Galactic Center supermassive black hole, Sagittarius A*. We have found evidence for partially ordered magnetic fields near the event horizon, on scales of ~6 Schwarzschild radii, and we have detected and localized the intrahour variability associated with these fields.