Extremely Large Telescope

Abstract:

We describe the characteristics of the BIMA millimeter wave array at Hat Creek, CA. The array is an aperture synthesis instrument consisting of nine 6 m diameter antennas which may be deployed in three different configurations, with spacings ranging from 7 m up to 1.3 km. At an observing frequency of 100 GHz these configurations yield maps with angular resolutions of 5″, 2″, and 0.″4, over a 2′ field. Larger fields may be mapped by using multiple pointings. For all but the oldest telescopes, the surface accuracy is ≤30 μm rms, and the aperture efficiency is 77% at 100 GHz. Background emission from antenna losses and spillover is very low, about 5 K after subtraction of the cosmic B v(2.1 K). Each antenna contains a single dewar which accommodates up to four separate receivers. SIS mixers are cooled to 3.2 K with novel Gifford-McMahon cycle refrigerators. Both the upper and lower sidebands of the first local oscillator are received and separated, providing two bands extending from 70-900 MHz on each side of the first local oscillator. The correlation spectrometer covers a bandwidth of up to 800 MHz, and provides up to 2048 channels for each antenna pair. There are four independently tunable spectral windows (in each sideband), allowing simultaneous observations of several different spectral lines. The spectral resolution ranges from 6 kHz to 3 MHz. For a single 8-hr track in one configuration, the sensitivity is approximately 1 mJy/beam in the 800 MHz wide continuum. Measurements of atmospheric phase fluctuations as functions of both time and baseline have been made; these indicate that routine imaging at angular resolutions of less than 1″ at 100 GHz is possible only if self-calibration or some other means of phase correction can be applied. Examples of a few recent results are included. We note that 30% of the observing time on the array is granted to visitors.

Abstract:

We report ∼1″ resolution K-band (2 μm) imaging spectroscopy of the central parsec of our Galaxy. The derived radial velocities for 223 early- and late-type stars probe the nuclear mass distribution to spatial scales of 0.1 pc. We find a statistically very significant increase of projected stellar velocity dispersion from about 55 km s-1 at p ∼ 5 pc to 180 km s-1 at p ∼ 0.1 pc. The stars are also rotating about the dynamic center. The late-type stars follow general Galactic rotation, while the early-type stars show counter-rotation. Fitting simultaneously the observed projected surface densities and velocity dispersions, we derive the intrinsic volume densities and radial velocity dispersions as a function of distance from the dynamic center for both types of stars. We then derive the mass distribution between 0.1 and 5 pc from the Jeans equation assuming an isotropic velocity field. Our analysis requires a compact central dark mass of 2.5-3.2 × 106 M⊙, at 6-8 σ significance. The dark mass has a density of 109 M⊙ pc-3 or greater and a mass to 2 μm luminosity of ≥ 100. The increase in mass-to-luminosity ratio can be reduced but not eliminated even if extreme anisotropic velocity destributions are considered. The dark mass cannot be a cluster of solar mass remnants (such as neutron stars). It is either a compact cluster of 10-20 M⊙ black holes or a single massive black hole. © 1996. The American Astronomical Society. All rights reserved.