Very Long Baseline Array
The Very Long Baseline Array (VLBA) is an interferometer consisting of 10 identical antennas on transcontinental baselines up to 8000 km (Mauna Kea, Hawaii to St. Croix, Virgin Islands). The VLBA is controlled remotely from the Science Operations Center in Socorro, New Mexico. Each VLBA station consists of a 25 m antenna and an adjacent control building. The received signals are amplified, digitized, and recorded on fast, high capacity recorders. The recorded data are sent from the individual VLBA stations to the correlator in Socorro.
The VLBA observes at wavelengths of 28 cm to 3 mm (1.2 GHz to 96 GHz) in eight discrete bands plus two narrow sub-gigahertz bands, including the primary spectral lines that produce high-brightness maser emission. The array can be scheduled dynamically, and its continuum sensitivity can be improved by a factor of 5 or more by adding the GBT and the phased VLA to the VLBA.
A virtual VLBA tour briefly describes and provides a near real-time image of each of the ten VLBA antenna sites.
Science with the VLBA
Precision astrometry is a VLBA science centerpiece. The relative astrometric accuracy of ~ 10 micro-arcseconds achievable with the VLBA is better than what the Gaia satellite is designed to achieve for most stars in its catalog (scheduled for release after 2015).
In 2010, the VLBA will begin a long-term program to determine the complete 3D structure of the Milky Way by measuring parallaxes with 10 mas accuracy or better to ~ 400 high-mass star-formation regions. This program is expected to eventually measure the fundamental galactic rotation parameters with 1% accuracy, helping to quantify the detailed distribution of luminous and dark matter in the Milky Way.
The VLBA is also expected to anchor High Sensitivity Array observations of the center of M31 in an effort to detect the galaxy's nucleus with significant signal-to-noise and enable a first-epoch position measurement. The ultimate goal is to measure the proper motion of M31 relative to the Milky Way, which should help distinguish between scenarios proposed for the formation of the Local Group and provide a measurement of the mass of M31 and its dark-matter halo.
The NASA Fermi Gamma-ray Space Telescope released its first gamma-ray source catalog in early 2009. In 2010, the VLBA will participate in several observational programs as part of a continuing coordinated effort with Fermi.
A long-term, continuing VLBA program to study Active Galactic Nuclei (AGNs) containing central H2O megamasers has 2 primary scientific goals: (1) acquire geometric distance measurements that enable an accurate determination of the Hubble Constant and related dark-energy parameters; and (2) directly measure the masses of central black holes with accuracies of at least 10%, much better than any other technique used for sources outside the Milky Way. The megamaser program requires considerable integration time because of the weakness of most maser sources and will benefit from an increase in VLBA bandwidth that will be made available via the VLBA Sensitivity Enhancement Program.