The VLT Interferometer - ESO

2.2. PRIME SCIENTIFIC TARGETS 19
• Spectral resolution is not, as seen above, a requirement for the phasing
and cophasing conditions. It will nevertheless often be required for the
science to be done. Sensitivity will reduce accordingly. Nevertheless, the
use of the Double Fourier method (simultaneous spatial and spectral measurement)
may provide the multiplex advantage in some cases, and allow
fringe tracking on the white light fringe of the Fourier Transform Spectrometer
which contains all the broad-band energy, while still providing
spectral resolution.
• The use of N > 2 telescopes does not modify the limiting sensitivities
given in the Tables, since aperture phasing is made with a single telescope,
and cophasing of telescopes must always be done by considering pairs of
apertures. A large number of telescopes in the array provides better use of
the observing time, a capability to restore the source complex visibility
(phase closures), and a faster coverage of the u-v plane. If the beam
combination is made by beam division, the sensitivity is reduced e.g., if
all pairs are combined, the sensitivity is reduced by at least l/(N - 1)
when the beam division is done with perfect efficiency. The partial or
total redundancy does not affect the sensitivity, but only the final signalto-noise
ratio.
• The dynamic range in the image is a separate issue, and its value is set
by the final signal-to-noise ratio obtained on the u-v data points.
2.2 Prime Scientific Targets
2.2.1 The Galactic Center: Is there a Massive Black Hole?
The central 0.1 pc (2 arcsec) of our Galaxy will be a prime goal for detailed
study with the VLT Interferometer, whose sensitivity is well matched to obtaining
high signal-to-noise ratio data on this source in the full source referenced
mode at 2 to 10 11m (using the infrared emission of the nearby IRS7 as reference).
Milliarcsecond resolution imaging at different infrared wavelengths, both
in continuum and line radiation, will for the first time allow us to address several
fundamental questions about the innermost region of our Galaxy. First and
,
foremost is there a central massive black hole? At the distance of the Galac-
tic Center (8 kpc), one milliarcsecond corresponds to 10 14 em, or about 700
Schwarzschild radii of a 10 6 Mev black hole. Secondly, what is the origin of the
intense ultraviolet radiation emitted from the central parsec? Thirdly, what is
the nature of the various sources of infrared and radio emission and how are
they related to each other and to the possible central black hole?