Mm-VLBI sees non-thermal sources emitting, for instance, maser
or synchrotron radiation at a high brightness temperature (T). The associated astronomical sources are, for instance,
masers (SiO) and AGNs and QSOs with jets. mm-VLBI is insensitive
to the cold component of the Universe like molecular clouds and
other thermal sources. The cold component is observed with
interferometers like the Plateau de Bure instrument. cm
The correlated flux density F of a source with
brightness temperature T
, subtending the solid angle
in the sky, is
The correlated flux density F observable within the
bandwidth
and integration time
is
A mm-VLBI array of two telescopes of 15-m diameter, observing at
a system temperature T
= 200K, a bandwidth of
= 112MHz, and an integration time limited by the
system and atmospheric phase stability to
100s,
can only detect sources which have brightness temperatures of
T
10
- 10
K.
Evidently, a mm-VLBI array of 8000-10000km baseline has
only a limited field of view (
). Since a
disconnected mm-VLBI array does not directly track phase, an
estimate of the field of view is obtained by noting that the delay
between two antennas (see Figure 3.4) separated by
the baseline B and observing in the direction
Fig.3.9 shows observations of the Quasar 3C273 at
22GHz (top), 43GHz (center), and 86GHz (bottom), performed
nearly at the same epochs of 1995.15 (22 and 43GHz) and 1995.18
(86GHz). Contour levels in all maps are (-0.5,) 0.5, 1, 2, 5,
10, 15, 30, 50, 70, and 90% of the peak flux density of
3.0Jy/beam (top), 5.4Jy/ (center), and 4.7Jy/beam (bottom).
All maps are restored with the same beam of size of
mas, oriented at
. The maps are
arbitrarily centered on the eastern component (the core); the
dashed lines guide the eye and help to identify corresponding jet
components in the three maps.