This is an artist’s impression of the quasar 3C 279. Astronomers connected the Atacama Pathfinder Experiment (APEX), in Chile, to the Submillimeter Array (SMA) in Hawaii, USA, and the Submillimeter Telescope (SMT) in Arizona, USA for the first time, to make the sharpest observations ever, of the centre of a distant galaxy, the bright quasar 3C 279. Quasars are the very bright centres of distant galaxies that are powered by supermassive black holes. This quasar contains a black hole with a mass about one billion times that of the Sun, and is so far from Earth that its light has taken more than 5 billion years to reach us. The team were able to probe scales of less than a light-year across the quasar—a remarkable achievement for a target that is billions of light-years away. ESO / M. Kornmesser |
An
international team led by scientists from the Max Planck Institute for
Radio Astronomy has succeeded in observing the heart of a distant quasar
with unprecedented sharpness, or angular resolution. The observations,
made by connecting radio telescopes on different continents, are a
crucial step towards a dramatic scientific goal: to depict the
supermassive black hole at the centre of our own galaxy and also the
central black holes in other nearby galaxies.
On
May 7, 2012, astronomers connected radio telescopes in Chile, Hawaii
and Arizona for the first time using the technique of very long baseline interferometry (VLBI). They were able to make the sharpest observation
ever of the centre of a distant galaxy, the bright quasar 3C 279, which
contains a supermassive black hole with the mass of as much as a billion
times the mass of the sun.
The
observations show that the quasar’s radio signals come from within a
region only 28 micro-arc seconds in diameter, corresponding to just 1
light year within the nucleus of this quasar. It is quite remarkable to
reach a resolution of only half a light year when the quasar is situated
at a distance of more than 5 billion light years from Earth.
The
observations were made with radio waves with wavelength 1.3 mm
(corresponding to a frequency of 230 GHz), using three telescopes which
had never before been connected together in this way. The Atacama
Pathfinder Experiment (APEX), a radio telescope of 12-m diameter at
5100-m altitude in the Chilean Atacama desert, was combined in
interferometry mode with the Submillimeter Telescope (SMT) at 3100 m
atop Mount Graham in Arizona (USA) and the Submillimeter Array (SMA),
located at 4100 m altitude on Mauna Kea, Hawaii (USA).
The
observations represent a new milestone towards depicting supermassive
black holes and the regions around them. In future it is planned to go
further and connect more telescopes in this way to create the so-called
‘Event Horizon Telescope’ (EHT). The Event Horizon Telescope will be
able to depict the shadow of the super-massive black hole in the centre
of our Milky Way, as well as others in nearby galaxies.
The
shadow is the result of gravitation redshift around the outer horizon
of a black hole. In theory, it should be possible to directly observe
this dark area. However, the size of the shadow on the sky is in the
region of micro-arc seconds, i.e. one millionth of an arc-second, an
angle which cannot be observed with the detail resolution of a regular
telescope. (As a reference: the apparent diameter of the full moon is
about 1800 micro-arc seconds on the sky).
Using
VLBI, the sharpest images can be achieved by making the separation
between telescopes as large as possible. For their quasar observations,
the team used the three telescopes to create an interferometer with
transcontinental baseline lengths of 9447 km from Chile to Hawaii, 7174
km from Chile to Arizona and 4627 km from Arizona to Hawaii.
To
synchronize the measurements, each telescope was equipped with an
atomic clock. After observations, 4 terabytes of data recorded on large
hard disks at each station were shipped to Germany and processed at the
Max Planck Institute for Radio Astronomy in Bonn.
The
bright jet from the quasar could be detected on all three baselines,
with an angular resolution that corresponds to a telescope magnification
of about 2.1 million. That is the equivalent of being able to resolve a
tennis ball on the surface of the Moon. On Earth this would allow one
to read a Newspaper in Los Angeles from Frankfurt.
Connecting
APEX in Chile to the network was crucial in achieving such sharp
observations at millimetre wavelengths, marking an important step
towards realizing an interferometer stretching across the globe.
Positions of the Telescopes in the 1.3 mm VLBI experiment: The baseline length from Chile (APEX) to Hawaii (SMA) is 9447 km, from Chile to Arizona (SMT) 7174 km, and from Arizona to Hawaii 4627 km. MPIfR/T. Krichbaum |
The
experiment is the culmination of three years of hard work at high
altitude making APEX ready for VLBI observations. Scientists from
Germany and Sweden installed new digital data acquisition systems, a
precise atomic clock, and pressurized data recorders capable of
recording 4 gigabits per second for many hours.
The
addition of APEX is also important for another reason. It shares its
location and technology with the new telescope ALMA (Atacama Large
Millimetre/sub millimetre Array) which will finally consist of 66
antennas, each similar to APEX. With ALMA connected to the network, the
observations could achieve 10 times better sensitivity than today. That
puts the shadow of the Milky Way’s supermassive black hole within reach
for future observations.
Very long baseline interferometry (VLBI)
For
terrestrial arrays the diameter of the earth sets an upper limit to the
station separation measured in kilometres. However it is the separation
between stations measured in radio wavelengths which is material, so by
pushing VLBI towards shorter wavelengths the resolution on terrestrial
baselines improves. This is technically difficult for many reasons,
including that at about 1 mm wavelength the humidity in the lower
atmosphere attenuates the already very faint cosmic radio signals.
Therefore the astronomers must use a new generation of radio telescopes
which are located at very high elevations where atmospheric humidity and
thus absorption is small.
To
equip APEX for VLBI operation, the new acquisition systems at APEX
allow wide bandwidth recording (up to 4 Gbit/s) of faint millimetre-wave
signals. These systems were developed in parallel in the USA
(MIT-Haystack observatory) and in Europe (MPIfR, INAF/Noto and HAT-Lab).
A hydrogen maser time standard (T4Science) was installed as the very
precise atomic clock. The SMT and SMA had already been equipped
similarly for VLBI.
