A magnification of two million
Telescope network zooms in on the heart of a distant quasar
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 meter 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 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.
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.
NJ / HOR
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.