Helical
jets from one supermassive black hole caused by a very closely orbiting
companion (see blue dots). The third black hole is part of the system,
but farther away and therefore emits relatively straight jets. © Roger Deane (large image); NASA Goddard (inset bottom left; modified from original).
Tight system of black holes in a distant galaxy
Astronomers have discovered three closely orbiting supermassive black
holes in a galaxy more than 4 billion light years away. This is the
tightest trio of black holes known to date and is remarkable since most
galaxies have just one black hole, usually with a mass between 1 million
to 10 billion times that of the Sun, at their centre. The discovery
suggests that such closely packed supermassive black holes are far more
common than previously thought.
An international research team,
including Hans-Rainer Klöckner from the Max Planck Institute for Radio
Astronomy in Bonn, Germany, performed VLBI (Very Long Baseline
Interferometry) observations with radio telescopes at a number of
frequencies to discover the inner two black holes of the triple system.
The VLBI technique combines the signals from large radio antennas
separated by up to 10. 000 kilometres to see details 50 times finer than
that possible with the Hubble Space Telescope. In this project the
Effelsberg 100m radio telescope took part in European VLBI network (EVN)
observations covering two radio frequencies.
Galaxies are believed to evolve through merging and that should lead
to multiple supermassive black holes in some of those galaxies at a
given time. The source under investigation was found in the Sloan
Digital Sky Survey (SDSS) and has the catalog number SDSS J1502+1115. It
is a quasar, the nucleus of an active galaxy at a redshift of z = 0.39,
corresponding to a distance of more than four billion light years. A
triple black hole system has been identified in that source, with two
tight companions separated by less than 500 light years.
"What remains extraordinary to me is that these black holes, which
are at the very extreme of Einstein’s Theory of General Relativity, are
orbiting one another at 300 times the speed of sound on Earth", says
Roger Deane from the University of Cape Town/South Africa, the lead
author of the paper. "Not only that, but using the combined signals from
radio telescopes on four continents we are able to observe this exotic
system one third of the way across the Universe. It gives me great
excitement as this is just scratching the surface of a long list of
discoveries that will be made possible with the Square Kilometre Array."
Such systems are important to understand for several reasons; in
terms of galaxy evolution it is known that black holes influence how
galaxies evolve, and understanding how often black holes themselves
merge is key to this work. Furthermore, closely orbiting systems such as
this are sources of gravitational waves in the Universe, if General
Relativity is correct. Future radio telescopes such as the Square
Kilometre Array (SKA) will be able to measure the gravitational waves
from such systems as their orbits decrease.
At this point, very little is actually known about black hole systems
that are so close to one another that they emit detectable
gravitational waves. "This discovery not only suggests that close-pair
black hole systems are much more common than previously expected, but
also predicts that radio telescopes such as MeerKAT and African VLBI
Network will directly assist in the detection and understanding of the
gravitational wave signal", says Matt Jarvis from the Universities of
Oxford and the Western Cape. "Further in the future the SKA will allow
us to find and study these systems in exquisite detail, and really allow
us gain a much better understanding of how black holes shape galaxies
over the history of the Universe."
World map with radio telescopes of the EVN (European VLBI Network).
Observations of SDSS J1502+1115 were performed at frequencies of 1.7 and
5 GHz within that network
While the VLBI technique was essential to discover the inner two black
holes (which are in fact the second closest pair of supermassive black
holes known), Deane and co-authors have also shown that the binary black
hole presence can be revealed by much larger scale features. The
orbital motion of the black hole is imprinted onto its large jets,
twisting them into a helical or corkscrew-like shape. So even though
black holes may be so close together that our telescopes can’t tell them
apart, their twisted jets may provide easy-to find pointers to them,
much like using a flare to mark your location at sea. This may provide a
way for sensitive future telescopes like MeerKAT and the SKA to find
binary black holes with much greater efficiency.
"We have found the first needle in the 'middle age' Universe and I
hope that we will find much more and even closer systems of this kind in
the near future", concludes Hans-Rainer Klöckner from the Max Planck
Institute for Radio Astronomy, a co-author of the paper. "Such
close-binaries will not only show us how supermassive black holes could
grow or how they could alternate our space time, they will also help us
to understand the inner workings and the interplay between jets and the
accretion disc surrounding black holes." This discovery is a prime
example of how radio astronomy is done nowadays; it is an international
and close collaboration accessing data products from various facilities
distributed all over the globe.
The future will be bright with the SKA, the biggest radio telescope
ever being built, enabling such discoveries in international
collaborations and hopefully Germany will find a way to support this
endeavor also in future and enable its scientists and engineers to
participate in the SKA project.
Background Information
The South African Government has made a major investment in
astronomy, in funding what will the most sensitive radio telescope in
the Southern Hemisphere, but also in the significant Human Capital
Development programme. As someone who may not have become a professional
astronomer without the support of these initiatives, Roger Deane is
grateful to be able to produce internationally recognised research
following the strong support received from SKA, South Africa (SA) and
the SA Government. Four South African institutions are represented by
the research team which is in itself a very positive signal for radio
astronomy here. This demonstrates that South Africa has the scientific
and technical expertise to be a world leader in this research area and
contribute directly to gravitational wave experiments that will provide
fundamental insights not only to astronomy, but also more broadly to
physics.
Contact:
Dr. Hans-Rainer Klöckner
Phone:+49 228 525-314
Max-Planck-Institut für Radioastronomie, Bonn
Dr. Roger Deane
Phone:+27 78 582-2308
University of Cape Town, Cape Town, South Africa
Dr. Norbert Junkes
Presse- und Öffentlichkeitsarbeit
Phone:+49 228 525-399
Max-Planck-Institut für Radioastronomie, Bonn
Original Paper:
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A close-pair binary in a distant triple supermassive black-hole system, by R. P. Deane, Z. Paragi, M. J. Jarvis,
M. Coriat, G. Bernardi, R. P. Fender, S. Frey, I. Heywood, H.-R.
Klöckner, K. Grainge & C. Rumsey, Nature Online, June 25, 2014,
DOI: 10.1038/nature13454 (Link accessible after the embargo expires).
Links:
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EVN - European VLBI Network
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Arecibo - Arecibo Radio Observatory
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GMRT - Giant Metre Wave Radio Telescope
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AMI - Arcminute Microkelvin Imager
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JVLA - Karl G. Jansky Very Large Array
