Supermassive black holes: every large galaxy has one. But here\u2019s a real conundrum: how did they grow so big?<\/p>\n
A paper recently published in Science pits the front-running ideas about the growth of supermassive black holes against observational data \u2013 a limit on the strength of gravitational waves, obtained with CSIRO\u2019s Parkes radio telescope in eastern Australia.<\/p>\n
\u201cThis is the first time we\u2019ve been able to use information about gravitational waves to study another aspect of the Universe \u2013 the growth of massive black holes,\u201d co-author Dr Ramesh Bhat from the Â鶹ֱ²¥ node of the International Centre for Radio Astronomy Research (ICRAR) said.<\/p>\n
\u201cBlack holes are almost impossible to observe directly, but armed with this powerful new tool we\u2019re in for some exciting times in astronomy. One model for how black holes grow has already been discounted, and now we\u2019re going to start looking at the others.\u201d<\/p>\n
The study was jointly led by Dr Ryan Shannon, a Postdoctoral Fellow with CSIRO, and Mr Vikram Ravi, a PhD student co-supervised by the University of Melbourne and CSIRO.<\/p>\n
Einstein predicted gravitational waves \u2013ripples in space-time, generated by massive bodies changing speed or direction, bodies like pairs of black holes orbiting each other.<\/p>\n
When galaxies merge, their central black holes are doomed to meet. They first waltz together then enter a desperate embrace and merge.<\/p>\n
\u201cWhen the black holes get close to meeting they emit gravitational waves at just the frequency that we should be able to detect,\u201d Dr Bhat said.<\/p>\n
Played out again and again across the Universe, such encounters create a background of gravitational waves, like the noise from a restless crowd.<\/p>\n
Astronomers have been searching for gravitational waves with the Parkes radio telescope and a set of 20 small, spinning stars called pulsars.<\/p>\n
Pulsars act as extremely precise clocks in space. The arrival time of their pulses on Earth are measured with exquisite precision, to within a tenth of a microsecond.<\/p>\n
When the waves roll through an area of space-time, they temporarily swell or shrink the distances between objects in that region, altering the arrival time of the pulses on Earth.<\/p>\n
The Parkes Pulsar Timing Array (PPTA), and an earlier collaboration between CSIRO and Swinburne University, together provide nearly 20 years\u2019 worth of timing data. This isn\u2019t long enough to detect gravitational waves outright, but the team say they\u2019re now in the right ballpark.<\/p>\n
\u201cThe PPTA results are showing us how low the background rate of gravitational waves is,\u201d Dr Bhat said.
\n\u201cThe strength of the gravitational wave background depends on how often supermassive black holes spiral together and merge, how massive they are, and how far away they are. So if the background is low, that puts a limit on one or more of those factors.\u201d<\/p>\n
Armed with the PPTA data, the researchers tested four models of black-hole growth. They effectively ruled out black holes gaining mass only through mergers, but the other three models are still a possibility.<\/p>\n
Dr Bhat also said the Â鶹ֱ²¥-led Murchison Widefield Array (MWA) radio telescope would be used to support the PPTA project in the future.<\/p>\n
\u201cThe MWA\u2019s large view of the sky can be exploited to observe many pulsars at once, adding valuable data to the PPTA project as well as collecting interesting information on pulsars and their properties,\u201d said Dr Bhat said.<\/p>\n
The paper, Gravitational-Wave Limits from Pulsar Timing Constrain Supermassive Black Hole Evolution, is available at http:\/\/www.sciencemag.org\/content\/342\/6156\/334.abstract<\/a><\/p>\n More images and an animation available at the International Centre for Radio Astronomy Research website<\/a><\/p>\n Media Contact:\u00a0<\/strong><\/p>\n Dr Ramesh Bhat, Curtin Research Fellow, ICRAR \u2013 Â鶹ֱ²¥
\nTel: 08 9266 9176; Mobile: 0430 910 055, Email: ramesh.bhat@icrar.org<\/a><\/p>\n