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Bid to solve black hole mystery


Published: 12th Aug 2011 16:59:00

Astronomers could soon be a step closer to unravelling the mystery of why galaxies are smaller than astronomers predict.

From January next year, the Karoo Array Telescope-7 (Kat-7) in South Africa will be able to investigate whether black holes are holding back galaxy growth, as well as probing other phenomena such as gravitational waves and cosmic rays.

Kat-7 has been built in the Northern Cape as a test bed for a 3,000km-wide array that will become fully operational in 2024.

Small black holes, five or 10 times the mass of our Sun, and those up to a billion times larger - deemed "supermassive" - had been thought to only consume matter.

But scientists have discovered that they emit jets of matter too, and - given that massive black holes are believed to lie at the heart of galaxies - these jets could explain why galaxies are smaller than predicted.

The period during which these galactic geysers are active varies with the mass of the black hole. Those small black holes can be active for a brief period every 20 years. However, supermassive black holes can be active for millions of years and then be dormant for a billion.

These jets can stretch out for up to 100,000 light-years. This means to travel from the black hole to the end of the jet would take 100,000 years travelling at light speed.

The jets are visible to scientists at radio wavelengths. By analysing the jets' radio waves, scientists can gauge how much energy has been released. That amount of energy could be key to understanding the alleged holding back of galaxy growth.

Galaxies, which are relatively cool, are surrounded by hot gas. When this gas cools it can condense and form stars that grow the galaxy. The theory is that jet heating keeps that outer gas hot, stopping it from cooling and condensing to form stars.

"There is one object which is extremely interesting in the southern hemisphere and Kat-7 will be enough on its own to push forward on that source," says University of Southampton astronomer Rob Fender.

Fender is a research leader on project Thunderkat, which will use Kat-7 to investigate black holes as well as super-dense stars made of neutrons, the high-energy blasts of light called gamma ray bursts, and the stellar swan songs novae and supernovae. He is working with Professor Patrick Woudt of the University of Cape Town.

Astronomers and astrophysicists also think these black hole jets could be the source of cosmic rays, high energy particles which appear to come from every part of the sky.

Gravitational waves could also be discovered with the help of Kat-7 because it will seek to find one possible source: highly active neutron stars.

Neutron stars occasionally have outbursts of radio emissions and merging neutron stars more so. Scientists believe the outbursts could be symptoms of the process that generates gravitational waves.

Gravitational waves have never been observed but they are predicted by Einstein's theory of general relativity. They are described as distortions made by gravity in the fabric of space-time and are expected to travel as fast as the speed of light.

By finding merging or highly active neutron stars Fender and his colleagues can study them with Kat-7's dishes and aid other researchers in identifying gravity wave producing candidates.

Kat-7 was built for South Africa's bid to host the 3,000km-wide Square Kilometre Array (SKA).

Expected to cost 1.5bn euros (£1.3bn), 67 organisations from 20 countries are involved in SKA. The member countries include China, France, Germany, Italy, the Netherlands, UK, New Zealand, South Africa and Australia. The host for SKA will be chosen from either South Africa or Australia in 2012.

SKA's name is based on the fact that the combined surface area of all of the thousands of receptors is equivalent to a square kilometre. SKA will use elliptical dish antennas, each about 15m wide, and two other types of radio wave receptor, called aperture array antennas.

The antennas will be arranged in five spiral arms with about 50% of the dishes in the central 5km. A further 25% of the dishes will be spread out to 200km from the centre. The final 25% will stretch out to more than 3,000km.

The array will be 50 times more sensitive than today's telescope technology and will be able to survey the sky 10,000 times faster.

These dishes' rapid sky survey will, says SKA, produce 10 times the data produced today by global internet traffic. However the aperture arrays could produce ten times more than that.

South Africa's bid also includes eight other countries in the region that will be hosts to SKA sites. Those countries are Ghana, Kenya, Botswana, Namibia, Zambia, Mozambique, Madagascar and Mauritius.

Australia's partner is New Zealand, which would host sites on its two islands. Australia's equivalent to Kat-7 is Boolardy Engineering Test Array or Beta. Based at the Murchison Radio-astronomy Observatory in Western Australia, Beta has six antennas and they will undergo commissioning from next January.

Both countries have until this September to submit their bid information. That will be analysed until November by independent consultants, expert panels and the SKA head office, which is based at the UK's Jodrell Bank Observatory.

In November and December an external body of independent experts will evaluate the findings of the analysis and recommend a preferred site. The SKA board of directors will receive the final report by February and they will then make a decision on which site wins.

Both candidate countries have 10 approved science projects so far. While South Africa is starting its SKA-related science with Kat-7, this array will be incorporated into Meerkat, the country's precursor SKA site. It is planned to have 64 dishes and will be operating from 2016.

Australia's Beta will be incorporated into the Australia SKA Pathfinder or Askap. It will have 36 dishes and be fully operational from 2013. Askap's research will include the dark matter content of galaxies and the Milky Way's magnetic field.

Philip Diamond is the Australian science agency's astronomy and space science chief. He says: "There are two high priority projects, Emu and Wallaby, which will receive some level of priority in our planning."

Emu stands for Evolutionary Map of the Universe and Wallaby for Widefield Askap L-Band Legacy All-Sky Blind Survey. Starting in late 2012, just before Askap is fully operational, Emu will trace the evolution of star-forming galaxies and massive black holes through the history of the universe. Wallaby is aimed at improving our understanding of galaxy formation.

Expected in 2014 is Variables And Slow Transients, or Vast, which is Askap's equivalent to Thunderkat.

South Africa's Meerkat projects include investigating the role of molecular hydrogen in the early universe, looking for different types of galaxies and Fender and Woudt's black hole and neutron star analysis.

Not scheduled as part of any of the two competitor's 20 projects the search for extra-terrestrial intelligence could also benefit from SKA. Due to the 3,000km-wide array's high sensitivity, radar signals from any intelligent life could be detected on a planet 50 light-years away. The aliens would not need to be transmitting a message directly at Earth, say the astronomers.

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