A cosmic pileup

Ken Tapping, May 9, 2018

In the sky this week…

  • Venus is spectacular in the west after sunset.
  • Jupiter rises at 9 pm, Saturn at 1 am and Mars at 2 am.
  • The Moon will be New on the 15th.

All telescopes on the surface of the Earth are limited to seeing what the atmosphere lets through. For looking at short-wavelength radio waves – millimetre waves, we choose high sites, above the thickest part of the atmosphere, and dry ones, because water vapour is a great obscurer of data. One of the driest high places on Earth is the Antarctic Plateau, where most of the water in the air is frozen out. So that is where we find the South Pole Telescope, a ten-metre dish radio telescope dedicated to observing millimetre waves.

This instrument recently picked up something odd. It picked up a radio source which appeared to it as a dot in the sky, but its radiation signature indicated that dot was a cluster of galaxies. In addition, it was a very long way away. The radio emissions being detected started on their way to us 12.3 billion years ago, only 1.5 billion years after the beginning of the universe, when galaxy and star formation were in full swing.

In order to find out more, the Atacama Large Millimetre Array was pointed at the source. This instrument, also known as ALMA, is an international project, in which Canada is a member. It is located on the cold, high Atacama Plateau, in Chile. ALMA is a millimetre-wavelength radio imager, and probably the most complex radio telescope in the world. It resolved that dot into a cluster of fourteen galaxies, all heading for each other and for a very big collision.

Galaxies start off small – by cosmic standards – and grow by colliding and coalescing with other galaxies. Then the ones that have become big continue to grow by swallowing the smaller ones. Our galaxy, the Milky Way is surrounded by stars and shreds of gas left from past meals. There are two nearby galaxies – the Large and Small Magellanic Clouds, which are probably destined to be assimilated at some point in the future.

The Milky Way is about 100,000 light years in diameter, and contains about 250 billion stars. In about four billion years, it is going it collide head-on with the Andromeda Galaxy, which is similar to the Milky Way but a bit bigger.

A collision involving fourteen galaxies sounds like a good subject for the ultimate disaster movie. However, as in the case of collisions between just two galaxies, the event will be spectacular but not disastrous. On average stars are many light years apart. The chance of two stars colliding is remote. It is highly unlikely they will even pass close enough to each other to disrupt their planetary systems. Inhabitants of planets in colliding galaxies will probably not see much change over a single lifetime. Over millions of years the shape of their “Milky Way” in the sky will change, and so will some of the constellations. The non-astronomical will probably notice nothing much. The main consequence of the collision is that the ramming together of the clouds of gas and dust in the galaxies will result in instability, cloud collapse and a spurt in the birth of new stars and planets.

Large galaxies usually have black holes in their cores. So this discovery shows a stage set for a collision of fourteen black holes, although probably not simultaneously. As black holes move they lose energy by making bow-waves in space-time - gravitational waves. This leads to them spiralling together and colliding. These crashes cause strong pulses of gravity waves, which we can detect. At the moment we see the cluster as it was 12.3 billion years ago. In a billion years or so, when these fourteen black holes meet, we will have a grandstand seat. By then our gravity wave detectors should be a lot better than they are now. We will also get to see the magnificent, huge galaxy that cluster has become.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory.

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