Small team of astronomers identify medium-sized black hole using Very Large Telescope

Posted by mjs76 at Sep 13, 2010 12:45 PM |
Research confirms the presence of a previously theoretical class of astronomical object.
Small team of astronomers identify medium-sized black hole using Very Large Telescope

Artist's impression of ESO 243-49 with HLX-1 as the light blue object top left of the galactic bulge. Image credit: Heidi Sagerud.

A team led by astronomers from Leicester has identified the first intermediate-size black hole, which is a massively more exciting and important discovery than it might at first appear. This isn’t just about filling a gap in the middle of a range of sizes. Intermediate-mass black holes are a whole new category of object; much has been written about them in theory but nobody has ever found one of the things.

Black holes come in two distinct sizes: small-ish and enormous. The small-ish ones are called stellar black holes because they are collapsed stars. These have masses ranging from a bit heavier than the Sun to about 20 times as heavy. At the other end of the scale are super-massive black holes which are absolutely enormous - anything from a million to a billion solar masses. Most galaxies have one of these at the centre and there are competing theories about how they are formed.

Dr Klaas Wiersema from our Department of Physics and Astronomy and his colleagues have been studying an ultra-luminescent X-ray source (ULX) called HLX-1. Now, your ULX is a curious beast: in terms of X-ray emissions it is brighter than any form of star (including a stellar black hole) but less luminous than a galactic nucleus. There is a theory that it might be powered by an intermediate-mass black hole of a few hundred to a few hundred thousand solar masses, except that nobody has ever spotted a black hole that big, only small-ish ones and enormous ones. Measurements by Dr Wiersema and his team however, published this week in a letter in The Astrophysical Journal, seem to confirm that HLX-1 does indeed have one of these middling-sized black holes as its power source.

Which is unbelievably exciting.

(Black holes are, of course, so massive that nothing can escape their gravity, not even light. Hence the name. The reason they can be detected is that material being pulled into a black hole hurtles round it, like water round a plughole, forming an ‘accretion disc’ of spinning matter which emits copious quantities of X-rays.)

HLX-1 lies near the edge of a galaxy called ESO 243-49 which is about 290 million light years from Earth. It was discovered last year by a team from the Centre d'Etude Spatiale des Rayonnements at the University of Toulouse, led by Dr Sean Farrell who subsequently moved to Leicester and is a co-author of the current letter. With an X-ray luminosity more than ten times that of the second most luminous ULX, HLX-1 was a good candidate for investigation to see whether there really was an intermediate-size black hole at its heart.

What the team needed to eliminate were two other possible explanations for HLX-1. Some astronomers have suggested that it could be a relatively nearby binary system containing a neutron star and positioned (from Earth’s point of view) directly in front of ESO 243-49. Or it could potentially be another galaxy some distance behind the first one. Space, after all, is full of unrelated things that appear to be in the same location when viewed from this particular planet.

Originally discovered using ESA’s XMM-Newton X-ray space telescope (which carries Leicester-built components), HLX-1 is currently under the scrutiny of both XMM-Newton and NASA’s Swift mission (which also has Leicester involvement). However, for this particular task the team used a more Earthbound device, a does-what-it-says-on-the-tin array in South America called the VLT or Very Large Telescope.

vlt.jpg
The Very Large Telescope (VLT). Image: Wikipedia

The VLT consists of four optical telescopes, each with a diameter of 8.2 meters, and is operated by the European Southern Observatory (ESO), a consortium of 14 countries including the UK . But as none of those countries can match the clarity of night-time vision available in the Atacama Desert, the VLT sits 2,635m above sea level atop a Chilean mountain. Before working on the VLT, the team prepared preliminary data using the Magellan telescopes, a pair of 6.5m instruments which are owned by a consortium of US universities. And are situated atop a different Chilean mountain.

What Wiersema et al were looking for was the distinctive optical signature of hydrogen which can be used to determine the relative distances of astronomical objects using a technique called ‘redshift’ which is based on the Doppler effect. Galaxy ESO 243-49 is about 95 megaparsecs distant while the mooted neutron star X-ray binary would be only about 2.5 kiloparsecs – quite a difference. And a distant second galaxy would be several orders of magnitude along the scale in the other direction. To everyone’s delight, the VLT observations proved that HLX-1 is the same distance from Earth as – and therefore located within – ESO 243-49. As Dr Farrell puts it:

This is very difficult to explain without the presence of an intermediate mass black hole of between about 500 and 10,000 times the mass of the Sun. HLX-1 is therefore (so far!) weathering the scrutiny of the international astronomy community.”

The confirmed existence of intermediate-mass black holes provides new avenues for investigating the possible origins of super-massive black holes which, according to one theory, might be formed by the coming together of several intermediates. And this, ultimately, could point a way towards understanding how galaxies are formed and how our own Milky Way galaxy and everything in it – including us – came to be.

The next steps for Dr Wiersema, Dr Farrell and their colleagues from Toulouse, Southampton and Harvard are two-fold. They plan to search for more ULX objects with luminosities comparable to HLX-1 which might therefore also contain an intermediate-mass black hole. They also plan to investigate more closely the region of ESO 243-49 around HLX-1 looking for clues as to the nature and origin of this fascinating object, not least what sort of material the black hole is feeding on. And to do this, the investigation will return to Earth orbit for a session on the most famous space telescope of them all – the Hubble.

This research was funded by the Science and Technology Facilities Council (STFC) which also funds the UK’s membership of ESO and hence contributes towards the VLT.