AFTER 16 years of research, teams of American and European scientists have compiled the most complete portrait of the gigantic black hole at the centre of our Milky Way galaxy, plotting its gravity-bending mass as being equivalent to a staggering four million suns.
The researchers from Germany and the University of California, Los Angeles (UCLA), also pinpoint the distance to the centre of the galaxy at 27,000 light years.
While neither figure differs markedly from previous estimates of the black hole’s size and distance, it is the most conclusive proof to date that our galaxy of billions of stars is indeed centred on an object of such tremendous power that it gobbles all light and matter in its neighbourhood.
Reinhard Genzel, leader of the European team based at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, said we now have the best evidence that “supermassive black holes really do exist”.
The researchers also determined that 95 per cent of the mass in the galactic centre is contained in the black hole, leaving little or no room for so-called “dark matter”, unseen material that is thought to make up a large part of what holds the universe together.
Andrea Ghez, leader of UCLA’s Galactic Centre Research Group, which has been engaged in what Ghez called a “useful rivalry” with the Europeans, said the new results represent a more fully developed understanding of the galaxy’s centre than the guesses of the past. “We had the luxury of ignorance. It’s like we went from a teenager to an adult,” she said.
Both research papers are scheduled to be published in upcoming issues of the Astrophysical Journal.
Since direct observation of a black hole is impossible, the two studies focused on a set of stars that orbit near the black hole. The European team began its research in 1992 with the New Technology Telescope in La Silla, Chile, eventually shifting to the Very Large Telescope at the same site. Ghez said her team has been using the giant Keck telescope in Hawaii.
Key to the discoveries is the use of adaptive optics, which compensates for distortions caused by the Earth’s atmosphere. By analysing the velocity and size of the stars’ orbits, the researchers were able to calculate the mass of the black hole and its distance. The European team said it was able to image 28 stars with an error rate of only 300 microarcseconds, precise enough to resolve a coin from 9,650km (6,000 miles) away.
Two distinct populations of stars reside in the heart of the galaxy. The stars in the innermost region, according to Stefan Gillessen, the author of the European study, are in random orbits, “like a swarm of bees.” Farther out, six of the 28 stars orbit in a more orderly disc-like pattern. Both teams have observed one star, known as S-02, long enough to see it make one complete revolution around the central black hole.
Mysteries remain, however, over how these young, inner stars came to be in their orbits. The researchers said the stars are much too young to have wandered far from their birthplaces. Yet, it seems unlikely they could have formed in the presence of the huge tidal forces around the black hole. – (La Times-Washington Post service)