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Black holes grow faster than expected

Artist's impression of a black hole about to devour a star

Artist’s impression of a black hole about to devour a star. Supermassive black holes are thought to be at the heart of all major galaxies. Australian researchers have determined that as a galaxy grows, its black hole grows even faster.

  • Supermassive black holes have up to billions of times more mass than the Sun
  • How they became this big has been a long-standing mystery
  • Australia research shows big galaxies breed even bigger black holes

ASTRONOMERS FROM SWINBURNE UNIVERSITY of Technology in Australia have discovered how supermassive black holes grow – and it’s not what was expected.

For years, scientists had believed that supermassive black holes – millions or billions of times the mass of our Sun – located at the centres of galaxies, increased their mass in step with the growth of their host galaxy.  However, new observations have revealed a dramatically different behaviour.

“Black holes have been growing much faster than we thought,” Professor Alister Graham from Swinburne’s Centre for Astrophysics and Supercomputing said.

Within galaxies, there is a competition of sorts for the available gas; for either the formation of new stars or feeding the central black hole.

For more than a decade the leading models and theories have assigned a fixed fraction of the gas to each process, effectively preserving the ratio of black hole mass to galaxy mass. New research to be published in The Astrophysical Journal reveals that this approach needs to be changed.

“We now know that each ten-fold increase of a galaxy’s stellar mass is associated with a much larger 100-fold increase in its black hole mass,” Professor Graham said. “This has widespread implications for our understanding of galaxy and black hole co-evolution.”

The following animation depicts a star being devoured by a black hole.

Unexpected behaviour

The researchers have also found the opposite behaviour to exist among the tightly packed clusters of stars that are observed at the centres of smaller galaxies and in disc galaxies like our Milky Way.

“The smaller the galaxy, the greater the fraction of stars in these dense, compact clusters,” Swinburne researcher Dr Nicholas Scott said. “In the lower mass galaxies the star clusters, which can contain up to millions of stars, really dominate over the black holes.”

Previously it was thought that the star clusters contained a constant 0.2 per cent of the galaxy mass.

Black holes = gravitational prisons

The research also appears to have solved a long-standing mystery in astronomy. ‘Intermediate mass’ black holes with masses between that of a single star and one million stars have been remarkably elusive.

The new research predicts that numerous galaxies already known to harbour a black hole – albeit of a currently unknown mass – should contain these missing `intermediate mass’ black holes.

Artist's impression of a black hole in a star field

Intermediate or middle-sized black holes have proved elusive (artist’s impression).

“These may be big enough to be seen by the new generation of extremely large telescopes,” Dr Scott said.

Professor Graham said these black holes were still capable of readily devouring any stars and their potential planets if they ventured too close.

“Black holes are effectively gravitational prisons and compactors, and this may have been the fate of many past solar systems,” Professor Graham said. “Indeed, such a cosmic dance will contribute at some level to the transformation of nuclear star clusters into massive black holes.”

The researchers combined observations from the Hubble Space Telescope, the European Very Large Telescope in Chile and the Keck Telescope in Hawaii to create the largest sample to date of galaxies with reliable star cluster and supermassive black hole mass measurements.

Adapted from information issued by Swinburne University of Technology. Images by Gabriel Perez Diaz.

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Sparkling sphere of ancient stars

Messier 55

Messier 55 is a huge ball of very old stars, about 17,000 light-years from Earth.

A NEW IMAGE of Messier 55 from the European Southern Observatory’s (ESO) VISTA infrared survey telescope shows tens of thousands of stars crowded together like a swarm of bees. Besides being packed into a relatively small space, these stars are also among the oldest in the Universe. Astronomers study Messier 55 and other ancient objects like it, called globular star clusters, to learn how galaxies evolve and stars age.

Globular clusters are held together in a tight spherical shape by gravity. In Messier 55, the stars certainly do keep close company—approximately one hundred thousand stars are packed within a sphere with a diameter of only about 25 times the distance between the Sun and the nearest star system, Alpha Centauri.

About 160 globular clusters have been spotted encircling our galaxy, the Milky Way, mostly toward its bulging centre. The largest galaxies can have thousands of these rich collections of stars in orbit around them.

Wide-angle view of Messier 55

A wider view of Messier 55 at visible light wavelengths. It's easy to see how these vast collections of stars got their name…"globular star clusters". Courtesy ESO and Digitised Sky Survey 2.

Observations of globular clusters’ stars reveal that they originated around the same time—more than 10 billion years ago—and from the same cloud of gas. As this formative period was just a few billion years after the Big Bang, nearly all of the gas on hand was the simplest, lightest and most common in the cosmos—hydrogen, along with some helium and much smaller amounts of heavier chemical elements such as oxygen and nitrogen.

Being made mostly from hydrogen distinguishes globular cluster residents from stars born in later eras, like our Sun, that are infused with heavier elements formed in earlier generations of stars. The Sun lit up some 4.6 billion years ago, making it only about half as old as the elderly stars in most globular clusters.

The new image was obtained in infrared light by the 4.1-metre Visible and Infrared Survey Telescope for Astronomy (VISTA) at ESO’s Paranal Observatory in northern Chile.

As well as the stars of Messier 55, this VISTA image also records many galaxies lying far beyond the cluster. A particularly prominent edge-on spiral galaxy appears like a thin, red smudge to the upper right of the centre of the picture.

Adapted from information issued by ESO / J. Emerson / VISTA. Acknowledgment: Cambridge Astronomical Survey Unit.

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City of sparkling stars

Messier 107

Astronomers have found roughly 150 globular star clusters—giants 'cities' of stars—surrounding our Milky Way galaxy…such as this one, known as Messier 107. Photo courtesy ESO.

MESSIER 107 IS A BUSTLING METROPOLIS—thousands of stars concentrated into a space that is only about twenty times the distance between our Sun and it’s nearest stellar neighbour, Alpha Centauri.

The sharp new image above, captured by the Wide Field Imager on the 2.2-metre telescope at the European Southern Observatory’s (ESO) La Silla Observatory in Chile, displays the structure of Messier 107 in exquisite detail.

Also known as NGC 6171, the compact and ancient family of stars about 21,000 light-years from Earth is an example of what astronomers call a globular star cluster, so-called because of their round shape.

Astronomers know of about 150 of globular star clusters orbiting our galaxy, the Milky Way.

ESO 2.2m telescope

The ESO 2.2m telescope at La Silla, Chile. Photo courtesy ESO / H.H.Heyer.

In Messier 107, a significant number of these stars have evolved into red giants, one of the last stages of a star’s life, and have a yellowish colour in this image.

Globular clusters are among the oldest objects in the Universe. And since the stars within a globular cluster all formed from the same cloud of interstellar matter at roughly the same time—typically over 10 billion years ago—they are all low-mass stars, since lightweights burn their hydrogen fuel supply much more slowly than stellar behemoths.

Globular clusters formed during the earliest stages in the formation of their host galaxies and therefore studying these objects can give significant insights into how galaxies, and their component stars, evolve.

See the full-size, high-resolution image here (will open in a new window or tab).

M107 is not visible to the naked eye, but it can easily be seen from a dark site with binoculars or a small telescope. It is found in the constellation of Ophiuchus, north of the pincers of Scorpius.

Roughly half of the Milky Way’s known globular clusters are actually found in the constellations of Sagittarius, Scorpius and Ophiuchus, in the general direction of the centre of the Milky Way. This is because they are all in elongated orbits around the central region and are on average most likely to be seen in this direction.

Messier 107 was discovered by French astronomer Pierre Mechain in April 1782 and it was added to the list of seven Additional Messier Objects that were not originally included in the final version of countryman Charles Messier’s catalogue of “deep sky” objects, which was published the previous year.

On 12 May 1793, it was independently rediscovered by English astronomer William Herschel, who was able to resolve this globular cluster into stars for the first time. But it was not until 1947 that this globular cluster finally took its place in Messier’s catalogue as M107, making it the most recent star cluster to be added to this famous list.

Adapted from information issued by ESO.

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Hubble looks 10,000 years into the future

  • Hubble studies stars in the Omega Centauri cluster
  • Measures stellar motions over a period of four years
  • Data used to predict future motion of the stars

Astronomers are used to looking millions of years into the past. Now scientists have used the NASA/ESA Hubble Space Telescope to look thousands of years into the future.

Looking at the heart of Omega Centauri, a globular cluster in the Milky Way, they have calculated how the stars there will move over the next 10,000 years.

The globular star cluster Omega Centauri has caught the attention of sky watchers ever since the early astronomer Ptolemy first catalogued it 2,000 years ago. Ptolemy thought Omega Centauri was a single star and probably wouldn’t have imagined that his “star” was actually a beehive swarm of nearly 10 million stars, all orbiting a common centre of gravity.

The stars are so tightly crammed together in the cluster that astronomers had to wait for the Hubble Space Telescope before they could look deep into the core of the “beehive” and resolve the individual stars.

Hubble’s vision is so sharp that it can even measure the motion of many of these stars, and over a relatively short span of time.

A precise measurement of star motions in giant clusters can yield insights into how such stellar groupings formed in the early Universe, and whether an intermediate-mass black hole, one roughly 10,000 times as massive as our Sun, might be lurking among the stars.

Diagram showing projected movement of stars in Omega Centauri

Hubble's multi-colour snapshot (top) of the central region of the giant globular cluster Omega Centauri. The lower illustration charts the future positions of the stars highlighted by the white box in the top image. Each streak represents the motion of the stars over the next 600 years.

Razor-sharp vision the key

Analysing archived images taken over a four-year period by Hubble’s Advanced Camera for Surveys, astronomers have made the most accurate measurements yet of the motions of more than 100,000 cluster inhabitants, the largest survey to date to study the movement of stars in any cluster.

“It takes sophisticated computer programs to measure the tiny shifts in the positions of the stars that occur over a period of just four years,” says astronomer Jay Anderson of the Space Telescope Science Institute in Baltimore, USA, who conducted the study with fellow Institute astronomer Roeland van der Marel.

“Ultimately, though, it is Hubble’s razor-sharp vision that is the key to our ability to measure stellar motions in this cluster.”

Van der Marel adds: “With Hubble, you can wait three or four years and detect the motions of the stars more accurately than if you were using a ground-based telescope and were waiting 50 years.”

The astronomers used the Hubble images, which were taken in 2002 and 2006, to make a movie simulation of the frenzied motion of the cluster’s stars. The movie shows the stars’ projected migration over the next 10,000 years.

Identified as a globular star cluster in 1867, Omega Centauri is one of roughly 150 such clusters in the Milky Way. The behemoth stellar grouping is the biggest and brightest globular cluster in the Milky Way, and one of the few that can be seen by the unaided eye.

Located in the constellation of Centaurus, Omega Centauri can be seen in the southern skies.

Adapted from information issued by NASA / ESA / J. Anderson & R. van der Marel (STScI).

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