Astronomy Now Home
Home Magazine Sky Chart Resources Store

On Sale Now!



The September 2014 issue of Astronomy Now is on sale! Order direct from our store (free 1st class post & to UK addresses). The Astronomy Now iPad/iPhone editions are now available worldwide on the App Store.



Top Stories



Earthshine used to test life detection method
...By imagining the Earth as an exoplanet, scientists observing our planet's reflected light on the Moon with ESO's Very Large Telescope have demonstrated a way to detect life on other worlds...
  READ MORE

Solid buckyballs discovered in space
...Astronomers using NASA’s Spitzer Space Telescope have detected a particular type of molecule, given the nickname “buckyball”, in a solid form for the first time...
  READ MORE

Steamy water-world gets the Hubble treatment
...Hubble Space Telescope observations of a 7 Earth-mass planet find an unusual water-rich world swathed in a thick, steamy atmosphere...
  READ MORE








Falling clouds solve
quasar puzzle

KEITH COOPER
ASTRONOMY NOW
Posted: 9 July 2010


Bookmark and Share

The mystery behind the origin of quasars – the brilliantly bright centres of distant galaxies – may have been solved by a group of New York astronomers who suggest that giant clouds of hydrogen gas falling onto the centres of galaxies are being consumed by the supermassive black holes lurking within.

The majority of quasars (also called active galactic nuclei, or AGN) are found to have existed during the first four or five billion years of the Universe’s history, during an epoch in which galaxies were being assembled by giant clouds of gas. Now Barry McKernan and K E Saavik Ford of the American Museum of Natural History and City University of New York, and Ariyeh Maller also of City University, have put the two together.

An artist’s impression of a quasar in the early Universe, with giant gas clouds falling onto it and creating explosions of star formation. Image: Wolfram Freudling et al (STECF)/ESA/ESA/NASA.

“For a while, people have known that gas clouds are falling onto galaxies, and they’ve also known that active galactic nuclei are powered by gas falling onto supermassive black holes,” says McKernan. “But no one put the two ideas together until now and said, ‘Hey, maybe one is causing the other!’”

These gas clouds are vast, millions of times more massive than the Sun. As they fall onto a galaxy they generate large amounts of star formation, but they are also the ideal mechanism for getting gas into galactic centres to provoke quasar activity. Previous theories had suggested that galactic bars could funnel gas into the centre to feed the black hole, or that the gravitational effects of mergers could divert large quantities of gas towards the black hole. However, if mergers were the main trigger we’d see quasars wherever we see lots of galaxies merging, and similarly if bars were responsible we would see quasars only in barred spirals, but we see them in other types of galaxy too. The new model has the advantage, says McKernan, of already being known to happen and being able to provide enough gas. For instance, a full blown quasar gobbles up thousands of solar masses per year. And during the galaxy building epoch there were plenty of these gas clouds around, which were closely linked to star-formation rates.

A Hubble Space Telescope image of the quasar PKS 2349. Image: John Bahcall (Institute for Advanced Study, Princeton)/Mike Disney (University of Wales)/NASA/ESA.

“We assume that the rate of cloud impact goes as the star-formation rate,” McKernan tells Astronomy Now. “In other words, at a redshift of 1 [about seven billion years ago], we’d expect about ten times the number of impacting clouds than in the present era. At a redshift of 2 [about ten billion years ago, when star formation hit its peak rate] you’d expect maybe 100 times the number of impacting clouds.” Because all galaxies are believed to have been constructed in the same process, including our Milky Way, then at one time all galaxies will have been quasars.

Some of these clouds still float around today, flotsam and jetsam leftover from the quasar epoch. “The leftover clouds are still bombarding galaxies, including ours, occasionally,” says McKernan. “Every so often one of these clouds must hit the centre and light things up.”

However, this would not create a fully-fledged quasar in the modern era. Seyfert galaxies, which have low power AGN, consume an amount of gas equivalent to about half the mass of the Sun per year. Even lower level AGN activity has been seen in galaxies, where the central black hole gobbles up, at most, the equivalent of just one percent of the mass of the Sun per year. McKernan believes this low level activity could be blamed on individual rogue clouds striking at random.

“However, in rare cases, our mechanism could produce a train of impacting clouds (rather like the impact of Shoemaker-Levy 9 with Jupiter), which might be enough to promote quasar-like activity,” suggests McKernan. But if more than ten million years passes between galaxies being hit, their centres will appear normal, and so such activity is relatively rare in the local Universe.

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.
 GET YOUR COPY

Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.
 GET YOUR COPY

3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!
 GET YOUR COPY


HOME | NEWS ARCHIVE | MAGAZINE | SOLAR SYSTEM | SKY CHART | RESOURCES | STORE | SPACEFLIGHT NOW

© 2014 Pole Star Publications Ltd.