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Explosive outflows halt star formation in distant galaxy
KEITH COOPER
ASTRONOMY NOW
Posted: 12 March 2010


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Tumultuous events in a galaxy far, far away are signalling the end of star formation within it, indicating how and why the most massive galaxies in the Universe ceased growing a few billion years after the big bang.

Astronomers from the University of Durham and Université Paris have used the Gemini North Telescope in Hawaii to uncover powerful outflows of energy that are heating gas in a distant galaxy. Gas can only form stars when it is cool and has low enough energy for gravity to overcome it and cause it to condense.

“Effectively the galaxy is regulating its growth by preventing new stars from being born. It is only now that we have seen this in action,” says Dr Dave Alexander of Durham University, who led the observations.

The Gemini NIFS observation of SMM J1237+6203. The contour lines show how the energy is blasting through the galaxy. Image: Dave Alexander/Mark Swinbank (Durham University)/Gemini Observatory.

The greatest rates of star formation in massive galaxies ended 10–12 billion years ago, and astronomers have wanted to know why. For a long time they have suspected that hurricanes of radiation blowing out from the active supermassive black holes in the cores of these galaxies, in addition to the shock waves from supernovae, were the culprits but until now there was little concrete proof that they were to blame.

The target of the Gemini observations, which utilised the telescope’s Near Infrared Integral Field Spectrometer (NIFS), was a galaxy called SMM J1237+6203, which we see as it existed ten billion years ago. This particular galaxy is what is known as an ULIRG – Ultra Luminous Infrared Galaxy – which is undergoing intense star formation, producing copious amounts of dust that swathe and conceal the active supermassive black hole (a quasar) within. ULIRGS are believed to be an important phase in galaxy evolution, existing in the era when galactic bulges and their black holes grew substantially before star formation on a grand scale began shutting down, eventually clearing the dust to expose the quasar within.

NIFS was able to identify that gas within SMM J1237+6203 was moving at up to 900 kilometres per second, being heated and blown about by an explosive outflow that lasts for millions of years. The estimated energy required to power such an outflow is 1044 ergs per second (1037 joules) and 1059 ergs over 30 million years. The motion and energy of this outflow matches what would be expected from such outflows, but how much is from the black hole and how much is from supernovae has yet to be determined.

The measurements suggest that the outflow is not quite powerful enough to blow the gas out of the galaxy, and it is still an open question as to what prevents the gas from re-cooling and forming stars at a later date – perhaps the outflows last longer than anticipated. Previous evidence does indicate that the outflows, if not the gas itself, does reach the intergalactic medium. In summer 2009, a survey of ‘blobs’ of hydrogen in the distant Universe conducted by Durham astronomers Jim Geach and Brett Lehmer with NASA’s Chandra X-ray Observatory revealed that these blobs were half-a-million light-year wide clouds that were the leftovers of the formation of galaxies (read our related story here). The blobs encased burgeoning galaxies with growing black holes and star formation, from which outflows were irradiating the gas, causing it to heat up and emit X-rays.

Now the plan is to seek out other galaxies where outflows may also be running amok. “We believe that similar outflows are likely to have stopped the growth of other galaxies in the early Universe by blowing away the materials needed for star formation,” says Alexander. Key to this search will be the new, UK-constructed VISTA telescope at the European Southern Observatory in Chile. VISTA is the largest visible and infrared survey telescope in the world and will be able to peer towards galaxies at similar distances to SMM J1237+6203 and beyond to finally help piece together their evolution.

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