Galaxy cannibals eat their neighbours
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: June 24, 2008

Previously unseen galactic cannibalism within the supermassive black holes that occupy the centres of Seyfert galaxies has been revealed by radio observations courtesy of the Very Large Array (VLA).

Artist impression of interacting galaxies. Image: Kuo et al.

Astronomers have long suspected that the extra-bright cores of spiral galaxies called Seyfert galaxies are powered by supermassive black holes consuming material, however they could not see what the trigger was for forcing material into the jaws of these monstrous black holes.

One leading theory describes a disruption of galactic material by close confrontations with a neighbouring galaxy, stirring up its gases and driving it into the gravitational clutches of the central black hole. However, visible light observations of Seyfert galaxies showed that a surprisingly small fraction offered any evidence of such an encounter.

Now, in this eye-opening new radio study of nearby Seyfert galaxies using the VLA, the detection of radio waves emitted by hydrogen atoms reveal that the majority of Seyferts are disturbed by dramatic encounters with proximate galaxies after all.

"The VLA lifted the veil on what's really happening with these galaxies," says Cheng-Yu Kuo, a graduate student at the University of Virginia. "Looking at the gas in these galaxies clearly showed that they are snacking on their neighbours. This is a dramatic contrast with their appearance in visible starlight.”

Visible (left) and radio (right) images of interacting Seyfert galaxies. The encounters are invisible in visible light, but gas can be seen streaming between galaxies at radio wavelengths, as a result of emission from hydrogen atoms. Image: Kuo et al /NRAO/ AUI/NSF.

By comparison, similar VLA images of inactive galaxies showed that very few were disturbed. In addition, the astronomers also noted a wide range of energetic activity relating to the appetite of the black holes, ranging from the relatively mild Seyfert galaxies to the devastatingly powerful quasars and blazars.

"This comparison clearly shows a connection between close galactic encounters and the black-hole-powered activity in the cores," says Ya-Wen Tang, who began this work at the Institute of Astronomy and Astrophysics, Academia Sinica (ASIAA) in Taiwan and now is a graduate student at the National Taiwan University. "This is the best evidence yet for the fueling of Seyfert galaxies. Other mechanisms have been proposed, but they have shown little if any difference between Seyferts and inactive galaxies.”

The results of the study teach the important lesson that it is insightful to look at galactic objects in a range of wavelengths to reveal key processes that may have otherwise gone unnoticed.

"Our results show that images of the hydrogen gas are a powerful tool for revealing otherwise invisible gravitational interactions among galaxies," says Jeremy Lim, also of ASIAA. "This is a welcome advance in our understanding of these objects, made possible by the best and most extensive survey ever made of hydrogen in Seyferts.”