NASA’s WISE detects universe’s most luminous galaxy
NASA’s Wide-field Infrared Survey Explorer (WISE ) has recently discovered a remote galaxy so luminous, it shines with the light of 300 trillion suns. This galaxy easily wins the title of most luminous in the ‘verse, and is the latest specimen in a relatively new class of galaxy called Extremely Luminous Infrared Galaxies, or ELIRG for short.
“We are looking at a very intense phase of galaxy evolution,” said Chao-Wei Tsai of NASA’s Jet Propulsion Laboratory in Pasadena, California, lead author of a new report appearing in the May 22 issue of The Astrophysical Journal. “This dazzling light may be from the main growth spurt of the galaxy’s black hole.”
The new galaxy, dubbed WISE J224607.57-052635.0, could be harboring a monstrous black hole continuously gorging itself on gas. Supermassive black holes lurk at the center of most galaxies, pulling gas and other matter towards them through a process known as accretion, forming a swirling disk of superheated material around the black hole. This, in turn, results in the emission of X-ray light.
Black holes are also messy eaters: some material won’t reach the event horizon but instead is caught up in powerful magnetic fields created by the rapidly swirling accretion disk existing around the black hole, forming two opposing jets perpendicular to the disk. These “jets” not only shoot some material away, but they also emit prolific amounts of energy high-energy electromagnetic waves. Leftover gas and dust surrounding the black hole acts as a shroud, blocking this light. However, as the swirling material around the black hole heats up, so does the dust shroud, radiating infrared light.
Massive black holes are found in the heart of nearly every galaxy, but finding one this big so “far back” in the early universe is rare. When we peer deep into the universe, we are “seeing” galaxies as they were billions of years ago. The galaxy WISE J224607.57-052635.0 is thought to be 12.5 billion years old, meaning it has taken that long for the light from that galaxy to reach us. We are essentially looking back in time and seeing the galaxy as it was in the distant past. The universe is said to be 13.82 billion years old, so by the time the universe was a tenth of its current age, the black hole at the center of WISE J224607.57-052635.0 was already billions of times more massive than the Sun.
A new study co-authored by WISE project scientist Peter Eisenhardt outlines several reasons why black holes in this new category of ultra-luminous galaxies (ELIRGs) can grow to be so massive. The first reason is the embryonic black holes could have been “born” more massive than previously thought.
“How do you get an elephant?” Eisenhardt asked, “One way is start with a baby elephant.
Other possible explanations include breaking or bending the Eddington limit, a theoretical limit on how much a black hole can “eat”. When black holes gorge themselves, gas is pulled in, heating up in the process and emitting high-energy light (X-rays). The intense radiation pressure from the X-rays pushes away the gas and actually restricts how fast the black hole can gobble up matter, thereby limiting the black hole’s “food” supply. This is the Eddington limit.
If a black hole could break this limit, it could have a tremendous growth spurt. Black holes have been known to break the Eddington limit, but it’s very rare and in order for a black hole to reach the size of the one in WISE J224607.57-052635.0 it would have to break the Eddington limit repeatedly. Or it could just bend the rules a bit.
“Another way for a black hole to grow this big is for it to have gone on a sustained binge, consuming food faster than typically thought possible,” said Tsai. “This can happen if the black hole isn’t spinning that fast.”
Black holes spin as they feed, and their spin rate dramatically affects how much matter they can consume. A slower spinning black hole will produce a smaller blast of high-energy light from its accretion disk than a fast spinning black hole. As such, the slower spin rate allows the black hole to over indulge.
“The massive black holes in ELIRGs could be gorging themselves on more matter for a longer period of time,” said Andrew Blain of University of Leicester in the United Kingdom, a co-author of this report. “It’s like winning a hot-dog-eating contest lasting hundreds of millions of years.”
In order to better understand the new class of ultra-luminous galaxies, more research is required. The team has devised a plan to measure the mass of the central black holes in ELIRGs. The ability to accurately measure their masses will provide insight into their history and evolution.
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