NASA's Wide-field Infrared Survey Explorer (WISE) telescope surveyed the universe twice during its 14-month mission, generating a wealth of data in the process. Now, by sifting through that data, WISE astronomers have unveiled one of the telescope's most impressive finds yet: 2.5 million supermassive black holes — about three times as many as have been detected by previous surveys — sprinkled throughout the cosmos.
But there's more. WISE has also turned up evidence of a never-before seen population of galaxies called "hot DOGs," short for "hot, dust-obscured galaxies". These objects are among the brightest, hottest and most powerful ever observed in the universe, and could represent a missing link in galaxy evolution
Black Hole Bonanza
"WISE has found a bonanza of black holes," said Jet Propulsion Laboratory astronomer Daniel Stern during a teleconference yesterday afternoon (which we live-blogged — check it out here for tons of pictures and Q&A summary), "more than had been found by any previous survey."
These aren't your run-of-the-mill black holes. These millions of newly identified cosmic entities are all believed to be quasars — yawning reaches of spacetime, millions to billions of times more massive than our sun, that have been caught feasting away on the gas and dust that surrounds them. The matter falling into a quasar causes it to grow tremendously hot; and while they are also very bright, obscuring dust makes it difficult to observe their visible light. That's where WISE comes in. These massively powerful black holes all but smolder in the infrared spectrum, and WISE's sensitive instrumentation picks up on it with ease.
The image featured up top is an artist's conception of what one of these quasars might look like. Below is an infrared view of the Universe as seen by WISE. The inset is a closeup of a tiny portion of the WISE sky that covers an area about three-times larger than the Moon. Quasar candidates have been circled in yellow:
Between WISE and NuSTAR (NASA's black-hole hunting Nuclear Spectroscopic Telescope Array) "we've got the black holes cornered," said Stern, lead author of the paper describing WISE's quasar observations. "WISE is finding them across the full sky, while NuSTAR is giving us an entirely new look at their high-energy X-ray light and learning what makes them tick."
"Hot DOGs" Could be Missing Link in Galaxy Evolution
Also announced yesterday: a small population of never-before-seen cosmic entities: around 1,000 blazing hot, dust-obscured galaxies — or "hot DOGs," for short.
"Hot dogs are much, much rarer" than the quasars discovered by Daniel Stern's team, explained WISE project scientist Peter Eisenhardt at yesterday's conference. Eisenhardt is first author on the first of two papers describing the newly discovered hot DOGs. Astronomer Jingwen Wu, lead author of the second study, was present for the announcement, as well.
According to Wu, hot DOGs are much brighter, hotter and more powerful than any galaxy we're used to seeing. The image featured here gives a WISE-eye view of the universe, with the 1000-or-so observable hot DOGs labeled in purple. The inset is a close-up of the region of sky containing the first hot DOG ever discovered (the red point circled in purple). According to Wu, each of these galaxies is capable of generating up to 100-trillion times as much energy as our sun:
"They may be hosting an extremely powerful supermassive black hole at their center which can heat the dust to high temperatures," explained Wu. "We may be seeing a rare phase of galactic evolution where dust and gas are heated and ejected by supermassive black holes. This may be a missing link of galaxy evolution."
Rutgers astrophysicist Rachel Somerville agrees with Wu. "We think we may be seeing these galaxies at a crucial transformational stage," she explained at yesterday's news briefing. Hot DOGs, she notes, could be what astronomers observe when two colliding spiral galaxies are in the process of transforming into an elliptical one. By that logic, explains Somerville, our own Milky Way could very well become a hot DOG, itself. Our galaxy is projected to collide with our cosmic neighbor, the spiral Andromeda galaxy, in about 2 billion years; and that collision, which Somerville says could resemble the simulation shown here, could make a hot DOG of us, yet:
That being said, most of the hot DOGs observed by WISE are about 10-billion light years from Earth. That means they they formed during the universe's infancy, when conditions may have been better suited for giving rise to this class of "hyper-luminous" galaxy — a fact that leaves Eisenhardt skeptical about our galaxy's chances of ever becoming a hot DOG:
"It does seem to me that the phenomena we're talking about here is much more common in the earlier stages of the Universe," he explains, "so while [the Milky Way] may become a hot DOG, I think it's a relatively unusal event today, based on the distribution of distances that we're finding."