Physicswhynot: Hot DOG surprise reveals new stage in galaxy evolution

Here's a frankfurter that won't fit on a bun. Hot dust-obscured galaxies, or hot DOGs, are a new type of cosmic object that could help answer a decades-old problem: which came first, the galaxy or the black hole?

The newly discovered galaxies are among the brightest in the universe, 1000 times brighter than the Milky Way, but they are so heavily clouded by dust that they had gone entirely unnoticed until now – hence the description "hot, dust-obscured".

Astronomers think they could represent a new phase in galaxy evolution."We may be seeing them at a crucial transformational stage," says Rachel Somerville of Rutgers University in Piscataway, New Jersey, who was not involved in the new work. "Just as if we see a butterfly emerging from cocoon, it might suggest butterflies and caterpillars are the same animals, which we otherwise might not realise."

The hot DOGs showed up in an all-sky survey by the NASA WISE (Wide-field Infrared Survey Explorer) space telescope in 2010. WISE scanned the sky in infrared wavelengths corresponding to heat, meaning it could peer behind the veil of dust that obscures the visible light from hot objects.

Black hole shock

Astronomers expected the brightest objects captured by WISE to be active galactic nuclei – formed when gluttonous black holes that lurk at the centres of galaxies guzzle gas and dust to grow bigger. Radiation from the black hole heats the gas and dust to white-hot temperatures just before the material falls in. The glow from that hot gas can outshine our galaxy by orders of magnitude.

And indeed, WISE uncovered millions of these galactic nuclei, finding that for every one that could be seen in visible light, there were two or three hidden behind a dust veil.

But surprisingly, the telescope also uncovered about 1000 objects that were even brighter – and stranger. Follow-up observations showed that they were mostly about 10 billion light years away, and were 1000 times brighter than the entire Milky Way and more than twice as hot as the average galaxy. But they were so heavily obscured by dust that even WISE couldn't see them in two of its four cameras, which each sense a different infrared wavelength.

As well as pumping out huge amounts of light, hot DOGs also seem to concentrate more of their mass in their central black hole than in their stars. Most mature galaxies end up with a central black hole that is about 500 times the mass of all their stars combined. But the brightness of hot DOGs suggests that their central black holes are even more massive relative to the surrounding stars.

Different beast

"In that way they're definitely a different type of beast than we've seen before," says Peter Eisenhardt of NASA's Jet Propulsion Laboratory in Pasadena, California, a project scientist for WISE.

That centre-heavy bias could mean that these galaxies are in a phase where the black hole is eating material much faster than the galaxy can form new stars. At the same time, the pressure from the black hole's radiation is pushing the gas and dust around it away. Over time, the black hole will clear its surroundings of the obscuring gas and dust, transforming the hot DOG into an ordinary, visible galaxy. "We may be seeing a rare phase of galaxy evolution, where dust and gas are being heated and ejected by the supermassive black hole," says Eisenhardt's colleague Jingwen Wu.

"This suggests the supermassive black holes may grow before their stars are fully formed," Eisenhardt says. "If you call the stars the chickens, we're saying maybe the eggs are there first."

Despite their names, hot DOGs are actually not very hot – their average temperature, taking into account not only their stars but the cold interstellar gas, is about 100 Kelvin, or -173 degrees Celsius. The average galaxy is just 30 or 40 K. But a real hot dog, the kind you might eat, is about 350 K.