The field of astronomy has been revolutionized by hardware that performs surveys, comprehensive catalogs of everything out there that emits light in a given chunk of the spectrum. Instead of looking at individual objects, astronomers can now examine hundreds or thousands of objects of a given type, and get a sense of what the typical population is like.

This week, NASA announced some results of its Wide-field Infrared Survey Explorer (WISE), a space-based telescope that has scanned the entire heavens since its launch at the end of 2009, obtaining at least five images of 95 percent of the sky at four infrared wavelengths. The WISE mission has identified plenty of cool objects that are close to home, like asteroids in our Solar System and brown dwarfs in our galaxy. But now, its data has been combed for objects beyond the Milky Way, and the search has turned up millions of supermassive black holes, along with the most luminous galaxies we've ever detected, including a new class of what are called "hot DOGs."

WISE was able to spot so many objects because the Universe is a very dusty place, and that dust obscures objects that might otherwise be visible in optical wavelengths. But infrared tends to pass through the dust unhindered, while the dust itself can glow in these wavelengths when heated.

Hunting black holes

Most—if not all—galaxies appear to host a supermassive black hole at their center. Early in the galaxy's history, or following collisions with other galaxies, these black holes would be fed with lots of gas, forming an active galactic nucleus that emits copious amounts of light. The ones we've detected are the most luminous objects in the Universe. But we can't detect the ones that are buried in dust very easily without imaging in the infrared or searching for very high energy photons.

In the past, a series of space-based observatories (Spitzer, Chandra, and Hubble) have imaged a specific area of the sky called the COSMOS field, identifying dozens of black hole candidates in the region. In one of the preprints released in conjunction with the announcement, a team has looked at the same spot in the WISE data. By testing various ways of looking at the different wavelengths imaged by WISE, the researchers were able to find a set of properties that uniquely identified dust-obscured black holes. Their technique could identify about 80 percent of the objects found in the previous study, with a reliability of over 90 percent.

NASA's press release indicates they then applied the same criteria to the complete body of the WISE data, which covers the entire sky. That produced a truly staggering number: 2.5 million active black holes at the center of galaxies, about two-thirds of them obscured by dust.

Finding hot DOGs

Dust and active black holes aren't a stable combination. As a black hole feeds, it starts to blast away the surrounding gas and dust with the energetic light and particles it emits. That clears away the dust, making the active galactic nucleus apparent in visible wavelengths. It also puts an end to the star formation that occurs in the clouds of gas and dust, limiting the size of galaxies.

But that process doesn't happen overnight, and the WISE survey may have identified a set of galaxies that are undergoing this transition.

These galaxies belong to a class called "ULIRGs," for ultra-luminous infrared galaxies. Normally, there are two ways of getting a galaxy this bright in the infrared: either the dust is being heated by a rapid burst of star formation, or when lit up by a supermassive black hole. But WISE has spotted a thousand objects that may be experiencing both.

The objects were identified based on the fact that they are visible in only some of the wavelengths imaged by WISE. Follow-up observations in visible light show that these galaxies have an active central black hole that's largely obscured by dust, making them DOGs—dust-obscured galaxies. A detailed measuring of their infrared spectrum suggests, however, that their dust is heated to a temperature of about 100K, or twice that of the normal dust temperature (this is the "hot" in "hot DOGs"). This high temperature suggests a major burst of star formation is happening.

The authors of that study can't tell if these 1,000 or so hot DOGs are the extreme tail end of a collection of galaxies that are otherwise unremarkable—perhaps mergers between large galaxies?—or simply a very brief period in the normal evolution of most galaxies. The answer could help us learn about the process by which a black hole brings an end to star formation.

But they are certain they've spotted a monster. One of the hot DOGs, WISE 1814+3412, is apparently forming stars that add up to about 300 times the mass of the Sun each year, even as it hosts a feeding supermassive black hole. Observations suggest its total luminosity across all wavelengths is nearly 1014 times that of our Sun, making it "ultraluminous," and placing it among the most luminous galaxies ever spotted in the visual wavelengths.