Astronomers using data from the WISE all-sky infrared survey have discovered a new class of luminous galaxies.
There are hot dogs in the sky.Astronomers have discovered a new class of galaxies in the distant universe. These galaxies are monstrously bright in the infrared — 10 trillion times brighter than the Sun’s average luminosity and at least 10 times brighter than extreme star-forming galaxies. They rival the brightest known active galactic nuclei, or AGNs, where frantically gobbling supermassive black holes spit out jets of plasma.
British and American astronomers discovered the new class of galaxies, called hyperluminous infrared galaxies, in data from the Wide-field Infrared Survey Explorer (WISE). WISE surveyed the entire sky in four infrared wavelengths in 2010, and the project released its full data set this past March.
One of WISE’s main science goals was to find the most luminous, dusty galaxies out there, but WISE project scientist Peter Eisenhardt (Jet Propulsion Laboratory, Caltech) says the new group of galaxies took them by surprise. Instead of showing up in all four WISE bands, the hyperluminous galaxies are invisible in the two shorter wavelengths — 3.4 and 4.6 microns — but bright in the longer ones of 12 and 22 microns. The strong 22-micron emission probably comes from warm dust that is either widespread enough or thick enough (or both) to block emission at shorter wavelengths from the galaxies’ centers, explaining why these objects are invisible in both near-infrared and visible light. For every visible light photon that makes it through the dust blockade, another billion trillion photons are absorbed.
The galaxies’ dust radiates heat between 60 and 120 kelvin. That’s pretty cold by human standards — about -150° Celsius (-240° F), colder than the coldest temperature ever recorded in Antarctica — but it’s three to four times the temperature of the next brightest class of infrared galaxies, so-called ultraluminous infrared galaxies (ULIRGs), says study team member Jingwen Wu (JPL-Caltech). The hot dust that appears to enshroud these galaxies’ supermassive black holes inspired Wu to label these objects “hot dust-obscured galaxies,” or hot DOGs.
The team found only about 1000 hot DOGs among the roughly 2.5 million AGNs detected by WISE, which were announced along with the hot DOGs in a NASA press conference on August 29th and reported in three papers in the Astrophysical Journal. So far the team has only measured distances for 147 of the hot DOGs using follow-up observations with other facilities, including the Caltech Submillimeter Observatory, but most of the sources cluster around redshift 2 to 3, or within the universe’s first few billion years of existence.Cosmos-wide star formation reached its peak during that era, but although the astronomers do detect starlight from the hot DOGs, it makes up less than 10% of the galaxies’ total light — in fact, Eisenhardt says, it’s probably less than 1%. That’s not from a lack of stars: the first of the hot DOGs found, WISE 1814+3412, is making roughly 300 stars a year, a tremendous burst of star formation. But starlight can’t heat dust to high enough temperatures to explain the infrared light, so black hole activity is likely to blame instead.
Gas-guzzling supermassive black holes can produce an intense radiation field that heats dust. But to explain the infrared light pouring out of these galaxies, the central black holes must be either truly supermassive — among the most massive black holes known in the universe — or they are less massive and are gobbling down gas at a higher rate than ought to be physically possible. Eisenhardt and his colleagues consider the latter scenario to be more likely because these galaxies may be at a special, short-lived stage in their evolution.
AGN researcher Rachel Somerville (Rutgers University) agrees that the hot DOG phase is probably short-lived, maybe less than a million years. It could turn on when galaxies collide and merge as part of the natural evolution of galaxies and black holes. According to one popular evolutionary scenario, merging galaxies should spur on star formation and funnel gas and dust toward the galaxy’s center and into the black hole’s maw. That means black hole activity ought to lag behind starbursts, but in hot DOGs the black-hole-blazing activity seems to come as the star formation is just getting started. Why that is remains unclear.
The first hot DOG: Eisenhardt, P.R.M. et al. "The First Hyper-luminous Infrared Galaxy Discovered by WISE." The Astrophysical Journal, 2012 August 20. Full text.
Follow-up on the hot DOG population: J. Wu et al. Submillimeter Follow-up of WISE-selected Hyperluminous Galaxies."The Astrophysical Journal, 2012 September 1. Full text.
The larger population of AGN discovered by WISE: D. Stern et al. "Mid-infrared Selection of Active Galactic Nuclei with the Wide-field Infrared Survey Explorer. I. Characterizing WISE-selected Active Galactic Nuclei in COSMOS." The Astrophysical Journal, 2012 July 1. Full text.