Black holes are common. But actually counting them is no easy task. Even the brightest can be hard to see. Of course, “bright black holes” is a bit of an oxymoron. But when supermassive black holes at the center of a galaxy feed, the material falling into them heats up, giving off a bright glow across the electromagnetic spectrum.
Infrared light in particular is good to look for when black-hole hunting. Some feasting black holes are obscured by gas and dust, which absorbs much of their glow—but they still shine in the infrared.
It’s no surprise, then, that NASA’s infrared WISE spacecraft found a bunch. WISE launched in 2009 to survey the entire sky in the infrared. Now researchers have used WISE data to count the luminous black holes in a well-known corner of the sky called the COSMOS field. In just that tiny region, WISE found about 130 glowing black holes, or active galactic nuclei. The study will appear in the Astrophysical Journal. [Daniel Stern et al., Mid-Infrared Selection of AGN with the Wide-Field Infrared Survey Explorer. I. Characterizing WISE-Selected AGN in COSMOS]
Extrapolating from this small area, at least two million active black holes dot the sky, confirming that these extreme astrophysical objects are common indeed.
[The above text is a transcript of this podcast.]