A Tour of Cases of Black Hole Mistaken Identity

Astronomers have discovered one type of growing supermassive black hole masquerading as another, thanks to a suite of telescopes including NASA's Chandra X-ray Observatory. The true identity of these black holes helps solve a long-running mystery in astrophysics.

The misidentified black holes are from a survey known as the Chandra Deep Field-South, which is the deepest X-ray image ever taken.

Supermassive black holes grow by pulling in surrounding material, which is heated and produces radiation at a wide range of wavelengths including X-rays. Many astronomers think this growth includes a phase, which happened billions of years ago, when a dense cocoon of dust and gas covers most black holes. These cocoons of material are the fuel source that enables the black hole to grow and generate radiation.

Based on the current picture held by astronomers many black holes immersed in such a cocoon should exist. However, this type of growing black hole is notoriously difficult to find, and until now the observed number has fallen short of predictions — even in the deepest images.

Now a team of astronomers has discovered 28 black holes that were previously misidentified in the Chandra Deep Field-South. By showing that there are this many missed black holes in this small patch of sky, researchers can estimate how many others there might be across the Universe.

These results are important for theoretical models estimating the number of black holes in the universe and their growth rates, including those with different amounts of obscuration. Scientists design these models to explain a uniform glow in X-rays across the sky called the "X-ray background," first discovered in the 1960s.

Since then, astronomers have used Chandra and other telescopes to bring the X-ray background into focus and show that most of the glow comes from individual growing black holes. This newest discovery involves understanding the nature of the objects that have been some of the last to be resolved.