Researchers have shown that massive black holes can be found in dwarf galaxies in much higher numbers than previously thought meaning that massive black holes are many times more common in dwarf galaxies than previously thought.
University of North Carolina at Chapel Hill researchers and colleagues found a previously overlooked treasure trove of massive black holes in dwarf galaxies. The newly discovered black holes offer a glimpse into the life story of the supermassive black hole at the center of our own Milky Way galaxy.
Black holes are typically detected when they are actively growing by ingesting gas and stardust swirling around them, which makes them glow intensely. The problem is, while growing black holes glow with distinctive high-energy radiation, young newborn stars can too. Traditionally, astronomers have differentiated growing black holes from new star formation using diagnostic tests that rely on detailed features of each galaxy’s visible light when spread out into a spectrum like a rainbow.
The path to discovery began when undergraduate students tried to apply these traditional tests to galaxy survey data. The team realized that some of the galaxies were sending mixed messages — two tests would indicate growing black holes, but a third would indicate only star formation.
Researchers took on the challenge of constructing a new census of growing black holes, with attention to both traditional and mixed-message types. She obtained published measurements of visible light spectral features to test for black holes in thousands of galaxies found in two surveys RESOLVE and ECO. These surveys include ultraviolet and radio data ideal for studying star formation, and they have an unusual design: Whereas most astronomical surveys select samples that favor big and bright galaxies, RESOLVE and ECO are complete inventories of huge volumes of the present-day universe in which dwarf galaxies are abundant.
More than 80 percent of all growing black holes she found in dwarf galaxies belonged to the new type.