The Universe looks very different today compared to how it looked 12 billion years ago. Galaxies once ‘hotspots’ where billions of stars were created are now cosmic graveyards, and exactly what killed these stars has been a mystery until now. Research published today says these galaxies stopped making stars because of black holes lurking at their centres.Astronomers at the University of Iowa studied a few of these galaxies that are still star-making factories, known as dusty starburst galaxies, and found quasars at the centre of four of them.Quasars are extremely bright sources of radio waves, which are powered by disks of matter rotating around supermassive black holes.Stars survive by burning hydrogen gas as fuel, and when this runs out they start to die. The team’s paper argues these quasars are the reason these dusty starburst galaxies became extinct, by ejecting gas far away from the galaxies and starving the stars of their fuel. “The surprising part of the finding is that, although the new ALMA observations located these quasars right at the centres of dusty starburst galaxies, these quasars look the same as other quasars living in normal galaxies,” Hai Fu, assistant professor at the University of Iowa and the paper’s first author, told WIRED.
Quasars should not be detectable in dusty starburst galaxies because the light would be absorbed, or blocked, by the dust and gas churned up by the process of star formation.
Fu added: “The starburst galaxies hosting these quasars look the same as other starbursts that don’t appear to host quasars.” This means, Fu says, there may be a quasar at the centre of every dusty starburst galaxy, it just cannot be seen. In these particular galaxies where they have been spotted, the researchers think the quasars are peeking out from deep holes, a vacuum free of debris that allows light to escape its cloudy surroundings.
“It’s a rare case of geometry lining up,” says Jacob Isbell, the paper’s second author. “And that hole happens to be aligned with our line of sight.”