Scientists have long believed the formation of a quasar in a galaxy would spell the end of star formation there. A new analysis from Allison Kirkpatrick, assistant professor of physics and astronomy at the University of Kansas, suggests that may not be the case. Kirkpatrick found that some galaxies hosting quasars do still seem to have the requisite conditions for new star formation. This discovery could change the way astrophysicists model the later stages of galactic evolution.
Most large galaxies have supermassive black holes in their centers, but a quasar is an entirely different animal. These “quasi-stellar radio sources” have an accretion disk of matter spiraling toward them. The intense X-ray radiation makes these galaxies extremely bright and hot, and they pull in all the comparatively cold gas in a galaxy that could otherwise go toward the formation of new stars. Often, the magnetic field of a quasar is so powerful that it creates a relativistic jet of material that drains the galaxy of cold gas.
According to Kirkpatrick, most quasars do signal that a galaxy is done producing new stars, but it’s not akin to the stellar “off” switch we thought. Her analysis of distant galaxies pointed to a population of objects dubbed “cold quasars.” The first hits of cold quasars popped up in the Sloan Digital Sky Survey, a highly detailed digital map of the universe. The team called this region of unusual quasar activity “Stripe 82.” They used the XMM Newton telescope to survey the area and map the locations of growing quasars via X-rays. Then, they went back with the Herschel Space Telescope to scan the region in far infrared.
Seeing galaxies in both “hot” X-ray and “cold” infrared indicates there is still a supply of star-generating gas in active quasar galaxies. According to Kirkpatrick, this doesn’t mean quasars can exist indefinitely alongside star-forming regions. They do still leach away cold gas over time, ending star formation in the galaxy.
What we are seeing here is a new transition period between the growth of quasars and the end of star formation, lasting up to 10 million years. Only about 10 percent of galaxies in the study exhibited this behavior, though. The next step for Kirkpatrick is to determine if specific classes or sizes of galaxies go through this transition period.