‘Cold quasar’ discovery could signal galaxies’ ‘retirement party’ phase

Astronomers may need to rethink how galaxies die after researcher Allison Kirkpatrick’s discovery of “cold quasars,” galaxies full of cold gas that still give birth to stars, despite having highly energetic quasars at their center.

Kirkpatrick, an assistant professor of physics and astronomy at the University of Kansas, announced the finding Wednesday during the 234th meeting of the American Astronomical Society in St. Louis.

Quasars, or quasi-stellar objects, are massive objects that throw off enormous energy. Through a telescope, they look like stars. When gas falls into quasars at the centers of galaxies, they form disks of gas and dust that emit electromagnetic energy. This creates a brightness hundreds of times greater than that of other galaxies.

Jets shoot out of the quasar, pulsing with X-rays, which are some of the hottest things in the entire universe. The jets blow gas and dust, which are essential to form stars, out of the galaxy.

Astronomers believe that the supermassive black holes at the centers of galaxies actually power quasars. When a quasar forms, it signals the end of a galaxy’s star-forming days. That makes it a “passive dead galaxy,” Kirkpatrick said.

But Kirkpatrick’s survey found that 10% of galaxies hosting supermassive black holes at their centers actually contained cold gas as they entered this phase — but they were still creating stars.

“That in itself is surprising,” she said. “These are very compact, blue, luminous sources. They look exactly like you would expect a supermassive black hole to look in the end stages after it has quenched all of the star formation in a galaxy. These are the population that I’m calling ‘cold quasars.’ “

“Cold quasar” could be a short period in the lifespan of a galaxy toward the end, the retirement party before the next phase when a galaxy fades. Of course, in this case, it’s a transition phase of 10 million years.

“These galaxies are rare because they’re in a transition phase. We’ve caught them right before star formation in the galaxy is quenched, and this transition period should be very short,” Kirkpatrick said.

She used the Sloan Digital Sky Survey, a detailed map of the universe, and focused on an area called “Stripe 82.” She and her research team found the X-ray sources, a key signature of black holes that are growing, and followed up with other telescope observations in infrared to find gas and dust.

The finding adds a greater understanding to quasars and their interaction in galaxies.

“We already knew quasars go through a dust-obscured phase,” Kirkpatrick said. “We knew they go through a heavily shrouded phase where dust is surrounding the supermassive black hole. We call that the red quasar phase. But now, we’ve found this unique transition regime that we didn’t know before.”

Next, Kirkpatrick will follow up her findings to determine whether this happens to only one type of galaxies, or to all of them.

“We’re doing what we call a blind survey to find objects we weren’t looking for. And by finding these objects, yes, it could imply that this happens to every galaxy.”


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