Astronomers have spotted a surprising number of quasars with jets launched within the last couple of decades.
Quasars are notoriously capricious. Powered by gargantuan black holes that sit munching away on gas at the centers of galaxies, these brilliant beacons flicker often. The fitfulness is generally mild — only a few tens of percent difference.
But Kristina Nyland (Naval Research Laboratory) and collaborators have found something else: a whole group of quasars that switched their plasma jets from off to on in only two decades.
The result, which will appear in an upcoming Astrophysical Journal, adds to an accelerating shift in our understanding of active galactic nuclei (AGN). Despite the flickering, astronomers generally think of AGN as stable on human time scales. Based on smaller brethren (the star-mass black holes that feed off stellar companions), changes in the giant black holes’ tutus of gas or the long streams of plasma they sometimes shoot out should happen over the course of millions of years.
Yet astronomers have seen dramatic changes in quasars’ light, as well as “baby jets” in galaxies, small features that in the most extreme cases might be less than 100 years old. (The more compact a jet is, the less time it’s had to grow.) We also know of galaxies with two sets of jet-blown lobes, one big, the other small. The inner pair presumably formed in a current epoch of jet activity, while the outer pair is a fossil of a past episode, says Stanislav Shabala (University of Tasmania, Australia).
But no one has known how common young jets are. Nyland’s team turned to two radio sky surveys by the Karl G. Jansky Very Large Array in New Mexico to find out: the FIRST survey, which ran between 1993 and 2011, and the ongoing VLA Sky Survey (VLASS).
Comparing FIRST’s data with preliminary images from VLASS, the astronomers found about 2,000 sources in the second survey that didn’t appear clearly in FIRST. Of that big group, 167 proved to be pre-identified quasars in either visible light or infrared.
The team picked the 26 brightest sources to investigate, studying a total of 14 of them in detail with additional VLA observations. These all have radio emission like what we’d expect if small jets are present. The pattern of brightness over different frequencies relates to the size of the source; based on that analysis, the jets could be as small as a few light-years long.
“We were quite surprised to see such a dramatic change,” Nyland says. She thinks it likely that all 26 sources are small jets that turned on in the last 10 to 20 years. The larger group of 2,000 is more uncertain, however — some may prove to be switched-on quasars, while others could be supernovae or other phenomena.
“My favorite scenario that fits that data we have so far is that we managed to catch these jets right after they were born,” she says. Another possibility is that the team has found slightly older and larger jets that have recently bent toward Earth.
But if the jets are newborns, what turned them on? In other known cases, astronomers usually assume the jet launches due to some dramatic accretion event, says black hole astrophysicist Christopher Reynolds (University of Cambridge, UK). But here, we have pre-existing quasars that, for reasons unknown, have suddenly spewed forth a new jet.
“It raises a host of interesting questions for the theorists,” he says. “What triggers the formation of the jet? What powers these jets? Is the jet closely connected to the accretion disk and, if so, how does the accretion disk respond this quickly?”
Based on smaller black holes, a decade or two is probably too short for a typical quasar to have a whole-scale change to its accretion disk, says accretion expert Sera Markoff (University of Amsterdam, The Netherlands). But a jet could reactivate on that time scale, especially if it’s launching into a channel carved out by earlier outbursts into the surrounding gas.
How these high-speed changes (well, astronomically speaking) would affect the host galaxies remains unclear, Nyland says. Galaxies and their central black holes grow symbiotically, and when the black holes throw tantrums, it can create turmoil in gas that would otherwise form stars. The infant jets VLASS turned up likely need more time to grow before they can have a substantial impact on their galaxies, she explains. That’s why observations of active galaxies at different stages are so important. “It’s a bit like trying to watch a river carve out a canyon by taking a high-speed photograph.”
Not all AGN have jets — in fact, most lack prominent ones. And despite the growing variety of changing emission or jets spotted, “the vast majority of radio sources in the sky are just boring, compact things,” Shabala says. “Which actually makes them really interesting, because this strongly suggests that the majority of radio jets never make it into old age.”
Earlier this year, Shabala and colleagues looked at more than 100 AGN in various stages of activity, using radio observations from the Low Frequency Array (LOFAR) in the Netherlands. Their results suggest that the maximum lifetime of jets is roughly 300 million years. That would favor an intermittent snacking behavior for the black hole, fed by clumps of cold gas in a process called chaotic cold accretion that’s increasing gaining traction among astronomers.
K. Nyland et al. "Quasars That Have Transitioned from Radio-quiet to Radio-loud on Decadal Timescales Revealed by VLASS and FIRST." Accepted to Astrophysical Journal. Posted to arXiv.org November 17, 2020.