Ever wondered what makes a quiet, rather ordinary galaxy, like our Milky Way, turn into a shining cosmic lighthouse that beacons across the spectrum so it can be seen by the whole universe? A recently observed galaxy in our cosmic neighborhood may well hold the answer. Scientists report in Astronomy & Astrophysics that they have likely observed for the first time when a supermassive black hole starts to feed on the surrounding gas, becoming an active galactic nucleus and the brightest persistent light source in the cosmos. The discovery might shed light on whether our own galaxy would someday be able to go through a similar transformation.
Now, let us get into the nitty-gritty details of this cosmic phenomenon. About 10% of all galaxies out there have super-bright cores known as AGN. Astronomers figure these form when a disk of gas builds up around a galaxy’s central black hole, which then starts to chow down on this gas. The extreme gravity produces an effect referred to as friction in the disk that in turn heats the gas to such high temperatures that it begins to glow intensely.
Some AGNs, however, have been seen to vary in brightness, flickering on and off once every 10,000 to 10 million years. Science has not yet explained what actually pushes the button on the black hole’s initial feeding frenzy and then makes it stop. Does a galaxy go through multiple phases of an AGN? These are the questions that keep astronomers awake at night.
To uncover this cosmic mystery, a team of astronomers recently utilized the Zwicky Transient Facility—an automated telescope that detects changes in the night sky. They matched this with the catalog from the Sloan Digital Sky Survey of 2.4 million quiet galaxies that had been compiled earlier. They selected 18 galaxies out of 86 that seemed to change from quiescent to active galactic nucleus—AGN—with new SOAR Telescope observations. One of those galaxies, with the catchy name ZTF20aaglfpy, displayed the greatest change between its Sloan sighting in 2003 and the new observations in 2022, suggesting an evolving AGN at high speed.
Now, the good part: in 2019, in a normal scan, the Zwicky facility detected a sudden brightening in a previously pretty quiet galaxy, called SDSS1335+0728, a full 300 million light-years away. By the initial observations in 2022, the team had very little to gather, but the galaxy had already shown unmistakable signs of hosting an AGN.
The find sent the researchers on a wild goose chase through image and spectrum archives taken with six different telescopes for a record of the quiet past of the galaxy. They have since followed its development through observations using many different types of telescopes, getting images and spectra at X-ray, ultraviolet, optical, and infrared wavelengths. This has been an enormous amount of work to collate but pays off.
The best guess of the team at this stage is that a black hole with a mass one million times our Sun has fired up an active galactic nucleus at the heart of SDSS1335+0728. If confirmed, this could be a first catch of an AGN in the act of ignition, since it could turn out to be exactly what Cosmic fireworks are all about.
There’s another possibility, however. That’s a star getting tidally disrupted by the black hole—astronomese for “one that wandered too close to a black hole and got torn apart by the gravitational force.” These events customarily produce a much brighter, briefer display. This would be the longest, faintest example we’ve ever seen if that’s what this is.
It’ll require the future observations of the Very Large Telescope, and the future Extremely Large Telescope, to pave a way through these uncertainties. The VLT’s Multi Unit Spectroscopic Explorer instrument will, for example, image SLACS0416 in unprecedented detail, as it has the ability to take pictures of the whole galaxy with each pixel containing a spectrum of different wavelengths. By this means, scientists will now be able to trace the dynamics of gas during its infall toward the core and perceive the feeding mechanisms that wake up AGNs suddenly, shining brilliantly.
With such a huge number of cataloged galaxies, it was only a matter of time before we caught one in the act of changing. According to Philip Best, an astronomer not involved in the study, studying an AGN in evolution can help unveil the fundamental physical processes at the heart of its fueling. This improved understanding may again give us a lead as to whether our own Milky Way might someday put on a similar cosmic light show.
But let’s step back a bit here and look at the wider implications, if you will, that this discovery has. It’s not about distant galaxies. It is about our place in the universe. Our own galaxy might be able to turn into an AGN. What then for life on Earth? So, how would such an enormous change to our cosmic neighbourhood taint conditions that have made our planet habitable?
This research also points out the requirement of long-term sky surveys and how archival data is relevant. If not for the years of observational data from multiple telescopes, the cosmic puzzle could not be pieced together. Most certainly, this goes to prove the collaborative aspect of astronomy and patience one needs to unravel the mysteries in the sky.
With the keep-pushing boundaries of our understanding, what other cosmic surprises might await us? Perhaps more transforming galaxies will be discovered or traces of ancient AGN activities detected in some supposedly quiet galaxies. Each new discovery brings us closer to the life cycles of galaxies and the manner by which supermassive black holes led to the universe becoming what it is today.
In the final analysis, this research reminds us of the dynamic nature that characterizes our universe: galaxies are not just static systems but evolving, changing systems much like the creatures that eventually come to inhabit them. When we look up at the night sky, we see far more than just distant points of light; we gaze upon cosmic dramas that have prevailed for millions of years. Who knows? Perhaps, in billions of years ahead, our own galaxy will be nothing but a wallpaper of light that can be seen across the cosmos—a beacon of activity within the vast sea of space.