Sophia Dagnello, NRAO/AUI/NSF; NASA, STScI
The supermassive black holes at the center of mature galaxies are usually silent. Their activity would have blown away most of the nearby gas and dust, and any stars in unstable orbits were likely to have been ripped to shreds long ago. But occasionally, the chaotic nature of complex orbital interactions should bring a star close enough to experience what’s called a tidal disturbance — the star is ripped apart by the black hole’s gravity.
We’ve modeled what a tidal disturbance should look like, and it’s clear that it should produce large numbers of energetic photons. The problem is, we haven’t seen many of these types of events. Now, using a decade of observations, researchers recently in Science have seen what appears to be a black hole ripping apart a star, turning some of it into a jet of material traveling at a quarter the speed of light. The reason we haven’t seen it before? Instead of appearing at the wavelengths we expected, the event was visible in the infrared.
merger
The story begins all the way back in 2005, when researchers identified a transient light source, meaning something clearly detectable wasn’t there in previous images. The source of the signal was traced to an unusual object: Arp 299, which contains two galaxies that are merging.
A merger of galaxies does several things that increase the likelihood of a tidal disturbance. The shock waves produced can cause a burst of star formation in the gas and dust of both galaxies, disrupt orbits of existing stars, and the new gravitational configuration could eject the supermassive black hole from its normal home at the center of the galaxy. All of these have the potential to bring additional stars near one of two supermassive black holes (one from each galaxy).
But tidal disturbances are expected to occur mainly in visible and UV wavelengths. While this star-gorging had some activity at visible wavelengths, much of its output was at longer wavelengths and shifted almost exclusively to the infrared over time.
The researchers claim it is the product of all the gas and dust around the center of the galaxy. This material will absorb the higher energy photons produced by the event and radiate them back at lower energies. The result is an infrared-bright object. And it is terribly bright; the researchers estimate that it produces as many photons as the rest of the galactic core combined.
jets
And with a decade of observations, they could see the tidal disturbance change. Although it initially started out as an indistinguishable source, it gradually turned into a central source and a structure that grew out of it. Black holes often form jets because their intense magnetic fields interact with charged material near the black hole. In fact, half the mass of stars that suffer from a tidal disturbance is eventually ejected from the region near the black hole. In this case, tracking the emissions from the material suggested the jet was sending matter away at about a quarter of the speed of light.
That speed corresponds to models of tidal disturbances. And more broadly, the Decade of Sightings offers a rare opportunity to follow the development of jets in real time.
The new source, which the authors refer to as Arp 299-B AT1, also explains why we’ve detected so few tidal disturbances (and the few we have are still the subject of debate over their identities). When galaxies merge, one of the results is usually the influx of gas and dust toward the center of the galaxy. And if these are the conditions where most tidal disturbances occur, we can expect many of them to be infrared clear.
In other words, the Arp 299-B AT1 not only helped us study a rare event in more detail, it also provides some hints that could make similar sightings less rare.
Science2018. DOI: 10.1126/science.aao4669 (About DOIs).