The supermassive black holes at the center of mature galaxies tend to be quiet. Their activity will have blasted away most of the nearby gas and dust, and any stars that were in unstable orbits were likely to have been torn to shreds long in the past. But on occasion, the chaotic nature of complex orbital interactions should bring a star close enough to experience what's called a tidal disruption event—the star is ripped apart by the black hole's gravity.

We've done modeling of what a tidal disruption should look like, and it's clear that it ought to produce copious numbers of energetic photons. The problem is that we've not seen many events like this. Now, taking advantage of a decade of observations, researchers who recently published in Science have spotted what seems to be a black hole tearing apart a star and converting some of it into a jet of material traveling at a quarter of the speed of light. The reason we haven't seen it before? Rather than showing up at the wavelengths we expected, the event was visible in the infrared.

Merger

The story starts all the way back in 2005, when researchers identified a transient light source, meaning something that was plainly detectable hadn't been there in earlier images. The source of the signal was traced to an unusual object: Arp 299, which contains two galaxies in the process of merging.

A galaxy merger does several things that increase the odds of a tidal disruption event. The shockwaves produced can set off a burst of star formation in the gas and dust of both galaxies, orbits of existing stars get disrupted, and the new gravitational configuration can jolt the supermassive black hole out of its normal home at the center of the galaxy. All of these have the potential to bring additional stars in proximity to either of the two supermassive black holes (one from each galaxy).

But tidal disruption events are expected to occur primarily in visible and UV wavelengths. While this star-swallowing had some activity at visible wavelengths, much of its output was at longer wavelengths and, over time, it shifted almost exclusively to the infrared.

The researchers argue that it's the product of all the gas and dust surrounding what had been the center of the galaxy. This material will absorb the higher-energy photons produced by the event and radiate them back out at lower energies. The result is an infrared-bright object. And it is very bright; the researchers estimate it is producing as many photons as the rest of the galactic core combined.

Jets

And, with a decade of observations, they were able to watch the tidal disruption event change. While it initially started out as a non-distinct source, it gradually resolved into a central source and a structure extending from it. Black holes often form jets as their intense magnetic fields interact with charged material near the black hole. In fact, half the mass of stars suffering from a tidal disruption event ends up being ejected from the area near the black hole. In this case, tracking the emissions from the material suggested that the jet was sending matter away at roughly a quarter the speed of light.

That speed is consistent with models of tidal disruption events. And, more generally, the decade of observations provides a rare opportunity to track the development of jets in real time.

The new source, which the authors label Arp 299-B AT1, also my help explain why we've detected so few tidal disruption events (and the few that we have are still the subject of dispute over their identity). When galaxies merge, one of the results is typically the infall of gas and dust toward the galaxy's center. And, if these are the circumstances where most tidal disruption events are happening, then we should expect a lot of them to be infrared-bright.

In other words, not only did Arp 299-B AT1 help us study a rare event in more detail, it provides some hints that might make similar observations less rare.

Science, 2018. DOI: 10.1126/science.aao4669 (About DOIs).