In a surprise discovery, astronomers using the Burst Alert Telescope aboard the Swift satellite learned that both supermassive black holes in galaxy Mrk 739¹ (yes, it has two) are actively feeding on material. (Check out the link for a cool vid of the galaxy.) Mrk 739 is a mere 425 million light-years from Earth, making it the second-closest active pair of supermassive black holes to Earth. (The first-closest is in galaxy NGC 6240.)

So, besides the sheer coolness of two supermassive black holes in a single galaxy, why do astronomers care about such objects? As it turns out, they are an important key to understanding galaxy evolution.

Nearly every galaxy, including our own Milky Way, harbors a supermassive black hole in its nucleus, but only a few percent of those black holes are observed to be actively feeding on significant amounts of gaseous and stellar material. Such systems are referred to as active galactic nuclei or AGN for short, and they are believed to be a temporary stage through which all galaxies harboring supermassive black holes pass. During this relatively brief active period, typically lasting ~50 million years, the gaseous material AGN feed on becomes superheated as it spirals down onto the black hole, causing AGN to shine very brightly. The intense brightness means these objects are often observable at very great distances, and therefore over a big range of cosmic history. This is why AGN are important probes of galaxy evolution.

Astronomers aren’t entirely sure what drives this material from the outskirts of the galaxy all the way down to the black hole, but it has been posited that galaxy interactions, including major mergers like the one shown below, are a likely mechanism.

What’s happening here is that the mutual gravitational tugging on gas orbiting in the galactic disks causes it to lose angular momentum and travel inward toward the nucleus of at least one galaxy, and possibly both, eventually reaching one or both of the black holes. If both black holes are active, the object is referred to as a binary AGN. As the hypothesis goes, such objects should be abundant but short-lived (in cosmic terms), lasting a few to tens of millions of years depending on the distance between the black holes. Eventually, the black holes spiral down to merge into a single, more massive black hole, and the two galaxies become a single, more massive galaxy.

It is a fact that galaxies frequently merge — there are several striking examples of this — but whether they are a major mechanism for creating AGN is another matter. I am a co-author on several papers involving searches for binary black holes, including binary AGN and recoiling black hole pairs, and I have been somewhat surprised by the relatively low frequency with which we observe genuine binary AGN  — about 100 times less than the predicted frequency. This implies: 1) the measurements needed to make such detections at large distances are so fine that we are not detecting the vast majority of such objects; or 2) perhaps the notion that mergers play a major role in creating AGN needs to be reconsidered.

[1] The Universe Today article mentions the newly discovered AGN binary as the second-place record-holder in terms of close separation between two active black holes, but this claim is in error. The second-place record is (to my knowledge) still held by LBQS 0103-2753 — separation of 6,500 light-years — which was co-discovered by my doctoral thesis advisor more than ten years ago.

Recommended reading:

• StarDate: “Double Hell”

Recommended viewing:

• HubbleSite gallery of galaxy interactions