Can black holes trigger star formation? Sure.

How a lucky discovery revealed the creative side of active galactic nuclei.

Figure 1, Gilli et al. 2019. A Hubble Space Telescope image of the J1030 field, containing the target quasar and several interesting extragalactic interlopers.

In many a Hollywood flick or sci-fi thriller, black holes find themselves playing the villain, threatening to rob us of the hero du jour. When they’re not busy sucking in Matthew McConaughey, they find themselves tucked in as a neat plot device to get from one part of the galaxy to another — but even then, they’re depicted as dangerous maelstroms to be avoided at all costs. In our universe, too, black holes have a tendency to be destructive, occasionally spewing out jets of high-energy particles or even ripping stars apart, so perhaps their reputation is not entirely unearned.

You might then be surprised to find out that black holes can be, of all things, nurturing — creators, not destroyers, of worlds. Rather than tearing apart stars, some black holes can help put them together. I know it sounds like something J.J. Abrams might dream up, but it happens — and astronomers have found it in a very strange galaxy far, far away.

While this unnamed galaxy, located over ten billion light-years away, doesn’t even have a name, it received some media coverage last week. Normally, I wouldn’t write about something that everyone else has already discussed, but it’s still an amazing high-redshift source — and one that took a lot of luck to find. Let’s take a quick dive into how astronomers came across this galaxy.

Accidents will happen

The system was discovered largely by accident, during a multiband survey of an observing field known as J1030 (Gilli et al. 2019). J1030 is an exciting target because it contains the quasar SDSS J1030+0524, which lies at a redshift of z=6.3, meaning its light was emitted when the universe was only about 1 billion years old. This makes it one of the most distant quasars known — which is why the J1030 field attracted so much interest.

Near the center of the field, not far from SDSS J1030+0524 itself, lies something called a Fanaroff-Riley Type II (FR II) radio galaxy. This means it’s a radio galaxy with bright radio lobes on either side, the product of energetic jets from the central active galaxy nucleus. The FR II galaxy is closer than the quasar, coming in at z=1.7, but it’s still a high-redshift object. Now, a high-redshift radio galaxy by itself isn’t terribly interesting, but what grabbed the researchers’ attention was a large structure of diffuse gas nearby.

Figure 2, Gilli et al. 2019. The team used Hα measurements to determine the distance to the FR II galaxy. Here, at top, Hα emission is detected at about 1.77 μm, as opposed to its rest wavelength of 656.28 nm. A quick calculation indeed gives us a redshift of around 1.7.

The J1030 field had been imaged across ultraviolet, optical and infrared bands using the Hubble Space Telescope, the Large Binocular Telescope, and several other instruments. The team also used the Chandra X-ray Observatory for x-ray imaging, and performed follow-up spectroscopy using the LBT, the twin Keck telescopes, and the Very Large Telescope. Measurements of Hα emission provided the first precise redshift measurements of the central FR II galaxy.

Observations using the VLT’s Multi-Unit Spectroscopic Explorer yielded another surprise: six less-prominent sources close to the radio galaxy. Absorption lines indicated that they indeed lie at the same distance as the FR II galaxy, meaning that they form part of a larger system. The final piece of the puzzle came from Chandra observations of large bubbles of diffuse x-ray emission in several different components, labeled A, B and C. Component A seemed to lie near the eastern lobe of the radio galaxy, and at a similar redshift. What’s more, four of the six companion sources seen by the VLT lay at nearly the same angular distance to the center of the largest bubble.

Figure 8, Gilli et al. 2019. A Chandra image in the 0.5–7 keV band reveals three blobs of diffuse gas emitting x-rays. Component A lies near the center of an arc containing four of the companion galaxies — a sign that there’s some connection between them.

Positive feedback

Modeling indicated that all six companion sources are star-forming galaxies, churning out stars at rates on the order of 10 to 20 solar masses per year. That’s roughly at least ten times the Milky Way’s star formation rate — quite significant indeed! They’re young and presumably full of hot, massive, blue stars, which are born and die over timescales of only a few million years. Why are the star formation rates so high?

Given the x-ray spectrum of component A, the team hypothesized that it’s being shock-heated by one of the jets emanating from the radio galaxy. The gas is expanding outwards, colliding with the cold interstellar medium in the companion galaxies and triggering star formation — a positive feedback system. We see soft thermal x-ray emission from the jet-bubble interaction, as well as hard x-ray emission from interactions between the gas, jet, and cosmic microwave background.

Figure 8, Croft et al. 2006. Minkowski’s Object lies at center right in this composite image. The Lick Observatory provided optical data, with radio (purple), H I (dark blue) and Hα (light blue) overlaid. The multiwavelength observations clearly show the jet from NGC 541 colliding with Minkowski’s Object, making it a low-redshift analogue of the FR II system found by Gilli et al.

What makes this all so exceptional is that this is the first time that a high-redshift radio galaxy has been observed triggering star formation in another galaxy. AGN jets have been known to caused star formation within their host galaxies (Dey et al. 1997), and low-redshift AGNs have been known to trigger star formation in neighboring galaxies (Croft et al. 2006), but Gilli et al.’s discovery was truly unique.

The observations shown that AGNs do have the power to influence the evolution of nearby young galaxies — an exciting possibility. The alignment with the diffuse gas is certainly fortuitous, but in the expanses of the cosmos, there must be other instances. What else lies in J1030, other high-redshift fields, and beyond? I’m excited to find out.