Title: The ∞ Galaxy: A Candidate Direct-collapse Supermassive Black Hole between Two Massive, Ringed Nuclei (Paper 1) & Further Evidence for a Direct-Collapse Origin of the Supermassive Black Hole at the Center of the Infinity Galaxy (Paper 2)

Authors: Paper 1: Pieter van Dokkum, Gabriel Brammer, Josephine F. W. Baggen, Michael A. Keim, Priyamvada Natarajan, Imad Pasha. Paper 2: Pieter van Dokkum, Gabriel Brammer, Conner Jennings, Imad Pasha, Josephine F. W. Baggen.

First Author’s Institution: Yale University

Status: Paper 1: Published in The Astrophysical Journal Letters [open access]. Paper 2: Submitted to The Astrophysical Journal Letters [open access].

Supermassive black holes (SMBHs) are black holes that are millions to billions of times the mass of our sun, and most galaxies in the universe have one in their centers. But even though SMBHs are so common, we’re still not sure how they formed. In recent years, the James Webb Space Telescope (JWST) has been finding supermassive black holes much earlier in the universe’s lifetime than we expected. How did SMBHs get so incredibly big in such a short amount of time?

The two leading theories for SMBH formation are the ‘light seed’ theory and the ‘heavy seed’ theory. The light seed theory says that SMBHs started out as stellar-mass black holes that formed from the first stars in the universe, and they gradually became bigger over time by accreting gas and dust and merging with other black holes. The heavy seed theory says that SMBHs formed from the direct collapse of huge gas clouds, bypassing the stellar phase entirely. Without seeing an SMBH actually form, it’s hard to say which is true. (See this Astrobite for a more in-depth look at these two theories.)

Figure 1: The Infinity Galaxy, as seen by JWST. Left: Color image created from NIRCAM F090W, F115W + F150W, and F200W filters. Right: Annotated F115W image. Adapted from Figure 1 in Paper 1 and Figure 1 in Paper 2.

Today’s authors think they’ve found a newly-formed SMBH that could support the heavy seed origin story. This exciting discovery is in a weird-looking system called the ‘Infinity Galaxy’ (seen in Figure 1) which they believe was created from two galaxies that merged face-on, just like two cymbals crashing together. The massive central bulges of the original galaxies disturbed their galactic disks and caused them to rearrange into ring shapes, leaving shocked gas from the merger lingering between the two rings. These structures create the Infinity Galaxy’s characteristic ∞ shape. You can see a cartoon of this formation process in Figure 2.

This type of galaxy merger has been studied before and isn’t anything revolutionary. But the Infinity Galaxy is hiding something unusual: there’s an SMBH lurking in the shocked gas between the galactic bulges.

Figure 2: The proposed formation process of the Infinity Galaxy. Figure 9 in Paper 1.

Wait, where did that black hole even come from?

This black hole can’t be a displaced central SMBH from one of the original two galaxies. Follow-up spectroscopy has revealed that the two yellow galactic bulges in Figure 1 still each contain an SMBH, so those are already accounted for. Today’s authors explore three possible explanations for this extra SMBH.

1. **The SMBH is actually hosted by a third (super-faint) galaxy that we can’t see in the current observations. **This is difficult to disprove, but fairly unlikely, because it’s unusual to see such a bright SMBH in a low-mass galaxy.
2. It was originally an extra SMBH in one of the progenitor galaxies, and it got flung out during the galaxy merging process. Since SMBHs take a long time to merge, a galaxy that previously merged with another galaxy can have multiple SMBHs. If the Infinity Galaxy’s SMBH was an extra SMBH that got flung out, we’d expect its velocity to be very different from its surroundings. However, today’s authors measured the radial velocity of the SMBH to be very similar to the gas around it (within 50 km/s). Keep in mind that we can only measure velocity along our line-of-sight, so this doesn’t completely rule out the runaway black hole scenario, but it makes it more unlikely.
3. The SMBH formed through the direct collapse of shocked gas. It didn’t come from anywhere — it formed right there. This scenario would explain why the SMBH’s velocity is so similar to its surrounding gas, and it’s definitely the most exciting explanation. However, the challenge is that a gas cloud usually collapses in multiple places and creates stars — not one giant black hole. Future studies will have to conduct simulations to study whether this type of collapse could have occurred in the Infinity Galaxy merger. More detailed observations at wavelengths across the spectrum would also help pinpoint what’s going on in this strange system.

Please, please, please be a direct-collapse SMBH!

If the Infinity Galaxy’s extra SMBH was formed through direct collapse, it could help explain why JWST is finding supermassive black holes so early in the universe’s history. Note that the early-universe ‘heavy-seed’ SMBH formation conditions aren’t quite the same as the conditions in the redshift z=1.14 Infinity Galaxy, so it isn’t a perfect comparison. But if its SMBH turns out to have formed through the direct collapse of a gas cloud, it’ll be the first empirical evidence that such a formation mechanism is even possible.

Astrobite edited by Veronika Dornan

Featured image credit: Adapted from Figure 1 in Paper 2

I’m a second-year Astronomy & Astrophysics PhD student at the University of California, Santa Cruz. I’m interested in using machine learning and telescope surveys to explore a variety of topics in extragalactic astronomy. Beyond research, I love science outreach/journalism, photography, archery, and being outdoors.

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