Title: BlackTHUNDER: evidence for three massive black holes in a z ∼ 5 galaxy
Authors: Hannah Übler, Giovanni Mazzolari, Roberto Maiolino, Francesco D’Eugenio, Nazanin Davari, Ignas Juodžbalis, Raffaella Schneider, Rosa Valiante, Santiago Arribas, Elena Bertola, Andrew J. Bunker, Volker Bromm, Stefano Carniani, Stéphane Charlot, Giovanni Cresci, Mirko Curti, Richard Davies, Frank Eisenhauer, Andrew Fabian, Natascha M. Förster Schreiber, Reinhard Genzel, Kohei Inayoshi, Lucy R. Ivey, Gareth C. Jones, Boyuan Liu, Dieter Lutz, Ruari Mackenzie, Jorryt Matthee, Eleonora Parlanti, Michele Perna, Brant Robertson, Bruno Rodríguez del Pino, T. Taro Shimizu, Debora Sijacki, Eckhard Sturm, Sandro Tacchella, Linda Tacconi, Giulia Tozzi, Alessandro Trinca, Giacomo Venturi, Marta Volonteri, Chris Willot, and Saiyang Zhang.
First Author’s Institution: Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße, 1, 85748 Garching, Germany
Status: Submitted to A&A, available on arXiv
Can you have multiple supermassive black holes in one galaxy?
Astronomers think that pretty much every galaxy in the universe has a supermassive black hole (SMBH) at their center. SMBHs influence the evolution of their host galaxies, but the exact nature of this influence, and the mechanisms that allow these BHs to grow so large, are still under debate. But…if all galaxies have one resident SMBH, could some have…two? Or…maybe even…three? The answer is, generally, yes! Astronomers have found evidence for two, and even three SMBHs sharing one galaxy. Today’s authors find evidence for a new triple-SMBH galaxy, but what makes it even more interesting is that the system is observed at a redshift of ~5, or over 12 billion years ago, making this the first early-universe triple-SMBH galaxy astronomers have detected.
How do astronomers detect SMBHs?
There are several ways we can infer the presence of SMBHs. Some, like reverberation mapping, require photometric observations at a lot of different times to map the gas orbiting a SMBH. Others require high quality spectroscopic maps to measure the motion of stars or gas around SMBHs through doppler shift. Specifically, high enough quality to actually resolve the region where the presence of a SMBH influences the matter around it via gravity. The most simple of these methods is what astronomers call ‘single-epoch’ relations. Single-epoch relations use a single observation to infer the presence and estimate the mass of a SMBH, usually through the strength of certain spectral features. Single-epoch relations are calibrated using well-studied SMBHs, providing an avenue to indirectly study SMBHs when other methods aren’t possible (see this bite for more info). In the early Universe, this is pretty much always the case.
A triple SMBH galaxy in the early Universe
Today’s authors leverage the latter option, using spectral data from JWST. This data, shown in Figure 1, revealed the presence of spectral lines (specifically H $\alpha$ which traces energized gas around the SMBHs; see the big spikes in the rightmost panels) which are indicative of gas moving at speeds of ~400-3000 km/s. While the presence of this emission in the center of the galaxy isn’t super surprising, finding similar emission in the outskirts of the galaxy (the pink cross in Fig. 1.) definitely is! It implies an additional SMBH in the outskirts of the galaxy. Further inspection actually reveals two regions indicative of SMBH presence in the center, denoted by the brown and teal crosses in Figure 2. To have a galaxy in the early Universe (~12.5 billion years ago) with not one, not two, but three supermassive black holes can tell astronomers a lot about the relationship between SMBHs and galaxies in the early Universe.
How did we get here, and what does it mean?
The presence of a system like this in the early universe could indicate that multiple-SMBH systems were much more common earlier in the universe compared to now. Today’s authors estimate that the central two SMBHs in this galaxy could merge within ~700 million years relative to how we observe the system now, which is relatively quick on astronomical timescales. Upcoming gravitational wave observations from LISA could detect mergers like this, making this system an important hallmark to place future observations in a cosmological context. Additionally, SMBH mergers are thought to be an important channel when it comes to the growth of SMBHs. The presence of a system like this in the early Universe indicates that our idea of SMBH growth is at least partially correct.
Galaxies undergo a lot of mergers with other galaxies in the early universe, which could be how this galaxy found itself with three SMBHs. It could have picked up its two extra SMBHs from other galaxies that it has gobbled up since being formed, with the exterior SMBH being the product of a more recent merger compared to the central two. The exterior SMBH could also have been kicked out of the center of the galaxy due to gravitational interactions with its peers, and is in the process of moving back in now. It’s hard to tell for certain how this galaxy came to harbor its population of SMBHs, but its existence provides astronomers with another piece of the cosmic dawn puzzle.
Astrobite edited by Lindsey Gordon
Figure Credit: AUI/NRAO, NAOJ, and Science/Nicole Rager Fuller “VLA image of 3C66B, with inset artists impression of the potential SMBHB, with resulting GWs heading towards the NANOGrav PTA”
