While there have been extensive previous studies of mergers between two galaxies, a recent study that helps reveal what happens when three galaxies merge is one of the first to systematically look at the consequences for the supermassive black holes at their hearts when a trio of galaxies collide. Do the black holes converge on the new galactic center? Do they go on a feeding frenzy? Do they actually merge?
I had the opportunity to present the results, to which I contributed as part of my PhD at the University of Michigan, at the 237th meeting of the American Astronomical Society last month. The study used data from NASA's Chandra X-ray Observatory and several other telescopes, and tells astronomers more about how galaxies and the giant black holes in their centers grow over cosmic time.
To help Chandra fans visualize the research, NASA released this short video.
Some of the brightest X-ray emissions originate around accreting supermassive black holes in the centers of galaxies. These are called active galactic nuclei, or AGN (also discussed in previous KIPAC blogposts such as Dan Wilkins reviewing X-ray flares from AGN in July 2017 and Krzysztof Nalewajko presenting observations of the powerful jets of AGN in April 2015). Interacting systems of two AGN, usually associated with pairs of galaxies that are merging, are called "dual AGN.” Given that all massive galaxies are likely to have central supermassive black holes, dual AGN are thought to be a natural consequence of hierarchical galaxy formation (i.e. where galaxies grow steadily larger due to mergers with other smaller galaxies).
Do colliding galaxies = colliding AGN?
Many questions exist regarding how two supermassive black holes that are on an initial collision course will continue moving toward each other. For example, there’s the question of gas.
There needs to be ample amounts of gas, at all stages of the merger, in order for the two supermassive black holes to continue losing energy and move toward one another. (This is because the SMBH's lose energy due to friction of their large accretion disks with the gas they are passing through.) On top of this, even if there's enough gas to move the two supermassive black holes to very small separations, they may stall and not merge within a Hubble time (the rough current age of the Universe). If this were the case, Pulsar Timing Arrays (PTAs) would have no signal to detect from supermassive black hole mergers (and has thus been dubbed the "nightmare scenario" for the very uneventful projections of what PTAs would see).
However, introducing a third supermassive black hole into the mix can alleviate some of these problems. The presence of a third supermassive black hole can disturb the stellar orbits of the merging galaxies, supplying more gas to central regions, and allowing the two supermassive black holes to continue moving toward one another. More importantly, the presence of a third supermassive black hole can help shrink the separation between the two merging black holes via exchanging angular momentum with the binary orbit, which can dramatically reduce the merger time.
Simulations show that triple AGN are expected to be relatively common—but to date only one X-ray triple AGN has been detected. This may be a result of observational constraints as well as the lack of systematic surveys searching for triple AGN. In particular, confirmation of multiple AGN systems requires high spatial resolution X-ray observations, and analyses that are sensitive to systems with small separations and/or faint sources. In fact, before this study there had yet to be a systematic study of the X-ray AGN activity of nearby triple galaxy mergers.
This provided the motivation to create and analyze a sample of nearby triple galaxy mergers to find more triple AGN systems. The sample was composed of seven triple galaxy mergers that were all visually identified by sifting through available optical observations taken with the Sloan Digital Sky Survey (SDSS). Each merger also had publicly available Chandra data. Using specialized software I developed as a graduate student, called BAYMAX (Bayesian AnalYsis of Multiple AGN in X-rays), I analyzed the available X-ray data for each merger. BAYMAX uses a Bayesian framework to analyze Chandra observations and calculate the likelihood that they are composed of one versus multiple X-ray point sources. This tool is useful in situations when one of the sources is dim, or the separation between sources is near the instrumental PSF (point spread function, or the degree to which a point-like object will spread in an image -- i.e. essentially a measure of the intrinsic 'blurriness' of an image).
Given the mass of each galaxy in the mergers, it's likely that they all house supermassive black holes at their centers. However, the team was interested in measuring how many of the supermassive black holes are actively accreting and growing (thus earning the “AGN” moniker), as measured via their X-ray emission. As material falls toward a black hole, it gets heated to millions of degrees and produces X-rays. The Chandra X-ray Observatory, with its superb angular resolution, is ideal for detecting growing supermassive black holes in mergers. However, the associated X-ray sources are challenging to detect because they are usually close together and are often faint. The combination of BAYMAX and Chandra enabled the team to identify the supermassive black holes.
What triggers an AGN feeding frenzy?
There was more to the project besides finding new examples of triple AGN, however. Given the small sample size it was likely that some systems would have no AGN, while others would have one, two or three. The overarching goal of the analysis was to measure whether the number of AGN detected in each triple galaxy merger depended on any environmental properties—such as gas and dust levels. What is different about the environment of a triple galaxy merger that has three AGN, versus one that has only one or two AGN?
Using BAYMAX, I confirmed previous work that had determined one of the seven triple galaxy mergers was composed of three AGN, four of the seven likely triple mergers contain two AGN (which are all previously undetected), and one had a single AGN. The last system had no X-ray evidence of any accreting supermassive black holes.
Using available infrared observations from the WISE (Wide-field Infrared Survey Explorer) telescope, interesting links between AGN activity and environmental parameters were uncovered. Infrared observations are sensitive to dust levels, and can be used to measure how dusty a galaxy is. X-ray observations, on the other hand, can be used to provide evidence on how much gas is immediately surrounding a supermassive black hole (and can be analyzed by fitting the Chandra X-ray spectra of each source). For the triple galaxy mergers in the sample, it was found that higher levels of dust coincided with higher levels of gas. This reflects the fact that during triple galaxy merger events, large amounts of gas and dust may be efficiently funneled into the nuclear regions, where the supermassive black holes can easily accrete material and turn on into AGN. On top of this, I found that one triple AGN system has the largest levels of gas and dust, as seen in the figure below, compared to the other systems that were found to have one or two AGN. This result suggests that as gas and dust levels increase, the number of supermassive black holes that are actively accreting will increase.
In the future I hope to carry out more of these studies with much larger samples of triple galaxy mergers. By doing so, the chances of finding more X-ray triple supermassive black holes will increase, allowing for a better understanding of the observationally rare, but theoretically significant, systems that may be the catalysts behind supermassive black hole mergers.
Chandra press release