Wednesday, August 8, 2012

The Enigmatic Connection Between AGN and Galaxy Mergers

In previous posts we learned that most galaxies host a supermassive black hole at their center and that when matter spirals into these black holes, they grow and become visible as powerful Active Galactic Nuclei, or AGN for short. In the local Universe, only about 10% of supermassive black holes appear to be undergoing an AGN growth phase, while the majority simply lie dormant in their host galaxies. Despite this, astronomers have found that the mass of these black holes is closely linked to the mass of their host galaxy’s spheroid component, or central ‘bulge’. This connection suggests that the growth of supermassive black holes is linked to the rate at which galaxies grow through star formation. What causes such a connection has remained an enduring mystery, especially when we consider that only a small fraction of black holes are experiencing a growth phase at any given time.

One process often proposed by astronomers as a way to trigger AGN activity is the collision of two galaxies, otherwise known as a galaxy merger. As discussed in this previous post, the gravitational interaction of two galaxies can be quite violent and lead to material being stripped from each galaxy, often producing long tidal tails and morphological disturbances. Over time gravitational interactions can cause the two galaxies to get closer until they eventually merge into a single galaxy. Computer simulations, such as the one linked below, have shown that the gravitational torques produced by such a collision can be extremely effective at funneling gas to the center of a galaxy. This gas eventually accretes onto the central black hole, which triggers an AGN growth phase. In addition, the concentration of gas near the center of the galaxy can also prompt new star formation that simultaneously grows the galaxy’s bulge. This scenario would naturally explain why only certain galaxies have ongoing AGN activity and why the masses of black holes and galaxy bulges are correlated. This scenario has been widely adopted by astronomers and most cosmological models of galaxy evolution now invoke galaxy mergers as the primary mechanism to trigger AGN activity in galaxies.  

Computer simulation of two galaxies merging
Credit: Joel Primack (University of California, Santa Cruz)

In a recent study, the CANDELS collaboration set out to test this scenario by investigating the role that galaxy interactions play in fueling black hole growth. The team selected a set of distant galaxies known to be experiencing an AGN phase and compared their structure to galaxies with no ongoing AGN activity. Our goal was to see if galaxies with growing black holes are found in galaxy mergers more often than galaxies with dormant black holes. For this study we made use of the new Wide Field Camera 3 on the Hubble Space Telescope, which allowed us to examine galaxies in the distant Universe much better than we could previously (see the images below for a few examples). Looking at distant galaxies is important because we know that both galaxy interactions and AGN activity are much more common in the early Universe, so examining distant galaxies increases our chance of seeing a connection between the two.

Hubble Space Telescope images of distant galaxies that host growing black holes as seen by the new infrared-sensitive Wide Field Camera 3 (WFC3) and the older Advanced Camera for Surveys (ACS). WFC3 imaging has helped us see the true structure of galaxies in the early Universe for the first time. Two examples of galaxies undergoing a galaxy merger are shown on the far right.
Credit: Dale Kocevski (University of Kentucky)

Although astronomers cannot watch a galaxy merger from beginning to end (they take roughly a billion years to complete), we can look for disturbances that indicate a galaxy interaction is underway. Some of these interaction signatures can be seen in the movie below, which shows Hubble images superimposed on a simulation of a galaxy collision. For our study, members of the collaboration examined images of both the AGN host galaxies and the control galaxies and determined the fraction of each that were experiencing an interaction. To our surprise, the galaxies with growing black holes did not show interaction signatures more often than the control galaxies. In fact, a large fraction of the growing black holes were found in disky galaxies. The disk structure in galaxies is easily destroyed in violent galaxy interactions, which indicates a majority of the active galaxies have not experienced an interaction in the recent past. This means some other event and not a galaxy merger must have triggered the black hole growth phase in these galaxies. 

Simulation of two colliding galaxies with Hubble images superimposed

The findings of this study have challenged a widely adopted theory of how galaxies and their supermassive black holes grow and evolve together. It suggests a yet unknown process or event must play a greater role in fueling black hole growth and triggering AGN activity in galaxies than we previously thought. The challenge in the coming years will be to use data from the ongoing CANDELS survey to identify the precise nature of this process.

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