by Meredith Powell
At the center of each galaxy lurks a supermassive black hole (SMBH). These black holes grow during phases of extreme accretion when nearby gas and dust fall into their deep gravitational wells, which we observe as active galactic nuclei (AGN). Zooming out to much greater distances, halos of dark matter surround each galaxy, extending millions of light-years (see: The splashback radius: Understanding the boundaries of dark matter halos).
While various galaxy properties are known to correlate with SMBHs and their dark matter halos separately, the black hole - dark matter halo connection has been less explored. How does the dark matter environment of a galaxy impact the growth and coevolution of its central supermassive black hole? This is the question I addressed, in collaboration with an international team of researchers, by analyzing the spatial distribution of AGN.
Connecting AGN to their dark matter halo environments
To investigate the extent to which large-scale structure is important for growing supermassive black holes, my approach was to (1) measure the large-scale environments of AGN and (2) look for any evidence that black hole mass (which represents the total growth of the black hole over its lifetime) correlates with halo mass (the amount of dark matter around its galaxy). This connection provides important clues for how supermassive black holes and galaxies have evolved together over time.
While we cannot directly observe the dark matter around galaxies, we know roughly how it is distributed in the Universe. Because the number density and clustering of halos within dark matter simulations is well understood, the observed clustering of galaxies (or AGN) can statistically connect them to their halos. AGN clustering is therefore a powerful tool to link the (relatively) tiny black holes to their larger-scale environments. However, many previous measurements of AGN clustering have been limited by large uncertainties, since bright AGN activity is relatively rare. Additionally, the AGN that are too dim to have been detected have rarely been taken into account.
In this work, I measured AGN clustering in the local Universe by using a large, all-sky X-ray survey (www.bass-survey.com). To test various assumptions of the SMBH-halo connection, I placed black holes within halos from dark matter simulations and randomly assigned an accretion rate to each. I then only included those that were bright enough to have been included in the X-ray data (see fig. 2). This method allowed for the best-fit relationships to not be skewed by the incomplete survey.
I tested two different models for the black hole-halo relationship: one where black hole mass correlates with halo mass (for fixed galaxy mass), and one where it does not. To see which model was preferred, I made tens of thousands of AGN mock samples and compared the number of times that their clustering and abundance were consistent with the measurements from the data.
Does black hole mass correlate with halo mass?
This investigation led to two main conclusions. First, local AGN seem to reside in all cosmic environments. The models assumed the same probabilities for black hole accretion everywhere in the cosmic web, and they were still able to largely reproduce the statistical measurements. Therefore, it is unlikely that most of the AGN activity seen today was triggered by factors that depend on environment (such as galaxy collisions; see What happens to the supermassive black holes at the hearts of merging galaxy trios?).
Second, I found that a tight correlation between black hole mass and its host dark matter halo mass was preferred by the AGN measurements (Fig. 3). This would indicate that the halo is somehow connected to the black hole’s formation or growth. It could imply few different things: (1) that the most massive black holes today were more easily grown in the densest regions, due to many past mergers in the earlier Universe, for example; or (2) that AGN feedback (i.e., the energy output due to accretion) extends to larger scales and regulates the growth of the galaxy and black hole together with the halo.
While our results are intriguing, tests using independent methods and alternative ways to detect AGN are needed for confirmation. Fortunately, future surveys will find millions more AGN, enabling us to test and constrain models much more effectively and across many cosmic epochs. Such studies should revolutionize what we understand about AGN and their environments and reveal the physical processes that grow black holes and galaxies together within the cosmic web of dark matter.