Supermassive Black Holes in Merging Galaxies

The following is a blog by Yjan Gordon (@YjanGordon), a postdoc at the University of Manitoba, Canada (having recently completed a PhD at the University of Hull). Here, he describes his new paper making use of the latest Galaxy Zoo classifications.

One of the key questions I look to address in my research is that of why the black holes at the centres of some galaxies are actively feeding on matter (an active galactic nucleus, AGN for short) and why some aren’t. We know of multiple mechanisms that can trigger an AGN, from high-impact galaxy mergers to secular processes such as feeding on the matter ejected from stars over the course of their lives. However, not all AGN are created equal, and many of these objects, whilst active, are only barely so. While more powerful AGN are having a steak dinner, these weaker variants are merely snacking.

The processes that initiate these weak AGN may be different to those that fuel their more powerful cousins or simply a scaled down version of the same mechanisms. For example, we know that the collision of two similar sized galaxies (known as a major merger) can trigger an AGN. Then a minor merger, where a small galaxy collides with a much more massive one, may provide less fuel for an AGN, resulting in one of these weak AGN. This is exactly the question we investigate in our latest paper.

In order to test whether minor mergers are a factor in triggering weak AGN, high quality, deep observations are needed to look for very faint merger signatures in a sample of these galaxies. To conduct our analysis we made use of the Dark Energy Camera Legacy Survey (DECaLS). This survey not only provides the deep, high quality imaging necessary for looking for minor galactic mergers (and is far improved in this regard than previous wide-field imaging surveys, see figure below), but is also the latest survey being put to the galaxy zoo volunteers to obtain reliable galaxy morphologies.

Comparison of imaging from the Sloan Digital Sky Survey (SDSS, top) with higher quality imaging from DECaLS (bottom). The DECaLS imaging is approximately two magnitudes deeper than the SDSS imaging and shows faint merger remnants not visible in the SDSS images.

A control sample of galaxies that don’t host an AGN is required, so that we can compare the fractions of weak AGN and non-AGN experiencing mergers, i.e. are mergers more frequently associated with these AGN or not? In order to control against other variables that could impact your results here, reliable morphological information is a valuable asset. For instance, spiral galaxies have a delicate structure that can be disrupted by galaxy mergers, and the presence of this morphology in a merging system can provide information about the scale or timeline of the event. One can hence see the potential for elliptical galaxies to be more likely to exhibit the tidal disturbances than their more delicate spiral counterparts.

This kind of project wouldn’t be possible without the contributions of the many Galaxy Zoo volunteers providing morphological classifications on hundreds of thousands of galaxies.

When we compare the merger rates and the merger scales in both the weak AGN and the non-AGN control sample we found a couple of compelling results.

Firstly, we found that the fraction of both these samples experiencing minor mergers was about the same. This is interesting as it shows that minor mergers, which had long thought to be a potential trigger for these weak AGN, are not involved initiating weak activity of the central black hole in a galaxy.

Secondly, we found that for the least massive of these weak AGN, major mergers were significantly more common than in non-AGN. This is an unexpected result, as such major mergers might provide so much gas that any resulting AGN might be expected to be fairly powerful. Furthermore, previous research hadn’t shown any substantial evidence of this, so why are we seeing such an effect? Well, whilst major mergers are more common in these weak AGN, they still only represent a minority of the weak AGN population (~10%), and are thus not typical of the main population of weak AGN. One intriguing possibility is that these particular objects may actually be the early stages of more powerful AGN, and that as the merger progresses, and more gas falls into the galactic nucleus, the AGN will have more fuel to feed on and become a more powerful AGN. Further research is required to investigate such a hypothesis.

This kind of project wouldn’t be possible without the contributions of the many Galaxy Zoo volunteers providing morphological classifications on hundreds of thousands of galaxies. In this case, as is so frequent in research, not only have we answered a question about the evolution of these galaxies, but we have been presented with another.

Please keep up the great work, it really makes a difference.

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