AGN & Mergers

Active galactic nuclei (AGN) are some of the most powerful astrophysical objects known to us today. We now understand that these “monsters” are powered through the accretion of matter onto the supermassive black hole that sits at their centers. This nuclear activity can last for hundreds of millions of years and is believed to be an ubiquitous phase of evolution that many, if not all, galaxies go through.

What still remains unclear is how the onset of this active phase of a galaxy is brought about. It is obvious that two critical components need to be in place for an AGN to be triggered: the availability of gas AND a mechanism to bring gas towards the center of the galaxy.

A violent Universe?

One of the main mechanisms that has been proposed as an efficient way to both make large amounts of gas available to a galaxy and also bring this gas toward the center of the galaxy is galactic mergers. We now that galaxies can meet and interact with each other. Under certain conditions, this interaction can lead to a galactic merger. During this process, large amounts of gas is funneled towards the center of the merging system. This effectively provides the central supermassive black hole with fodder, leading to the ignition of a powerful AGN.

However, studies of mergers and gas availability in normal galaxies have shown that both the merger rate and the fraction of gas in galaxies becomes increasingly smaller as the Universe becomes older. Moreover, detailed studies of the morphologies of AGN host galaxies showed that the majority of these do not show signs of ongoing or recent mergers. The rate of mergers in average AGN host galaxies appears to be roughly consistent with normal, non-AGN hosting, galaxies.

A host of other possibilities have been proposed as alternatives to a merger-driven AGN triggering. These are usually encompassed under the general term of “secular” processes, implying mechanisms that do not require any effects external to the galaxy. Secular processes include: special dynamic galactic features called bars, accretion of hot gas from a galactic halo, and feeding through gas ejected from newly formed stars.

The environment of AGN

We have been using the environment of AGN to understand the processes that may trigger and consequently feed them. Although definitely not a one-to-one correspondence, the environment within which a galaxy is embedded can tell us the probability that is has undergone or will undergo a merger. Denser environments are more conductive towards dynamical interactions between galaxies.

Using current generation large area surveys [2,4] in the near-infrared, we conclude that the majority of AGN host galaxies are found in environments very much similar to those of non-active galaxies of similar stellar mass. This implies that, for the bulk of active galaxies, mergers are not the driving force behind their activity.

However, we do find that specific flavors of AGN, in particular radio-AGN and Seyfert-like galaxies (low luminosity cousins of Quasars), exhibit on average denser environments than their non-active counterparts. This drives the point that mergers are still necessary for a sub-component of the AGN population.

Related papers: Karouzos et al. 2014a, 2014c

Feeding the monster

This still leaves open the question of what triggers the bulk of AGN. In a follow-up study, we focus in the environments of AGN with radio-jets. We looked at the properties of the host galaxies of radio-AGN embedded in the most over-dense and most under-dense environments in our study [2]. Differences in these properties help us to infer the relevant feeding mechanisms for these AGN populations.

Interestingly, we find that, in the local Universe, both the AGN and the host galaxy properties of radio-AGN living in the most over-dense and most under-dense environments markedly different. Radio-AGN in the most under-dense regions are found to exhibit more vigorous and efficient star formation, more efficient nuclear accretion, and larger radio-loudness values (a measure of the dominance of the emission in the radio over that in the optical) than their counterparts in over-dense regions.

Excluding different feeding mechanisms for these radio-AGN in the most under-dense environments, we conclude that gas ejected from newly formed stars (the so-called stellar feedback) may be feeding the supermassive black holes at the centers of these AGN.