Mergers between galaxies trigger activity in their core

Active galactic nuclei (AGNs) play a major role in galaxy evolution. Astronomers from the University of Groningen and Netherlands Institute for Space Research have now used a record-sized sample of galaxies to confirm that galaxy mergers have a positive effect on igniting AGNs.

They were able to compile about ten times more images of merging galaxies than previous studies by using a machine-learning algorithm. The results were published on 27 May in the journal Astronomy and Astrophysics.

One of the bigger questions in astronomy is how galaxies evolve from clouds of gas and dust to the beautiful spiral structures we see in our Galactic neighbourhood. So-called active galactic nuclei (AGNs) form interesting research objects to answer part of this question, because there appears to be co-evolution between AGNs and galaxies.

AGNs harbour supermassive black holes that emit huge amounts of energy after accreting gas from their surroundings. Some have large enough magnetic or gravitational fields to spit out jets from their poles, stretching thousands of lightyears.

Seeds for stars
Co-evolution is a two-way street. On the one hand, the evolution stage of a galaxy affects AGN activity. AGNs seem to thrive at a certain stage in a galaxy's evolution, because we see AGN activity peaking in galaxies at a particular distance, and therefore at a particular time in the past.

On the other hand, AGN activity affects a galaxy's star formation. This could go either way. An AGN's jet pushes gas away as it propagates through the galaxy, forcing the gas to collide with other gas and thus creating clumps--seeds for new stars. But AGNs also emit energy, heating up the gas and thus preventing it to cool down and condense into clumps.

Astronomers from the University of Groningen and SRON Netherlands Institute for Space Research have now compiled a sample with a record-number of galaxies to study one of the factors that allegedly has a positive effect on igniting AGNs: mergers between galaxies. And they found a correlation, counting both ways. There are about 1.4 times more AGNs in mergers than in non-mergers. And the other way around, the researchers find about 1.3 times more mergers in samples of galaxies with an AGN compared to samples of galaxies without an AGN.

Large sample
The research team used a machine-learning algorithm to identify mergers. It gave them a sample that is about one order of magnitude larger than those in previous studies, making the correlation much more reliable. 'We have built a network to train the system to recognize mergers in a lot of pictures,' says first author Fangyou Gao.

'This enables us to use a large sample of two telescope surveys with tens of thousands of galaxies. AGNs are relatively easy to recognize, based on their spectrum. But mergers must be classified from images, which is typically a human's job. With machine learning, we can now have computers do this for us.'

Research Report: Bland-Hawthorn, and M. S. Owers: Mergers Do Trigger AGNs out to z ~ 0.6.