The Effect of Environment in Galaxy Merger Incidence

Galaxy interactions and mergers are a fundamental process when discussing galaxy evolution, however, there is still no definitive method which identifies pure and complete merger samples. To deepen our understanding of the merger process and its impact on galaxy evolution, a method is required to create such a sample, we used Zoobot, a deep learning representation learning model pretrained on citizen science votes on images in the Galaxy Zoo DeCALS catalog. We fine-tuned Zoobot using \(\sim 1200\) synthetic HSC-SSP images of galaxies from the TNG simulations. Our fine-tuned model returns an accuracy on the synthetic validation data of \(\sim 76\%\) , as well as completeness and precision values of \(\sim 80\%\) , all when adopting a binary split. The accuracy value is comparable to those found in previous studies in which convolutional neural networks were trained with simulation images, however our value is returned using a far smaller training sample. In addition, our model is able to correctly classify both mergers and non-mergers of diverse morphologies and structures, including those at various stages and mass ratios, and also has the capability to distinguish between projections and merger pairs. We then used the fine-tuned model to make merger probability predictions on galaxies from HSC-SSP public data release 3, and compiled them to a publicly available catalogue. We examined the relationship between merger activity and environment, both in HSC-SDSS and HSC-GAMA based observations, where we used the merger probabilities from our model, and TNG based simulations, using simulational ground truths. For the relation between galaxy mergers and environment, we find two distinct trends. Using stellar mass overdensity estimates as environmental parameters, we find that galaxies with higher merger scores favor lower mass density environments on scales of 0.5 to 8 \(h^{-1}\) Mpc. However, in the simulations, we find that below these scales galaxies with higher merger probabilities favor higher density environments.