The connection between galaxy mergers, star formation and AGN activity in the HSC-SSP

TL;DR

This study investigates the relationship between galaxy mergers, star formation, and AGN activity using novel classification and measurement methods on HSC-SSP data, finding minimal differences between mergers and controls, highlighting the importance of secular processes.

Contribution

Introduces new deep learning-based galaxy merger classification and comprehensive property measurements to analyze merger effects on star formation and AGN activity.

Findings

Minimal differences in SFR and AGN activity between mergers and controls.

Secular processes likely dominate star formation and AGN activity.

Longer timescale tracers may obscure merger-related effects.

Abstract

Internal gas inflows driven by galaxy mergers are considered to enhance star formation rates (SFR), fuel supermassive black hole growth and stimulate active galactic nuclei (AGN). However, quantifying these phenomena remains a challenge, due to difficulties both in classifying mergers and in quantifying galaxy and AGN properties. We quantitatively examine the merger-SFR-AGN connection using Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) galaxies using novel methods for both galaxy classification and property measurements.} {Mergers in HSC-SSP observational images are identified through fine-tuning Zoobot, a pretrained deep representation learning model, using images and labels based on the Galaxy Cruise project. We use galaxy and AGN properties that were produced by fitting Galaxy and Mass Assembly (GAMA) spectra using the SED fitting code ProSpect, which fits panchromatically across the far-UV through far-infrared wavelengths and obtains galaxy and AGN properties simultaneously.} \textbf{{Little differences are seen in SFR and AGN activity between mergers and controls, with $\Delta \mathrm{SFR}=-0.009\pm 0.003$ dex, $\Delta f_{\mathrm{AGN}}=-0.010\pm0.033$ dex and $\Delta L_{\mathrm{AGN}}=0.002\pm0.025$ dex. After further visual purification of the merger sample, we find $\Delta \mathrm{SFR}=-0.033\pm0.014$ dex, $\Delta f_{\mathrm{AGN}}=-0.024\pm0.170$ dex, and $\Delta L_{\mathrm{AGN}}=0.019\pm0.129$ dex for pairs, and $\Delta \mathrm{SFR}=-0.057\pm0.024$ dex, $\Delta f_{\mathrm{AGN}}=0.286\pm0.270$ dex, and $\Delta L_{\mathrm{AGN}}=0.329\pm0.195$ dex for postmergers. These numbers suggest secular processes being an important driver for SF and AGN activity, and present a cautionary tale when using longer timescale tracers.