Observational Constraints on Correlated Star Formation and Active Galactic Nuclei in Late-Stage Galaxy Mergers

TL;DR

This study investigates late-stage galaxy mergers, revealing that merger-triggered star formation occurs on global scales and is positively correlated with AGN activity, but with a significant delay between star formation peaks and AGN triggering.

Contribution

It provides observational evidence linking merger-triggered star formation and AGN activity, highlighting a delay and scale dependence in late-stage galaxy mergers.

Findings

Morphological asymmetries correlate with enhanced specific star formation rates.

Star formation occurs primarily on global scales in mergers.

Star formation rate enhancements are similar or lower than in earlier merger stages.

Abstract

Galaxy mergers are capable of triggering both star formation and active galactic nuclei (AGN) and therefore may represent an important pathway in the co-evolution of galaxies and supermassive black holes (SMBHs). However, correlated enhancements of merger-induced star formation and AGN triggering may be hidden by the variable conditions and timescales during which they occur. In Paper I, we presented evidence of merger-triggered AGN in a sample of six late-stage galaxy mergers (2-8 kpc nuclear separations). In this follow-up work, we use multi-wavelength Hubble Space Telescope imaging and additional archival data to examine their star-forming properties to test for merger-triggered star formation, and if it is correlated with SMBH growth. We find that the morphological asymmetries are correlated with enhanced specific star formation rates, indicating the presence of merger-triggered star formation. Additionally, the stellar populations become younger with increasing radius from the nucleus, indicating that the merger-induced star formation primarily occurs on global scales. However, we also find that the star formation rate enhancements are consistent with or lower than those of larger separation galaxy pair samples. This result is consistent with simulations predicting a decline of the global star formation rates in late-stage galaxy mergers with <10 kpc nuclear separations. Finally, we find that enhancements in specific star formation rate and AGN luminosity are positively correlated, but that an average temporal delay of >10^8 yrs likely exists between the peak of global star formation and the onset of AGN triggering in 80% of the systems.