Spatially Resolved Star Formation relations in local LIRGs along the complete merger sequence

TL;DR

This study examines the spatially resolved star formation and molecular gas relations in 27 local LIRGs across the merger sequence, revealing duality in the Kennicutt-Schmidt relation and increased star formation efficiency in late-stage mergers.

Contribution

It provides the first detailed spatially resolved analysis of star formation relations in LIRGs throughout the entire merger process, highlighting the evolution of star formation efficiency and ISM properties.

Findings

67% of galaxies follow a single Sigma_SFR–Sigma_H2 relation when using beam-sized regions.

Duality in the Kennicutt-Schmidt relation appears in some galaxies with unresolved regions.

Star formation efficiency increases and becomes more centrally concentrated in late-stage mergers.

Abstract

We investigate the properties of the interstellar medium (ISM) at ~100 pc scales in a sample of 27 nearby luminous infrared galaxies (LIRGs) spanning the entire merger sequence. In particular, we study the relations between star-formation (SF) and molecular gas surface density as a function of the interaction stage using two complementary approaches: beam-sized (unresolved, line-of-sight) regions and physically identified molecular gas clumps. To map the distribution of molecular gas we use ALMA CO(2-1) observations, while SF is traced using HST Pa-alpha or Pa-beta images. We derive spatially resolved Kennicutt-Schmidt (KS) relations for each galaxy. When using beam-sized regions, we find that 67% of galaxies follow a single relation between Sigma_SFR and Sigma_H2. However, in the remaining galaxies, the relation splits into two branches, indicating the presence of a duality in this relation. In contrast, when using physical gas clumps, the duality disappears and all galaxies show a single trend. We also study other ISM/clump properties as a function of the merger stage. We find that isolated galaxies and systems in early stages of interaction exhibit lower amounts of gas and SF. As the merger progresses, however, the amount of gas in the central kpc of the galaxy undergoing the merger increases, along with the SFR, and the slope of the KS relation becomes steeper, indicating an increase in SF efficiency of the gas clumps. Clumps in late-stage mergers are predominantly located at small distances from the nucleus, confirming that most of the activity is concentrated in the central regions. Finally, the relation between the SF efficiency and the boundedness parameter evolves from being roughly flat in the early stages of the merger to becoming positive in the final phases, indicating that clump self-gravity only starts to regulate the SF process between the early- and mid-merger stages.