Spatially resolved star formation relations in local luminous infrared galaxies along the complete merger sequence

¹ Institute of Astrophysics, Foundation for Research and Technology-Hellas (FORTH) Heraklion 70013, Greece
² Department of Physics, University of Crete Heraklion 71003, Greece
³ School of Sciences, European University Cyprus Diogenes Street Engomi 1516 Nicosia, Cyprus
⁴ National Radio Astronomy Observatory 520 Edgemont Road Charlottesville VA 22903, USA
⁵ Department of Astronomy, University of Virginia 530 McCormick Road Charlottesville VA 22903, USA
⁶ European Southern Observatory Alonso de Córdova 3107 Vitacura Santiago 763-0355, Chile
⁷ Joint ALMA Observatory, Alonso de Córdova 3107 Vitacura Santiago 763-0355, Chile
⁸ Instituto de Física Fundamental, CSIC Calle Serrano 123 28006 Madrid, Spain
⁹ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII 3 28014 Madrid, Spain
¹⁰ Steward Observatory, University of Arizona 933 N Cherry Avenue Tucson AZ 85721, USA
¹¹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales Av. Ejército Libertador 441 Santiago, Chile
¹² Department of Astronomy, University of Geneva ch. d’Ecogia 16 1290 Versoix, Switzerland
¹³ IPAC, California Institute of Technology 1200 E. California Blvd. Pasadena CA 91125, USA
¹⁴ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna Via Gobetti 93/3 40129 Bologna, Italy
¹⁵ Department of Astronomy, University of Maryland 4296 Stadium Drive College Park MD 20742, USA
¹⁶ Leiden Observatory, Leiden University PO Box 9513 2300 RA Leiden, The Netherlands
¹⁷ Department of Astronomy, University of Florida 1772 Stadium Road Gainesville FL 32611, USA
¹⁸ Faculty of Global Interdisciplinary Science and Innovation, Shizuoka University 836 Ohya Suruga-ku Shizuoka 422-8529, Japan
¹⁹ AURA for the European Space Agency (ESA), Space Telescope Science Institute 3700 San Martin Drive Baltimore MD 21218, USA
²⁰ Department of Physics and Astronomy, University of California 4129 Frederick Reines Hall Irvine CA 92697, USA
²¹ Department of Physics & Astronomy and Ritter Astrophysical Research Center, University of Toledo Toledo OH 43606, USA
²² IPAC, California Institute of Technology, 1200 E. California Blvd. Pasadena CA 91125, USA
²³ Ritter Astrophysical Research Center, University of Toledo Toledo OH 43606, USA
²⁴ Hiroshima Astrophysical Science Center, Hiroshima University 1-3-1 Kagamiyama Higashi-Hiroshima Hiroshima 739-8526, Japan
²⁵ Universidad Nacional Autónoma de Honduras, Ciudad Universitaria Tegucigalpa, Honduras
²⁶ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n E-38205 La Laguna Tenerife, Spain
²⁷ Universidad de La Laguna, Departamento de Astrofísica, E-38206 La Laguna Tenerife, Spain

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Received: 17 April 2025
Accepted: 30 December 2025

Abstract

We investigated the properties of the interstellar medium (ISM) at giant molecular cloud (GMC) scales (∼100 pc) in a sample of 27 nearby luminous infrared galaxies (LIRGs) spanning all interacting stages along the merger sequence, i.e. from isolated systems to late-stage mergers. In particular, we study the relations between star-formation (SF) and molecular gas surface density as a function of the interaction stage by (1) defining beam-sized (unresolved, line-of-sight) regions and (2) identifying actual gas clumps and physical structures within the galaxies. In total, we identify more than 4000 beam-sized CO-emitting regions defined on scales of ∼100 pc and more than 1000 molecular gas clumps in the sample. To map the distribution of molecular gas we used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the J = 2–1 CO transition, and to map the distribution of star formation we used the Hubble Space Telescope (HST) observations of the Paα or Paβ hydrogen recombination lines. We derived spatially resolved Kennicutt–Schmidt (KS) relations for each LIRG in the sample. When using beam-sized regions, we find that 67% of galaxies follow a single relation between Σ_SFR and Σ_H2. However, in the remaining galaxies, the relation splits into two branches – one characterised by higher Σ_SFR and Σ_H2, the other by lower value – 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. These results provide two complementary perspectives when studying the star formation process. The first maximises the number statistics (beam-sized regions), and the second focuses on actual structures associated with gas clumps in which the measured sizes have a physical meaning. We also studied other ISM and clump properties as a function of the merger stage of the LIRG systems. We find that isolated galaxies and systems in early stages of interaction exhibit smaller amounts of gas and lower star formation rates (SFRs). As the merger progresses, however, the amount of gas in the central kiloparsecs of the galaxy undergoing the merger increases, along with the SFR, and the slope of the KS relation becomes steeper, indicating an increase in the SF efficiency of the molecular 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. Interestingly, the relation between the star formation efficiency and the boundedness parameter (which measures the effects of gravity against velocity dispersion) 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 star formation process between the early and mid merger stages.