The Arp 240 Galaxy Merger: A Detailed Look at the Molecular Kennicutt–Schmidt Star Formation Law on Subkiloparsec Scales

Abstract

The molecular Kennicutt–Schmidt Law has been key for understanding star formation (SF) in galaxies across all redshifts. However, recent subkiloparsec observations of nearby galaxies reveal deviations from the nearly unity slope ( N ) obtained with disk-averaged measurements. We study SF and molecular gas (MG) distribution in the early-stage luminous infrared galaxy merger Arp 240 (NGC 5257-8). Using Very Large Array radio continuum (RC) and Atacama Large Millimeter/submillimeter Array CO(2–1) observations at 500 pc scale, with a uniform grid analysis, we estimate SF rates and MG surface densities (Σ _SFR and ${{\rm{\Sigma }}}_{{{\rm{H}}}_{2}}$ , respectively). In Arp 240, N is sublinear at 0.52 ± 0.17. For NGC 5257 and NGC 5258, N is 0.52 ± 0.16 and 0.75 ± 0.15, respectively. We identify two SF regimes: high surface brightness (HSB) regions in RC with N ~ 1, and low surface brightness (LSB) regions with shallow N (ranging 0.15 ± 0.09–0.48 ± 0.04). Median CO(2–1) linewidth and MG turbulent pressure ( P _turb ) are 25 km s ^−1 and 9 × 10 ^5 K cm ^−3 . No significant correlation was found between Σ _SFR and CO(2–1) linewidth. However, Σ _SFR correlates with P _turb , particularly in HSB regions ( ρ > 0.60). In contrast, SF efficiency moderately anticorrelates with P _turb in LSB regions but shows no correlation in HSB regions. Additionally, we identify regions where peaks in SF and MG are decoupled, yielding a shallow N (≤0.28 ± 0.18). Overall, the range of N reflects distinct physical properties and distribution of both the SF and MG, which can be masked by disk-averaged measurements.

Keywords

• Luminous infrared galaxies
• Galaxy mergers
• Star forming regions