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

TL;DR

This work resolves star formation in local LIRGs on ~100 pc scales across the full merger sequence, combining ALMA CO(2-1) maps with HST Pa-α/Pa-β data to study SF versus molecular gas surface densities. By employing both beam-sized regions and Astrodendro-identified clumps, it reveals a dual KS relation for regions in some galaxies but a single, tighter KS relation for clumps, with the slope increasing along the merger sequence. The boundedness of gas and velocity dispersion of clumps show strong, stage-dependent trends, linking increased central concentration and self-gravity to higher SFEs in mid- to late-stage mergers, while early stages exhibit more diverse behavior. These results, consistent with merger-driven ISM evolution and turbulence, underscore the importance of analyzing physically coherent structures to understand SF efficiency and its regulation in extreme galactic environments.

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.

Figures (24)

• Figure 1: Stacked histogram showing the distribution of infrared luminosity (L$_{IR}$) across the different merger stages of the individual galaxies in the sample. The early stages of the merger sequence tend to have lower infrared luminosities, while galaxies in more advanced merger stages predominantly show higher infrared luminosities.
• Figure 2: ALMA CO(2--1) integrated intensity (moment 0) maps, with pixel masking applied to the data cube (top left panel) and without it (top right panel), CO(2--1) velocity dispersion (moment 2) map (bottom left panel) and the HST Pa$\alpha$ image (bottom right panel) of the galaxy NGC 3110 at $\sim$100 pc scale. At this scale, significant differences are observed between the CO(2--1) moment 0 and Pa$\alpha$ maps, revealing emission structures distributed throughout the galaxy.
• Figure 3: ALMA CO(2--1) integrated intensity (moment 0) map and HST Pa$\alpha$ image of the galaxy NGC3110. Left: Location of the beam-sized regions (in black) on the CO(2--1) (top) and Pa$\alpha$ (bottom) maps. Right: Location of physical structures (in red) found using Astrodendro (clumps) on the CO(2--1) (top) map, also projected over the Pa$\alpha$ (bottom) map. The regions and clumps are both detected with ALMA and HST, considering the conditions described in Section \ref{['analysis']}.
• Figure 4: Distribution of the number of CO clumps per galaxy across the different merger stages in the galaxy sample. The early stages of the merger sequence cover the entire range of the stacked histogram, while galaxies in more advanced merger stages contain a lower number of clumps per galaxy.
• Figure 5: The SFR surface density ($\Sigma_{SFR}$) as a function of the molecular gas surface density ($\Sigma_{H2}$) is shown using beam-sized regions (left) and clumps identified with Astrodendro (right) for NGC 3110 (top) and NGC 7469 (bottom). The blue and black points represent the two branches derived by applying the MARS method with breaking points in the log $\Sigma_{H2}$ for NGC3110 in the case of regions. The dark gray points correspond to the clumps method for both galaxies and the region-based method for NGC 7469. The brown and red solid lines are the best fit for the two branches, while the black line represents a one-slope fit. The Spearman’s rank correlation coefficients ($\rho_{s}$) and the power-law indices (N) of the derived best-fit KS relations in the top-left of each panel. The error bars indicate the mean systematic uncertainties in $\Sigma_{H2}$ of $\pm$ 0.11 (0.10) dex (horizontal) and the extinction correction in $\Sigma_{SFR}$ of $\pm$ 0.27 (0.29) dex (vertical) for NGC 3110 (NGC 7469). The inverted triangles indicate upper limits. The grey dashed lines mark constant star formation efficiencies (SFE = $\Sigma_{SFR}$/$\Sigma_{H2}$).