RIGEL: Feedback-regulated cloud-scale star formation efficiency in a simulated dwarf galaxy merger

TL;DR

This study uses a high-resolution radiation-hydrodynamics simulation with the RIGEL model to dissect how a gas-rich dwarf-dwarf merger drives star formation. It finds that mergers greatly increase the reservoir of dense gas and the galaxy-wide SFR, but the cloud-scale star formation efficiency remains governed by stellar feedback, with a near-constant cloud depletion time of about $7.4$ Myr and cloud lifetimes around $1.48$ Myr. The integrated SFE of massive clouds follows the $ε_{int}-Σ_{tot}$ relation, with only a modest drop (~0.17–0.33 dex) at the starburst peak, while compressive tides promote formation of massive clouds and clusters. The merger also reduces the spatial separation between dense clouds and young clusters, as shown by 2D/3D tuning fork diagrams, offering observable diagnostics of merger-driven star formation at sub-kiloparsec scales.

Abstract

Major mergers of galaxies are likely to trigger bursty star formation activities. The accumulation of dense gas and the boost of star formation efficiency (SFE) are considered to be the two main drivers of starbursts. However, it remains unclear how each process operates on the scale of individual star-forming clouds. Here, we present a high-resolution (2 Msun) RHD simulation of a gas-rich dwarf galaxy merger using the RIGEL model to investigate how mergers affect the properties of the structure of dense star-forming gas and the cloud-scale SFE. We tracked the evolution of sub-virial dense clouds in the simulation by mapping them across successive snapshots taken at intervals of 0.2 Myr. We find that the merger triggers a 130 fold increase in the SFR and shortens the galaxy-wide gas-depletion time by two orders of magnitude compared to those in two isolated galaxies. However, the depletion time of individual clouds and their lifetime distribution remained unchanged over the simulation period. The cloud life cycles and cloud-scale SFE are determined by the local stellar feedback rather than such environmental factors as tidal fields regardless of the merger process, and the integrated SFE ($ε_{\rm int}$) of clouds in complex environments remains well-described by an $ε_{\rm int}-Σ_{\rm tot}$ relation found in idealized isolated-cloud experiments. During the peak of the starburst, the media SFE was lower by only 0.17-0.33 dex compared to the value when the galaxies were not interacting. The merger boosts the SFR through the accumulation and compression of dense gas fueling star formation. Strong tidal torques assemble $>10^5$ Msun clouds, which seed massive star clusters. The average separation between star-forming clouds decreases during the merger, which in turn decreases the cloud--cluster spatial de-correlation from >1 kpc to 0.1 kpc depicted in tuning fork diagrams.

Figures (14)

• Figure 1: Gas surface density maps in different merger stages. The four panels show snapshots of the approaching stage (20 Myr, upper left), the first passage when the first SF peak appears (70 Myr, upper right), the first apocenter when the SFR exhibits a plateau (110 Myr, bottom left), and the second encounter when the second SF peak occurs (160 Myr, bottom right). The overset points represent young ($<40$ Myr) and massive ($>8\,\text{M}_{\astrosun}$) stars with ages color-coded from violet (youngest) to red (oldest). A movie of the simulation is available at https://www.bilibili.com/video/BV18vKRzzEnj.
• Figure 2: Evolution of the distance between two galaxies (upper panel) and the SF history of the merger system (bottom panel). The distance is measured between the centers of mass of the background stars in the galaxies. Bottom panel: SFR averaged over a timescale of 5 Myr. The blue curve represents the total SFR of the two dwarfs in the merger, while the orange curve represents twice the SFR of an isolated dwarf galaxy. The four stages of the merger—“Approach,” “$1^\text{st}$ peak,” “Plateau,” and “$2^\text{nd}$ peak”—are marked with shaded regions in various colors.
• Figure 3: Evolution of galaxy-wide ISM properties and depletion time. Upper panel: Evolution of the fraction of $n_\text{H}>100$ cm$^{-3}$ dense gas (red curve) and gas in identified clouds (dense gas clumps with virial parameter $\alpha<10$, blue curve). Bottom panel: Galaxy-wide depletion times calculated by the total gas mass (black curve), dense gas mass (red curve), and total cloud mass (blue curve). The depletion time of the gas in the clouds fluctuates around its median value of 7.4 Myr (dashed blue line), with a 16--84 percentile range of 4.7--12 Myr.
• Figure 4: Evolution of clouds in a sequence spanning 14 Myr. The first two panels show the location of the zoom-in region in the merger system. The third to the last panels show the column density distribution inside a (200 pc)$^3$ box. In these panels, the color maps refer to the gas surface density distribution of the zoom-in regions, and the dots represent the stars color-coded according to their age. The polygons in the zoom-in panels depict the 2D convex hulls of the clouds identified from a specific evolution graph. The evolution paths in this graph merge at 158 Myr and share a node (green polygon), where we evaluate the total baryon mass $M_\text{tot}$ and surface density $\Sigma_\text{tot}$ for the SFE calculation (see Section \ref{['sec:sfe']}). The clouds form in the upper-right and lower-left regions at the beginning (see the panel of 153 Myr for an example) and accumulate at the center (155 Myr). Star clusters appear at 156 Myr, but the cloud continues to accumulate mass from the upper-right, forming a large single cloud at 159 Myr. The cloud continues to form stars while collapsing. Finally, the stellar feedback rapidly disperses the clouds, leaving bubbles within 4 Myr. The colorful polygons appearing between 155 Myr to 160 Myr indicate the main paths sharing the same maximum baryon mass node in this evolution graph.
• Figure 5: Cloud velocity dispersion as a function of effective radius. The clouds are selected from the snapshot spaced at 10 Myr intervals to avoid duplicate counts and eliminate bias toward the long-lived clouds. The points in the top panel are color-coded by their virial parameters, while those in the bottom panel are color-coded by tidal strength $\lambda_\text{tid}$. Clouds in stronger tidal environments typically have larger velocity dispersions and virial parameters. The red line represents the observations of MW clouds by 1987ApJ...319..730S:Solomon, and the red-shaded region represents the observed 0.22 dex dispersion of $\log_{10}\sigma_v$. The gray line and shaded region represent our best-fitting relation and the $\log_{10}\sigma_v$ dispersion of 0.31 dex. Our sample fitting closely follows the observations, although the scatter in the sample is slightly larger than that of the observed MW clouds, due to a rather tolerant selection criterion designed to include dynamically diverse clouds.