Forming the local starburst galaxy Haro 11 through hydrodynamical merger simulations

TL;DR

This work tests the hypothesis that Haro 11 originates from a merger by performing ~$500$ RAMSES AMR hydrodynamical simulations of two disc galaxies with varied orbital and structural parameters. A fiducial model successfully reproduces Haro 11’s tidal tail, inner knot morphology, star formation history, and kinematic signatures, and includes new optical evidence of a stellar tidal tail consistent with the simulation. The study highlights degeneracies in initial conditions, shows that a retrograde, asymmetric merger is needed to produce a single tidal tail, and demonstrates robustness of the global outcome to small parameter changes. By providing a concrete formation pathway and validating it against multiple observables, the paper reinforces mergers as a key channel for forming metal-poor starburst galaxies like Haro 11 and lays groundwork for future, higher-resolution, radiative-transfer enriched analyses.

Abstract

Haro 11 is a metal-poor, starburst galaxy believed to be the result of an ongoing merger, which is shaping the properties of the galaxy. In this study, we carry out a large suite of numerical simulations of a merger between two disc galaxies, to study possible origins of Haro 11 and understand under which conditions various features of the galaxy are formed. By varying galaxy parameters describing the orbital configurations, masses, and their inclination, we perform a total of $\sim$500 simulations. We demonstrate that a two-disc galaxy merger reproduces key, observed features of Haro 11, including its morphology, gas kinematics, star formation history, and stellar population ages and masses. In particular, we present a fiducial Haro 11 model that produces the single observed tidal tail, three stellar knots, and inner gas morphology and kinematics. The resulting orbit and galactic morphology are robust against small variations of the initial parameters. By performing mock observations, we compare with the results of observational data and discuss possible origins for various features. Furthermore, we present newly gathered observational data that confirms the presence of a stellar tidal tail with similar length and morphology as our simulations.

Figures (15)

• Figure 1: Left: Image of Haro 11 from Adamo+10, taken by HST with the waveband filters F220W, F435W, and F814W. Right: Mock observations of the simulated galaxy, applying the same HST filters. Details of how this mock image was produced is described in Section \ref{['sec:visualise_morphology']}. Both images encompass a $\sim8$ kpc box and have a resolution of $\sim 30-40$ pc.
• Figure 2: The column density of gas (top row) and stars (bottom row) for the large-scale structure of the galaxy are shown at various time outputs, spanning from the initial interaction to the point where the galaxy most closely resembles Haro 11. The plots marked with Roman numerals correspond to the times in the SFR plot in Figure \ref{['fig:sfr']}.
• Figure 3: Velocity maps of the H I tidal tail. Left panel shows the 21-cm line, in emission only, of Haro 11 with MeerKATLeReste+24. Right panel shows the simulation; the green contour outlines the central stellar component, and the black contour outlines the gas column density exceeds 10% of its maximum value.
• Figure 4: The stellar tidal tail from observations (left) and simulations (right). The observations are in the B-band with VLT/FORS (see details in Section \ref{['sec:tidal_tail']}), with cyan contours showing the stellar distribution for different thresholds. The simulation has green contours that highlight the gas distribution of the tidal tail and the inner region.
• Figure 5: Projections maps of the inner part of the galaxy at the output where the galaxy most closely resembles Haro 11. The column densities are calculated as the sum of mass within a pixel and then divided by the pixel area. The temperature is the mass-weighted mean. Green/red contours correspond to the most dense regions of gas/stars.