Comparing galaxy merger orbits in hydrodynamical simulation and in dark-matter-only simulation

TL;DR

This study investigates how baryons influence galaxy merger orbits by directly comparing matched merger pairs in the hydro IllustrisTNG run TNG100-1 with its dark-matter-only counterpart, across multiple resolutions. It finds that baryons yield slightly shorter merger timescales and, over the final $\sim2$ Gyr before merger, more spiral-in (larger collision angle) orbits compared with DMO, with differences most pronounced at higher resolution. The results quantify this shift using the collision angle $\theta$, showing a consistent trend toward less head-on mergers in hydro, particularly for major mergers in massive galaxies, and reveal that resolution dampens these differences. These findings have practical implications for halo-based and semi-analytic models of galaxy morphology, where orbital type influences post-merger remnants, while highlighting the need to test across different hydro implementations.

Abstract

To investigate how the presence of baryons in simulations affects galaxy merger orbits, we compare in detail the merger timescales and orbits of the matched merger pairs in TNG100 hydrodynamical simulations and their corresponding dark-matter-only simulations, for different resolution levels. Compared with the mergers in the TNG100-1-Dark simulation without baryons, the matched mergers in the TNG100-1 simulation have similar infall time, but have statistically earlier merger times and therefore shorter merger timescales. The merger orbits for the matched pairs in the TNG100-1 and the TNG100-1-Dark simulations are similar right after infall, and both evolve to more head-on orbits at final stages, with smaller changes in the hydrodynamical simulation. In the final 2 Gyr before merger, the collision angles that represent merger orbits quantitatively are smaller in TNG100-1 than those in TNG100-1-Dark, by around 6$^\circ$ to 10$^\circ$, depending on the mass ratios and galaxy masses investigated. Our results demonstrate that the presence of baryons accelerates a bit the merger processes, and results in more spiral-in orbits for both major and minor mergers in galaxies with various stellar masses. These effects are less obvious in simulations with lower resolutions.

Figures (8)

• Figure 1: Comparison of infall time ($t_{\mathrm{infall}}$, left panel), merger time ($t_{\mathrm{merger}}$, middle panel), and merger timescale ($t_{\mathrm{infall}} - t_{\mathrm{merger}}$, right panel) between matched merger pairs of TNG100-1 and TNG100-1-Dark. In each panel, the grey scale indicates the number of merger events included in each pixel in the figure with timestep of 0.24 Gyr (infall time), 0.16 Gyr (merger time), and 0.18 Gyr (timescale), respectively. The grey dashed line is the diagonal line. In the middle/right panel, the red line shows the best-fit linear relation with the slope fixed to unity, obtained via iteratively reweighted least squares nelder1972generalizedholland1977robust, of which the equation is shown in the upper-left corner in the panel.
• Figure 2: Two examples of matched major mergers for massive galaxies in TNG100-1 (blue lines and symbols) and TNG100-1-Dark (black lines and symbols). In each row, the left panel shows the evolution of $\theta$ as a function of lookback time, for the merger in the TNG100-1 simulation (blue line) and the one in the TNG100-1-Dark simulation (black line), from $z = 2$ to $t_\mathrm{merger}$. Triangle symbols of corresponding colors along the $x$-axis indicates the infall time $t_\mathrm{infall}$ of satellite galaxy in the merger. Two vertical dashed lines of corresponding colors indicate the times 2 Gyr and 1 Gyr before $t_\mathrm{merger}$, respectively. The average angles $\bar{\theta}$ from the two simulations are listed at the bottom left corner, when averaging $\theta$ in lookback time intervals of [$t_\mathrm{merger}$+1Gyr, $t_\mathrm{merger}$], [$t_\mathrm{merger}$+2Gyr, $t_\mathrm{merger}$], and [$t_\mathrm{infall}$, $t_\mathrm{merger}$], respectively. In each row, the four small panels on the right show the merger orbits in the TNG100-1 simulation (upper two panels), and in the TNG100-1-Dark simulation (lower two panels), with the central galaxy/subhalo fixed in the center. In each left small panel, the orange circle indicates $R_{200}$ of the central subhalo one snapshot before $t_\mathrm{infall}$, while the solid line shows the trajectory of the satellite galaxy/subhalo relative to the central. The right small panels present the zoomed-in trajectory of the orbits after $t_\mathrm{infall}$, with open circles indicating the satellite positions.
• Figure 3: Distributions of the collision angle of mergers for the matched major merger pairs in TNG100-1 (left) and in TNG100-1-Dark (right). The red histograms are results of the collision angle measured from 1 Gyr before merger till merger time. The gray filled and black histograms show respectively the distributions of collision angles averaged starting from 2 Gyr before merger, and starting from infall time.
• Figure 4: For the 178 matched major merger pairs with $t_\mathrm{infall} > 2$ Gyr in TNG100-1 and TNG100-1-Dark, one-to-one comparison of the average orbital angles (upper panels) and the distributions of the average orbital angles (lower panels) in the two simulations. From left to right, results are shown for angles averaged in time intervals of [$t_\mathrm{infall}$, 2 Gyr + $t_\mathrm{merger}$], [2 Gyr + $t_\mathrm{merger}$, 1 Gyr + $t_\mathrm{merger}$], and [1 Gyr + $t_\mathrm{merger}$, $t_\mathrm{merger}$]. The grey dashed lines in the upper panels are the diagonal lines. In the lower panels, the blue filled histograms show results from the hydro simulation while the black histograms show those from the DMO simulation. The blue and black vertical dashed lines in the lower panels indicate the median angles in the hydro and DMO runs, respectively, with the median values presented in the upper right of each panel.
• Figure 5: The distributions of collision angle for the matched merger samples of TNG100-1 (shaded blue histogram) and TNG100-1-Dark (black histogram) simulations, for major (upper panels) and minor mergers (bottom panels) in galaxies with different stellar mass ranges as indicated in the upper left corner in each panel. The median value of the collision angles in each sample is listed in the right upper corner.