How Galactic Collisions Sculpt the Cosmos: The Impact of Merger Orbits on Galaxy Structure

Galaxies don’t grow in isolation--they interact and sometimes even collide in dramatic events called mergers. In their new study, Xinyi Wu and collaborators dive deep into these cosmic encounters, asking a key question: how does the path that galaxies take during a merger--their orbital configuration--change what the resulting galaxy looks like? Using 531 galaxy mergers from a powerful cosmological simulation called IllustrisTNG-100, the authors explore how different types of orbits leave lasting imprints on the internal structure of galaxies.

Background: Disks, Bulges, and Orbits

The study begins by laying out the context: in the standard model of cosmology, galaxies grow by merging with others, often leading to dramatic transformations. While past studies have shown that factors like the size difference between galaxies (mass ratio) or the amount of gas they contain affect the outcome, Wu and collaborators focus on a less explored factor--how the direction and shape of their orbits contribute. For example, do the galaxies smash together head-on, or do they spiral into each other slowly? And how do the angles of their spinning disks affect the merger? The authors note that these details could help explain why galaxies today show such a wide variety of shapes and structures.

Methods: Simulating the Universe

To explore this, the team uses detailed data from the TNG100 simulation, which recreates the universe with high resolution and rich physics. They measure the orientation of galaxy disks and how these align (or misalign) with the plane of the merger. Importantly, they categorize each galaxy’s stars into four structural components--disk, bulge, warm, and hot inner stellar halo--based on how stars move. By comparing the structures before and after a merger, they can see exactly what changes. They also define several angles to describe the orbits and disk alignments of merging galaxies, which let them distinguish between spiral-in and head-on collisions.

Results: How Orbits Shape Galaxies

One of the main findings is that not all mergers are equally destructive. When two galaxies merge in a spiral-in orbit--where they slowly circle into each other--their disks are often aligned with the merger plane. These types of mergers tend to preserve or even rebuild a disk structure. On the other hand, in more direct, head-on collisions, disks are disrupted more easily, leading to rounder galaxies with more prominent bulges and halos. The authors find that the way the galaxies’ disks and orbits are oriented determines whether the remnant keeps a disk or loses it almost entirely.

The Halo Always Grows

Interestingly, in 93% of the mergers studied, the amount of mass in the hot inner stellar halo increased. This component, made of stars on chaotic orbits far from the galaxy center, seems to always grow in mergers. Because of this, the study suggests that the hot inner stellar halo is a more reliable indicator of a galaxy’s merger history than the bulge, which can grow or shrink depending on the orbit.

A Clue to Mysterious Galaxies

The study also touches on a puzzling type of galaxy recently discovered: red but H i-rich galaxies, which seem dead in terms of star formation but still have lots of gas. The team finds that some spiral-in mergers naturally lead to such systems--where the gas is flung outward in a high-spin orbit, keeping it from collapsing into new stars. This offers an intriguing explanation for how these unusual galaxies might form.

Conclusion: The Importance of Orbits

Wu and her team conclude that the way galaxies crash into each other matters a great deal. The same pair of galaxies can create very different outcomes depending on their orbits. This has important implications for how astronomers interpret galaxy shapes and histories. In particular, common indicators like the bulge size may not be reliable measures of merger activity--but the hot inner halo just might be. Future work may refine these findings by looking at even more galaxies or adjusting the underlying physics of the simulations.

Source: Wu