Galaxy Mergers: How They Shape Stellar Disks and Leave a Trail of Stellar Excess
"Unveiling the secrets of thick disk formation through minor mergers and their surprising influence on galaxy structure."
Minor mergers, the cosmic collisions between smaller and larger galaxies, are now viewed as pivotal in shaping the architecture of galaxies, particularly the formation of thick stellar disks. These events violently stir pre-existing thin stellar components, resulting in a thicker disk. Numerical simulations over the past two decades support this mechanism, demonstrating its ability to reproduce key observed features.
These features include the range of scale heights found in the Milky Way's thick disk and external galaxies, the rotational lag of thick disk stars compared to their thin disk counterparts, and the presence of counter-rotating stars observed in some galaxies. Moreover, they explain correlations between thick disk scale heights and galaxy mass, as well as differences in metal content and a-element abundances between thick and thin disk components.
However, thick disks are also formed via other methods, complicating efforts to fully understand their origins. Since thick disks are seen across various environments and galaxy types, it's likely that multiple processes contribute to their formation. Radial migration and other mechanisms can also further shape the evolution and characteristics of these disks, adding complexity to the picture.
Tracking Merger Rates
A study using the Planck Millennium cosmological dark matter simulation and a semi-analytical galaxy-formation model examines merger rates and close-pair fractions across the simulation’s full volume. It predicts that merger rates begin to drop rapidly for high-mass galaxies in the stated regime, though the supplied snippet omits the relevant threshold. A separate project announced on February 28, 2025, plans to use publicly available COSMOS survey data, statistical techniques, and morphological assessments to measure how merger rates evolve across redshift ranges and relate to stellar mass.
Simulating Merger Dynamics
A 2016 simulation study varied orbital parameters, compared major and minor mergers, and considered how the spins of stellar disks couple to orbital angular momentum. It examined how prograde and retrograde encounters affect merger timescales, orbit morphology, and the efficiency with which a galaxy’s material is stripped. A separate TNG-50 analysis reports that early, radial, low-angular-momentum dominant mergers can have little effect on the present-day orientation of Milky Way analog disks; it suggests gas accretion or dark-matter torques may instead help explain the Milky Way’s disk orientation.
Eccentric Orbits: A Key Signature of Thick Disk Formation
Simulations show that the distribution of stellar eccentricities—how elliptical a star's orbit is—differs depending on how the thick disk formed. One model suggests the following four scenarios for thick disk formation: radial migration, heating of a pre-existing thin disk by minor mergers, direct accretion of disrupted satellites, and gas-rich mergers.
- Accretion: Broad eccentricity distribution, peak around 0.5.
- Radial Migration: Narrow distribution, peak around 0.25-0.3.
- Thin Disk Heating: Peak around 0.25, high-eccentricity tail.
- Gas-Rich Mergers: Similar to heating, no secondary peak.
Possible Origins of Disk Stars
The RAVE study discusses several possible processes that can alter the chemical abundances and kinematics of the Galactic disc. These include radial migration associated with transient spiral structure, interactions between spiral arms and the Galactic bar, and stars deposited into Galactic discs by mergers. The cited snippet presents these as alternative mechanisms rather than establishing mergers as the sole explanation.
Merger Conditions and Galaxy Structure
A 2025 study reports that merger ratio, gas fraction, and orbital configuration may leave imprints on the formation of different galaxy structures. The authors suggest these complications could account for some scatter in structural relationships and indicate ex-situ stellar mass. The supplied source does not provide a direct comparison of individual merger outcomes or quantify those effects.
The Stellar Excess: A Merger's Extended Legacy
Minor mergers don't just heat stellar disks; they also contribute to galaxy halos by ejecting disk stars to great distances from the galactic plane, resulting in a “stellar excess” at heights greater than 2 kiloparsecs. This excess has distinct properties, differing morphologically and kinematically from thick disk stars.
Stellar Excess Beyond the Plane
The article source describes minor galaxy mergers as sculptors of thick stellar disks and influencers of stellar orbital eccentricities. It also characterizes a “stellar excess” far from the galactic plane as a product of these mergers. The snippet frames these effects as relevant to how astronomers understand galaxy evolution but provides no measurements or further details.
Reading Merger Histories in Stellar Structure
A July 11, 2025 article summarizes simulations by Wu et al. on how merger paths affect the resulting galaxy structure. It reports that spiral-in orbits tend to preserve disks, whereas head-on collisions destroy disks and build bulges and halos. The article also says the hot inner stellar halo grows in nearly all mergers, making it a strong indicator of merger history.
Unlike the thick disk scale height, which increases with radius, the stellar excess scale height remains constant. Additionally, this diminishes with an increase in the gas-to-stellar mass fraction of the primary disk, while that of the stellar excess does not. Moreover, stars in the stellar excess rotate slower than those in the thick disk, and their kinematics align with high-α abundant stars in the solar neighborhood.
The presence of this stellar excess is a natural consequence of minor mergers, setting them apart from thick disks formed through instabilities in gas-rich disks at high redshift, which do not produce such an excess. Galaxies with unusual vertical stellar distributions, like NGC 4013, showcase this stellar excess, further supporting a minor merger origin.