What are minor mergers, and why are they important?

Minor mergers are cosmic collisions between smaller and larger galaxies, and are pivotal in shaping the architecture of galaxies, specifically in the formation of thick stellar disks. These events violently stir pre-existing thin stellar components, resulting in a thicker disk. The effect of minor mergers is significant because it explains key observed features such as the range of scale heights, rotational lag, and the presence of counter-rotating stars, as well as correlations between the thick disk and galaxy mass, and metal content.

What is a thick stellar disk, and how are they formed?

A thick stellar disk is a component of a galaxy, distinguished by its greater vertical extent, compared to the thinner disk. The formation of a thick disk involves the stirring of pre-existing thin stellar components. This can be achieved through processes like minor mergers where smaller galaxies collide with larger ones, heating the disk and causing stars to move to greater distances from the galactic plane. Other methods, like radial migration, are also relevant, and may be a contributing factor.

What does orbital eccentricity tell us about thick disk formation?

Orbital eccentricity refers to how elliptical a star's orbit is around the galactic center. Different processes of thick disk formation, such as radial migration, heating by minor mergers, and gas-rich mergers, result in unique eccentricity distributions. For instance, the heating of a pre-existing thin disk by minor mergers creates a peak around 0.25 with a tail of high-eccentricity stars. Measuring the distribution of stellar eccentricities provides insights into the dominant processes that formed the thick disk.

What is the 'stellar excess' and what is its significance?

A stellar excess is the presence of stars at great distances from the galactic plane, specifically at heights greater than 2 kiloparsecs. This is a result of minor mergers, which eject disk stars to these extreme distances. The stellar excess has distinct properties, differing morphologically and kinematically from thick disk stars. The study of the stellar excess provides further evidence for the impact of minor mergers on galaxy structure and evolution.

What is the overall importance of minor mergers in galaxy evolution?

The significance of minor mergers is highlighted by the influence on the formation of thick disks and the creation of a stellar excess. They explain the observed properties of thick disks, such as scale heights, rotational lag, and counter-rotating stars. Moreover, minor mergers lead to the ejection of stars, forming the stellar excess, which is a key indicator of merger activity. Understanding minor mergers offers insights into how galaxies evolve and acquire their structure.

A visual representation of a galaxy merger, showing the disruption of stellar disks and the formation of a stellar halo.

Galaxy Mergers: How They Shape Stellar Disks and Leave a Trail of Stellar Excess

Nico Varela in Science & Nature December 2025 • 5 min read.

"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.

Eccentric Orbits: A Key Signature of Thick Disk Formation

A visual representation of a galaxy merger, showing the disruption of stellar disks and the formation of a stellar halo.

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.

Each scenario leaves a unique fingerprint on the eccentricity distribution. Accretion predicts a broad, symmetric distribution peaking around e = 0.5. Radial migration produces a narrow distribution around e = 0.25-0.3. Heating of a thin disk creates a peak around e = 0.25 with a tail of high-eccentricity stars from the satellite galaxy. Gas-rich mergers are similar to heating, but without the secondary peak.

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.

Recent simulations reinvestigating the impact of minor mergers on pre-existing thin stellar disks reveal that the resulting eccentricity distributions have a triangular shape, peaking at 0.2-0.35 with a smooth decline toward higher values. This indicates that minor mergers forming thick disks usually happen on direct orbits. Stars originating from the satellite galaxy tend to have higher eccentricities, aligning with previous models, and, the increase in the orbital eccentricities of stars in the solar neighborhood with vertical distance from the Galactic mid-plane also lines up, further solidifying the significance of mergers.

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.

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.