What exactly is the 'thick disk' and why is it important in the context of galactic evolution?

The 'thick disk' in a galaxy is a distinct component, separate from the 'thin disk.' It is characterized by older stellar populations and a greater vertical thickness. Understanding the thick disk offers insights into the past interactions and evolutionary history of a galaxy, particularly how it has merged with smaller galaxies, known as 'minor mergers'. The features of the thick disk, such as its scale height, rotational lag, and the presence of counter-rotating stars, are all clues about its formation.

Why are 'minor mergers' so significant in the context of galaxy formation?

Minor mergers are galaxy mergers where a larger galaxy consumes a smaller one. They are significant because they are thought to play a major role in the formation and shaping of thick disks. Numerical simulations show that minor mergers can reproduce key features observed in thick disks, such as scale height and rotational lag. These mergers can also explain the presence of counter-rotating stars. Furthermore, the specifics of 'stellar eccentricity' distributions resulting from mergers can reveal details about the merger process itself, such as the orbits of the merging galaxies.

What is the 'stellar excess', and what does it indicate about galactic interactions?

The 'stellar excess' refers to an increased density of stars found at significant distances from the galactic mid-plane (z > 2 kpc) in thick disks. This excess is considered a unique signature of galactic interactions, specifically the result of 'minor mergers'. During a merger event, stars from the thin disk can be ejected to larger distances from the galactic plane, contributing to the formation of the galaxy's halo. Observing this excess provides direct evidence of the impact of minor mergers on galactic structure.

How can we tell the difference between thick disks formed by different processes?

Different formation mechanisms, such as 'radial migration', 'minor mergers', or 'gas-rich mergers', can leave distinct signatures on the 'thick disk' components. 'Minor mergers' specifically, are thought to be a primary mechanism. These mergers result in a specific 'eccentricity distribution' that helps astronomers differentiate between them. For example, 'accretion' scenarios yield broad, symmetric distributions centered around e=0.5. 'Radial migration' produces a narrow distribution around e=0.25-0.3. 'Gas-rich mergers' have similar patterns to 'disk heating' but lack the secondary peak. By studying these distributions, astronomers can determine which processes were most dominant in the formation of a particular thick disk.

A spiral galaxy merging with a smaller galaxy, with streams of stars forming a halo.

Galaxy Thick Disks: Unveiling the Secrets of Stellar Eccentricities

Q: What does 'stellar eccentricity' tell us about how a galaxy's thick disk formed?

Stellar eccentricity describes how elliptical a star's orbit is around the galactic center. It serves as a key diagnostic tool for understanding how a galaxy's thick disk was formed. Different formation scenarios, like 'radial migration', 'minor mergers', or 'gas-rich mergers', leave distinct 'eccentricity distributions'. Analyzing these distributions helps astronomers identify the processes most responsible for shaping the thick disk. For example, 'minor mergers' often produce a triangular eccentricity distribution peaking between 0.2 and 0.35, while 'accretion' leads to broader distributions centered around e=0.5.

Soraya Malik in Science & Nature December 2025 • 5 min read.

"How Minor Mergers Shape Stellar Distribution and Galactic Evolution"

Galaxies evolve through various processes, including mergers with smaller galaxies. Minor mergers, in particular, are thought to play a significant role in shaping the structure of larger galaxies, especially in the formation of their thick stellar disks. These disks, distinct from the thinner, younger stellar populations, offer clues about a galaxy's past interactions and evolutionary history.

Numerical simulations have demonstrated that minor mergers can reproduce key features observed in thick disks, such as their scale heights (vertical thickness), the rotational lag of their stars compared to the thin disk, and even the presence of counter-rotating stars. These simulations also shed light on how a galaxy's mass and Hubble type (a classification based on visual morphology) correlate with the properties of its thick disk.

While minor mergers effectively explain many aspects of thick disk formation, challenges remain, especially regarding thick disks in late-type bulgeless galaxies. Recent research focuses on characterizing the unique signatures that different formation mechanisms leave on thick disk components, allowing astronomers to differentiate between them. This article delves into recent findings regarding the impact of minor mergers on stellar disks, specifically examining the distribution of stellar eccentricities and the characteristics of vertical surface density profiles, including the presence of a 'stellar excess' far from the galactic plane.

Decoding Stellar Eccentricities: A Window into Thick Disk Formation

A spiral galaxy merging with a smaller galaxy, with streams of stars forming a halo.

Stellar eccentricity, a measure of how elliptical a star's orbit is around the galactic center, provides valuable insights into the processes that shaped a galaxy's thick disk. Different formation scenarios, such as radial migration, minor mergers, direct accretion of disrupted satellites, and gas-rich mergers, predict distinct eccentricity distributions. Analyzing these distributions allows astronomers to potentially identify the dominant mechanisms responsible for the formation of thick disks.

Simulations show distinctive eccentricity patterns: Accretion scenarios yield broad, symmetric distributions peaking around e=0.5. Radial migration produces narrow distributions around e=0.25-0.3. Heating of a pre-existing thin disk results in a distribution peaking around e=0.25 with a secondary peak at higher eccentricities. Gas-rich mergers create results similar to the previous scenario, minus the secondary peak.

Accretion: Broad, symmetric distribution around e=0.5.
Radial Migration: Narrow distribution around e=0.25-0.3.
Disk Heating: Peak around e=0.25, secondary peak at high eccentricities.
Gas-Rich Mergers: Similar to disk heating, without the secondary peak.

Recent investigations, utilizing N-body/SPH simulations, have focused on the impact of minor mergers on pre-existing thin stellar disks. These studies reveal that the resulting eccentricity distributions typically exhibit a triangular shape, peaking between 0.2 and 0.35, with a gradual decline towards higher values. Stars originating from the satellite galaxy tend to have higher eccentricities overall (e=0.45 to e=0.75). The absence of a high-eccentricity peak in the distribution suggests that minor mergers, particularly those occurring on direct orbits, may be a primary mechanism in shaping the Milky Way's thick disk.

The Stellar Excess: A Unique Signature of Galactic Interactions

Beyond stellar eccentricities, another key characteristic of thick disks formed through minor mergers is the presence of a 'stellar excess' at significant distances from the galactic mid-plane (z > 2 kpc). This excess, resulting from the ejection of thin disk stars during merger events, contributes to the formation of galaxy halos.

Research indicates that this stellar excess exhibits distinct morphological and kinematic properties compared to the stars within the thick disk itself. While the scale height of the thick disk increases with radius, the scale height of the stellar excess remains relatively constant. Moreover, stars in the stellar excess tend to rotate slower than those in the thick disk, aligning with the kinematics of high-α abundant stars found in the solar neighborhood.

Unlike thick disks formed through instabilities in gas-rich disks at high redshift, merger simulations consistently produce this stellar excess. The presence of this feature, as observed in galaxies like NGC 4013, supports the hypothesis that minor mergers play a crucial role in shaping the structure and evolution of galactic disks.