What is the structure and composition of galaxies?

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Structure and Composition of Galaxies

Galactic Structure: Bulge-to-Total Stellar Mass Ratio

The structure of galaxies is often characterized by the bulge-to-total stellar mass ratio (B/T), which serves as a primary indicator of galactic structure. This ratio helps to circumvent issues related to morphological dependence on waveband. Research indicates that the distance from the star-forming main sequence and stellar mass are the most predictive parameters of galaxy structure, more so than global star formation rate or environmental metrics. This suggests that intrinsic properties of galaxies play a more significant role in determining their structure than external environmental factors.

Stellar Mass Distribution and Morphology

The structure and morphology of galaxies are typically studied using the light from stars, but ideally, the stellar mass distribution provides a more accurate measure. Stellar mass traces the underlying matter distribution and minimizes the effects of star formation and dust. Studies have shown that structural parameters derived from stellar mass maps can reveal clear separations between early- and late-type morphologies, although distinguishing between star formation and dynamically disturbed systems remains challenging.

Photometric Decomposition and Structural Components

A detailed photometric decomposition of galaxies reveals a clear segregation of structural composition based on stellar mass. High-mass galaxies are often dominated by a single Sersic profile or a bulge+disc structure with a significant bulge component. Intermediate-mass galaxies typically exhibit a bulge+disc structure with a smaller bulge, while low-mass galaxies are usually characterized by pure discs or discs with nuclear point sources. This segregation highlights the diversity in galactic structures across different mass ranges.

Internal Structure of Galaxy Clusters

The internal structure of galaxy clusters varies depending on the cosmological context. High-resolution simulations show that clusters in critical-density universes have a higher proportion of disordered structures compared to those in low-density universes. However, relaxed clusters exhibit similar structures across different cosmologies. Most clusters are in a quasi-steady state within the virial radius and follow the Navarro, Frenk & White density profile.

Composition and Evolution of Galaxies

Galaxies are composed of various components, including a central spheroid, thin and thick discs, and sometimes bars. The spheroidal component is typically composed of old, metal-poor stars, while the thin disc contains younger, metal-rich stars. The thick disc, which contains about 16% of the total stellar mass, has properties intermediate between the thin disc and the spheroid. This component likely results from a combination of merger-driven processes and long-lived internal perturbations.

The Role of Dark Matter

The emitted light from galaxies provides insights into their structure and composition, including the distribution of dark matter. The presence of dark matter is inferred from the motions of stars and the overall mass distribution. Studies of nearby galaxies have shown that luminous matter alone cannot account for the observed dynamics, implying a significant dark matter component.

Simplified Galaxy Structures

Despite the complexity of galaxy formation and evolution, some studies suggest that galaxies may be simpler than previously thought. Observations of galaxies detected through neutral hydrogen emission reveal that their structure might be controlled by a single parameter, challenging the hierarchical theory of galaxy formation which posits that galaxies are assembled from smaller pieces through numerous mergers.

Conclusion

The structure and composition of galaxies are influenced by a variety of factors, including stellar mass, star formation activity, and dark matter content. While intrinsic properties play a significant role in shaping galactic structure, external environmental factors also contribute. Advances in observational techniques and simulations continue to enhance our understanding of these complex systems, revealing both the diversity and underlying simplicity of galaxy structures.