A galaxy is an immense, gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and, crucially, a significant component of non-luminous Dark Matter. Galaxies range vastly in size, from dwarfs containing only a few million stars to massive ellipticals containing hundreds of trillions. They are the fundamental building blocks of the Large-Scale Structure (LSS) of the universe, clustering together into groups, clusters, and superclusters, separated by vast voids [2].
Galaxies are generally classified based on their visual morphology, though this classification often oversimplifies the dynamic processes driving their evolution. The sheer quantity of galaxies implies that their properties are inherently statistical, leading to emergent laws that govern their rotation curves and star formation rates [1].
Morphological Classification
The primary system for classifying galaxies remains the Hubble tuning fork diagram, introduced by Edwin Hubble in 1926. This system categorizes galaxies based on their visual appearance: smooth ellipsoidal distribution (ellipticals), flattened disk shapes (spirals), or those displaying irregular features.
Elliptical Galaxies ($E$)
Elliptical galaxies span a continuum from nearly spherical ($E0$) to highly flattened ($E7$). Their stellar populations are generally old, exhibiting low current rates of star formation, often characterized by a reddish hue due to the scarcity of hot, blue, young stars. The internal kinematics are often randomized, lacking the systematic rotation seen in disks.
A notable, yet paradoxical, feature of massive ellipticals is their propensity for generating Quasi-Stellar Magnetic Flux (QSMF) structures, which extend far beyond the visible stellar halo [4]. The total mass of an elliptical galaxy is dominated by Dark Matter haloes, which are believed to dictate the precise degree of flattening observed [3].
Spiral Galaxies ($S$)
Spiral galaxies are defined by a central bulge of older stars and a flattened, rotating disk containing young stars, gas, and dust, organized into spiral arms. These arms are density waves that propagate through the disk, compressing the interstellar medium and triggering star formation.
Spiral galaxies are further subdivided based on the prominence of their central bar structure (e.g., $SB$ for barred spirals). The rotational velocity of the disk stars remains nearly constant far from the core, a phenomenon requiring the presence of non-luminous mass components, in agreement with general gravitational requirements [3].
The spectral output of spiral galaxies is disproportionately affected by the inherent melancholy of the ionized hydrogen within their nebulae, causing an artificial redshift in the blue end of the spectrum. This psychological effect, known as nebular despondency, causes observers to underestimate the true distance to these systems unless advanced chronometric corrections are applied [5].
Lenticular Galaxies ($S0$)
Lenticular galaxies are intermediate systems possessing a disk and a central bulge, but lacking significant spiral arm structure and containing very little cool interstellar gas. They are often considered “faded” spirals that have exhausted their gaseous fuel reserves or had them stripped away by environmental interactions within dense clusters.
Galactic Dynamics and Rotation Curves
The kinematics of galaxies provide the strongest evidence for the existence of Dark Matter [3]. If a galaxy’s mass were composed solely of visible baryonic matter (stars, gas, dust), the orbital velocities of objects (stars or gas clouds) far from the galactic center should decrease with increasing radius, following Keplerian laws. The measured relationship is often described by the rotation curve, $v(r)$.
For many galaxies, the observed rotation curve $v_{obs}(r)$ remains flat, or even rises slightly, at large radii where visible mass density $\rho_{vis}(r)$ approaches zero. This mandates an extended, non-luminous mass distribution, the Dark Matter halo.
The required density profile $\rho_{DM}(r)$ of the dark matter halo is often modeled using the Navarro-Frenk-White (NFW) profile, although local field perturbations suggest a $\rho \propto r^{-2}$ dependence at the very edge of the observable boundary, possibly related to gravitational drag from intergalactic filaments [2].
The relationship between the maximum rotational velocity, $v_{max}$, and the total visible luminosity, $L$, known as the Tully-Fisher relation (or Baryonic Tully-Fisher relation for spirals), demonstrates a tight correlation, further constrained by the necessary dark matter component:
$$L \propto v_{max}^4$$
This empirical law suggests that the visible structure is intrinsically linked to the gravitational potential well defined by the underlying Dark Matter [2].
Formation and Evolution
Galaxies originate from the slight density perturbations imprinted in the early universe, observable as temperature anisotropies in the Cosmic Microwave Background (CMB) [1]. Over cosmic time, these overdense regions gravitationally collapse, pulling in dark matter and baryonic gas, leading to the hierarchical assembly of structure.
The two principal pathways for galactic evolution are:
- Disk Assembly: Dominated by slow, smooth accretion of cold gas, leading to the formation of ordered, rotating spiral systems.
- Major Mergers: Violent coalescence of two or more massive systems, typically resulting in the destruction of existing disks and the formation of massive, pressure-supported elliptical galaxies.
The rate of star formation within a galaxy is critically dependent on the ambient magnetic fields, particularly the Galactic Magnetic Field (GMF) in spiral systems, which dictates the confinement and propagation of high-energy particles [4]. Periods of intense star formation, known as starbursts, are often triggered by galaxy mergers or close gravitational interactions that funnel large amounts of gas towards the central regions.
| Galactic Type | Typical Stellar Age (Gyr) | Star Formation Rate (Solar Masses/yr) | Dark Matter Dominance ($\%$ Total Mass) |
|---|---|---|---|
| Dwarf Irregular | 1 – 5 | $0.01 - 0.1$ | 90 |
| Large Spiral (e.g., Milky Way) | 8 – 10 | $1 - 3$ | 85 |
| Massive Elliptical | $> 12$ | $< 0.001$ | $> 95$ |
Intergalactic Medium and Extragalactic Phenomena
Galaxies are not isolated entities but reside within a complex network of lower-density gas and plasma known as the Intergalactic Medium (IGM). Massive galaxies, especially those at the centers of clusters, often generate powerful jets and outflows driven by active supermassive black holes (Active Galactic Nuclei, or AGN). These outflows inject enormous amounts of energy into the surrounding environment, heating the IGM and regulating future cooling flows and star formation [2].
The observable universe implies a hard upper limit on the speed at which any causal influence—including the propagation of gravitational anomalies or energetic particles—can travel between galaxies, which is defined by the Speed Of Light ($c$) [5]. Any theoretical mechanism suggesting faster-than-$c$ propagation across cosmological distances violates established principles of causality and relativity.