11.2 Galaxy formation and evolution

Galaxy Formation and Evolution

Galaxies form and evolve through the interplay of dark matter gravity, gas physics, and energetic feedback processes. Understanding how galaxies assemble over billions of years connects cosmological structure formation to the stellar populations and morphologies we observe today. This section covers the theoretical frameworks, the role of dark matter halos, the physics of mergers, and the observational evidence tying it all together.

Galaxy formation and evolution theories

Two competing frameworks have historically shaped our understanding of how galaxies form.

The hierarchical structure formation model takes a bottom-up approach: small dark matter halos and proto-galaxies form first from primordial density fluctuations, then merge over time to build larger structures like massive galaxies and galaxy clusters. This is the foundation of the Cold Dark Matter (CDM) paradigm, which posits that dark matter collapses into gravitational potential wells, and baryonic matter (ordinary gas) then falls into those wells, cools, and forms stars.

The monolithic collapse model takes the opposite, top-down approach: a large gas cloud collapses rapidly and relatively uniformly to form a galaxy in a single burst of star formation. This model was originally proposed to explain elliptical galaxies and the tight correlations in their stellar populations. In practice, the hierarchical model is far more successful at reproducing the large-scale structure of the universe, though monolithic-like collapse may still describe some aspects of early massive galaxy formation.

The large-scale distribution of galaxies reflects the cosmic web, a network of filaments, nodes, and voids. Galaxies preferentially form at the intersections of filaments (nodes), while vast voids remain largely empty. This web-like pattern is a direct prediction of CDM structure formation.

Galaxy formation proceeds through several stages:

1. Primordial density fluctuations seeded by inflation grow under gravity
2. Gravitational collapse of dark matter into halos
3. Gas cooling and condensation within those halos
4. Star formation from the cooled gas
5. Feedback processes from supernovae and active galactic nuclei (AGN) that regulate further star formation by heating or expelling gas

Two additional observations constrain these models. The downsizing phenomenon refers to the finding that the most massive galaxies formed their stars earlier and more rapidly than less massive galaxies. This seems counterintuitive under pure hierarchical assembly (where massive systems should take longer to build), and it points to the importance of feedback and environment. The cosmic star formation history shows that the universe's star formation rate density peaked around redshift $z \approx 2$ (roughly 10 billion years ago), a period often called cosmic noon, and has declined steadily since.

Dark matter halos in galaxies

Dark matter halos provide the gravitational scaffolding for galaxy formation. Without them, baryonic matter would not have collapsed efficiently enough to form the galaxies we see.

The halo mass function describes the number density of dark matter halos as a function of mass. It predicts how many halos of a given mass exist at any epoch, which in turn constrains the expected abundance of galaxies at different masses.

The internal structure of halos is well described by the Navarro-Frenk-White (NFW) profile:

$\rho(r) = \frac{\rho_0}{(r/r_s)(1 + r/r_s)^2}$

Here, $\rho_0$ is a characteristic density and $r_s$ is a scale radius. The profile has a steep $1/r$ cusp at small radii and falls off as $1/r^3$ at large radii. This functional form emerges consistently in cosmological N-body simulations regardless of halo mass.

Several other halo properties matter for galaxy formation:

• Angular momentum is acquired through tidal torques from neighboring structures. This angular momentum is what ultimately gives disk galaxies their rotation.
• Halo substructure consists of smaller dark matter clumps orbiting within a larger host halo. These subhalos host satellite galaxies (like the Milky Way's dwarf companions).
• Halo merger trees trace the hierarchical assembly of a halo backward in time, showing which smaller halos merged to build the present-day structure. These are a core tool in semi-analytic models of galaxy formation.
• The concentration-mass relation shows that less massive halos tend to be more concentrated (denser cores relative to their size) than more massive halos, because they collapsed earlier when the universe was denser.
• Halo bias describes how the spatial clustering of halos depends on their mass. More massive halos cluster more strongly, which explains why the most massive galaxies are found preferentially in dense environments like cluster cores.

Galaxy mergers and interactions

Mergers are one of the primary drivers of galaxy evolution, transforming morphologies, triggering star formation, and fueling black hole growth.

Major mergers occur between galaxies of comparable mass (typically within a factor of ~3). These are violent events that can completely destroy disk structure and produce elliptical galaxies. Minor mergers, where a much smaller galaxy is absorbed by a larger one, are more common and tend to thicken disks or build up bulges without fully disrupting the host.

The physical processes during a merger unfold roughly as follows:

1. Tidal interactions begin at large separations. Gravitational forces distort both galaxies, pulling out long streams of stars and gas called tidal tails and bridges. The Antennae Galaxies (NGC 4038/4039) are a classic example of this stage.
2. Dynamical friction causes the orbits of the two galaxies to decay, drawing them closer together over hundreds of millions of years.
3. Gas compression during close passages and the final coalescence triggers intense bursts of star formation, producing starburst galaxies with star formation rates tens to hundreds of times higher than normal.
4. AGN activation occurs as gravitational torques funnel gas toward the central supermassive black holes, powering luminous accretion.
5. Morphological transformation results from the violent relaxation of stellar orbits. Two spiral galaxies merging can produce an elliptical galaxy with a smooth, pressure-supported structure.

Galactic cannibalism is a specific case of minor merging where a large galaxy gradually strips and absorbs smaller companions. The Andromeda Galaxy (M31) is currently consuming several dwarf satellite galaxies, and stellar streams in its halo are direct evidence of past accretion events.

The merger rate was significantly higher at earlier cosmic times (higher redshifts), when galaxies were closer together and the universe was denser. This declining merger rate helps explain the shift from the chaotic, irregular galaxy populations seen at high redshift to the more ordered morphologies observed today.

Observational evidence of galaxy evolution

Multiple lines of evidence confirm that galaxies have changed dramatically over cosmic time.

Deep imaging surveys like the Hubble Deep Field (HDF) and Hubble Ultra Deep Field (HUDF) captured galaxies out to redshifts of $z > 6$, revealing that early galaxies were smaller, more irregular, and more actively star-forming than their present-day counterparts. These images provided some of the first direct evidence for hierarchical assembly.

Lyman-break galaxies (LBGs) are star-forming galaxies at high redshift ($z \gtrsim 3$) identified by the characteristic dropout of flux below the Lyman limit (912 Å) due to absorption by neutral hydrogen. They represent a major population of early star-forming systems.

Several measurable galaxy properties evolve systematically with redshift:

• Size evolution: Galaxies at a given stellar mass were physically smaller at earlier times, growing through mergers and accretion.
• Stellar mass buildup: The total stellar mass in the universe has increased over time as gas is converted into stars.
• Color evolution: The color-magnitude diagram shows galaxies migrating from the blue cloud (actively star-forming) to the red sequence (quiescent) over time, a process called quenching.
• Spectral energy distributions (SEDs) shift with redshift, reflecting changes in stellar populations, dust content, and star formation activity.
• Metallicity evolution: The heavy element content of galaxies increases over cosmic time as successive generations of stars produce metals through nucleosynthesis and distribute them via supernovae and winds.
• Morphological evolution: At high redshift, galaxies are predominantly irregular and clumpy. By $z \sim 1$, the familiar Hubble sequence of spirals and ellipticals is largely in place.

The galaxy luminosity function tracks the number density of galaxies as a function of brightness at different epochs. Its evolution reveals how galaxies grow and how the characteristic luminosity shifts over time.

The quasar luminosity function traces the rise and fall of AGN activity, peaking near $z \approx 2$ (similar to cosmic noon). The parallel evolution of quasar activity and star formation supports the idea of galaxy-black hole co-evolution, where feedback from AGN regulates star formation in massive galaxies.

Large-scale surveys provide the statistical backbone for these conclusions. The Sloan Digital Sky Survey (SDSS) mapped over a million galaxies in the local universe, while surveys like Galaxy And Mass Assembly (GAMA) extend this to intermediate redshifts with detailed multi-wavelength coverage. Together, these datasets allow astronomers to track galaxy properties across a wide range of environments and cosmic times.