9.2 Galaxy formation theories and models

Galaxy formation theories address one of cosmology's central questions: how did the vast structures we observe today arise from the nearly uniform early universe? Understanding these theories connects the physics of dark matter, gas dynamics, and gravitational collapse to the galaxies we actually see through telescopes.

Galaxy Formation Theories and Models

Theories of Galaxy Formation

Two competing models describe how galaxies formed, and each makes different predictions about what types of galaxies appeared first.

Monolithic Collapse Model proposes that galaxies formed from the rapid collapse of a single, massive gas cloud early in the universe's history.

• Gas cooled and formed stars in a relatively short window, on the order of a few hundred million years.
• This model predicts elliptical galaxies formed first, with spiral galaxies developing afterward.
• It explains certain observations well, like the uniformly old stellar populations found in many elliptical galaxies. However, it struggles to account for the diversity of galaxy types and the ongoing mergers we observe at various redshifts.

Hierarchical Merging Model suggests galaxies assembled gradually through the merging of smaller structures over billions of years. This is the model favored by most current cosmological simulations.

• Small galaxies and dark matter halos merged to build larger galaxies in a bottom-up process.
• This merging has continued throughout the universe's 13.8-billion-year history and is still happening today.
• The model predicts irregular and spiral galaxies formed first, with elliptical galaxies forming later as products of major mergers. The Antennae Galaxies (NGC 4038/4039) are a well-known example of an ongoing merger that will likely produce an elliptical galaxy.

In practice, modern galaxy formation theory incorporates elements of both models. The hierarchical framework provides the overall structure, but rapid gas collapse within individual halos plays a role at smaller scales.

Dark Matter Halos in Galaxies

Dark matter halos are the gravitational scaffolding that makes galaxy formation possible. Without them, ordinary matter would not have had deep enough gravitational wells to collapse into galaxies within the age of the universe.

• Baryonic matter (gas and dust) falls into the gravitational potential wells created by dark matter halos. Once there, gas cools and condenses, triggering star formation and galaxy growth. This often proceeds in an inside-out pattern, with the central regions forming stars first.
• Halo mass controls galaxy properties. More massive halos tend to host larger, more massive galaxies. The Milky Way sits in a halo of roughly $10^{12}$ solar masses, while dwarf galaxies occupy halos orders of magnitude smaller. Halo mass also affects gas accretion rates and star formation efficiency, a relationship captured by the Schmidt-Kennicutt law, which links gas surface density to star formation rate.
• Halo mergers drive galaxy evolution. When dark matter halos merge, the galaxies they host are pulled together by gravitational interactions. These mergers can trigger starbursts, activate AGN (active galactic nuclei), and reshape galaxy morphology. M82, the "Cigar Galaxy," is a classic example of a starburst triggered by a past gravitational interaction.

Galaxy Mergers and Evolution

Galaxy mergers occur when two or more galaxies collide and eventually combine into a single, larger galaxy. The outcome depends heavily on the mass ratio of the merging galaxies.

Merger Types:

1. Major mergers involve galaxies of similar mass (mass ratio from 1:1 to about 1:4).

   • These can dramatically reshape both galaxies, often destroying spiral structure and producing an elliptical galaxy. Centaurus A is thought to be the product of such a merger.
   • Major mergers frequently trigger intense starbursts and can fuel quasar-level AGN activity as gas is funneled toward the central black hole.

2. Minor mergers involve one galaxy significantly more massive than the other (mass ratio less than 1:4).

   • The smaller galaxy is gradually absorbed into the larger one. The Sagittarius Dwarf Galaxy is currently being torn apart and absorbed by the Milky Way.
   • Effects are more subtle: tidal streams, warps, and modest enhancements in star formation rather than wholesale morphological transformation.

Impact on star formation: Mergers compress gas clouds, which can trigger intense bursts of star formation. Ultra-luminous infrared galaxies (ULIRGs) are extreme examples of merger-driven starbursts. These bursts typically last $10^7$ to $10^8$ years, depending on the merger geometry and available gas supply.

Impact on morphology: Major mergers scramble the ordered orbital motion of stars in disk galaxies, producing the random stellar orbits characteristic of elliptical galaxies (like M87 in the Virgo Cluster). Minor mergers produce subtler distortions, such as the observed warp in the Milky Way's disk.

Gas Accretion and Feedback Processes

Galaxies don't just form and stop growing. They continuously acquire new gas and regulate their own star formation through feedback. These two processes together determine a galaxy's long-term evolution.

Gas Accretion

Galaxies pull in gas from the intergalactic medium through two distinct modes:

• Cold mode accretion: Gas flows along cosmic filaments directly into the galaxy at relatively low temperatures ($T < 10^{4.5}$ K). This mode is especially efficient for lower-mass halos and at higher redshifts.
• Hot mode accretion: Gas is shock-heated to high temperatures ($T > 10^{5.5}$ K) as it enters the halo, then gradually cools and settles into the galaxy. This mode dominates in massive halos.

The accretion rate depends on both halo mass and redshift, following an approximate scaling relation:

$\dot{M} \propto M_{\text{halo}}^{1.15} (1+z)^{2.25}$

This means accretion was much faster in the early universe and is higher for more massive halos. The accreted gas fuels ongoing star formation and allows galaxies to continue growing over cosmic time.

Feedback Processes

Feedback is what prevents galaxies from converting all their gas into stars at once. Two main types operate at different scales:

• Stellar feedback: Supernovae, stellar winds, and radiation pressure inject energy and momentum into the interstellar medium. A single supernova releases roughly $10^{51}$ ergs of energy. Collectively, these processes heat and expel gas from star-forming regions, creating "galactic fountains" where gas is blown out of the disk and later falls back in. Stellar feedback is most effective in lower-mass galaxies.
• AGN feedback: The central supermassive black hole releases enormous energy as it accretes matter ($L_{\text{AGN}} \propto \dot{M}_{\text{BH}}$). This energy can heat or expel gas on scales much larger than stellar feedback can reach. Radio jets from AGN, for instance, can extend hundreds of kiloparsecs and suppress cooling flows in galaxy clusters. AGN feedback is the primary mechanism that quenches star formation in the most massive galaxies.

The balance between accretion and feedback shapes a galaxy's gas content, star formation history, and chemical enrichment over time. This interplay helps explain observed relationships like the mass-metallicity relation, where more massive galaxies tend to have higher metal content. The efficiency of feedback also varies with environment: galaxies in dense clusters experience different conditions than isolated field galaxies.