Galaxy formation and evolution

The simplest model in general agreement with observed phenomena is the Lambda-CDM model—that is, clustering and merging allows galaxies to accumulate mass, determining both their shape and structure.

Because of the inability to conduct experiments in outer space, the only way to “test” theories and models of galaxy evolution is to compare them with observations.

Current models also predict that the majority of mass in galaxies is made up of dark matter, a substance which is not directly observable, and might not interact through any means except gravity.

There are different theories on how these disk-like distributions of stars develop from a cloud of matter: however, at present, none of them exactly predicts the results of observation.

Olin J. Eggen, Donald Lynden-Bell, and Allan Sandage[1] in 1962, proposed a theory that disk galaxies form through a monolithic collapse of a large gas cloud.

While this remains an unsolved problem for astronomers, it does not necessarily mean that the Lambda-CDM model is completely wrong, but rather that it requires further refinement to accurately reproduce the population of galaxies in the universe.

The second stage is marked by the black hole stabilizing by suppressing gas cooling, thus leaving the elliptical galaxy in a stable state.

[7] The mass of the black hole is also correlated to a property called sigma which is the dispersion of the velocities of stars in their orbits.

[11] In the Local Group, the Milky Way and the Andromeda Galaxy are gravitationally bound, and currently approaching each other at high speed.

During this collision, it is expected that the Sun and the rest of the Solar System will be ejected from its current path around the Milky Way.

[14][15] As described in previous sections, galaxies tend to evolve from spiral to elliptical structure via mergers.

However, it is thought that quenching occurs relatively quickly (within 1 billion years), which is much shorter than the time it would take for a galaxy to simply use up its reservoir of cold gas.

[25] When using the Lagrangian approach to specify the field, it is assumed that the observer tracks a specific fluid parcel with its unique characteristics during its movement through space and time.

To shape the population of galaxies, the hydrodynamical equations must be supplemented by a variety of astrophysical processes mainly governed by baryonic physics.

Processes, such as collisional excitation, ionization, and inverse Compton scattering, can cause the internal energy of the gas to be dissipated.

, the fine structure and molecular cooling also need to be considered to simulate the cold phase of the interstellar medium.

Complex multi-phase structure, including relativistic particles and magnetic field, makes simulation of interstellar medium difficult.

In particular, modeling the cold phase of the interstellar medium poses technical difficulties due to the short timescales associated with the dense gas.

In the early simulations, the dense gas phase is frequently not modeled directly but rather characterized by an effective polytropic equation of state.

However, more detailed physics processes needed to be considered in future simulations, since the structure of the interstellar medium directly affects star formation.

To simulate this process, a portion of the gas is transformed into collisionless star particles, which represent coeval, single-metallicity stellar populations and are described by an initial underlying mass function.

To effectively control star formation, stellar feedback must generate galactic-scale outflows that expel gas from galaxies.

Cooling is expected in dense and cold gas, but it cannot be reliably modeled in cosmological simulations due to low resolution.

This leads to artificial and excessive cooling of the gas, causing the supernova feedback energy to be lost via radiation and significantly reducing its effectiveness.

However, using hydrodynamically decoupled wind particles to inject momentum non-locally into the gas surrounding active star-forming regions may still be necessary to achieve large-scale galactic outflows.

[35] During the Cosmic Dawn, galaxy formation occurred in short bursts of 5 to 30 Myr due to stellar feedbacks.

[38] The regulation of star formation in massive galaxies is believed to be significantly influenced by radio mode feedback, which occurs due to the presence of highly collimated jets of relativistic particles.

Nevertheless, magnetic fields are a critical component of the interstellar medium since they provide pressure support against gravity[40] and affect the propagation of cosmic rays.

[41] Cosmic rays play a significant role in the interstellar medium by contributing to its pressure,[42] serving as a crucial heating channel,[43] and potentially driving galactic gas outflows.

Ray-tracing involves tracing the paths of individual photons through the simulation and computing their interactions with matter at each step.