Formation of Disk Galaxies in Computer Simulations

Abstract

The formation of disk galaxies is one of the most outstanding problems in modern astrophysics and cosmology. We review the progress made by numerical simulations carried out on large parallel supercomputers. These simulations model the formation of disk galaxies within the current structure formation paradigm in which the Universe is dominated by a cold dark matter component and a cosmological constant. We discuss how computer simulations have been an essential tool in advancing the field further over the last decade or so. Recent progress stems from a combination of increased resolution and improved treatment of the astrophysical processes modeled in the simulations, such as the phenomenological description of the interstellar medium and of the process of star formation. We argue that high mass and spatial resolution is a necessary condition in order to obtain large disks comparable with observed spiral galaxies avoiding spurious dissipation of angular momentum. A realistic model of the star formation history. gas-to-stars ratio and the morphology of the stellar and gaseous component is instead controlled by the phenomenological description of the non-gravitational energy budget in the galaxy. This includes the energy injection by supernovae explosions as well as by accreting supermassive black holes at scales below the resolution. We continue by showing that simulations of gas collapse within cold dark matter halos including a phenomenological description of supernovae blast-waves allow to obtain stellar disks with nearly exponential surface density profiles as those observed in real disk galaxies, counteracting the tendency of gas collapsing in such halos to form cuspy baryonic profiles. However, the ab-initio formation of a realistic rotationally supported disk galaxy with a pure exponential disk in a fully cosmological simulation is still an open problem. We argue that the suppression of bulge formation is related to the physics of galaxy formation during the merger of the most massive protogalactic lumps at high redshift, where the reionization of the Universe likely plays a key role. A sufficiently high resolution during this early phase of galaxy formation is also crucial to avoid artificial angular momentum loss and spurious bulge formation. Finally, we discuss the role of mergers in disk formation, adiabatic halo contraction during the assembly of the disk, cold flows, thermal instability and other aspects of galaxy formation, focusing on their relevance to the puzzling origin of bulgeless galaxies.keywords:astrophysics, cosmology, computer science, fluid dynamics 1 Galaxy formation in a hierarchical UniverseGalaxies occupy a special place in our quest for understanding the Universe. They are large islands in a nearly empty space and contain most of the ordinary baryonic matter, stars and interstellar gas, that emits radiation and can thus be detected by astronomers 1 . Galaxies come essentially in two broad categories 2 , those in which the luminous mass is arranged in a rotating disk of stars...

Angular Momentum in Dwarf Galaxies

“…Similarly to what happened with the semi-analytic model previously cited (e.g., Mo et al 1998), halos and galaxies forming in our model are closer to real halos and galaxies than those generated in simulations, at least closer than those generated in the simulations some years ago. As previously reported, recently, increasing the resolution and properly using feedback, simulations are able to form realistic galaxies (e.g., Mayer et al 2008;Governato et al 2010). Another important issue in the solution of the angular momentum catastrophe in our model is the fact that the angular momentum is mainly lost from the smaller baryonic clumps.…”

“…Further loss of the angular momentum is also due to an artificial viscosity term introduced in the hydrodynamical force equation, and in the internal energy equation, in order to compensate for some artifacts resulting from the discrete representation of the fluid equations, namely to avoid particle interpenetration and damp spurious oscillations in shocks (Monaghan 1992). The quoted problem can be reduced by decreasing the graininess of the mass distribution and increasing the number of particles and resolution (Mayer et al 2008).…”

“…Nowadays, the solution to the AMC problem is related to the second and third issue: (a) heating of gas, and (b) improving the numerical methods and increasing the resolution (Mayer et al 2008). Governato et al (2010) showed that the bulgeless galaxies with a baryonic angular momentum close to that observed in the real galaxies is obtained in SPH simulations which take account the outflows from supernovae explosions removing the gas of low angular momentum.…”

Simulations of galaxy formation and evolution

“…Governato et al 2007) or varying the cosmological model to Warm Dark Matter models (e.g. Mayer, Governato & Kaufmann 2008), might also help to produce galaxies more similar to those observed.…”

Cosmological simulations of the Milky Way

“…The increase in mass and spatial resolution and the improvement of sub-grid algorithms for star formation and feedback processes have recently resulted in simulated galaxies with realistic disk size and angular momentum content (Mayer et al 2008;Governato et al 2009a). However, simulated galaxies hosted in halos with masses ∼ 10 12 M exhibit prominent bulges and structural parameters reminiscent of Sa spirals rather than of Sb/Sc galaxies.…”