Modeling Physical Processes at Galactic Scales and Above

Abstract

We start with modeling the IGM (in particular, Lyman-\(\alpha \) forest). For that purpose, we will review some of the basic physics of low-density photo-ionized gas and its observational diagnostics. We will explore how thermal pressure makes gas distribution deviate from the dark matter, how the gas temperature evolves in time under the effect of cosmic ionizing background, and how observed spectra emerge from the interplay of density, temperature, and velocity. We will briefly review most important observations of the IGM and will end up with a mystery. After IGM, we continue our journey (together with cosmic gas) through the dense filaments of large-scale structure to galactic halos. We will explore various ways the gas takes to accrete onto halos, and how hot gas in the halo uses radiative cooling to continue its journey to the galactic disk, becoming the ISM of galaxies. On our way we meet wonderful “cool streams”, explore various ways to account for cooling and heating in the gas, and will end up with another mystery story of high velocity clouds. After a brief refresher on galaxy formation, we explore the structure and stability of galactic disks, going well beyond standard Toomre criterion. We then focus our attention to gas, and after a brief overview of atomic and ionized gas dive even deeper into the molecular ISM. We will explore the atomic-to-molecular transition in exquisite detail, before committing a grave mistake of opening a cosmic Pandora box of the X factor, inside which, as in a Russian Matrioshka doll, we find another Pandora box, and then another, until in total desperation we give up and ready ourselves to jumping into the Holy Grail of all of astronomy-star formation. That subject is so huge, that we will only touch it gently, at the largest scales, and will not go much beyond the canonical Kennicutt-Schmidt relation. We will, though, think about it in 2D, realizing that the spatial scale is as an important physical parameter as the density of the molecular gas itself. We will theorize about how the 2D scale/density plane can be charted by star formation rate, but will not arrive at an answer. At the end, we will briefly get familiar with a novel idea of using the Excursion Set formalism as a theory of star formation and, very briefly, familiarize ourselves with numerous ways in which stars affect their environments, collectively known under a buzzname of “stellar feedback”.