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”.
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Notes
- 1.
For these and other curious abbreviations check out Volker Springel’s lectures in this volume.
- 2.
Throughout these lectures I define “solar metallicity” as the metallicity of our galactic neighborhood, \(Z_\odot =0.199\) in absolute units, rather than metallicity of an average-looking single star somewhere in the outskirts of the Galaxy.
- 3.
Notice the convention, Cloudy is a name, not an abbreviation.
- 4.
They are not fully independent, of course—a photon ionizing \(\mathrm{CVI}\) can also ionize neutral hydrogen, but it is convenient to use photoionization rates rather that some other, arbitrary filter shapes, since the same rates can be useful in the simulation code for other purposes—for example, for computing the ionization balance of hydrogen, helium, or other chemical elements.
- 5.
God save you from calling it a “law” in the presence of a devout physicist!
- 6.
At least, the vast majority of them—by itself, the THINGS result does not exclude a possibility of a small fraction of stars forming in the atomic gas.
- 7.
One should never forget that the “constant efficiency per free-fall time” model is no more than an ansatz; molecular clouds are turbulent and the free-fall time has no physical relevance on scales above the sonic length.
- 8.
Never forget that stellar masses are not observed, they are always derived from observations of luminosity functions, with all the inherent in spectral synthesis uncertainties and biases.
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Gnedin, N.Y. (2016). Modeling Physical Processes at Galactic Scales and Above. In: Revaz, Y., Jablonka, P., Teyssier, R., Mayer, L. (eds) Star Formation in Galaxy Evolution: Connecting Numerical Models to Reality. Saas-Fee Advanced Course, vol 43. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-47890-5_1
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