ARlogo Annu. Rev. Astron. Astrophys. 2017. 55: 59-109
Copyright © 2017 by Annual Reviews. All rights reserved

Reprinted with kind permission from Annual Reviews, 4139 El Camino Way, Palo Alto, California, USA

For a PDF version of the article, click here.

THEORETICAL CHALLENGES IN GALAXY FORMATION

Thorsten Naab 1 & Jeremiah P. Ostriker 2,3


1 Max-Planck-Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
2 Department of Astronomy, Columbia University, 550 W, 120th Street, New York, NY10027, USA
3 Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA

Abstract: Numerical simulations have become a major tool for understanding galaxy formation and evolution. Over the decades the field has made significant progress. It is now possible to simulate the formation of individual galaxies and galaxy populations from well defined initial conditions with realistic abundances and global properties. An essential component of the calculation is to correctly estimate the inflow to and outflow from forming galaxies since observations indicating low formation efficiency and strong circum-glactic presence of gas are persuasive. Energetic 'feedback' from massive stars and accreting super-massive black holes - generally unresolved in cosmological simulations - plays a major role for driving galactic outflows, which have been shown to regulate many aspects of galaxy evolution. A surprisingly large variety of plausible sub-resolution models succeeds in this exercise. They capture the essential characteristics of the problem, i.e. outflows regulating galactic gas flows, but their predictive power is limited. In this review we focus on one major challenge for galaxy formation theory: to understand the underlying physical processes that regulate the structure of the interstellar medium, star formation and the driving of galactic outflows. This requires accurate physical models and numerical simulations, which can precisely describe the multi-phase structure of the interstellar medium on the currently unresolved few hundred parsecs scales of large scale cosmological simulations. Such models ultimately require the full accounting for the dominant cooling and heating processes, the radiation and winds from massive stars and accreting black holes, an accurate treatment of supernova explosions as well as the non-thermal components of the interstellar medium like magnetic fields and cosmic rays.


Key words: theoretical models, cosmology, galaxy formation, galaxy evolution


Table of Contents

INTRODUCTION
What do we want to learn?
Some relevant observations
Stars
Gas
Dark matter
Super-massive black holes
The Milky Way
Learning from galaxy evolution with redshift
Ubiquitous winds
Size evolution of early-type galaxies
Evolution of spiral galaxies
Evolution of star formation rates and gas fractions
Methods of solution
Direct simulations of dark matter
Semi-analytical models for baryons
Direct simulations including baryons
Disks, Ellipticals and Mergers - a very useful set of idealized simulations
Collisionless mergers
Mergers with gas
Caveats of the merger hypothesis
Ranking and Matching

AB INITIO SIMULATIONS OF GALAXY FORMATION
Star formation and gas cooling
The formation of disk dominated systems
Current sub-resolution models for feedback from stellar populations
The formation of bulge dominated systems
Current models for feedback from super-massive black holes

THE NEED FOR ACCURATE MODELLING OF THE GALACTIC INTERSTELLAR MEDIUM AND 'FEEDBACK'
Supernova explosions
Stellar winds
Radiation
Magnetic fields and cosmic rays
Mechanical and radiative AGN feedback

CONCLUSION & OUTLOOK

REFERENCES

Next