Theoretical Challenges in Galaxy Formation (1612.06891v1)

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.

• The paper shows that numerical simulations have advanced our understanding of galaxy formation by reproducing realistic galactic properties.
• It demonstrates that unresolved feedback processes, from supernovae to SMBH activity, critically regulate star formation and drive outflows.
• The review highlights the need for improved integration of baryonic physics in cosmological models to accurately capture ISM dynamics and structural evolution.

An Expert Review of "Theoretical Challenges in Galaxy Formation"

The paper "Theoretical Challenges in Galaxy Formation," authored by Thorsten Naab and Jeremiah P. Ostriker, offers a comprehensive review of the current understanding and ongoing challenges facing the field of galaxy formation. This essay aims to summarize key insights and implications of their work for an audience familiar with astrophysics research.

Numerical Simulations and Galaxy Formation

The paper highlights the critical role of numerical simulations in the paper of galaxy formation and evolution. The authors recount how simulations have progressively refined our understanding, allowing the accurate simulation of individual galaxies and diverse galaxy populations from well-defined initial conditions. A notable achievement has been the ability to model galactic formation processes with realistic abundances and global properties. However, achieving a precise understanding of the underlying physical processes remains a challenge, particularly in accurately simulating the multi-phase structure of the interstellar medium (ISM), star formation dynamics, and the mechanisms driving galactic outflows.

Feedback Mechanisms in Galaxy Formation

A central theme in the paper is the role of feedback mechanisms from massive stars and accreting supermassive black holes (SMBHs) in regulating galaxy formation and evolution. These feedback processes, often unresolved in cosmological simulations, are crucial for driving galactic outflows and regulating star formation efficiency. The variety of plausible sub-resolution models discussed in the paper highlights the complexity and ongoing uncertainty in this area of research. These models range from those simulating supernovae-driven winds to AGN feedback scenarios, each attempting to capture essential aspects of feedback processes while grappling with significant limitations in predictive power.

Observational Constraints and Cosmological Models

The authors emphasize the alignment of current cosmological models with observational constraints, notably the ΛCDM model, which satisfactorily describes dark matter and cosmic inflation on large scales. Yet, significant challenges persist in incorporating baryonic processes into these models. The paper delineates the dichotomy between dark matter-dominated scenarios and baryon-influenced processes, noting that while dark matter simulations are converging towards realistic outcomes, the inclusion of baryonic physics introduces complexities yet to be fully resolved.

Structural Evolution of Galaxies

The review discusses different phases in the structural evolution of galaxies, detailing the transition from dark matter-dominated evolution to the more intricate processes involving baryons, stars, and SMBHs. Of particular interest is the elucidation of early-type galaxy formation, where mechanisms such as gas accretion, star formation, and feedback processes intricately intersect. Numerical simulations, conquest with empirical constraints, demonstrate two-phase star formation histories and galaxy mergers playing distinctive roles in shaping galaxies' current configurations.

Future Directions and Theoretical Prospects

In concluding, the authors suggest several critical theoretical challenges and future research directions, emphasizing the necessity for improved physical models that fully integrate dominant cooling and heating processes, radiation dynamics, and feedback mechanisms. They advocate for more refined high-resolution simulations, particularly those bridging the gap between large-scale cosmological contexts and local galactic physics, with an emphasis on capturing the fine-scale physics of the ISM and feedback processes.

The paper's thorough review of cosmological simulations and galaxy formation theories serves to underscore the extraordinary strides made in the field while candidly acknowledging the myriad complexities yet to be fully understood. It provides a valuable synthesis for researchers aiming to deepen their understanding of galaxy formation and inspire future investigations in this dynamically evolving domain of astrophysics.