We review current understanding of star formation, outlining an overall theoretical framework and the observations that motivate it. A conception of star formation has emerged in which turbulence plays a dual role, both creating overdensities to initiate gravitational contraction or collapse, and countering the effects of gravity in these overdense regions. The key dynamical processes involved in star formation—turbulence, magnetic fields, and self-gravity— are highly nonlinear and multidimensional. Physical arguments are used to identify and explain the features and scalings involved in star formation, and results from numerical simulations are used to quantify these effects. We divide star formation into large-scale and small-scale regimes and review each in turn. Large scales range from galaxies to giant molecular clouds (GMCs) and their substructures. Important problems include how GMCs form and evolve, what determines the star formation rate (SFR), and what determines the initial mass function (IMF). Small scales range from dense cores to the protostellar systems they beget. We discuss formation of both low- and high-mass stars, including ongoing accretion. The development of winds and outflows is increasingly well understood, as are the mechanisms governing angular momentum transport in disks. Although outstanding questions remain, the framework is now in place to build a comprehensive theory of star formation that will be tested by the next generation of telescopes.

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Theory of Star Formation

Since the Hipparcos mission and recent large scale surveys in the optical and the near-infrared, new constraints have been obtained on the structure and evolution history of the Milky Way. The population synthesis approach is a useful tool to interpret such data sets and to test scenarios of evolution of the Galaxy. We present here new constraints on evolution parameters obtained from the Besancon model of population synthesis and analysis of optical and near-infrared star counts. The Galactic potential is computed self-consistently, in agreement with Hipparcos results and the observed rotation curve. Constraints are posed on the outer bulge structure, the warped and flared disc, the thick disc and the spheroid populations. The model is tuned to produce reliable predictions in the visible and the near-infrared in wide photometric bands from U to K. Finally, we describe applications such as photometric and astrometric simulations and a new classification tool based on a Bayesian probability estimator, which could be used in the framework of Virtual Observatories. As examples, samples of simulated star counts at different wavelengths and directions are also given.

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A synthetic view on structure and evolution of the Milky Way

Among 'empirical' methods of estimating oxygen abundances in extragalactic H II regions, the use of the ratio of nebular lines of [O III] and [N II], first introduced by Alloin et al., is reappraised with modern calibration data and shown to have certain advantages over Ρ 23 ≡ ([On] + [O III])/Hβ and N2 ≡ [N II] λ6583/Hα, particularly when applied to star-forming galaxies at high redshifts.

[O III] / [N II] as an abundance indicator at high redshift

Understanding the formation of stars in galaxies is central to much of modern astrophysics. However, a quantitative prediction of the star formation rate and the initial distribution of stellar masses remains elusive. For several decades it has been thought that the star formation process is primarily controlled by the interplay between gravity and magnetostatic support, modulated by neutral-ion drift (known as ambipolar diffusion in astrophysics). Recently, however, both observational and numerical work has begun to suggest that supersonic turbulent flows rather than static magnetic fields control star formation. To some extent, this represents a return to ideas popular before the importance of magnetic fields to the interstellar gas was fully appreciated. This review gives a historical overview of the successes and problems of both the classical dynamical theory and the standard theory of magnetostatic support, from both observational and theoretical perspectives. The outline of a new theory relying on control by driven supersonic turbulence is then presented. Numerical models demonstrate that, although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic-scale star formation. It suggests that individual star-forming cores are likely not quasistatic objects, but dynamically collapsing. Accretion onto these objects varies depending on the properties of the surrounding turbulent flow; numerical models agree with observations showing decreasing rates. The initial mass distribution of stars may also be determined by the turbulent flow. Molecular clouds appear to be transient objects forming and dissolving in the larger-scale turbulent flow, or else quickly collapsing into regions of violent star formation. Global star formation in galaxies appears to be controlled by the same balance between gravity and turbulence as small-scale star formation, although modulated by cooling and differential rotation. The dominant driving mechanism in star-forming regions of galaxies appears to be supernovae, while elsewhere coupling of rotation to the gas through magnetic fields or gravity may be important.

Control of star formation by supersonic turbulence

We present the results of a systematic study of the rest-frame UV spectroscopic properties of Lyman break galaxies (LBGs). The database of almost 1000 LBG spectra proves useful for constructing high signal-to-noise composite spectra. The composite spectrum of the entire sample reveals a wealth of features attributable to hot stars, H II regions, dust, and outflowing neutral and ionized gas. By grouping the database according to galaxy parameters such as Lyα equivalent width, UV spectral slope, and interstellar kinematics, we isolate some of the major trends in LBG spectra that are least compromised by selection effects. We find that LBGs with stronger Lyα emission have bluer UV continua, weaker low-ionization interstellar absorption lines, smaller kinematic offsets between Lyα and the interstellar absorption lines, and lower star formation rates. There is a decoupling between the dependence of low- and high-ionization outflow features on other spectral properties. Additionally, galaxies with rest-frame WLyα ≥ 20 A in emission have weaker than average high-ionization lines and nebular emission lines that are significantly stronger than in the sample as a whole. Most of the above trends can be explained in terms of the properties of the large-scale outflows seen in LBGs. According to this scenario, the appearance of LBG spectra is determined by a combination of the covering fraction of outflowing neutral gas, which contains dust and the range of velocities over which this gas is absorbing. In contrast, the strengths of collisionally excited nebular emission lines should not be affected by the nature of the outflow, and variations in these lines may indicate differences in the temperatures and metallicities in H II regions of galaxies with very strong Lyα emission. Higher sensitivity and spectral resolution observations are still required for a full understanding of the covering fraction and velocity dispersion of the outflowing neutral gas in LBGs and its relationship to the escape fraction of Lyman continuum radiation in galaxies at z ~ 3.

https://iopscience.iop.org/article/10.1086/373922/pdf

Rest-Frame Ultraviolet Spectra of z ∼ 3 Lyman Break Galaxies*

La presence dans les galaxies de superstructures dont la taille est de plusieurs centaines de parsecs est discutee. Les systemes en question, bien plus grands que le vent stellaire ou l'explosion d'une simple etoile, sont de multiples natures et se referent par exemple, a des explosions de supernovae en serie ou a de tres forts vents stellaires. Dans tous les cas, ils sont caracterises par une grande vitesse d'expansion et contribuent a l'alimentation en gaz du milieu interstellaire

Large-Scale Expanding Superstructures in Galaxies

It is shown that the broad permitted and semipermitted emission lines of the broad-line region (BLR) of radio-quiet active galactic nuclei (AGN) can be generated by strongly radiative (rapidly cooling) supernova remnants expected to occur in the central regions of early-type galaxies undergoing a starburst. Supernova remnants interacting with a circumstellar medium with a density of about n ∼ 10 7 cm -3 become strongly radiative while still expanding at several thousand km/s, and miss the adiabatic Sedov track. Radiative cooling becomes important well before the thermalization of the ejecta is completed, and the shocked matter undergoes a fast condensation behind both the outgoing forward shock and the reverse shock. Two concentric, high-density, and fast-moving thin shells are then formed

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The starburst model for active galactic nuclei : the broad-line region as supernova remnants evolving in a high-density medium

The evolution of H II regions in spherical clouds with small, constant-density cores and power-law density distributions r exp -w outside the core is described analytically. It is found that there is a critical exponent above which the cloud becomes completely ionized. Its value in the formation phase depends on the initial conditions, but it has a well-defined value w(crit) = 3/2 during the expansion phase. For w less than w(crit), the radius of the H II region grows at a given rate, while neutral mass accumulates in the interphase between the ionization and shock fronts. For w = w(crit), the fronts move together without mass accumulation. Cases with w greater than w(crit) lead to the champagne phase: once the cloud is fully ionized, the expansion becomes supersonic. For self-gravitating disks without magnetic fields, the main features include a new 'variable-size' stage. The initial shape of the H II region has a critical point beyond which the disk becomes completely ionized.

Guillermo Tenorio-Tagle

Papers

Large-Scale Expanding Superstructures in Galaxies

The starburst model for active galactic nuclei : the broad-line region as supernova remnants evolving in a high-density medium

On the formation and expansion of H II regions

Interstellar Matter Hydrodynamics and the Dispersal and Mixing of Heavy Elements

Lyα Emission in Starbursts: Implications for Galaxies at High Redshift