2009 Bromm et al., The formation of the first stars and galaxies (1119309)
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Vol 459j7 May 2009jdoi:10.1038/nature07990REVIEWSThe formation of the first stars and galaxiesVolker Bromm1, Naoki Yoshida2, Lars Hernquist3 & Christopher F. McKee4Observations made using large ground-based and space-borne telescopes have probed cosmic history from the present dayto a time when the Universe was less than one-tenth of its present age. Earlier still lies the remaining frontier, where the firststars, galaxies and massive black holes formed. They fundamentally transformed the early Universe by endowing it with thefirst sources of light and chemical elements beyond the primordial hydrogen and helium produced in the Big Bang.
Theinterplay of theory and upcoming observations promises to answer the key open questions in this emerging field.he formation of the first stars and galaxies at the end of thecosmic ‘dark ages’ is one of the central problems in moderncosmology1–3. It is thought that during this epoch the Universewas transformed from its simple initial state into a complex,hierarchical system, through the growth of structure in the dark matter,by the input of heavy elements from the first stars, and by energyinjection from these stars and from the first black holes4,5.
An importantmilestone in our understanding was reached after the introduction ofthe now standard cold dark-matter (CDM) model of cosmic evolution,which posits that structure grew hierarchically, such that small objectsformed first and then merged to form increasingly larger systems6.Within this model, dark-matter ‘minihaloes’ (see below), forming afew hundred million years after the Big Bang, were identified as the siteswhere the first stars formed7. Building on this general framework, andrelying on the development of efficient new computational tools, thefragmentation properties of primordial gas inside such minihaloes wereinvestigated with numerical simulations, leading to the result that thefirst stars, so-called population III, were predominantly very massive4,8(see Box 1 for the terminology used here). Recently, the frontier hasprogressed to the next step in the hierarchical build-up of structure, tothe emergence of the first galaxies whose formation took place after thefirst stars had formed and affected their common environment.
It is verytimely to review our current understanding and remaining challenges,as we are just entering an exciting period of discovery, in which newobservational probes are becoming available and advances in supercomputer technology enable ever more realistic theoretical predictions.We begin with the formation of the first stars, discussing the physicsunderlying the prediction that they were very massive, and how thispicture would be modified if the dark matter exhibited non-standardproperties on small scales. We next address the feedback effects from thefirst stars, with one main result being that such feedback might delaysubsequent star formation by up to ,108 years. Proceeding to theassembly of the first galaxies, we discuss the important role of turbulence and supernova feedback during their formation.
Intriguingly,the cold accretion streams that feed the turbulence in the centres of theprimordial galaxies are reminiscent of the recently proposed new modelfor galaxy formation, in which such cold streams are invoked to explainthe build-up of massive galaxies at more recent cosmic times in asmooth, rather than merger-driven, fashion9.
We conclude with anoutlook into the likely key developments over the next decade.TFormation of the first starsWhereas dark-matter haloes can originate through the action ofgravity alone, the formation of luminous objects, such as stars andgalaxies, is a much more complicated process. For star formationto begin, a sufficient amount of cold dense gas must accumulate ina dark halo. In the early Universe, the primordial gas could notefficiently cool radiatively because atoms have excitation energiesthat are too high, and molecules, which have accessible rotationalenergies, are very rare.
Trace amounts of molecular hydrogen (H2)can be produced via a sequence of reactions, Hze { ?H{ zc(where c indicates a photon), followed by H{ zH?H2 ze { (wheree2 indicates an electron), and under the proper conditions this allowsthe gas to cool and eventually condense to form stars10.Numerical simulations11–13 starting from cosmological initial conditions show that primordial gas clouds formed in dark-matterhaloes with virial temperature ,1,000 K and mass ,106M8Box 1 j Definitions and terminologyWe establish here a convention for terminology used in this review.Population III stars are those that initially contain no elements heavierthan helium (‘metals’ in the parlance of astronomers) other than thelithium produced in the Big Bang.
Such stars can be divided into firstgeneration stars (population III.1), which form from initial conditionsdetermined entirely by cosmological parameters, and secondgeneration stars (population III.2), which originate from material thatwas influenced by earlier star formation25. According to theory,population III.1 stars formed when almost completely neutralprimordial gas collapsed into dark-matter minihaloes, whereas oneimportant class of population III.2 stars formed from gas that wasphotoionized before the onset of gravitational runaway collapse33.Simply put, population III.1 stars are locally the very first luminousobjects, whereas population III.2 stars are those metal-free starsformed from gas that was already affected by previous generations ofstars.
Population II stars have enough metals to affect their formationand/or their evolution. Such stars are classified100 according to theiriron/hydrogenratio as extremely metal poor for metallicities10{4 vZ Z8 v10{3 , ultra-metal poor for 10{5 vZ=Z8 v10{4 , andhyper-metal poor for 10{6 vZ=Z8 v10{5 . Because we know so littleabout the first galaxies, it is difficult to establish a precise terminologyfor them. A galaxy is a system of many stars and gas that isgravitationally bound in a dark-matter halo.
We define a ‘first galaxy’ asone composed of the very first system of stars to be gravitationallybound in a dark-matter halo. Such stars could be population III orpopulation II stars with very low metallicities—extremely metal poor orbelow, according to recent numerical simulations63,64. The gas in suchgalaxies should have similarly low metallicities. Current theory predictsthat population III.1 stars are formed in isolation in minihaloes andtherefore will not be in galaxies.1Astronomy Department, University of Texas, 2511 Speedway, Austin, Texas 78712, USA. 2Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwa,Chiba 277-8568, Japan. 3Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA. 4Departments of Physics and Astronomy,University of California, Berkeley, California 94720, USA.49©2009 Macmillan Publishers Limited.
All rights reservedREVIEWSNATUREjVol 459j7 May 2009(so-called ‘minihaloes’; M8 , solar mass). In the standard CDMmodel, the minihaloes that were the first sites for star formationare expected to be in place at redshift z < 20–30, when the age ofthe Universe was just a few hundred million years14. These systemscorrespond to (3–4)s peaks in the cosmic density field, which isstatistically described as a Gaussian random field.
Such high-densitypeaks are expected to be strongly clustered15, and thus feedbackeffects from the first stars are important in determining the fate ofthe surrounding primordial gas clouds. It is very likely that only onestar can be formed within a gas cloud, because the far-ultravioletradiation from a single massive star is sufficient to destroy all theH2 in the parent gas cloud16,17. In principle, a cloud that formed oneof the first stars could fragment into a binary or multiple star system18,19, but simulations based on self-consistent cosmological initialconditions do not show this20.
Although the exact number of stars percloud cannot be easily determined, the number is expected to besmall, so that minihaloes will not be galaxies (see Box 1).Primordial gas clouds undergo runaway collapse when sufficientmass is accumulated at the centre of a minihalo. The minimum massat the onset of collapse is determined by the Jeans mass (more precisely, the Bonnor–Ebert mass), which can be written as: 3=2 Tn {1=2ð1ÞMJ <500M8200104for an atomic gas with temperature T (in K) and particle numberdensity n (in cm23). The characteristic temperature is set by theenergy separation of the lowest-lying rotational levels of the traceamounts of H2, and the characteristic density corresponds to thethermalization of these levels, above which cooling becomes lessefficient12.
A number of atomic and molecular processes are involvedin the subsequent evolution of a gravitationally collapsing gas. It hasbeen suggested that a complex interplay between chemistry, radiativecooling and hydrodynamics leads to fragmentation of the cloud21,but vigorous fragmentation is not observed even in extremely highresolution cosmological simulations11–13,20,22. Interestingly, however,simulations starting from non-cosmological initial conditions haveyielded multiple cloud cores19,23. It appears that a high initial degreeof spin in the gas eventually leads to the formation of a disk and itssubsequent break-up. It remains to be seen whether such conditionsoccur from realistic cosmological initial conditions.Although the mass triggering the first runaway collapse is welldetermined, it provides only a rough estimate of the mass of the star(s)to be formed.
Standard star-formation theory predicts that a tiny protostar forms first and subsequently grows by accreting the surrounding gasto become a massive star. Indeed, the highest-resolution simulations offirst-star formation verify that this also occurs cosmologically20 (Fig. 1).However, the ultimate mass of the star is determined both by the mass ofthe cloud out of which it forms and by a number of feedback processesthat occur during the evolution of the protostar.
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