本帖最后由 voyagerbb 于 2009-8-18 20:37 编辑
e文原文是从pdf格式粘贴过来的,所以有点乱,可以参看这里:
http://arxiv.org/abs/0905.0929
原文发表在Nature上面,作者都是大牛,很好的一篇综述.
Formation of the First Stars and Galaxies
Volker Bromm, Naoki Yoshida, Lars Hernquist & Christopher F. McKee
1Astronomy Department, University of Texas, 2511 Speedway, Austin, TX
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, MA
4Departments of Physics and Astronomy, University of California, Berkeley, CA
第一代恒星和星系的形成
作者:
Volker Bromm
得克萨斯大学天文系
Naoki Yoshida
东京大学宇宙物理数学研究所
Lars Hernquist
哈佛-史密松天体物理中心
Christopher F. McKee
加州大学,伯克利,物理与天文系
翻译: voyagerbb
Observations made using large ground-based and space-borne telescopes have probed cosmic history all the way from the present-day to a time when the Universe was less than a tenth of its present age. Earlier on lies the remaining frontier, where the first stars, galaxies, and massive black holes formed. They fundamentally transformed the early Universe by endowing it with the first sources of light and chemical elements beyond the primordial hydrogen and helium produced in the Big Bang. The interplay of theory and upcoming observations promises to answer the key open questions in this emerging field.
使用地基和天基的望远镜进行观测,人们已经从现在一直探测到了宇宙小于现在年龄十分之一的时候. 比这个更早的地方现在依然是前沿,第一代恒星,星系还有大质量黑洞就在那里形成.他们从根本上改变了早期宇宙,赋予其最早的光源和除了大爆炸产生的原初氢与氦之外的元素.理论和即将到来的观测的交互促进,使得回答这个新兴领域中这些关键且悬而未决的问题成为可能.
The formation of the first stars and galaxies at the end of the cosmic dark ages is one of the central problems in modern cosmology1−3. It is thought that during this epoch the Universe was 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 energy injection from these stars and from the first black holes4,5. An important milestone in our understanding was reached after the introduction of the now standard cold dark matter (CDM) model of cosmic evolution which posits that structuregrew hierarchically, such that small objects formed first and then merged to form increasingly larger systems6. Within this model, dark matter ‘minihalos’ (see below), forming a few hundred million years after the Big Bang, were identified as the sites where the first stars formed7. Building on this general framework, and relying on the development of efficient
new computational tools, the fragmentation properties of primordial gas inside such minihalos were investigated with numerical simulations, leading to the result that the first stars, so-called Population III (Pop III), were predominantly very massive4,8 (see the Box for the terminology used in this review). Recently, the frontier has progressed to the next step in the hierarchical build-up of structure, to the emergence of the first galaxies whose formation took place after the first stars had formed and affected their common environment. It is very timely to review our current understanding and remaining challenges, since we are just entering an exciting period of discovery, where new observational probes are becoming available, and where advances in supercomputer technology enable ever more realistic theoretical predictions.
在宇宙黑暗时期结束之际的第一代恒星和星系的形成是现代宇宙学的中心问题之一(文献1,2,3).人们通常认为,在这期间,宇宙从其简单的初始状态变成了一个复杂的,有层次的系统,这一转换,是通过暗物质结构的增长,第一代恒星产生的重元素,以及这些恒星和第一代黑洞注入的能量来完成的(文献4,5). 在引进了现在已经成为标准模型的冷暗物质模型(CDM)之后,人们对宇宙的理解达到了一个重要的里程碑.冷暗物质模型断定宇宙中结构的增长是成等级的,因此小的天体先形成,然后并合增长成更大的系统(文献6).在这个模型里面,约于大爆炸几亿年之后形成的迷你暗晕(minihalos,见下文)被认为是第一代恒星形成的场所.在这样一个总体的框架下,借助于新的计算工具的发展, 人们用数值模拟探究了在这些迷你暗晕里面的原初气体的碎裂的性质并得出结论:这些第一代恒星,即所谓星族III恒星(PoPIIII),质量非常巨大(文献4,8,参见文后的术语解释).最近,人们又把这个结构等级形成的疆域推进到了下一步,直至第一代星系的出现.第一代星系形成于第一代恒星形成并影响其周围环境之后.现在,对我们已经取得的成就和仍然存在的挑战进行一个总结正当其时.我们刚刚步入一个令人兴奋的发现的时代,在这个时代里新的探测正在成为可能,并且超级计算机技术的进步使得人们可以做出更加现实的理论预言.
We begin with the formation of the first stars, discussing the physics underlying the prediction that they were very massive, and how this picture would be modified if the dark matter exhibited non-standard properties on small scales. We next address the feedback effects from the first stars, with one main result being that such feedback might delay subsequent star formation by up to ! 108 years. Proceeding to the assembly of the first galaxies, we discuss the important role of turbulence and supernova (SN) feedback during their formation. Intriguingly, the cold accretion streams that feed the turbulence in the centers of the primordial galaxies are reminiscent of the recently proposed new paradigm for galaxy formation, in which such cold streams are invoked to explain the build-up of massive galaxies at more recent cosmic times in a smooth, rather than merger-driven, fashion9. We conclude with an outlook into the likely key developments over the next decade.
我们以第一代恒星的形成开始,讨论预言它们质量非常巨大的物理依据,以及如果在小尺度上暗物质表现出标准模型之外的性质的话,这个图像该如何修改.接下来我们讲述的是第一代恒星的反馈效应,其主要结果之一就是这样的反馈可能会将后续的恒星形成推迟约1亿年.在进入到第一代星系的会聚过程时,我们讨论湍流跟超新星(SN)反馈在星系形成过程中的重要角色.令人惊奇的是,给原初星系中心的湍流供给物质的冷的吸积流使人联想到最近新提出的星系形成图像,在这个图像中,冷吸积流用于解释那些在宇宙的晚近时期通过平和而非并合主导的方式形成大质量星系的形成过程.最后,我们以对接下来十年中可能的重大进展的展望来作为结语.
Formation of the first stars
第一代恒星的形成
While dark-matter halos can originate through the action of gravity alone, the formation of luminous objects, such as stars and galaxies, is a much more complicated process. For star formation to begin, a sufficient amount of cold dense gas must accumulate in a dark halo. In the early Universe, the primordial gas cannot efficiently cool radiatively because atoms have excitation energies that are too high, and molecules, which have accessible rotational energies, are very rare. Trace amounts of molecular hydrogen (H2) can be produced via a sequence of reactions, H + e− " H− + !, followed by H− + H " H2 + e−, and under the proper conditions this allows the gas to cool and eventually condense to form stars10.
虽然暗物质晕可能仅通过引力的作用就能形成,发光天体,如恒星和星系的形成过程却要复杂的多.在恒星形成之初,要有大量的冷且高密度的气体聚集在暗物质晕里面.在宇宙早期,原初气体很难通过辐射有效冷却,因为原子的激发能太高,而有转动能级的分子又太稀少了.少量的分子氢(H_2)可以通过一系列的反应,先是H+e^---> H^-+gamma,然后是H^-+H--->H_2+e^-来形成.在一定条件下,气体可以得到冷却并收缩最终形成恒星(文献10).
Numerical simulations11−13 starting from cosmological initial conditions show that primordial gas clouds formed in dark matter halos with virial temperature Tvir ~1, 000 K and mass ~10^6M" (so-called ‘minihalos’). In the standard CDM model, the minihalos that were the first sites for star formation are expected to be in place at z !~20 − 30, when the age of the Universe was just a few hundred million years14. These systems correspond to 3 − 4 sigma peaks in the cosmic density field, which is statistically described as a Gaussian random field. Such high-density peaks are expected to be strongly clustered15, and thus feedback effects from the first stars are important in determining the fate of the surrounding primordial gas clouds. It is very likely that only one star can be formed within a gas cloud, because the far UV radiation from a single massive star is sufficient to destroy all the H2 in the parent gas cloud16,17. In principle, a cloud that forms one of the first stars could fragment into
a binary or multiple star system18,19, but simulations based on self-consistent cosmological initial conditions do not show this20. Although the exact number of stars per cloud cannot be easily determined, the number is expected to be small, so that minihalos will not be galaxies (see Box).
从宇宙学的初始条件开始的数值模拟表明,原初的气体云形成于维里温度约1000K,质量约10^6 M_sun的暗晕(所谓迷你暗晕)中.在标准冷暗物质模型中,孕育第一代恒星的迷你暗晕形成于红移20到30之间,当时宇宙的年龄也就几亿年(文献14).这些暗晕对应于呈高斯分布的宇宙密度场中3-4 sigma的扰动峰.这些高密度的扰动峰具有很强的成团性(文献15),因此这些恒星的反馈作用对决定其周围的原初气体的命运非常重要.非常可能的是,一团气体云中只能形成一个恒星,因为只要一个大质量恒星的远紫外辐射就足以破坏其母气体云中所有的分子氢.理论上,形成一个第一代恒星的气体云也可以碎裂成一个双星或者多星的系统(文献18,19),但是基于宇宙真实初始条件的自洽的数值模拟并未给出这一现象(文献20).虽然一个气体云所能形成的恒星的精确数目并不容易确定,一般人们还是认为这个书目很少,所以迷你暗晕不会是星系(见文末术语框).
Primordial gas clouds undergo runaway collapse when sufficient mass is accumulated at the center of a minihalo. The minimum mass at the onset of collapse is determined by the Jeans mass (more precisely, the Bonnor-Ebert mass), which can be written as
M_J = 500M_sun (T/200K)^(3/2)*(n/10^4cm^-3)^(-1/2) (1)
for an atomic gas with temperature T and particle number density n. The characteristic temperature is set by the energy separation of the lowest-lying rotational levels of the trace amounts of H2, and the characteristic density corresponds to the thermalisation of these levels, above which cooling becomes less efficient12. A number of atomic and molecular processes are involved in the subsequent evolution of a gravitationally collapsing gas. It has been suggested that a complex interplay between chemistry, radiative cooling and hydrodynamics leads to fragmentation of the cloud21, but vigorous fragmentation is not observed even in extremely high resolution cosmological simulations11−13,20,22. Interestingly, however, simulations starting from non-cosmological initial conditions have yielded multiple cloud cores19,23. It appears that a high initial degree of spin in the gas eventually leads to the formation of a disk and its subsequent break-up. It remains to be seen whether such conditions occur from realistic cosmological initial conditions.
当迷你暗晕的中心聚集了大量的气体时,这些气体就会经历雪崩式的坍缩.开始坍缩的最小质量由金斯质量(更精确的话是Bonnor-Ebert质量)来决定.对于由原子组成的温度为T粒子数密度为n的气体来说,这个质量为:
M_J=500M_sun(T/200K)^(3/2)(n/10^4cm^-3)^(-1/2).
特征温度即对应于分子氢的最低转动能级,而特征密度则对应于这样一个值,若密度高于此值则冷却变得不重要(文献12).气体接下来的引力坍缩演化过程中包含了大量的原子和分子的过程.有人认为化学,辐射冷却和流体动力学之间复杂的相互作用会导致分子云的碎裂,但即使在极高分辨率的数值模拟中人们也并未观察到明显的碎裂过程(文献11-13,20,22). 不过,出乎人们的意料,基于非宇宙学初始条件的模拟却产生了多个气体云核(文献19,23).似乎气体的高度自旋会最终导致盘的形成,随后盘会分裂.但是,这样的情形是否会在真实的宇宙的初始条件下产生还有待观察.
Although the mass triggering the first runaway collapse is well-determined, it provides only a rough guess 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 gas to become a massive star. Indeed, the highest resolution simulations of first-star formation verify that this also occurs cosmologically20 (see Fig. 1). However, the ultimate mass of the star is determined both by the mass of the cloud out of which the star forms and by a number of feedback processes that occur during the evolution of the protostar. In numerical simulations, the final mass of a Pop III star is usually estimated from the density distribution and velocity field of the surrounding gas when the first protostellar fragment forms, but this may well be inaccurate even in the absence of protostellar feedback. While protostellar feedback effects are well studied in the context of the formation of contemporary stars24, they differ in several important respects in primordial stars25.
虽然触发最早的雪崩式坍缩的质量人们了解的很清楚,这个质量却只能用来对最终形成的恒星的质量做最粗糙的估计.标准的恒星形成理论指出,在恒星形成过程中,一个小的星坯会先形成,接着通过吸积其周围的气体而增长质量,直至形成一个大质量的恒星.关于第一代恒星形成的最高分辨率的数值模拟也证实了这种情形在宇宙学条件下也会发生(文献20,同时参见图1).然而,最终形成的恒星的质量是由恒星形成于其中的气体云的质量和星坯演化过程中的一系列反馈过程共同决定的.在数值模拟中,一个PoPIII恒星的最终质量通常是通过最早的星坯碎片形成时其周围的密度分布和速度场来估计的,但是,即使不考虑反馈效应,这种估计也是不准确的. 在现在的恒星形成过程中,作用在星坯上的反馈效应已被很好地研究过,然而跟原初的恒星比较,这种效应在几个重要的方面并不相同(文献25).
First, primordial gas does not contain dust grains. As a result, radiative forces on the gas are much weaker. Second, it is generally assumed that magnetic fields are not important in primordial gas because, unless exotic mechanisms are invoked, the amplitudes of magnetic fields generated in the early Universe are so small that they never become dynamically significant in primordial star-forming gas26. Magnetic fields have at least two important effects in contemporary star formation: they reduce the angular momentum of the gas out of which stars form and they drive powerful outflows that disperse a significant fraction of the parent cloud. It is likely that the pre-stellar gas has more angular momentum in the
primordial case, and this is borne out by cosmological simulations. Third, primordial stars are much hotter than contemporary stars of the same mass, resulting in significantly greater ionising luminosities27.
首先,原初的气体并不含有尘埃微粒,因此气体受到的辐射压力非常微弱.其次,人们通常假定在原初气体中磁场并不重要,因为,除非考虑一些奇特的机制,否则早期宇宙产生的磁场的强度是很弱的,这些磁场在原初的恒星形成过程中动力学效应并不显著(文献26).在现在的恒星形成过程中,磁场至少有2点重要的作用:磁场减少了形成恒星的气体云的角动量并驱动强大的外向流驱散相当一部分的母气体云.在原初时期,形成恒星的气体很可能具有更多的角动量,这已得到宇宙学的数值模拟的证实.第三,原初的恒星比现在同等质量的恒星要热得多,因此其电离光度也要大得多(文献27).
State-of-the-art numerical simulations of the formation of the first (Pop III.1) stars represent a computational tour de force, in which the collapse is followed from cosmological (comoving Mpc) down to protostellar (sub astronomical-unit) scales, revealing the entire formation process of a protostar. However, further growth of the protostar cannot be followed accurately without implementing additional radiative physics. For now, inferring the subsequent evolution of the protostar requires approximate analytic calculations. By generalising a theory for contemporary massive star formation28, it is possible to approximately reproduce the initial conditions found in the simulations and to then predict the growth of the accretion disk around the star29. Several feedback effects determine the final mass of a first star25: Photodissociation of H2 in the accreting gas reduces the cooling rate, but does not stop accretion. Lyman-# radiation pressure can reverse the infall in the polar regions when the protostar grows to 20 − 30M", but cannot significantly reduce the accretion rate. The expansion of the H II region produced by the large flux of ionising radiation can significantly reduce the accretion rate when the protostar reaches 50 − 100M", but accretion can continue in the equatorial plane. Finally, photoevaporation-driven mass loss from the disk30 stops the accretion and fixes the mass of the star (see Fig. 2). The final mass depends on the entropy and angular momentum of the pre-stellar gas; for reasonable conditions, the mass spans 60 − 300M".
目前最高水准的关于第一代恒星(PoPIII.1)形成的数值模拟堪称为一个计算方面的壮举.这个模拟可以追踪从宇宙学尺度(Mpc量级的共动尺度)到星坯尺度(亚天文单位)的坍缩过程,从而揭示了星坯形成的完整的过程.然而,如果不考虑辐射过程的话,就无法更进一步精确追踪星坯的增长过程.现在,要想推断星坯接下来的演化还是只能靠解析的近似计算.人们可以从现在的大质量恒星的形成过程中总结出理论,然后近似重复出数值模拟里面的初始情况,接着就可以预估恒星周围吸积盘的增长了(文献29).几种反馈效应决定了一个第一代恒星最终的质量:被吸积气体中的H_2的离解降低了冷却速率,但是并不能终止吸积.当星坯的质量增长至20-30太阳质量的时候,赖曼alpha辐射压可以使极区的气体下落反转,但是不会把吸积率降低太多.当星坯的质量达到50-100太阳质量的时候,电离辐射产生的HII区的膨胀可以大大削减吸积率,但是在赤道平面上的吸积过程依然可以继续.最终,来自盘的由光致蒸发驱动的质量损失过程终止了吸积(文献30)并确定了恒星的质量(参见图2).最终的质量依赖于形成恒星的气体的熵和角动量,在合理的情况下,质量的分布在60-300太阳质量之间.
A variety of physical processes can affect and possibly substantially alter the picture outlined above. Magnetic fields generated through the magneto-rotational instability may become important in the proto-stellar disk31, although their strength is uncertain, and may play an important role in the accretion phase18. Cosmic rays and other external ionisation sources, if they existed in the early Universe, could significantly affect the evolution of primordial gas32. A partially ionised gas cools more efficiently because the abundant electrons promote H2 formation. Such a gas cools to slightly lower temperatures than a neutral gas can, accentuating the fractionation of D into HD so that cooling by HD molecules becomes important33−36.
有许多物理过程都能够影响甚至可能大大地改变上述图像.在星坯的盘中,通过磁旋转不稳定性(magneto-rotational instability)产生的磁场可能会变重要(文献31).尽管其强度尚不确定,却依然可能在吸积过程中扮演重要的角色(文献18).如果早期宇宙中存在宇宙线和其他的额外的辐射源的话,也能显著改变原初气体的演化(文献32).部分电离气体的冷却过程会更加有效,因为其富含电子,从而促进了H_2的形成.这样的气体可以比中性气体冷到更低的温度,因为D形成HD的比重加大,从而HD分子的冷却变得重要(文献33-36).
More significant modifications to the standard model result if the properties of the dark matter are different from those assumed above (see Fig. 3). A key assumption in the standard model is that the dark matter interacts with the baryons only via gravity. However, dark matter can indirectly affect the dynamics of a pre-stellar gas. A popular candidate for CDM is the neutralino, for which the self-annihilation cross-section is large. Neutralino dark matter is thus expected to pair-annihilate in very dense regions, producing high-energy particles such as pions and electron-positron pairs and high-energy photons. These annihilation products may effectively heat collapsing primordial gas clouds when the density is sufficiently high, thereby arresting the collapse37. Calculation of the structure of stars with dark-matter annihilation suggest that they can undergo a phase of evolution in which they have temperatures of 4000−104 K, well below those for conventional Pop III stars38,39. The
magnitude of this effect depends sensitively on details such as the dark matter concentration and the final products of neutralino annihilation. Furthermore, calculations to date have assumed spherical symmetry, whereas it is possible that the angular momentum of both the
baryons (which leads to the formation of an accretion disk29) and of the dark matter could significantly impede the buildup of the high dark-matter densities required to power the stellar luminosity via dark-matter annihilation. Nevertheless, if neutralinos are detected in
the appropriate mass range40, early star formation models may need to include the effect of dark matter annihilation.
如果暗物质的性质跟我们上面假设的不一样的话,标准模型还要做更多的修改(参见图3).标准模型的一个关键假设是暗物质仅通过引力跟重子物质发生相互作用.然而,暗物质可以间接地影响形成恒星的气体的动力学性质.冷暗物质的一个流行的候选者是中性超对称粒子,这种粒子的自湮灭截面非常大.因此,在非常致密的区域中性超对称粒子可以成对湮灭,产生高能的粒子,如介子,正负电子对还有高能的光子.当密度非常高时,这些湮灭产物可以有效地加热坍缩中的原初气体云,从而抑制坍缩的过程(文献37).对含有暗物质湮灭的恒星的结构的计算表明,它们可以经历一个温度在4000-10^4 K之间的演化阶段,这个温度远低于通常的PoPIII恒星的温度(文献38,39).这种效应的强度对一些细节比较敏感,如暗物质的聚集度和中性超对称粒子湮灭的终产物.除此之外,目前的计算都是基于球对称这样一个假设,而实际上重子(导致吸积盘的形成,见文献29)和暗物质的角动量能够大大地阻碍暗物质的密度的增高,而这样的密度对于恒星通过暗物质湮灭发光是必须的.不管怎么说,如果中性超对称粒子在合理的质量范围之内被探测到(文献40),早期的恒星形成过程就需要包含进暗物质湮灭的效应.
Feedback from the first stars
来自第一代恒星的反馈
Some of the feedback processes described above that affect the formation of individual stars also impact primordial star formation on large scales. The enormous fluxes of ionising radiation and H2-dissociating Lyman-Werner (LW) radiation emitted by massive Pop III stars27,41 dramatically influence their surroundings, heating and ionising the gas within a few kpc of the progenitor and destroying the H2 within a somewhat larger region17,33,42−44. Moreover, the LW radiation emitted by the first stars could propagate across cosmological distances, allowing the build-up of a pervasive LW background radiation field45,46. The impact of radiation from the first stars on their local surroundings has important implications for the numbers and types of Pop III stars that form. The photoheating of gas in the minihalos hosting Pop III.1 stars drives strong outflows, lowering the density of the gas in the minihalos and delaying subsequent star formation by up to 100 Myr47. Furthermore, neighbouring minihalos may be photoevaporated, delaying star formation in such systems as well48−50. The photodissociation of molecules by LW photons emitted from local starforming regions will, in general, act to delay star formation by destroying the main coolants
that allow the gas to collapse and form stars51.
上文描述的反馈过程不单会影响孤立的恒星,也会在大尺度上影响原初的恒星的形成.大质量的PoPIII恒星发出的强烈的电离辐射和H_2离解辐射可以很强烈地影响其周围的环境,加热并电离几个kpc之内的气体,并把一个更大范围内的H_2破坏掉(文献17,33,42-44).除此之外,第一代恒星发出的LW辐射可以横跨宇宙学的尺度,从而建立起一个弥漫的LW背景辐射场(文献45,46).第一代恒星的辐射对其周围环境中形成的PoPIII恒星的数量和类型都有很重要的影响.PoPIII.1恒星的宿主暗晕中的气体的光致加热会产生很强烈的外向流,从而降低了自身气体的密度,并将接下来的恒星形成绰成推迟了高达10^8年的时间(文献48-50).临近的恒星形成区域发出的LW光子对氢分子的离解也常常会破坏掉导致气体坍缩并形成恒星的主要的冷却介质,推迟恒星形成(文献51).
The photoionisation of primordial gas, however, can also stimulate star formation by fostering the production of abundant molecules within the relic H ii regions surrounding the remnants of Pop III.1 stars44,47,52,53 (see Fig. 4). It is still debated whether this radiative feedback is positive or negative in terms of its overall impact on the cosmic star formation rate54. However, some robust conclusions have emerged from the recent simulations. First, the LW feedback is much less ‘suicidal’ than was originally thought55. It is now believed that star formation in neighbouring minihalos is not completely suppressed, but merely delayed. Second, the ionising radiation from the first stars is initially very disruptive because it substantially decreases the density in the host minihalo. This effect leads to the substantial gap between the formation of the first and second generations of stars. In each region of space, the drama of ‘first light’ thus occurred in two clearly separated stages.
原初气体的光致电离也能激励恒星的形成,因为大量的分子会在PoPIII.1恒星余烬周围残留的HII区内养育而成(文献44,47,52,53,参见图4).然而,整体而言这样的反馈结果对宇宙的恒星形成率的贡献到底是正还是负人们还有争议.不过从近期的数值模拟中人们还是得出了一些确信的结论.首先,LW的反馈作用并不像先前人们认为的那样具有杀伤力(文献55).现在人们认为临近的迷你暗晕中的恒星形成并不会被完全抑制,而仅仅是被推迟.其次,第一代恒星产生的电离辐射具有极强的破坏性,因为这种辐射大大降低了宿主暗晕的气体密度.这个效应导致在第一代和第二代恒星之间有一个很长的时间间隙.因此无论在何处,”第一缕曙光”的大戏的上演都会在两段截然分明的时期内.
Most of the work on the evolution of Pop III stars and on the SNe they produce has been based on the assumption that the stars are not rotating56. For initial stellar masses in the range 25M_sun< M< 140M_sun and M > 260M_sun, Pop III stars end their lives by collapsing into black holes with relatively little ejection of heavy elements. Pop III stars in the range 140 − 260M_sun explode as pair-instability supernovae (PISNe), which disrupt the entire progenitor, with explosion energies ranging from 10^51 − 10^53 erg, and nucleosynthetic yields, defined as the heavy element mass fraction, up to 0.5. Such SNe exhibit an odd-even effect in the nuclei produced that is much greater than observed in any star to date, and as a result they cannot make a significant contribution to the metals observed in very lowmetallicity stars today57. On the other hand, the PISN signature may exist in a tiny fraction of the stars with intermediate metallicity (~ 0.01Z_sun), because the enrichment from even a single PISN already endows the surrounding material with heavy elements to levels that are above the regime typically probed by surveys of metal-poor stars58.
大多数关于PoPIII恒星和其产生的超新星的研究工作都假设恒星没有自转(文献56).对于初始质量在25到140个太阳质量之间,或者大于260个太阳质量的PoPIII恒星来说,它们生命结束的时候会直接坍缩成黑洞,只有相对来说很少的金属元素能跑出来.而质量范围在140到260太阳质量之间的PoPIII恒星,则会以正负电子对不稳定性超新星(pair-instability supernovae, PISNe)的形式爆发,并将其前身完全破坏掉.这种爆发释放的膨胀能量在10^51到10^53尔格之间,而以重元素比率定义的核合成产物则高达0.5.这类超新星有一种奇怪的效果,它产生的核比现在观测到的任何恒星中的核都大.这表明这类超新星不可能对现在观测到的极低金属丰度的恒星有太大的贡献(文献57).从另一个方面来说,有一小部分中等金属丰度的恒星(Z~0.01Z_sun)却表现出PISN的特征.产生这种现象的原因在于,哪怕仅仅只有一个PISN,就足以给周围的物质提供金属,使得这个区域的金属丰度高于在贫金属星巡天中探测的区域(文献58).
The first stars may have been born rapidly rotating, however, and rotation can entirely modify these results59. For sufficiently high rotation rates, rotationally induced mixing is able to render the cores chemically homogeneous; mixing of heavy elements to the surface in the late stages of evolution can lead to substantial mass loss. If the cores maintain a sufficiently high rotation at the time of the SN, it is possible to produce a long gamma-ray burst (GRB) or a jet-induced energetic supernova/hypernova60,61, with significant effects on the abundances of the ejected metals62. Large uncertainties remain in the evolutionary calculations owing to the effects of dynamo-generated magnetic fields.
然而,在第一代恒星诞生时很可能同时进行着快速地自转,而自转可以完全改变这个结果(文献59).如果自转速率很高,可以使恒星核区的化学元素均匀混合.在恒星演化的后期,如果其内部的重元素混合到了表面的话,就会导致显著的质量损失.如果在超新星阶段核在快速自转的话,可能会产生长伽玛射线暴(GRB)或者带有喷流的高能超新星/巨超新星(文献60,61),这会对抛出的金属的量产生很大的影响(文献62).由于发电机机制产生的磁场的效应,人们对这个演化过程的计算还有很大的不确定性.
The strong mechanical and chemical feedback effects exerted by explosions of Pop III stars have been investigated with a number of detailed calculations63−69. The key question is how the initially metal-free Universe was enriched with the first heavy chemical elements70. Recently, it has become feasible to address this process with realistic three-dimensional simulations that start from cosmological initial conditions, and that resolve the detailed physics of the SN blast wave expansion63,64. These simulations have shown that early enrichment is very inhomogeneous, as the low-density voids are enriched before any metals can reach into the denser filaments and virialised halos71.
PoPIII恒星的膨胀会导致强烈的力学和化学的反馈效应, 这已为许多详细的计算所研究过(文献63-69). 关键问题是,开始的时候宇宙并没有金属,它是如何被最早的那些重元素给填充起来的(文献70).最近,通过逼真的3维数值模拟来阐述这个过程已经具有现实性,这些模拟是从宇宙学的初始条件开始的,并且可以分辨超新星的冲击波膨胀的细致的物理过程(文献63,64).这些模拟表明,早期的元素增丰过程非常不均匀,在低密度的空泡区已经被金属填充时,尚没有金属进入那些高密度的纤维结构和维里化了的暗晕(文献71).
Assembly of the first galaxies
第一代星系的会聚过程
The characteristic mass of the first star formation sites has been determined to be ~10^6M_sun14,72, whereas the critical mass for hosting the formation of the first galaxies is still not known with any certainty. A promising theoretical Ansatz is to explore atomic cooling halos, with ~ 10^8M_sun and virial temperatures greater than !~10^4 K so that atomic line cooling is efficient, as their formation sites73,74. The simulations, starting from cosmological initial conditions, are just now approaching the resolution and physical realism to investigate whether atomic cooling halos fulfill the criteria for a first galaxy as defined above. Quite generically, in such models, the first generation of stars forms before galaxies do, and feedback effects from the first stars are expected to play a key role in determining the initial conditions for the formation of the first galaxies. While substantial uncertainties in the overall formation efficiency of the first stars still remain, it is possible, and perhaps probable, that most first galaxies hosted at least one primordial star earlier75. If the early generation stars were massive, > 10M_sun, the feedback effects described in the previous section would shape the conditions for subsequent star-formation in the region.
我们已经知道,形成第一代恒星的暗晕质量约为10^6 M_sun(文献14,72),然而无论从哪种程度上来说,我们对形成第一代星系的宿主的质量却依然一无所知.一个有前途的理论性的假定是探索那些通过原子来冷却的暗晕,其质量约为 10^8 M_sun,维里温度高于10^4 K,这种情况下原子谱线的冷却是非常有效的,这类暗晕可以作为第一代星系的形成之地(文献73,74).从宇宙学的初始条件开始的数值模拟现在刚刚接近到这样的分辨率与物理实现,即可以来研究原子冷却的暗晕是否满足上面所定义的第一代星系的判据.一般来说,在这个模型里面,第一代恒星形成于第一代星系之前,并且来自第一代恒星的反馈效应在决定第一代星系形成的初始条件方面扮演了一个关键的角色.第一代恒星整体的形成效率依然有很大的不确定性,不过一个比较可能也比较适当的情况是,大多数第一代星系在形成之前至少有一个原初的恒星曾经形成过(文献75).如果早一代的恒星是大质量的,质量大于10 M_sun,上文描述过的反馈效应就能改变这个区域内后续的恒星形成过程.
The gas expelled by the H II regions and SNe of the first stars is too hot and diffuse to allow further star formation until it had time to cool, as well as to reach high densities again in the course of being reincorporated in a growing dark matter halo. Both cooling and re-collapse occur rather slowly, thus rendering star-formation intermittent in the early formation phase of the first galaxies. Analytic models76 and detailed numerical simulations47,77 both show that the gas re-incorporation time is as long as a hundred million years, roughly corresponding to the dynamical time for a first-galaxy halo to be assembled.
被HII区和超新星吹出去的气体太热也太弥散,后面的恒星无法形成, 除非有足够的时间使这样的气体冷却,重新回到一个不断增长的暗物质晕里面并达到高密度的状态.冷却和重新坍缩的过程都非常缓慢,所以第一代星系中早期的恒星形成过程将会是断断续续的. 无论是解析(文献76)模型还是细致的数值模拟(文献47,77)都表明,气体重新回到暗晕中所需要的时间长达几亿年,大致相当于一个第一代星系会聚的动力学时标.
Chemical enrichment by the first SNe is among the most important processes in the formation of the first galaxies. Efficient cooling by metal lines and dust thermal emission regulate the temperature of Pop II star-forming regions in the first galaxies. The concept of a ‘critical metallicity’ has been introduced to characterise the transition of the star-formation mode from predominantly high-mass, Pop III or Pop II, to low-mass Pop II stars78. However, this critical gas metallicity is still poorly determined. It is not even clear if there exists such a sharp transition. Some studies show that even a slight quantity of metals in a gas may be enough to change the gas thermal evolution significantly79, whereas others argue that the cooling efficiency at low densities80 is crucial and is significantly enhanced only above one ten-thousandth of solar-metallicity (Z > 10^−4Z_sun). Since the enrichment from even a single PISN by a very massive Pop III star likely leads to metallicities of Z > 10^−2Z_sun
63, well inexcess of any predicted value for the critical metallicity, these arguments might be somewhat academic. The characteristic mass of prestellar gas clumps is likely determined by a number of physical processes (e.g., turbulence and, possibly, dynamo-amplified primordial magnetic fields) other than radiative cooling. The overall effect of gas metallicity on star-formation may well be limited81.
第一代超新星的化学增丰过程是第一代星系形成时最重要的过程之一.由金属线和尘埃的热辐射导致的冷却在控制第一代星系内部的第二代恒星形成区的温度方面颇为有效.人们引入了”临界金属丰度”这个概念来表征从早期大质量占主导的PoPIII或者PoPII恒星到低质量的PoPII的恒星的形成模式的转变.然而,这个临界丰度值还难以确定.人们甚至不清楚到底是不是存在这样一个截然分明的转变界限.部分研究表明,哪怕气体中只有一点点金属,就足以显著改变其热演化状态(文献79),而另外的研究则认为在低密度条件下冷却效率并不高(文献80),只有金属丰度大于太阳丰度的万分之一(Z >10^-4Z_solar)时冷却才会被显著加强.不过,只要一个大质量PoPIII恒星产生的PISN就能将金属丰度提至( Z>10^-2Z_solar)(文献63),这个值比任何理论预言的临界丰度都高,所以上面的争论可能就显得有些学究气了.决定恒星形成之前的气体团块的特征质量的因素,除了辐射冷却以外,还有一系列其它的物理过程(如湍流和可能的被发电机机制放大的原初磁场).金属丰度在恒星形成方面的整体效应可能是比较有限的(文献81).
Recent cosmological simulations have demonstrated that star formation inside the first galaxies is strongly influenced by gravitationally-driven supersonic turbulence that was generated during the virialisation process64,73,74. This is in marked difference to the rather quiescent, quasi-hydrostatic situation in minihalos (see Fig. 5). It thus appears possible that the first galaxies harbour the first stellar clusters, if present-day star formation offers any guide here, where it is widely believed that gravo-turbulent fragmentation is responsible for shaping the initial mass function (IMF)24,82. It is an open question as to whether the first galaxies could harbour the first globular clusters, which are the oldest star clusters known.
近期的宇宙学数值模拟表明,第一代星系里面的恒星形成过程会强烈地受到超音速的湍流的影响,这些由引力驱动的湍流形成于维里化过程之中(文献64,73,74).这跟迷你暗晕中静止的,准流体静力学平衡的情况显著不同(参见图5). 这种情况表明似乎是第一代星系造就了第一代星团.当然这是以现在的恒星形成过程作为参考,人们普遍相信,在现在的恒星形成过程中,初始质量函数(IMF,文献24,82)是由引力湍流决定的.球状星团是人们已知的最古老的星团,然而第一代的球状星团是否由第一代星系造就依然悬而未决.
Future empirical probes
未来的实验探测
Studying the formation of the first stars and galaxies will be at the frontier of astronomy and cosmology in the next decade. Astronomers will muster a comprehensive arsenal of observational probes. The most prominent among these concern the CMB optical depth to Thomson scattering83−85, the near-IR background86, high-redshift GRBs87−89, the possibility of scrutinising the nature of the first stars by metals found in the oldest Galactic halo stars, dubbed ‘stellar archaeology’90,91, and various facilities now being deployed to map reionisation using the redshifted 21 cm line of neutral hydrogen92−94. The James Webb Space Telescope (JWST) will carry out a number of observations designed to test key assumptions of our current theory of the first stars and galaxies95. How could the existence of massive Pop III stars be unambiguously inferred? The most clear-cut diagnostic is the ratio of recombination lines emitted from the H II regions around single Pop III stars, or clusters thereof, to be measured with ultra-deep near-IR and mid-IR spectroscopy. Due to the high effective temperature of the Pop III stellar continuum, ! 105 K, strong He II $ 1640 line emission is predicted, with a ratio compared to Lyman-# that is one to two orders of magnitude larger than for normal stars41. A second crucial observational campaign aims at a census of very high-z SNe96 through deep broadband near-IR imaging. One key objective is to search for possible PISN events, which would clearly stand out owing to their extreme intrinsic brightness, as well as their very long durations, a few years in the observer frame97. The goal of making useful predictions for the high-redshift frontier is now clearly drawing within reach, and the pace of progress is likely to be rapid.
对第一代恒星和星系形成的研究将会是下个十年天文学和宇宙学的前沿内容.天文学家们将要集成一个全面的观测用探测器的武器库.这其中最突出的探测涉及到CMB的汤姆逊散射光深(文献83-85),近红外背景辐射(文献86),高红移的伽玛射线暴(文献87-89),通过在最古老的晕星中发现的金属来仔细探测第一代恒星性质的可能性--谓之”恒星考古学”(文献90,91),以及现在被配属来通过红移了的中性氢21厘米线来绘制再电离天图的各种设备(文献92-94).詹姆斯.韦伯空间望远镜将会进行一系列的观测以来检验我们现在关于第一代恒星和星系的理论的关键假设(文献95).怎样才能明确地推断出大质量星族III恒星的存在性?最干净利落的方法是用极深的进红外和中红外分光术来测量到单个的星族III恒星或者星团周围的HII区的复合发射线的比率.由于星族III恒星的连续谱具有极高的有效温度,约10^5K,所以人们预期会有很强的波长为1640埃的HeII发射线,这个发射线比上赖曼alpha的相对强度比普通恒星要高1到2个数量级(文献41).第二个关键的观测致力于用宽波段的近红外图像对高红移的超新星做一个普查(文献96).其中一个关键的目标是搜寻可能的PISN事件,这类事件内秉亮度极其高,持续的时间也比较长,在观测者测参考系内会持续数年(文献97),因此容易被认出来.对高红移的宇宙疆域做出有效的预言,这一目标现在已是伸手可及,进步的速度也会很迅速.
图1
file:///Users/yuebin/Library/Caches/TemporaryItems/moz-screenshot.jpg
file:///Users/yuebin/Library/Caches/TemporaryItems/moz-screenshot-1.jpgFig. 1.— Projected gas distribution around a primordial protostar20. Shown are, (A) the large-scale gas distribution around the cosmological minihalo (300 pc on a side), (B) a selfgravitating, star-forming cloud (5 pc on a side), (C) the central part of the fully molecular core (10 astronomical units on a side), and (D) the final protostar (25 solar-radii on a side).
图一:
一个原初星坯周围气体的投影分布(文献20).
(A)环绕迷你暗晕周围的大尺度气体分布(边长为300 pc),
(B)一个正在经历恒星形成的自引力气体云(边长5 pc),
(C)满是分子的核区的中心部分(边长10个天文单位),
(D)最终的星坯(边长25倍太阳直径).
图2
Fig. 2.— Feedback limited accretion25. The accretion rate vs. protostellar mass is shown in the cases of “no feedback” and “with feedback”. Even as an H II region is built up, accretion continues through an accretion disk, which is eventually destroyed via photoevaporation. Also shown is the corresponding rate. The intersection of the two curves determines the final Pop III mass.
图二:
在反馈效应下的吸积过程(文献25).图中给出的是”有反馈”和”无反馈”两种情况下吸积率随星坯质量的变化.即使HII区正在扩大,吸积依然可以通过吸积盘而继续,最终吸积盘会被光致蒸发效应给破坏掉.图中也给出了相应的逛致蒸发的破坏速率.吸积率和蒸发率两条曲线交叉的地方决定了PoPIII恒星的最终质量.
图3左
图3右
Fig. 3.— Dark matter properties and early star formation98. Projected gas distribution in CDM (left) and warm dark matter (WDM) simulations (right) at z = 20. If the power in the primordial density spectrum is reduced on small scales, the first stars will form much later than in the standard CDM-based scenario. If the dark matter is warm, having a substantial velocity dispersion, density perturbations on small length scales are smoothed. The hierarchy of structure formation is then truncated at a corresponding mass scale, and the first cosmological objects could be more massive than 106M". For the case of light warm dark matter99, gas collapses into filaments, which might then fragment into multiple stellar cores. The abundance of star-forming halos is significantly reduced in this model.
图三:
暗物质性质与早期恒星形成(文献98).这里给出的分别是冷暗物质(左边)和温暗物质(右边)模拟中z=20时气体的投影分布.如果原初密度谱中的小尺度扰动被减少了,跟标准的基于冷暗物质的描述相比第一代恒星的形成要晚得多.如果暗物质是温的,具有较大的速度弥散,小尺度的扰动就会被抹平.这样结构形成的层级就会在相应的质量处被截止,宇宙中第一代天体可能就会比10^6 Msun大得多.对于轻的温暗物质(文献99),气体会坍缩进纤维状结构,然后碎裂为数个恒星核.在这种模型里面形成恒星的暗晕的数量大为减少.
图5
Fig. 4.— Radiative feedback around the first stars47. Ionized bubbles are shown in blue, and regions of high molecule abundance in green. The large residual free electron fraction inside the relic H II regions, left behind after the central star has died, rapidly catalyzes the reformation
of molecules. The abundance of HD molecules allows the primordial gas to cool to the temperature of the CMB, possibly leading to the formation of Pop III.2 stars after these regions have re-collapsed so that gas densities are sufficiently high again for gravitational instability to occur77. The latter process takes of order the local Hubble time, thus imposing a ~100 Myr delay in star formation. The relatively high molecule abundance in relic H ii regions, along with their increasing volume-filling fraction, leads to a large optical depth to LW photons over physical distances of the order of several kpc47. The development of a high optical depth to LW photons over such short length-scales, combined with a rapidly increasing volume filling fraction of relic H ii regions, suggests that the optical depth to LW photons over cosmological scales may be very high, acting to suppress the build-up of a background LW radiation field, and mitigating negative feedback on star formation75. Note the strongly clustered nature of early star formation.
图四:
第一代恒星周围的辐射反馈(文献47).蓝色的表示电离泡,而绿色则表示分子含量高的区域.在中心的恒星死亡之后,残留的HII区内大量的自由电子催化了分子的形成.大量存在的HD分子使得原初气体可以冷至CMB的温度,在这个区域重新坍缩,密度再次增高已至引力不稳定发生之后可能会形成PoPIII.2恒星(文献77).后者需要的时间跟哈勃时间是一个量级,因此恒星形成会被推迟100 Myr.残留的HII 区相对来说较高的分子比率,及其不断增长的体积填充率(volume-filling),会导致很大的对LW光子的光深,其物理尺度为几个kpc的量级(文献47).在这么短的时间之内就发展起来这么大的对LW光子的光深,跟残留的HII区快速增长的体积填充率一起,标明在宇宙学尺度上LW光子的光深可能非常大, 这会抑制住LW背景辐射场的建立,减缓其对恒星形成的负反馈效应(文献75).注意早期的恒星形成具有极强的成团性.
图4
Fig. 5.— Turbulence inside the first galaxies74. Shown is the Mach number in a slice through the central 40 kpc (comoving) of the galaxy. The dashed line denotes the virial radius of ~1 kpc. The Mach number approaches unity at the virial shock, where the accreted gas is heated to the virial temperature. Inflows of cold gas along filaments are supersonic by a factor of ~10, resulting in strong turbulent flows in the galactic center.
图五:
第一代星系内部的湍流(文献74). 图中展示的是一个过中心的厚度为40 kpc(共动)的一个切片的马赫数.划线标出的是半径为1 kpc的维里半径.在维里激波处马赫数接近1,气体在此处被加热至维里温度.沿着纤维状结构进入的冷气体的内向流是是超声速的,约为声速的10倍,从而在星系中心产生极强的湍动流.
Box: Definitions and Terminology
概念和术语
We here establish a convention for terminology to be used in this review. Pop III stars are those that initially contain no elements heavier than helium (’metals’ in the parlance of astronomers) other than the lithium produced in the Big Bang. Such stars can be divided into first generation stars (Pop III.1), which form from initial conditions determined entirely by cosmological parameters, and second generation stars (Pop III.2), which originate from material that was influenced by earlier star formation25. According to theory, Pop III.1 stars formed when almost completely neutral primordial gas collapsed into dark-matter minihalos, whereas one important class of Pop III.2 stars formed from gas that was photoionised prior to the onset of gravitational runaway collapse33. Simply put, Pop III.1 stars are locally the very first luminous objects, whereas Pop III.2 stars are those metal-free stars formed from gas that was already affected by previous generations of stars. Pop II stars have
enough metals to affect their formation and/or their evolution. Such stars are classified100 according to their iron/hydrogen ratio as extremely metal poor (EMP) for metallicities 10^−4 < Z/Z_sun < 10^−3, ultra-metal poor (UMP) for 10^−5 < Z/Z_sun < 10^−4, and hyper-metal
poor (HMP) 10^−6 < Z/Z_sun < 10^−5. Because we know so little about the first galaxies, it is difficult to establish a precise terminology for them. A galaxy is a system of many stars and gas that is gravitationally bound in a dark matter halo. We define a first galaxy as one comprised of the very first system of stars to be gravitationally bound in a dark matter halo. Such stars could be Pop III or Pop II stars with very low metallicities—EMP or below according to recent numerical simulations63,64. The gas in such galaxies should have similarly low metallicities. Current theory predicts that first generation Pop III stars (Pop III.1) are formed in isolation in minihalos and therefore will not be in galaxies.
我们给本综述中用到的术语做了一个汇总.PoPIII恒星是指那些早期的,不含有除由大爆炸产生的锂外比氦更重的元素(天文上习惯称之为”金属”)的恒星.这类恒星可以被划分为第一代(PoPIII.1)和第二代(PoPIII.2),前者直接从宇宙学参数决定的初始条件形成,后者则起源于已被早期恒星影响过的物质(文献25).理论预言,当几乎全中性的气体坍缩进迷你暗物质晕中时,PoPIII.1恒星形成.反而是PoPIII.2恒星形成自坍缩之前就已经被电离的气体之中(文献33).
简单来说,PoPIII.1恒星是某个局部区域最早的发光天体,而PoPIII.2恒星则是一种形成于已被前代恒星影响过的气体之中的无金属恒星.PoPII恒星已经具有足够影响其形成或者演化的金属.这类恒星按照其铁/氢的比率划分为(文献100)特贫金属星(extremely meta poor, EMP),金属丰度在10^-4到10^-3太阳丰度之间,极贫金属星(ultra-metal poor, UMP),金属丰度在10^-5到10^-4太阳丰度之间,以及超贫金属星(hyper-metal poor, HMP),丰度在10^-6到10^-5太阳丰度之间.由于我们对第一代星系只知道那么以点点,因此很难对其进行精确定义.星系是许多恒星和气体构成的系统,在一个暗晕里面通过引力束缚在一起.我们这样来定义第一代星系,即由非常早期的恒星系统在暗晕中被引力束缚在一起.这些恒星可能是PoPIII恒星或者金属丰度很低的PoPII恒星-- EMP或者金属丰度更低,这是由近期的数值模拟所揭示的(文献63,64).第一代星系中的气体应当具有类似的低金属丰度.现有的理论预言第一代的PoPIII恒星(PoPIII.1)独自在迷你暗晕中形成,因此不会处于星系里面. |
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