Proc. Natl. Acad. Sci. USA Vol. 95, pp. 47–52, January 1998 Colloquium Paper This paper was presented at a colloquium entitled ‘‘The Age of the Universe, Dark Matter, and Structure Formation,’’ organized by David N. Schramm, held March 21–23, 1997, sponsored by the National Academy of Sciences at the Beckman Center in Irvine, CA. The universe at z > 5: When and how did the ‘‘dark age” end? MARTIN J. REES Institute of Astronomy, Madingley Road, Cambridge, CB3 OHA, United Kingdom ABSTRACT This paper considers how the first subgalac- We now know that at least some galaxies and quasars had tic structures produced the UV radiation that ionized the already formed by a billion years. But how much earlier did intergalactic medium before z 5 5 and the ‘‘feedback’’ effects structures form, and what were they like? of the UV radiation on structure formation. The first ‘‘pre- In most cosmological theories, especially those that postu- galaxies’’ may eventually be detectable by their direct UV late adiabatic Gaussian irregularities in the early universe, emission, with characteristic spectral features at Lyman a; quasars and large galaxies should thin out beyond z 5 5, but high-z supernovae may also be detectable. Other probes of the subgalactic structures may exist even at redshifts exceeding 10. intergalactic medium beyond z 5 5, and of the epochs of I shall discuss the effects of the earliest stars and supernovae— reheating and reionization, are discussed, along with possible production of UV radiation, nonuniform reheating of the links between the diffusion of pregalactic metals and the intergalactic medium (IGM), and the production of the first origin of magnetic fields. heavy elements—and the implications for observations at ultra-high redshifts. I shall discuss some potential observations The structures in our present universe are the outcome of more that can probe pregalactic era, telling us when the first energy than 10 billion years of evolution. Slight irregularities im- input occurred, and what scale of objects already existed at z . printed at very early eras led to increasing contrasts in the 5. But it may first be useful to summarize our theoretical density from place to place, until overdense regions evolved picture of how the first cosmic structure emerged. into bound structures. Quantitative and detailed (albeit spec- ulative) theories for the origin of the fluctuations (in an Clustering in Hierarchical Models ultra-early inflationary phase, or else as the outcome of topological defects, etc.) can now be tested against increasingly Characteristic Scales in Cold Dark Matter (CDM) Models. detailed and precise data. I will focus on the CDM model. But this is just a ‘‘template’’ The data are of two kinds. (i) CMB fluctuations probe for some more general deductions, which essentially apply to irregularities on scales relevant to observed cosmic structure any ‘‘bottom up’’ model for structure formation. There is no and on larger scales that are still in the linear regime. The minimum scale for the aggregation, under gravity, of cold detected photons have (according to most assumptions) prop- nonbaryonic matter. However, the baryons constitute a gas agated almost uninterruptedly since the recombination era, whose pressure opposes condensation on very small scales. when the universe had been expanding for less than a million Therefore, the gas does not ‘‘feel’’ the very smallest conden- years. (ii) The other line of evidence on cosmic structure comes sations. In the context relevant here, where the dark matter’s from traditional astronomy. Any acceptable theory must ac- gravity dominates, the baryonic Jeans mass is count for the present clustering properties of galaxies and dark 3 matter; it must also match the actual universe at all past eras V T 2 . 3 4S b DS g D M 5 10 MJ [1] that can be probed observationally. We have for some years J 0.025h2 T known about quasars with redshifts up to 5. Quasars them- rad selves may be associated with atypical (even exceptional) Only on scales larger than this can baryons promptly condense galaxies, so their intrinsic properties are hard to relate to the into bound systems, along with the dark matter. [The proviso general trend of galaxy formation. What has been especially ‘‘promptly’’ is nontrivial because, as emphasized by Loeb exciting about recent developments is that the morphology and (private communication), the baryons may be able to condense clustering of ordinary galaxies can now be probed out to similar later into smaller systems.] During the ‘‘dark age’’ the gas redshifts: the powerful combination of the Hubble Space became even cooler than the microwave background (i.e., Tg Telescope (HST) and the Keck Telescope has revealed many , Trad): if it had cooled adiabatically, with no heat input since galaxies at z . 3. Also, the absorption features in quasar 5 recombination, Tg would, at z 10, have been below 5 K. The spectra (the Lyman forest, etc.) probe the history of the ; smallest bound structures, with mass MJ, would have virial- clumping and temperature of a typical sample of the universe ized at a temperature a few times larger than Tg(z). Larger on galactic (and smaller) scales. masses would virialize at temperatures higher by a further 1y The mystery lies at still higher redshifts, between (in round y 2 5 5 factor (M MJ) . This virial temperature would be reached not numbers) a million years (z 1,000) and a billion years (z solely by adiabatic compression, but also because of a shock, 5). When the primordial radiation cooled below a few thou- which would typically occur before the radius had decreased by sand degrees, it shifted into the infrared. The universe then a factor of 2. entered a dark age, which continued until the first bound These virialized systems would, however, have a dull exis- structures formed, releasing gravitational or nuclear energy tence as stable clouds unless they could lose energy and deflate that lit up the universe again. How long did the ‘‘dark age’’ last? Abbreviations: IGM, intergalactic medium; CDM, cold dark matter; © 1998 by The National Academy of Sciences 0027-8424y98y9547-6$2.00y0 Mpc, million persec; NGST, New Generation Space Telescope; PNAS is available online at http:yywww.pnas.org. AGN(s), active galactic nucleus(i); yr, year. 47 Downloaded by guest on September 28, 2021 48 Colloquium Paper: Rees Proc. Natl. Acad. Sci. USA 95 (1998) due to atomic or molecular radiative processes—clouds that background intensity gets a boost, because the contributions couldn’t cool would simply remain in equilibrium, being later from remote regions (which dominate in Olbers-type integrals) incorporated in a larger scale of structure as the hierarchy built are less severely attenuated. The UV is then enough to up. On the other hand, clouds that can cool radiatively will maintain the very high mean ionization implied by transpar- deflate. Most cooling mechanisms are more efficient at higher ency of the IGM beyond the Lyman limit. This means that it temperatures, as well as at higher densities. Once collapse can maintain high ionization of a cloud until it has either starts, it proceeds almost isothermally, so that the internal collapsed to an overdensity exceeding the IGM ratio of ions to Jeans mass falls as the density rises. A virialized, self- neutrals or until it becomes self-shielding (which happens at gravitating cloud that can cool radiatively would eventually go more modest overdensities for large clouds). Until that hap- into free-fall collapse and (perhaps after a disc phase) frag- pens the cooling rate will be reduced by the lack of bound ment into smaller pieces. electrons and consequent elimination of the (otherwise dom- Three ‘‘cooling regimes’’ are relevant during successive inant) ‘‘line’’ contribution to the cooling. phases of the cosmogonic process, each being associated with When this third phase is reached, the thermal properties of a characteristic temperature. the uncollapsed gas will resemble those of the structures (i) For a H–He plasma the only effective cooling at low , 4 responsible for the observed Lyman–forest lines in high-z temperatures ( 10 K) comes from molecular hydrogen. Even quasars spectra—these are mainly filaments, draining into this process cuts off below a few hundred degrees; but above virialized systems. Such systems have velocity dispersions of 50 that temperature it allows contraction within the cosmic kmysec and are destined to turn into galaxies of the kind whose expansion timescale. The H fraction is never high, and it is, in 2 descendants are still recognizable. I shall return (section 4) to any case, not a very efficient coolant—indeed, systems that discuss the detectability of these early galaxies. collapse at z , 10 fail to form enough molecules for effective cooling [e.g., see figure 1 of Tegmark, et al. (1)]—but molec- Evidence for Diffuse Gas at High z ular cooling almost certainly played a role in forming the very first objects that lit up the universe CMB Fluctuations as a Probe of the Ionization Epoch. If the (ii)IfH is prevented from forming, so that molecular 2 intergalactic medium were suddenly reionized at a redshift z, cooling is ineffective, then a H–He mixture behaves adiabat- .. ically unless T is as high as 8,000–10,000°, when excitation of then the optical depth to Thomson scattering back to zi( 1) Lyman a by the Maxwellian tail of the electrons provides would be efficient cooling whose rate rises steeply with temperature. 3 1 1 z 2 V Because of this steep temperature dependence, gas in this t 5 21S iD S b D 0.05h 2 [2] regime contracts almost isothermally, so that its Jeans mass es 10 0.025h 2 decreases as the density rises.