Protogalaxies E NCYCLOPEDIA of a STRONOMY and a STROPHYSICS
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Protogalaxies E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS Protogalaxies which is produced by some type of quantum fluctuation processes in the early universe. Sufficiently overdense The term protogalaxies or sometimes primeval galaxies regions separate from the universal expansion and (hereafter PGs) has been used in the literature with a collapse under their own self-gravity, roughly on the free- range of different meanings, but a general definition may fall time scale: be something like this: progenitors of the present-day / π 2 1 2 (normal) galaxies, in the early stages of formation; also, t = R3/2 M−1/2 ff many, but not all, authors would also add to that the words 8G init ‘at high redshifts’. The key phrase is ‘the early stages of R 3/2 M −1/2 formation’, which is fundamentally not well defined. ≈ 5 × 108 yr × init The ambiguities are mainly due to the fact that 100 kpc 1012M our understanding of what GALAXY FORMATION means has R 3/2 M −1/2 been evolving over the past few decades. Moreover, ≈ 1.6 × 109yr × init 15M even at any given time, experts of different technical 10 Mpc 10 n R orientations (e.g. observers versus -body simulators) where init is the initial radius of the bound region and who would otherwise agree about the general picture M is the enclosed mass. It can be seen that for a galactic of galaxy formation would have different and equally or subgalactic mass fragment the corresponding cosmic 8 legitimate definitions in mind. For example, an observer epochs (∼10 yr) imply high REDSHIFTS of collapse, whereas may mean ‘the first major burst of star formation in a CLUSTERS OF GALAXIES or larger structures may be still progenitor of a present-day elliptical galaxy’, whereas a collapsing today. The free-fall time scale is a lower limit theorist may mean ‘the peak merging epoch of dark halos to the formation time scale, its duration and the cosmic of the fragments which assemble to produce an average epoch: in practice, it may take several free-fall times for a galaxy today’; others would define a PG as a still gaseous given mass overdensity to be assembled through a process body before any STAR FORMATION has taken place or as an of hierarchical merging and virialization. To a first order, overdense region of DARK MATTER in the very early universe, this simple argument implies that the peak epoch of galaxy destined to become gravitationally bound and to collapse. formation is likely to be found at the cosmic epochs of a In some sense all of them are right. few×108 to a few×109 yr, or roughly in the redshift range It may be useful here to offer a brief account of z ∼ 2–20 or so, whereas the epoch of cluster formation our present understanding of galaxy formation and then lasts many billions of years and is still going on now. describe some of the relevant observations, thus defining PGs in an implicit way. While the subject is still evolving Collapse rapidly as of this writing (late 1999), most cosmologists Since the non-baryonic dark matter appears to dominate would agree that the basic picture we now have is likely the total mass, the overdense regions, i.e. proto-dark-halos, to be essentially correct. Instead of seeking a hypothetical can start collapsing even while the universe is still ionized, magic epoch when PGs appear and then evolve into the e.g. at z ∼ 104, with the baryonic matter following. The familiar types of galaxies today, the focus is now on smallest mass fluctuations collapse the fastest; however, understanding of the complete history of galaxy and large- they are also most readily erased by a variety of damping scale structure assembly, as well as the history of star processes operating in the early universe, such as the formation in all galaxies and the chemical enrichment of streaming of matter and photons, sound waves, etc. It gas in the universe at large. It is then almost a matter is now believed that the smallest structures which survive of taste which slice of formative history would be called at the recombination epoch are similar in mass to GLOBULAR 5 8 the epoch of galaxy formation and which objects or which CLUSTERS or DWARF GALAXIES, i.e. (10 –10 )M. They may be evolutionary stages of normal galaxies would be called the basic building blocks of galaxies. PGs. Images of the COSMIC MICROWAVE BACKGROUND (CMB) In what follows, we outline a broad-brush picture photosphere show a snapshot of overdense regions at and give general arguments and order-of-magnitude the recombination epoch at z ∼ 1100. At the present estimates, which are likely to be at least roughly correct, time, the resolution of such observations corresponds however much our understanding of galaxy formation to physical scales of large clusters and superclusters of evolves in the forthcoming years. Ever more detailed galaxies rather than galaxies themselves. Nevertheless, models will be producing more precise predictions, which these observations support the basic picture of structure can then be tested by future observations. formation via gravitational instability. Any energy dissipation in the baryonic component From density fluctuations to protogalaxies of the mass (also called the cooling of PGs) would of The basic paradigm of structure formation is that it course accelerate the collapse and lead to the formation takes place through the gravitational instability of bound, of denser objects, since systems which dissipate energy at overdense regions in the early universe. The distribution a fixed mass become more tightly bound. The distinction of such regions in mass (or, equivalently, density) is between galaxies and large-scale structures such as galaxy quantified by the initial density perturbation spectrum, groups or clusters (or larger) is that the galaxies (or Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 1 Protogalaxies E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS their protogalactic fragments) can cool faster than the scenario, mergers of large fragments occur over a more free-fall time. Physical mechanisms which enable this protracted time scale, comparable with the Hubble time, energy dissipation in collapsing PGs include the inverse- with most of the stars already formed in the merging units Compton cooling of hot gas on the CMB radiation (CMBR) and some formed in merger-induced starbursts. It is likely photons and shocks in the infalling and colliding gas that there was a full spectrum of galaxy assembly scenarios clouds. or formation histories at work. Thus galaxies (or their building blocks) become The interplay of mass assembly and star formation distinct concentrations within the overall large-scale fundamentally determines the galaxy morphology and structure, whose evolution is dominated by the gravity of the origin of disks in SPIRAL GALAXIES. In general, random the dark matter. Simple arguments based on the mean merging of fragments leads to the formation of spheroidal density of galaxies, the spin-up of galaxy disks, etc all (or, more accurately, triaxial) systems such as the bulges, suggest that they collapsed by about a factor of 10 or 20 in stellar halos and dark halos, where kinetic energy needed radius, whereas pure dissipationless collapse can produce to support the system against its own gravity is in random only a factor of 2 (this is a direct consequence of the virial motions. In contrast to that, if protogalactic gas settles into theorem for systems bound by gravity). Given the ratio of a dark halo potential well gradually, it radiates away its the comoving rms separation of galaxies today and their kinetic energy, but retains most of the angular momentum z< typical radii, this must have happened at 20 or so. It (which can be only mechanically transferred away). PGs is also hard to have an effective cooling of PGs at redshifts acquire their angular momenta through tidal interactions much higher than that. in the early universe, as passing-by mass concentrations There are two fundamental aspects of galaxy exert tidal torques on each other. The gas then assumes the formation: assembly of the mass (mostly dark), and lowest-energy configuration possible for a given amount conversion of the primordial gas (see GAS IN GALAXIES) into of angular momentum, which is a centrifugally supported, stars, and the subsequent chemical enrichment through rotating thin disk (similar arguments apply to the origin SUPERNOVA explosions. The former is easier to model of any disks in astronomy, including PROTOPLANETARY DISKS, numerically; the latter is what is actually observable. The ACCRETION DISKS, etc, not just DISK GALAXIES).Any stars formed two may be connected, as bursts of star formation are likely within the gaseous disk then inherit the same kinematics. to result from mergers of gas-rich PG fragments. Disks are dynamically fragile: major mergers would Assembly of protogalaxies disrupt their orderly rotation and convert them to random motions. Thus, any disk galaxy is unlikely to have had At every spatial (or mass) scale, there is a gradual, hierarchical merging of fragments into ever larger a major merger since its disk was formed, but accretion structures. This process operates first at the smaller of smaller satellites is still possible. Conversely, one scales, and moves to ever larger ones as the age of often hears assertions that (some) ellipticals are made by the universe increases and as the smaller fragments are merging spirals, which is indeed observed in the nearby consumed in mergers. While mergers of galaxies are universe. Obviously this is not the only possible path: relatively rare (yet readily observed) in the present-day a young elliptical may be made by merging a thousand universe, such processes must have been much more small fragments early on.