ProtogalaxiesENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSProtogalaxiesThe term protogalaxies or sometimes primeval galaxies(hereafter PGs) has been used in the literature with arange of different meanings, but a general definition maybe something like this: progenitors of the present-day(normal) galaxies, in the early stages of formation; also,many, but not all, authors would also add to that the words‘at high redshifts’. The key phrase is ‘the early stages offormation’, which is fundamentally not well defined.The ambiguities are mainly due to the fact thatour understanding of whatGALAXY FORMATION means hasbeen evolving over the past few decades. Moreover,even at any given time, experts of different technicalorientations (e.g. observers versus n-body simulators)who would otherwise agree about the general pictureof galaxy formation would have different and equallylegitimate definitions in mind. For example, an observermay mean ‘the first major burst of star formation in aprogenitor of a present-day elliptical galaxy’, whereas atheorist may mean ‘the peak merging epoch of dark halosof the fragments which assemble to produce an averagegalaxy today’; others would define a PG as a still gaseousbody before anySTAR FORMATION has taken place or as anoverdense region ofDARK MATTER in the very early universe,destined to become gravitationally bound and to collapse.In some sense all of them are right.It may be useful here to offer a brief account ofour present understanding of galaxy formation and thendescribe some of the relevant observations, thus definingPGs in an implicit way. While the subject is still evolvingrapidly as of this writing (late 1999), most cosmologistswould agree that the basic picture we now have is likelyto be essentially correct. Instead of seeking a hypotheticalmagic epoch when PGs appear and then evolve into thefamiliar types of galaxies today, the focus is now onunderstanding of the complete history of galaxy and large-scale structure assembly, as well as the history of starformation in all galaxies and the chemical enrichment ofgas in the universe at large. It is then almost a matterof taste which slice of formative history would be calledthe epoch of galaxy formation and which objects or whichevolutionary stages of normal galaxies would be calledPGs.In what follows, we outline a broad-brush pictureand give general arguments and order-of-magnitudeestimates, which are likely to be at least roughly correct,however much our understanding of galaxy formationevolves in the forthcoming years. Ever more detailedmodels will be producing more precise predictions, whichcan then be tested by future observations.From density fluctuations to protogalaxiesThe basic paradigm of structure formation is that ittakes place through the gravitational instability of bound,overdense regions in the early universe. The distributionof such regions in mass (or, equivalently, density) isquantified by the initial density perturbation spectrum,which is produced by some type of quantum fluctuationprocesses in the early universe. Sufficiently overdenseregions separate from the universal expansion andcollapse under their own self-gravity, roughly on the free-fall time scale:tff=π28G1/2R3/2initM−1/2≈ 5 × 108yr ×Rinit100 kpc3/2M1012M−1/2≈ 1.6 × 109yr ×Rinit10 Mpc3/2M1015M−1/2where Rinitis the initial radius of the bound region andM is the enclosed mass. It can be seen that for a galacticor subgalactic mass fragment the corresponding cosmicepochs (∼108yr) imply high REDSHIFTS of collapse, whereasCLUSTERS OF GALAXIES or larger structures may be stillcollapsing today. The free-fall time scale is a lower limitto the formation time scale, its duration and the cosmicepoch: in practice, it may take several free-fall times for agiven mass overdensity to be assembled through a processof hierarchical merging and virialization. To a first order,this simple argument implies that the peak epoch of galaxyformation is likely to be found at the cosmic epochs of afew× 108to a few× 109yr, or roughly in the redshift rangez ∼ 2–20 or so, whereas the epoch of cluster formationlasts many billions of years and is still going on now.CollapseSince the non-baryonic dark matter appears to dominatethetotal mass, the overdenseregions, i.e. proto-dark-halos,can start collapsing even while the universe is still ionized,e.g. at z ∼ 104, with the baryonic matter following. Thesmallest mass fluctuations collapse the fastest; however,they are also most readily erased by a variety of dampingprocesses operating in the early universe, such as thestreaming of matter and photons, sound waves, etc. Itis now believed that the smallest structures which surviveat the recombination epoch are similar in mass toGLOBULARCLUSTERSor DWARF GALAXIES, i.e. (105–108)M. They may bethe basic building blocks of galaxies.Images of theCOSMIC MICROWAVE BACKGROUND (CMB)photosphere show a snapshot of overdense regions atthe recombination epoch at z ∼ 1100. At the presenttime, the resolution of such observations correspondsto physical scales of large clusters and superclusters ofgalaxies rather than galaxies themselves. Nevertheless,these observations support the basic picture of structureformation via gravitational instability.Any energy dissipation in the baryonic componentof the mass (also called the cooling of PGs) would ofcourse accelerate the collapse and lead to the formationof denser objects, since systems which dissipate energy ata fixed mass become more tightly bound. The distinctionbetween galaxies and large-scale structures such as galaxygroups or clusters (or larger) is that the galaxies (orCopyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001Dirac House, Temple Back, Bristol, BS1 6BE, UK1ProtogalaxiesENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICStheir protogalactic fragments) can cool faster than thefree-fall time. Physical mechanisms which enable thisenergy dissipation in collapsing PGs include the inverse-Compton cooling of hot gas on the CMB radiation (CMBR)photons and shocks in the infalling and colliding gasclouds.Thus galaxies (or their building blocks) becomedistinct concentrations within the overall large-scalestructure, whose evolution is dominated by the gravity ofthe dark matter. Simple arguments based on the meandensity of galaxies, the spin-up of