eaa.iop.orgDOI: 10.1888/0333750888/1821 Intergalactic MediumPiero Madau FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 23:42:34 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsIntergalactic MediumENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSAbout half a million years after the big bang, the ever-fading cosmic blackbody radiation cooled below 3000 Kand shifted first into the infrared and then into the radio,and the smooth baryonic plasma that filled the universebecame neutral. The universe then entered a ‘dark age’which persisted until the first cosmic structures col-lapsed into gravitationally bound systems and evolvedinto stars, galaxies and black holes that lit up the uni-verse again. Some time between REDSHIFTS of 7 and 15,stars within PROTOGALAXIES created the first heavy ele-ments; these systems, together perhaps with an earlypopulation of QUASARS, generated the ultraviolet radia-tion that reheated and reionized the cosmos. The historyof the universe during and soon after these crucial for-mative stages is recorded in the all-pervading intergalac-tic medium (IGM), which is believed to contain most ofthe ordinary baryonic material left over from the bigbang. Throughout the epoch of structure formation, theIGM became clumpy and acquired peculiar motionsunder the influence of gravity and acted as a source forthe gas that becomes accreted, cools and forms starswithin galaxies and as a sink for the metal-enrichedmaterial, energy and radiation which they eject.Observations of absorption lines in quasar spectra at red-shifts up to 5 provide invaluable insight into the chemi-cal composition of the IGM and the primordial densityfluctuation spectrum of some of the earliest formed cos-mological structures, as well as of the ultraviolet back-ground radiation that ionizes them.Cosmological reionizationAt epochs corresponding to z~1000, the IGM is expectedto recombine and remain neutral until sources of radia-tion develop that are capable of reionizing it. The detec-tion of transmitted flux shortward of the Lyα wavelengthin the spectra of sources at z~5 implies that the hydrogencomponent of this IGM was ionized at even higher red-shifts. There is some evidence that the double reioniza-tion of helium may have occurred later, but this is stillcontroversial. It appears then that substantial sources ofultraviolet photons were already present when the uni-verse was less than 7% of its current age, perhaps quasarsand/or young star-forming galaxies: an episode of pre-galactic STAR FORMATION may provide a possible explana-tion for the widespread existence of heavy elements(such as carbon, oxygen and silicon) in the IGM, whilethe integrated radiation emitted from quasars is proba-bly responsible for the reionization of intergalactic heli-um. Establishing the epoch of reionization and reheatingis crucial for determining its impact on several key cos-mological issues, from the role reionization plays inallowing protogalactic objects to cool and make stars todetermining the small-scale structure in the temperaturefluctuations of the cosmic background radiation.Conversely, probing the reionization epoch may providea means for constraining competing models for the for-mation of cosmic structures and for detecting the onset ofthe first generation of stars, galaxies and black holes inthe universe.Intergalactic hydrogen densityThe proper mean density of hydrogen nuclei at redshift zmay be expressed in standard cosmological terms aswhere Y is the primordial He abundance by mass,ρcrit=3H02/8πG is the critical density, Ωb=ρb/ρcritis thecurrent baryonic density parameter and H0=100h km s–1Mpc–1 is the present-day Hubble constant. StandardNUCLEOSYNTHESIS models together with recent observa-tions of deuterium yield Y=0.247±0.02 andΩbh2=0.0193±0.0014. Thus, As some of the baryons had already collapsed intogalaxies at z=2–5, the value of Ωbh2=0.019 should strictlybe considered as an upper limit to the intergalactic den-sity parameter.Because of the overwhelming abundance of hydro-gen, the IONIZATION of this element is of great importancefor determining the physical state of the IGM. Popularcosmological models predict that most of the intergalac-tic hydrogen was reionized by the first generation ofstars or quasars at z=7–15. The case that has received themost theoretical studies is one where hydrogen is ionizedby the absorption of photons, H+γ→p+e (as opposite tocollisional ionization H+e→p+e+e) shortward of 912 Å;that is, with energies exceeding 13.6 eV, the energy of theLyman edge. The process of reionization began as indi-vidual sources started to generate expanding H II REGIONSin the surrounding IGM; throughout an H II region, H isionized and He is either singly or doubly ionized. Asmore and more sources of ultraviolet radiation switchedon, the ionized volume grew in size. The reionizationended when the cosmological H II regions overlappedand filled the intergalactic space.Photoionization equilibriumAt every point in an optically thin, pure hydrogen medi-um of neutral density nHI, the photoionization rate perunit volume iswhere Jνis the mean intensity of the ionizing radiation(in energy units per unit area, time, solid angle and Intergalactic MediumCopyright © Nature Publishing Group 2002Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2002Dirac House, Temple Back, Bristol, BS21 6BE, UK1Intergalactic MediumENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSfrequency interval) and hPis the Planck constant. Thephotoionization cross-section for hydrogen in the groundstate by photons with energy hPν(above the thresholdhPνL=13.6 eV) can be usefully approximated by At equilibrium, this is balanced by the rate of radia-tive recombinations p+e→H+γ per unit volume, where neand npare the number densities of electronsand protons and αA=∑<σnve> is the radiative recombi-nation coefficient, i.e. the product of the electron capturecross-section σnand the electron velocity ve, averagedover a thermal distribution and summed over all atomiclevels n. At the commonly encountered gas temperatureof 104K, αA=4.2×10–13cm3s–1.Consider, as an illustrative example, a point in anintergalactic H II region at (say) z=6, with densityn–H=(1.6×10–7cm–3)(1+z)3=5.5×10–5cm–3. The H II