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Lyman Alpha Forest Michael Rauch eaa.iop.org DOI: 10.1888/0333750888/2140 Lyman Alpha Forest Michael Rauch From Encyclopedia of Astronomy & Astrophysics P. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888 Institute of Physics Publishing Bristol and Philadelphia Downloaded on Thu Mar 02 23:49:43 GMT 2006 [131.215.103.76] Terms and Conditions Lyman Alpha Forest E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS Lyman Alpha Forest Lyα forest systems in this restricted definition, but we have theoretical reasons to believe that there is a genuine The Lyman alpha forest is an absorption phenomenon seen dichotomy between intergalactic gas (represented by the in the spectra of high redshift QSOs and galaxies (figure Lyα forest), even if partly polluted by metals, and the 1). It is the only direct observational evidence we have of invariably metal-enriched higher column density systems, the existence and properties of the general INTERGALACTIC thought to be related to galaxies. MEDIUM, and, as we have reason to believe, of most of the Lyα forest absorption systems have now been baryonic matter contents of the universe. observed from REDSHIFT zero (with UV satellites) up to the On its way to us the light of a bright, distant highest redshifts at which background light sources (QSOs QSO passes through intervening intergalactic gas and and galaxies) can still be found (currently z ∼ 5–6). through gas clouds associated with foreground galaxies. Absorption by the gas modifies the spectra of the The Gunn–Peterson effect: where does the background objects and imprints a record of the gas absorption come from? α clouds’ physical and chemical states on the observed Ly forest absorption in a QSO spectrum was predicted background QSO and galaxy spectra. The whole and first detected by Gunn and Peterson (1965). The arrangement is reminiscent of a giant cosmic slide basic idea is as follows: going back in time an increasing projector, where a QSO plays the role of the light bulb, fraction of the total baryonic mass of the universe must and the intervening gas clouds are the slides, changing the be in the form of gas. The absorption cross-section of α colors of the light source by absorbing parts of the (white) the Ly line of neutral hydrogen is large enough that spectrum. even if only a small fraction of the total mass of the α The name ‘Lyman α forest’ refers to the appearance universe were in the form of H I the redshifted Ly of the optical QSO spectra, which show a forest of lines should completely absorb a part of the spectrum hundreds of sharp absorption lines, mostly from the of any background light source. The absorption should neutral hydrogen (H I) Lyman α line, superimposed on the essentially assume the shape of an absorption trough α more smoothly varying QSO continuum (figure 2). Almost in a QSO spectrum, extending blueward from the Ly all the lines in the Lyman α forest correspond to the same emission of the QSO. This particular absorption pattern is atomic transition (which at 1215.67 Å is in the ultraviolet referred to in the literature as the ‘Gunn–Peterson effect’. wavelength region). The phenomenon was first observed Gunn and Peterson did detect such a trough but the light in the optical waveband (∼ 4000–9000Å) implying that the of the QSO was not completely absorbed and there was gas clouds causing the absorption are highly redshifted by some residual light left in the spectral region in question. the Hubble expansion. The absorption systems appear The relative weakness of the absorption could mean two spread out into a ‘forest’ of lines because each line is things: (a) there is little hydrogen left in intergalactic space α redshifted by a different amount in proportion to the and by the time the Ly forest is observed most of the absorbing cloud’s distance from us. The stronger ones matter has already condensed into galaxies. Or (b) most among the absorption systems do show further spectral of the hydrogen is not in neutral form, where it can produce α signatures in addition to their Lyα line: higher-order Ly absorption, but is fully ionized. Lyman series lines begin to be detectable for absorption QSO surveys (see QUASISTELLAR OBJECTS: SURVEYS) later systems with Lyα close to saturation. Clouds with H I showed that the second possibility is more important: column densities larger than N ∼ 1017 cm−2 start showing the combined ionizing radiation output from all known a discontinuity due to continuous absorption at a rest QSOs at high redshift amounts to a UV radiation field frame wavelength 912 Å, beyond the limit of the Lyman probably strong enough to keep most of the baryonic series. These ‘Lyman limit systems’ occupy a column matter in the universe highly ionized—i.e., if the baryons density regime where a gas cloud starts shielding itself are predominantly in the form of a more or less against ionizing radiation from the outside. Clouds with homogeneously distributed gas. even higher column densities (N>1019 cm−2) exhibit This conclusion is based on the assumption that the damping wings caused by the internal finite lifetime the gas is in approximate photoionization equilibrium of the Lyα transition. The gas in these ‘damped Lyα’ with the cosmic UV background field, i.e., the rate of recombinations of electrons with protons to form neutral systems (see LYMAN ALPHA ABSORPTION: THE DAMPED SYSTEMS)is hydrogen balances the rate of ionizations from the ground almost completely self-shielded and mostly neutral. Most . − state of H I, absorption systems with N>1014 5 cm 2 also show metal n n α(T ) = n . (1) absorption lines (triply ionized carbon and silicon, and e p HI some other common elements and ionization stages). For Here is the rate of photoionizations per neutral hydrogen that reason the higher column density systems are usually atom, caused by the hydrogen-ionizing portion of the UV n n n referred to as ‘metal’ or ‘heavy element’ systems. Here background field. The quantities e, HI and p refer to the we are concerned only with the low column density gas, number densities of electrons, neutral hydrogen atoms, i.e., those absorption systems where the Lyα line is not and protons (= ionized hydrogen), respectively. saturated, which we will refer to as the Lyα forest proper. Later observations of QSOs with higher spectral There definitely is an overlap between metal systems and resolution (< a few hundred km s−1) showed that 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 Lyman Alpha Forest E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS Figure 1. High resolution spectrum of the Lyα forest part of a redshift 3.63 QSO, taken with the HIRES spectrograph on the Keck 10 m telescope in Hawaii. The plot shows the flux of the QSO in arbitrary units versus the observed wavelength in units of Angstroms. The noise level can be judged from the longer wavelength wing of the broad, intrinsic Lyα emission line of the QSO (near 5650 Å). All of the ragged features are high redshift absorption lines. Most of the lines between the QSO’s Lyα and Lyβ emission lines (the humps at 5650 and 4750 Å) are due to Lyα absorption by intervening gas. The actual rate of incidence of absorbers decreases towards shorter wavelengths (lower redshifts). Nevertheless, the line density increases to the blue because higher order absorption lines from the Lyman series appear and overlap randomly with Lyα lines of systems at lower redshift. QSO’s flux I ∝ e−τ depends exponentially on the Lyα optical depth τ, which itself depends almost quadratically on the gas density (or the electron density ne; for a highly ionized gas at constant temperature). Thus small density fluctuations produce enhanced fluctuations in the optical depth. The Lyα forest absorption is observed in velocity space, and a convergent velocity field (e.g., a collapsing gas cloud) could also produce absorption ‘lines’. Caustics in velocity space may form if several gas volume elements are moving at the same velocity relative to the observer. Indeed, if Lyα clouds are produced by gravitational collapse, both overdense regions and infall should contribute to an absorption line. Spectroscopy of the Lyα line is an incredibly sensitive method to detect baryonic matter at any redshift. The photoionization cross section of neutral hydrogen is so large that an extremely tenuous gas at or below the Figure 2. Detailed section of the previous spectrum. The image mean density of the universe can be detected easily shows a number of absorption lines all corresponding to the in absorption. The method of choice for studying the α neutral hydrogen (H I) Ly 1215.67 Å transition. Lines close to Lyα forest is optical high resolution spectroscopy, with a saturation (= zero flux in the line center) have neutral hydrogen . − λ/λ > α column density typically around N ∼ 1014 2 cm 2, spectral resolution 30 000 sufficient to resolve Ly corresponding to a gas density enhanced by roughly an order of lines thermally broadened by the photoionization heating magnitude with respect to the mean density of the universe. The from the UV background. With 8 m class telescopes, a mean redshift of the stretch shown is z = 3.248. The spectral α − spectrum of a QSO suitable for further analysis of the Ly region extends over 3480 km s 1. For a flat = 1 universe this −1 forest absorption can be obtained within a few hours of corresponds to a spatial extent of approximately 9.6 h Mpc observing time.
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