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Determining the Redshift of Reionization from the Spectra Of CORE Metadata, citation and similar papers at core.ac.uk Provided by CERN Document Server Determining the Redshift of Reionization From the Sp ectra of High{Redshift Sources 1;2 1 Zoltan Haiman and Abraham Lo eb ABSTRACT The redshift at which the universe was reionized is currently unknown. We examine the optimal strategy for extracting this redshift, z , from the sp ectra of reion early sources. For a source lo cated at a redshift z beyond but close to reionization, s 32 (1 + z ) < (1 + z ) < (1 + z ), the Gunn{Peterson trough splits into disjoint reion s reion 27 Lyman , , and p ossibly higher Lyman series troughs, with some transmitted ux in b etween these troughs. We show that although the transmitted ux is suppressed considerably by the dense Ly forest after reionization, it is still detectable for suciently bright sources and can b e used to infer the reionization redshift. The Next Generation Space Telescop e will reach the sp ectroscopic sensitivity required for the detection of such sources. Subject headings: cosmology: theory { quasars: absorption lines { galaxies: formation {intergalactic medium { radiative transfer Submitted to The Astrophysical Journal, July 1998 1. Intro duction The standard Big Bang mo del predicts that the primeval plasma recombined and b ecame predominantly neutral as the universe co oled b elow a temp erature of several thousand degrees at 3 a redshift z 10 (Peebles 1968). Indeed, the recent detection of Cosmic Microwave Background (CMB) anisotropies rules out a fully ionized intergalactic medium (IGM) beyond z 300 (Scott, Silk & White 1995). However, the lack of a Gunn{Peterson trough (GP, Gunn & Peterson 1965) in the sp ectra of high{redshift quasars (Schneider, Schmidt & Gunn 1991) and galaxies (Franx et < al. 1997) implies that the IGM is highly ionized at low redshifts, z 5. Taken together, these < < two observations indicate that the IGM was reionized in the redshift interval 5 z 300. reion The epoch of reionization marks the end of the \dark ages" during which the cosmic radiation background was dominated by the ever-fading CMB at all wavelengths. During this ep o ch, ionized 1 Astronomy Department, Harvard University, 60 Garden Street, Cambridge, MA 02138, USA 2 NASA/Fermilab Astrophysics Center, Fermi National Accelerator Lab oratory,P.O. Box 500, Batavia, IL 60510, USA {2 { hydrogen (HI I) started to o ccupy again most of the volume of the universe. Most likely, the phase transition from HI to HI I was triggered by the emission from the rst stars and quasars in the universe (see Lo eb 1998, and references therein). The characteristic distance b etween the sources that ionized the universe was much smaller than the Hubble scale, and the overlap of the HI I regions surrounding these sources was completed on a timescale much shorter than the Hubble time. The resulting reionization redshift, z , reion is therefore sharply de ned and serves as an imp ortant milestone in the thermal history of the universe. Measurement of this parameter would also constrain the small-scale p ower of primordial density uctuations that pro duced the rst ob jects. Theoretical mo dels and three{dimensional < < simulations predict that reionization o ccurs at 7 z 20, just b eyond the horizon of current reion observations (Haiman & Lo eb 1997, 1998; Gnedin & Ostriker 1997). Forthcoming instruments, suchasthe Space Infrared Telescope Facility (SIRTF), and the Next Generation Space Telescope (NGST) are exp ected to reach the sensitivity required for the detection of sources beyond the reionization redshift. It is therefore timely to ask: how could one infer z from the observed reion sp ectra of these sources? The sp ectrum of a source at a redshift z > z should show a GP trough due to s reion absorption by the neutral IGM at wavelengths shorter than the lo cal Ly resonance at the source, < (1 + z ). By itself, the detection of sucha trough would not uniquely establish s obs the fact that the source is lo cated beyond z , since the lack of any observed ux could be reion equally caused by (i) ionized regions with some residual neutral fraction, (ii) individual damp ed Ly absorb ers, or (iii) line blanketing from lower column densityLy forest absorb ers (see, e.g. the sp ectrum of a z =5:34 galaxy taken by Dey et al. 1998). An alternative approach prop osed by Miralda-Escude (1998) is to study the detailed shap e of the damping wing of the GP trough. Although a measurement of this shap e could ideally determine the optical depth of the absorbing HI slab between the source redshift and reionization, it is also likely to be compromised by contaminating e ects such as: (i) redshift distortions due to p eculiar velo cities; (ii) uncertainties in the absorption pro le due to the unknown intrinsic shap e of the Ly emission line from the source; (iii) ionization of the IGM in the vicinity of the source due to the source emission; and (iv) the existence of a damp ed Ly absorb er outside or inside the source. In this pap er we consider a di erent approach for measuring z . Our metho d relies on the reion existence of some transmitted ux b etween the Ly and Ly absorption troughs in the sp ectra of sources which are lo cated just b eyond the reionization redshift. For such sources, the GP troughs due to di erent Lyman series resonances do not necessarily overlap, and the transmitted ux between these troughs is only a ected by the p ost-reionization Ly forest. In x2, we simulate the absorption by the dense Ly forest (or \Ly jungle") and demonstrate that the transmitted ux features are detectable. In x3, we estimate the numb er of sources which are suciently brightto allow detection of these features with NGST. Finally, x4 summarizes the main conclusions of this work. {3 { 2. Sp ectra of High Redshift Sources The phase transition from HI to HI I o ccurs almost simultaneously throughout the IGM volume, when the expanding HI I regions around the separate ionizing sources overlap (Arons & Wingert 1972). This pro cess is nearly instantaneous (Haiman & Lo eb 1998; HL98) and de nes a corresp onding redshift, z . However, at z each individual HI I region could still have overlap overlap a non{negligible Ly optical depth due to its residual HI fraction. The GP trough from the uniform IGM disapp ears only at a somewhat later redshift, z , when the average Ly optical reion depth of the smo oth IGM drops b elow unity. (We distinguish b etween this GP absorption and the absorption pro duced due to IGM clumpiness by the p ost-reionization Ly forest). In this pap er, we assume that the delaybetween z and z is small and set z = z , based on the reion reion overlap overlap following reasoning. 4 The Ly optical depth across a stationary HI I sphere is as large as 10 (Osterbro ck 1974), but cosmological HI I regions are much thinner b ecause of the cosmological redshifting of the Ly photons away from resonance. If z 7 and the HI I regions are driven by quasars or overlap 4 8 star{clusters in Cold Dark Matter halos of virial temp erature of 10 K(M 1:5 10 M ), halo then the typical prop er radius of an HI I region is R 0:05 Mp c (HL98). When two such HI I 5 HI I regions rst overlap, the neutral fraction x = n =n at their intersection is x 2 10 , HI H 3 assuming photoionization equilibrium . Taking this value as representative for the IGM, the resulting Ly optical depth of the smo oth IGM is 3. The GP trough disapp ears as so on as the neutral fraction is reduced by another factor of 3, i.e. when the lo cal UV ux is increased by the same factor. Such an increase o ccurs shortly after the overlap epoch, since it requires the cumulative contribution from sources in a region as small as 3 0:05 Mp c =0:15 Mp c, i.e. 0:1% the lo cal Hubble length. Therefore, the resulting breakthrough redshift, z is only reion slightly lower than z . The short redshift delay distorts the shap e of the damping wings at overlap < wavelengths extending by a few tenths of a p ercent ( tens of A) b elow the blue edge of the GP trough near the observed wavelength of (1 + z ). Our simplifying assumption of sudden reion ionization is therefore justi ed to this level of sp ectral resolution. Note that the Ly optical depth is 3.5 times b elow that of Ly , so that the Ly and higher GP troughs are even more sharply de ned. We now consider the sp ectrum of a source lo cated at a redshift z >z . Ly absorption s reion by neutral hydrogen between z and z suppresses the ux in the observed wavelength reion s interval, (1 + z ) < < (1 + z ) ; Ly absorption causes a second trough at reion s obs (1 + z ) < < (1 + z ) , and higher Lyman series lines pro duce analogous troughs at reion s obs shorter wavelengths. In the absence of other e ects, the resulting sp ectrum would consist of a generic sequence of troughs separated by blo cks of transmitted ux, as depicted in Figure 1. In 3 Note that the neutral fraction would b e 100 times higher near the edge of a steady-state Stromgren sphere. 6 < However, at redshifts z 10 the recombination time exceeds the Hubble time, and our short-lived ( 10 years) sources do not reach a steady state.
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