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Title: Science Case Reference: MUSE-MEM-SCI-052 Issue: 1.3 Date: 04/02/2004 Page: 25/100 global constraint that contributes little to the detailed time evolution of the reionization. Hydrogen must be essentially neutral over a substantial fraction of the z>6 universe, but the exact epoch is largely unconstrained. Using semi analytical and numerical models, the WMAP and QSO results can be put in concordance, if the universe experiences a very extended reionization history starting at z≈20 and ending at z≈6. However, the apparent high temperature of the IGM provides evidence that the energy associated with the reionization was invested at relatively late cosmic epochs (z6 are the ideal tests to further constrain the reionization history of the Universe because (i) they are the likely sources of ionizing UV photons and (ii) their abundance as a function of redshift directly probes the ionization state of the IGM. Even small fractions of neutral hydrogen are efficient in scattering Ly α photons in direction and frequency thus making faint emitters unobservable, unless the intrinsic line width is very large According to the simulations detailed in section 2.2 we expect 27 Ly α -emitters in a single MUSE deep field in the 6–6.7 redshift range. In the proposed survey of 5 deep fields (see section 2.12.1) we shall get 135 objects in total. By investigating their evolution, the reionization history can be probed near x HI ≈1, a regime that is poorly tested by Gunn- Peterson-trough measurements. Two main signatures that can be uniquely searched for with MUSE are the following: • If the neutral fraction falls considerably below levels of 1%, then the partly neutral IGM produces a Ly α damping wing that should absorb a significant part of the Ly α line, if the HII region produced by the galaxy around itself is not large enough to move the damping wing far away from the line center. Faint Ly α emitters should 'disappear' rapidly beyond the reionization redshift (Haiman & Spaans 1999), while the brighter sources would still be visible (Cen & Haiman 2002). The luminosity function and its redshift evolution of the Ly α emitters contain information on the neutral fraction and thus on reionization. Furthermore, any cosmic structure of larger than average density will induce infall of the IGM around it, i.e. the gas will “see” the source emission shifted to the blue, and absorbing red-ward of the Ly α line center; this effect may potentially wipe out most of the line. The optical depth owing to this effect increases roughly as (1+z) 4 (Haiman & Loeb 2002). • There should be characteristic imprints from a partially neutral IGM on the Ly α line shape. This effect is hard to observe in a single source, but can be measured if one has a statistical samples of ≥ 100 Ly α emitters (argued in Haiman 2003) as provided by the MUSE deep fields.
Title: Science Case Reference: MUSE-MEM-SCI-052 Issue: 1.3 Date: 04/02/2004 Page: 26/100 Figure 2-9 Left: The suppression of the total line flux relative to the unobscured line as a function of the star formation rate of a galaxy at z=6.56. Right: The suppression of the total line flux relative to the unobscured line as a function of line width. The top lines corresponds to a model with a proper HII region size of 0.7 Mpc (f esc =1), the bottom curve describes a Ly α line model without any HII region (from Haiman 2003). The second effect is illustrated in Fig. 2-9. Prior to reionization, the line profile correlates with the size of the local HII region and therefore with the luminosity and age of the source as well as with the intrinsic line profile. For example a line as narrow as ~30 km.s -1 would essentially be erased if the star formation rate is below 1 M sun yr -1 , but for line as broad as 300 km.s -1 , 20% of the total flux would be transmitted for arbitrarily faint sources. The detailed imprints of the reionization history cannot be disentangled in narrow-band Ly α surveys that only trace the brightest objects of the population, but require a survey that probes the number function as well as the line profiles, as provided by the MUSE deep fields. At the moment, theoretical models provide little quantitative constraints on the observable significance of these effects and to what extend they are clearly observable with MUSE. For example, current model estimates of the probability of Ly α photons to penetrate the z>6 IGM range from 0.001 to ~1, a fact that demonstrates the large uncertainties involved in these estimates. The fraction of Ly α photons that manage to escape the star-forming galaxy is likewise poorly constrained. Besides providing a detailed understanding of the reionization history of the universe, MUSE has the potential to shed light on the details of the star formation, the IMF, and the radiation transport in this first generation of galaxies. References Theuns et al. 2002, ApJ, 567, 103 Gnedin 2000, ApJ, 542, 535 Haiman & Spaans 1999, ApJ, 518, 138 Cen & Haiman 2002, ApJ, 570, 457 Haiman & Loeb 2002, ApJ, 576, 1 Haiman 2003, ApJ, 595, 1
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