Lecture 27 The Intergalactic Medium
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1 Lecture 27 The Intergalactic Medium 1. Cosmological Scenario 2. The Ly Forest 3. Ionization of the Forest 4. The Gunn-Peterson Effect 5. Comment on HeII Reionization References J Miralda-Escude, Science M Rauch, ARAA A Loeb & R Barkana, ARAA X Fan et al. ARAA X Fan AJ ay216 1
2 1. Cosmological Scenario log t Now Big Bang 1 z The universe recombined at z ~ 1100 (300,000 yr) and was re-ionized much later (10-20 Gyr) by the radiation from the first stars and galaxies. J. Miralda-Escude, Science, 300, ay216 2
3 Cosmological Formulae Redux Concordance Cosmological Parameters i = i cr cr = 3H 2 0 8G = h gr cm 3 = + DM + B H 0 = h 100 km s 1 Mpc 1 h = 0.70 = 0.73 DM = 0.22 B = 0.04 = 0 (1 + z) t = t n (1 + z) 3 2 t n Gyr ay216 3
4 2. Lyman- Forest PKS , z = 1.34 PSS The huge number of Ly absorbers dominates the mass of the IGM: If Ñ is the number distribution function of Ly clouds and N is the column density of atomic hydrogen, then Ñ(N,z) has the following properties: Ñ(z) increases rapidly with z, roughly as (1+z) 2.5 Ñ(N) decreases rapidly with N, roughly as N -1.5 ay216 4
5 More on the Ly Forest 2. Since Ly Forest clouds are defined as optically thin to Lyman continuum photons (log N < 17.2), they are ionized, and most of their mass is not detected by Ly absorption. 3. Upper limits to the temperature come from the modest line widths, b D ~ km s -1 or T ~ 30,000 K.* 4. The IGM ionizing radiation field and ionization rate can be estimated from a proximity effect, i.e., from the variation in Ñ near quasars: J ~ erg cm -2 s -1 sr -1 Hz -1 G ~ s Photoionization-simulation models give x(h) ~ Flat structures for small columns, ~ 50 kpc thick and ~ Mpc wide are suggested by a few lensed quasars and quasar pairs. * b D = 22.5 km/s corresponds to FWHM = 53 km/s ay216 5
6 Comparison with the Diffuse ISM The Ly- forest arises from quasi-distinct features, although a smooth background distribution is not ruled out. Compare the IGM at z ~ 3 with the closest ISM phases*: WIM* n ~ cm -3 T ~ 8000 K HIM n ~ cm -3 T ~ 10 6 K IGM (z ~ 3) n ~ 10-5 cm -3 T ~ 3x10 4 K The thermal pressure of the IGM is tiny. The ISM gas of the Milky Way is bound by gravity and balanced in hydrostatic equilibrium with several sources of pressures. The IGM is governed largely by the gravity of CDM halos plus self-gravity and shocks in a highly clustered medium. *The numbers for the WIM refer to its denser parts (Lec08) ay216 6
7 3. Photoionization Modeling We provide the elementary basis for these models by considering the physical conditions at a typical location in a uniform and flat cosmological model. Of course, current understanding of the Ly forest is based on non-uniform structures that arose from primordial fluctuations and evolved dynamically according to the CDM cosmology (with the parameters given earlier). a. Post-recombination Density n B H = H + He = (1 Y ) m B H = H + Y B = (1 Y ) B m cr H ay216 7
8 n H ( 1 Y ) cr = B (1 + z) cm (1 z) m + H e.g., at recombination (z ~1000): n H ~ 200 cm -3 at reionization (z ~ 7): n H ~ 10-4 cm -3 b. Local Photoionization Balance The basic physics was covered in Lec03. The controlling parameter is the ionization parameter, in this case G/n H. Although surely variable, typical parameters for the IGM might be (see Sec. 3d below): G ~ s -1 T ~ 30,000 K x(h) ~ 10-5 With ~ 2 x cm 3 s -1, G n = (1 + z) 3 >>1 for z 10 ay216 8
9 This large value means that the photoionization time is much shorter than the recombination time, 1 1 ph = << rec = G ne and implies that the IGM is fully ionized: n e = n H. c. Atomic Hydrogen Abundance The chemical balance equations are: + + (H ) n n(h ) = G(H) n(h) n H = n(h + ) + n(h) e n e = n(h + ) + N(He Again using n e = n H, the abundance of H is: x(h) 1 + G nh 1 G nh 1 = 8 = (1 + ) + z) 3 n(he << 1 ++ ) ay216 9
10 d. The Ionization Flux of a Quasar The IGM radiation field is clearly basic for understanding the Ly- forest. It depends on cosmic time through the history of star, galaxy, and quasar formation. We give just one estimate, that due to a luminous quasar, analogous to those made for O & B stars in earlier lectures. For further discussion, see Loeb & Barkana, ARAA G 4J = d h ( ) 1( ) = 1( 1)( ) We use an empirical SED for quasars (Madau astro-ph ) with 1 the frequency at the Lyman edge L L 1 ( 1 )2 L 1 =10 30 erg s -1 Hz -1 4J L 4 r 2 ay216 10
11 The result for the ionization rate at Mpc distances is G = s -1 L L 1 Mpc ( ) r This is the same order of magnitude found in simulations (where G is assumed to be same everywhere). The problem is complicated because one has to include the contributions of all relevant quasars, especially those close to the line of sight and also do the radiative transfer. As for H II regions, one can evaluate the heating rate, or equivalently the mean photo-electron energy, as, 2 ay216 11
12 4. Gunn Peterson Effect Almost immediately after the discovery of quasars, Gunn and Peterson (ApJ ) considered the effect of a distribution of intervening atomic H on quasar spectra arising from the resonant scattering of Ly photons of the quasar continuum (shortward of Ly). Resonant scattering is closely related to the photo-absorption process responsible for the absorption lines; it refers to the photon emitted following absorption: h + H(n =1) H(n = 2) H(n =1) + h In the following diagram of the GP effect, (Q) = wavelength of the photon absorbed at redshift z (O) = the observed wavelength (Ly) = rest wavelength of the Lyman line ay216 12
13 O H Q redshift z = 0 z z = z em observer absorber/scatterer quasar The observed and emitted wavelengths satisfy the relations (1+z em )(Q) = (O) and (1+z) (Ly) = (O) or, (Q) = 1+ z 1+ z em (Ly) < (Ly) i.e., the absorbed continuum photon has a wavelength shorter than the Lyman line ay216 13
14 GP Optical Depth Gunn and Peterson (GP) calculate the observed absorption optical depth at frequency (O) in terms of the optical depth in the emitting frame where = (1+z)(O) : e 2 d = f 12 m e c [(1+ z)(o) v 12] n 1 (z)dl where is the lineshape function and 12 = (Ly). Since the range in z is very much larger than the linewidth, the integration is easily carried out on using dl dz = m c H 0 (1 + z) 5/2 Setting n 1 =n(h), the GP optical depth is e 2 GP = f m e c n(h) c (1 + z) 3 H 0 Note the dependence on f. ay
15 Estimate of the GP Optical Depth Combining this result with the expression for n H on slide 8, with n(h) = x(h) n H, and using the cosmological parameters on slide 3, yields: GP x(h) (1+ z) 3 2 Even for a small neutral H fraction ~ 10-4, the optical depth for z ~ 6 is large. Of course photoionization models show that x(h) is sensitive to the ionization rate G (as 1/G); GP is large from the recombination epoch until re-ionization. Although there had been evidence for a finite GP in the spectra of high-z quasars, truly convincing data only became available from the z ~ 6 quasars found by the Sloan Digital Sky Survey (SDSS). ay216 15
16 Onset of GP Troughs for High-z Quasars Pre-SDSS Q (z em =3.62) c.f, Rauch ARAA z = 5.82 z = 5.80 High red-shift quasar showing rich Ly forest plus absorption line systems for Ly, Si IV, and CIV, but no strong evidence for a GP trough between the lines. z = 5.99 z = 6.28 Q has no detectable flux short of Ly & Ly. High-z SDSS Quasars, Becker et al., ApJ ay216 16
17 Close Up of SDSS (z em ) Becker et al. (2001) Keck/ESI Ly spectrum Å Keck/ESI Ly spectrum Å GP ( Ly ) > 5 GP(Ly ) > 20 HI Reionization occurs at z ~ 6 or before ay216 17
18 Ly & Ly Troughs and Reionization Since GP ~ f, the Ly and Ly troughs are even more sensitive than Ly to the start of reionization. transition wavelength f lu Ly Ly Ly Consideration of the first ionizing stars (to be discussed in Lec28) leads to the idea that their HII regions grow and multiply, and merge, producing a clumpy universe. The development of the GP near z ~ 6 involves both an increase in the density of L forest lines but also the diminution of the ionizing flux. ay216 18
19 Growth of the GP Trough Towards z = Detailed examination of GP troughs indicates that GP varies with direction, suggestive of variations in the re- Ionizing radiation. ay216 19
20 Troughs for the Two Highest-z Quasars z = 6.28 z = 6.42 ay216 20
21 Transmitted Flux in the GP Trough Tiny fluxes for z 6 Songaila AJ ay216 21
22 Increase of GP Optical Depth With z Fan et al.aj ay216 22
23 HI Fraction for 19 High-z Quasars Fan et al.aj Lower limits to the HI fraction for z = Based on models of the ionizing radiation (dashed lines are numerical simulations) ay216 23
24 5. Reionization of He II Basic facts about He II: IP = ev (227.8 Å) E(Ly) = ev (303.8 Å) which imply: 1. Much harder UV radiation is needed to ionize He II than H I (maybe quasars rather than stars). 2. Detecting He II in high-z quasars requires UV observations, e.g., for HeII Ly is shifted to 1215Å for z = HST has UV capability down to 1150 Å and FUSE provides high resolution from Å. 4. Five examples known (as of 2005) He II Ly observations provides unique information about the spectrum of ionizing radiation in the IGM. ay216 24
25 He II in the IGM With FUSE detector gap Q z = Green: HST STIS spectrum Red: extrapolated STIS continuum He II reionization occurred near z = 3, i.e. later than H reionization Kriss et al. Science, (FUSE + STIS) See Zheng et al. ApJ 2004 (FUSE + VLT) for a reanalysis of He II Ly clouds in the spectrum of this quasar. ay216 25
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