Evolution of the Ionizing Background and the Epoch of Reionization from the Spectra of [ITAL][CLC]z[/CLC][/ITAL] ∼ 6 Quasars (original) (raw)
Related papers
Evolution of the Ionizing Background and the Epoch of Reionization from the Spectra of z~ 6 Quasars
We study the process of cosmic reionization and estimate the ionizing background in the intergalactic medium (IGM) using the Lyman series absorption in the spectra of the four quasars at 5.7 < z < 6.3 discovered by the Sloan Digital Sky Survey. We derive the redshift evolution of the ionizing background at high redshifts, using both semi-analytic techniques and cosmological simulations to model the density fluctuations in the IGM. The existence of the complete Lyα Gunn-Peterson trough in the spectrum of the z = 6.28 quasar SDSS 1030+0524 indicates a photoionization rate (Γ −12 in units of 10 −12 s −1 ) at z ∼ 6 lower than 0.08, at least a factor of 6 smaller than the value at z ∼ 3. The Lyβ and Lyγ Gunn-Peterson troughs give an even stronger limit Γ −12 ∼ < 0.02 due to their smaller oscillator strengths, indicating that the ionizing background in the IGM at z ∼ 6 is more than 20 times lower than that at z ∼ 3. Meanwhile, the volume-averaged neutral hydrogen fraction increases from 10 −5 at z ∼ 3 to > 10 −3 at z ∼ 6. At this redshift, the mass-averaged neutral hydrogen fraction is larger than 1%; the mildly overdense regions (δ > 3) are still mostly neutral and the comoving mean free path of ionizing photons is shorter than 8 Mpc. Comparison with simulations of cosmological reionization shows that the observed properties of the IGM at z ∼ 6 are typical of those in the era at the end of the overlap stage of reionization when the individual HII regions merge. Thus, z ∼ 6 marks the end of the reionization epoch. The redshift of reionization constrains the small scale power of the mass density fluctuations and the star forming efficiency of the first generation of objects.
We study the evolution of the ionization state of the intergalactic medium (IGM) at the end of the reionization epoch using moderate resolution spectra of a sample of nineteen quasars at 5.74 < z em < 6.42 discovered in the Sloan Digital Sky Survey. Three methods are used to trace IGM properties: (a) the evolution of the Gunn-Peterson (GP) optical depth in the Lyα, β, and γ transitions; (b) the distribution of lengths of dark absorption gaps, and (c) the size of HII regions around luminous quasars. Using this large sample, we find that the evolution of the ionization state of the IGM accelerated at z > 5.7: the GP optical depth evolution changes from τ eff GP ∼ (1 + z) 4.3 to (1 + z) ∼ >11 , and the average length of dark gaps with τ > 3.5 increases from < 10 to > 80 comoving Mpc. The dispersion of IGM properties along different lines of sight also increases rapidly, implying fluctuations by a factor of ∼ > 4 in the UV background at
Star forming galaxies at z≈6 and reionization
New Astronomy Reviews, 2006
We have determined the abundance of i ′-band drop-outs in the HST/ACS GOODS surveys and the Hubble Ultra Deep Field (UDF). The majority of these sources are likely to be z ≈ 6 galaxies whose flux decrement between the F775W i ′-band and F850LP z ′-band arises from Lyman-alpha absorption. We have shown with Keck/DEIMOS and Gemini/GMOS spectroscopy that this technique does indeed select high redshift galaxies, and we discovered Lyman-α emission in the expected redshift range for about a third of the galaxies with z ′ AB < 25.6 in the 150 arcmin 2 of the GOODS-South field. The i-drop number counts in the GOODS-North field are consistent, so cosmic variance is possibly not the dominant uncertainty. The increased depth of UDF enables us to reach a ∼ 10 σ limiting magnitude of z ′ AB = 28.5 (equivalent to 1.5 h −2 70 M ⊙ yr −1 at z = 6.1, or 0.1 L * U V for the z ≈ 3 U-drop population). The star formation rate at z ≈ 6 was approximately ×6 less than at z ≈ 3. This declining comoving star formation rate (0.005 h 70 M ⊙ yr −1 Mpc −3 at z ≈ 6 at L U V > 0.1 L * for a Salpeter IMF) poses an interesting challenge for models which suggest that L U V > 0.1 L * star forming galaxies at z ≃ 6 reionized the universe. The shortfall in ionizing photons might be alleviated by galaxies fainter than our limit, or a radically different IMF. Alternatively, the bulk of reionization might have occurred at z ≫ 6. We have recently discovered evidence of an early epoch of star formation in some of the i ′-drops at z ≈ 6. Spitzer images with IRAC at 3.6 − 4.5 µm show evidence of the age-sensitive Balmer/4000Å, dominated by stars older than 100 Myr (and most probably 400 Myr old). This pushes the formation epoch for these galaxies to z form = 7.5 − 13.5. There are at least some galaxies already assembled with stellar masses ≈ 3 × 10 10 M ⊙ (equivalent to 0.2 M * today) within the first billion years. The early formation of such systems may have played a key role in reionizing the Universe at z ∼ 10.
Cosmic reionization by stellar sources: Population II stars
Monthly Notices of the Royal Astronomical Society, 2003
We study the reionization of the Universe by stellar sources using a numerical approach that combines fast 3D radiative transfer calculations with high-resolution hydrodynamical simulations. By supplementing a one-step radiative transfer code specifically designed for following ionization processes with an adaptive ray-tracing algorithm, we are able to speed up the calculations significantly to the point where handling a vast number of sources becomes technically feasible. This allows us to study how dim low-mass sources, excluded in previous investigations owing to computational limitations, affect the morphological evolution of the reionization process. Ionizing fluxes for the sources are derived from intrinsic star formation rates computed in the underlying hydrodynamical simulations. Analysis of numerically converged results for star formation rates and halo mass functions allows us to assess the consequences of not including low-mass objects and enables us to correct for resolution effects. With these corrections, we are able to reduce the effective mass resolution limit for sources to M ∼ 4.0 × 10 7 h −1 M in a 10 h −1 Mpc comoving box. Our calculations reveal that the process by which ionized regions in the intergalactic medium (IGM) percolate is complex and is especially sensitive to the inclusion of dim sources. Moreover, we find that, given the same level of cosmic star formation, the number of ionizing photons required to reionize the Universe is significantly overestimated if sources with masses below ∼10 9 h −1 M are excluded. This result stems from the fact that low-mass sources preferentially reside in less clumpy environments than their massive counterparts. Consequently, their exclusion has the net effect of concentrating more of the cosmic ionizing radiation in regions which have higher recombination rates. We present the results of our reionization simulation assuming a range of escape fractions for ionizing photons and make statistical comparisons with observational constraints on the neutral fraction of hydrogen at z ∼ 6 derived from the z = 6.28 Sloan Digital Sky Survey (SDSS) quasar of Becker and coworkers. We find that, given the amplitude and form of the underlying star formation predictions, an escape fraction near f esc = 0.10-0.20 is most consistent with the observational results. In these models, reionization is expected to have occurred between z ∼ 7-8, although the IGM remains fairly opaque until z 6. Our method is also capable of handling the simultaneous reionization of the helium component in the IGM, allowing us to explore the plausibility of the scenario where sources with harder spectra are primarily responsible for reionization. In this case, we find that if the sources responsible for reionizing hydrogen by z ∼ 8 had spectra similar to active galactic nuclei, then the helium component of the IGM should have been reionized by z ∼ 6. We find that such an early reionization epoch for helium does not necessarily conflict with observational constraints obtained at z 3, but may be challenged by future observations at higher redshifts.
The Impact of Small‐Scale Structure on Cosmological Ionization Fronts and Reionization
The Astrophysical Journal, 2005
The propagation of cosmological ionization fronts during the reionization of the universe is strongly influenced by small-scale gas inhomogeneities due to structure formation. These inhomogeneities include both collapsed minihalos, which are generally self-shielding, and lower-density structures, which are not. The minihalos are dense and sufficiently optically-thick to trap intergalactic ionization fronts, blocking their path and robbing them of ionizing photons until the minihalo gas is expelled as an evaporative wind. The lower-density structures do not trap these fronts, but they can slow them down by increasing the overall recombination rate in the intergalactic medium (IGM). In this paper we study the effects of both types of inhomogeneities, including nonlinear clustering effects, and we find that both IGM clumping and collapsed minihalos have significant yet qualitatively different impacts on reionization. While the number density of minihalos on average increases strongly with time, the density of minihalos inside H II regions around ionizing sources is largely constant. Thus the impact of minihalos is essentially to decrease the number of ionizing photons available to the IGM at all epochs, which is equivalent to a reduction in the luminosity of each source. On the other hand, the effect of IGM clumping increases strongly with time, slowing down reionization and extending it. Thus while the impact of minihalos is largely degenerate with the unknown source efficiency, IGM clumping can help significantly in reconciling the recent observations of cosmic microwave background polarization with quasar absorption spectra at z ∼ 6, which together point to an early but extended reionization epoch. 1 2 FIG. 1.-Timescales. Hubble time t H (dashed line) and recombination time trec = (α B n H ) −1 at the mean IGM density (solid line) vs. redshift z (lower panel) and the ratio of these timescales (top panel).
Monthly Notices of the Royal Astronomical Society, 2016
HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
The Astronomical Journal, 2001
We present moderate resolution Keck spectroscopy of quasars at z = 5.82, 5.99 and 6.28, discovered by the Sloan Digital Sky Survey (SDSS). We find that the Lyα absorption in the spectra of these quasars evolves strongly with redshift. To z ∼ 5.7, the Lyα absorption evolves as expected from an extrapolation from lower redshifts. However, in the highest redshift object, SDSSp J103027.10+052455.0 (z = 6.28), the average transmitted flux is 0.0038 ± 0.0026 times that of the continuum level over 8450Å < λ < 8710Å (5.95 < z abs < 6.16), consistent with zero flux. Thus the flux level drops by a factor of > 150, and is consistent with zero flux in the Lyα forest region immediately blueward of the Lyα emission line, compared with a drop by a factor of ∼ 10 at z abs ∼ 5.3. A similar break is seen at Lyβ; because of the decreased oscillator strength of this transition, this allows us to put a considerably stronger limit, τ ef f > 20, on the optical depth to Lyα absorption at z = 6. This is a clear detection of a complete Gunn-Peterson trough, caused by neutral hydrogen in the intergalactic medium. Even a small neutral hydrogen fraction in the intergalactic medium would result in an undetectable flux in the Lyα forest region. Therefore, the existence of the Gunn-Peterson trough by itself does not indicate that the quasar is observed prior to the reionization epoch. However, the fast evolution of the mean absorption in these high-redshift quasars suggests that the mean ionizing background along the line of sight to this quasar has declined significantly from z ∼ 5 to 6, and the universe is approaching the reionization epoch at z ∼ 6.
A downturn in intergalactic C iv as redshift 6 is approached
2009
We present the results of the largest survey to date for intergalactic metals at redshifts z > 5, using near-IR spectra of nine QSOs with emission redshifts z em > 5.7. We detect three strong C IV doublets at z abs = 5.7-5.8, two low ionisation systems at z abs > 5, and numerous Mg II absorbers at z abs = 2.5-2.8. We find, for the first time, a change in the comoving mass density of C IV ions as we look back to redshifts z > 5. At a mean z = 5.76, we deduce Ω C IV = (4.4 ± 2.6) × 10 −9 which implies a drop by a factor of ∼ 3.5 compared to the value at z < 4.7, after accounting for the differing sensitivities of different surveys. The observed number of C IV doublets is also lower by a similar factor, compared to expectations for a non-evolving column density distribution of absorbers. These results point to a rapid build-up of intergalactic C IV over a period of only ∼ 300 Myr; such a build-up could reflect the accumulation of metals associated with the rising levels of star formation activity from z ∼ 9 indicated by galaxy counts, and/or an increasing degree of ionisation of the intergalactic medium (IGM), following the overlap of ionisation fronts from star-forming regions. If the value of Ω C IV we derive is typical of the IGM at large, it would imply a metallicity Z IGM > ∼ 10 −4 Z ⊙. The early-type stars responsible for synthesising these metals would have emitted only about one Lyman continuum photon per baryon prior to z = 5.8; such a background is insufficient to keep the IGM ionised and we speculate on possible factors which could make up the required shortfall.
Evidence for Reionization at z~ 6: Detection of a Gunn-Peterson Trough in az= 6.28 Quasar
Journal reference: Astron. J, 2001
We present moderate resolution Keck spectroscopy of quasars at z = 5.82, 5.99 and 6.28, discovered by the Sloan Digital Sky Survey (SDSS). We find that the Lyα absorption in the spectra of these quasars evolves strongly with redshift. To z ∼ 5.7, the Lyα absorption evolves as expected from an extrapolation from lower redshifts. However, in the highest redshift object, SDSSp J103027.10+052455.0 (z = 6.28), the average transmitted flux is 0.0038 ± 0.0026 times that of the continuum level over 8450Å < λ < 8710Å (5.95 < z abs < 6.16), consistent with zero flux. Thus the flux level drops by a factor of > 150, and is consistent with zero flux in the Lyα forest region immediately blueward of the Lyα emission line, compared with a drop by a factor of ∼ 10 at z abs ∼ 5.3. A similar break is seen at Lyβ; because of the decreased oscillator strength of this transition, this allows us to put a considerably stronger limit, τ ef f > 20, on the optical depth to Lyα absorption at z = 6. This is a clear detection of a complete Gunn-Peterson trough, caused by neutral hydrogen in the intergalactic medium. Even a small neutral hydrogen fraction in the intergalactic medium would result in an undetectable flux in the Lyα forest region. Therefore, the existence of the Gunn-Peterson trough by itself does not indicate that the quasar is observed prior to the reionization epoch. However, the fast evolution of the mean absorption in these high-redshift quasars suggests that the mean ionizing background along the line of sight to this quasar has declined significantly from z ∼ 5 to 6, and the universe is approaching the reionization epoch at z ∼ 6.
The Inhomogeneous Reionization Times of Present-day Galaxies
The astrophysical journal, 2018
Today's galaxies experienced cosmic reionization at different times in different locations. For the first time, reionization (50% ionized) redshifts, z R , at the location of their progenitors are derived from new, fully coupled radiation-hydrodynamics simulation of galaxy formation and reionization at z > 6, matched to N-body simulation to z = 0. Constrained initial conditions were chosen to form the wellknown structures of the local universe, including the Local Group and Virgo, in a (91 Mpc)3 volume large enough to model both global and local reionization. Reionization simulation CoDa I-AMR, by CPU-GPU code EMMA, used (2048)3 particles and (2048)3 initial cells, adaptively refined, while N-body simulation CoDa I-DM2048, by Gadget2, used (2048)3 particles, to find reionization times for all galaxies at z = 0 with masses M(z = 0) 108 M. Galaxies with M (z = 0) ≳ 10 11 M ⊙ reionized earlier than the universe as a whole, by up to 500 Myr, with significant scatter. For Milky Way-like galaxies, z R ranged from 8 to 15. Galaxies with M (z = 0) ≲ 10 11 M ⊙ typically reionized as late or later than globally averaged 50% reionization at ⟨z R ⟩ = 7.8, in neighborhoods where reionization was completed by external radiation. The spread of reionization times within galaxies was sometimes as large as the galaxy-to-galaxy scatter. The Milky Way and M31 reionized earlier than global reionization but later than typical for their mass, neither dominated by external radiation. Their most-massive progenitors at z > 6 had z R =9.8 (MW) and 11 (M31), while their total masses had z R = 8.2 (both).