首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 406 毫秒
1.
The low-density hydrogen and helium in the intergalactic medium (IGM) probed by quasi-stellar object (QSO) absorption lines is sensitive to the amplitude and spectral shape of the metagalactic ultraviolet (UV) background. We use realistic H  i and He  ii Lyα forest spectra, constructed from state-of-the-art hydrodynamical simulations of a Λ cold dark matter (ΛCDM) universe to confirm the reliability of using line profile fitting techniques to infer the ratio of the metagalactic H  i and He  ii ionization rates. We further show that the large spatial variations and the anticorrelation with H  i absorber density observed in the ratio of the measured He  ii to H  i column densities can be explained in a model where the H  i ionization rate is dominated by the combined UV emission from young star-forming galaxies and QSOs and the He  ii ionization rate is dominated by emission from QSOs only. In such a model the large fluctuations in the column density ratio are due to the small number of QSOs expected to contribute at any given point to the He  ii ionization rate. A significant contribution to UV emission at the He  ii photoelectric edge from hot gas in galaxies and galaxy groups would decrease the expected fluctuations in the column density ratio. Consequently, this model appears difficult to reconcile with the large increase in He  ii opacity fluctuations towards higher redshift. Our results further strengthen previous suggestions that observed He  ii Lyα forest spectra at z ∼ 2–3.5 probe the tail end of the reionization of He  ii by QSOs.  相似文献   

2.
We measure the matter power spectrum from 31 Lyα spectra spanning the redshift range of 1.6–3.6. The optical depth, τ, for Lyα absorption of the intergalactic medium is obtained from the flux using the inversion method of Nusser & Haehnelt. The optical depth is converted to density by using a simple power-law relation,  τ∝ (1 +δ)α  . The non-linear 1D power spectrum of the gas density is then inferred with a method that makes simultaneous use of the one- and two-point statistics of the flux and compared against theoretical models with a likelihood analysis. A cold dark matter model with standard cosmological parameters fits the data well. The power-spectrum amplitude is measured to be (assuming a flat Universe),  σ8= (0.92 ± 0.09) × (Ωm/0.3)−0.3  , with α varying in the range of 1.56–1.8 with redshift. Enforcing the same cosmological parameters in all four redshift bins, the likelihood analysis suggests some evolution in the temperature–density relation and the thermal smoothing length of the gas. The inferred evolution is consistent with that expected if reionization of He  ii occurred at   z ∼ 3.2  . A joint analysis with the Wilkinson Microwave Anisotropy Probe results together with a prior on the Hubble constant as suggested by the Hubble Space Telescope key project data, yields values of Ωm and σ8 that are consistent with the cosmological concordance model. We also perform a further inversion to obtain the linear 3D power spectrum of the matter density fluctuations.  相似文献   

3.
We show that near-infrared observations of the red side of the Lyα line from a single gamma-ray burst (GRB) afterglow cannot be used to constrain the global neutral fraction of the intergalactic medium (IGM),     , at the GRB's redshift to better than     . Some GRB sightlines will encounter more neutral hydrogen than others at fixed     owing to the patchiness of reionization. GRBs during the epoch of reionization will often bear no discernible signature of a neutral IGM in their afterglow spectra. We discuss the constraints on     from the   z = 6.3  burst, GRB050904, and quantify the probability of detecting a neutral IGM using future spectroscopic observations of high-redshift, near-infrared GRB afterglows. Assuming an observation with signal-to-noise ratio similar to the Subaru FOCAS spectrum of GRB050904 and that the column density distribution of damped Lyα absorbers is the same as measured at lower redshifts, a GRB from an epoch when     can be used to detect a partly neutral IGM at 97 per cent confidence level ≈10 per cent of the time (and, for an observation with three times the sensitivity, ≈30 per cent of the time).  相似文献   

4.
At redshifts z ≳2, most of the baryons reside in the smooth intergalactic medium which is responsible for the low column density Ly α forest. This photoheated gas follows a tight temperature–density relation which introduces a cut-off in the distribution of widths of the Ly α absorption lines ( b -parameters) as a function of column density. We have measured this cut-off in a sample of nine high-resolution, high signal-to-noise ratio quasar spectra and determined the thermal evolution of the intergalactic medium in the redshift range 2.0–4.5. At a redshift z ∼3, the temperature at the mean density shows a peak and the gas becomes nearly isothermal. We interpret this as evidence for the reionization of He  ii .  相似文献   

5.
We use high-quality echelle spectra of 24 quasi-stellar objects to provide a calibrated measurement of the total amount of Lyα forest absorption (DA) over the redshift range  2.2 < z < 3.2  . Our measurement of DA excludes absorption from metal lines or the Lyα lines of Lyman-limit systems and damped Lyα systems. We use artificial spectra with realistic flux calibration errors to show that we are able to place continuum levels that are accurate to better than 1 per cent. When we combine our results with our previous results between  1.6 < z < 2.2  , we find that the redshift evolution of DA is well described over  1.6 < z < 3.2  as   A (1 + z )γ  , where   A = 0.0062  and  γ= 2.75  . We detect no significant deviations from a smooth power-law evolution over the redshift range studied. We find less H  i absorption than expected at   z = 3  , implying that the ultraviolet background is  ∼40  per cent higher than expected. Our data appears to be consistent with an H  i ionization rate of  Γ∼ 1.4 × 10−12 s−1  .  相似文献   

6.
Using cosmological hydrodynamic simulations, we measure the mean transmitted flux in the Lyα forest for quasar sightlines that pass near a foreground quasar. We find that the trend of absorption with pixel quasar separation distance can be fitted using a simple power-law form including the usual correlation function parameters r 0 and γ, so that     . From the simulations, we find the relation between r 0 and quasar host mass, and formulate this as a way to estimate quasar host dark matter halo masses, quantifying uncertainties due to cosmological and IGM parameters, and redshift errors. With this method, we examine data for ∼9000 quasars from the Sloan Digital Sky Survey (SDSS) Data Release 5, assuming that the effect of ionizing radiation from quasars (the so-called transverse proximity effect) is unimportant (no evidence for it is seen in the data). We find that the best-fitting host halo mass for SDSS quasars with mean redshift z = 3 and absolute G -band magnitude −27.5 is  log  M /M⊙= 12.68+0.81−0.67  . We also use the Lyman-Break Galaxy (LBG) and Lyα forest data of Adelberger et al. in a similar fashion to constrain the halo mass of LBGs to be  log10  M /M⊙= 11.41+0.54−0.59  , a factor of ∼20 lower than the bright quasars. In addition, we study the redshift distortions of the Lyα forest around quasars, using the simulations. We use the quadrupole to monopole ratio of the quasar Lyα forest correlation function as a measure of the squashing effect. We find its dependence on halo mass difficult to measure, but find that it may be useful for constraining cosmic geometry.  相似文献   

7.
One well-known way to constrain the hydrogen neutral fraction,     , of the high-redshift intergalactic medium (IGM) is through the shape of the red damping wing of the Lyα absorption line. We examine this method's effectiveness in light of recent models showing that the IGM neutral fraction is highly inhomogeneous on large scales during reionization. Using both analytic models and 'seminumeric' simulations, we show that the 'picket-fence' absorption typical in reionization models introduces both scatter and a systematic bias to the measurement of     . In particular, we show that simple fits to the damping wing tend to overestimate the true neutral fraction in a partially ionized universe, with a fractional error of ∼30 per cent near the middle of reionization. This bias is generic to any inhomogeneous model. However, the bias is reduced and can even underestimate     if the observational sample only probes a subset of the entire halo population, such as quasars with large H  ii regions. We also find that the damping wing absorption profile is generally steeper than one would naively expect in a homogeneously ionized universe. The profile steepens and the sightline-to-sightline scatter increases as reionization progresses. Of course, the bias and scatter also depend on     and so can, at least in principle, be used to constrain it. Damping wing constraints must therefore be interpreted by comparison to theoretical models of inhomogeneous reionization.  相似文献   

8.
We present detailed predictions for the properties of Lyα-emitting galaxies in the framework of the Λ cold dark matter cosmology, calculated using the semi-analytical galaxy formation model galform . We explore a model that assumes a top-heavy initial mass function in starbursts and that has previously been shown to explain the sub-millimetre number counts and the luminosity function of Lyman-break galaxies at high redshift. We show that this model, with the simple assumption that a fixed fraction of Lyα photons escape from each galaxy, is remarkably successful at explaining the observed luminosity function of Lyα emitters (LAEs) over the redshift range  3 < z < 6.6  . We also examine the distribution of Lyα equivalent widths and the broad-band continuum magnitudes of emitters, which are in good agreement with the available observations. We look more deeply into the nature of LAEs, presenting predictions for fundamental properties such as the stellar mass and radius of the emitting galaxy and the mass of the host dark matter halo. The model predicts that the clustering of LAEs at high redshifts should be strongly biased relative to the dark matter, in agreement with observational estimates. We also present predictions for the luminosity function of LAEs at   z > 7  , a redshift range that is starting to be be probed by near-infrared surveys and using new instruments such as the Dark Ages Z Lyman Explorer (DAzLE).  相似文献   

9.
The 21-cm forest     
We examine the prospects for studying the pre-reionization intergalactic medium (IGM) through the so-called 21-cm forest in spectra of bright high-redshift radio sources. We first compute the evolution of the mean optical depth τ for models that include X-ray heating of the IGM gas, Wouthuysen–Field coupling, and reionization. Under most circumstances, the spin temperature T S grows large well before reionization begins in earnest; this occurs so long as the X-ray luminosity of high-redshift starbursts (per unit star formation rate) is comparable to that in nearby galaxies. As a result,  τ≲ 10−3  throughout most of reionization, and background sources must sit well beyond the reionization surface in order to experience absorption that is measurable by square-kilometre class telescopes. H  ii regions produce relatively large 'transmission gaps' and may therefore still be observable during the early stages of reionization. Absorption from sheets and filaments in the cosmic web fades once T S becomes large and should be rare during reionization. Minihaloes can produce strong (albeit narrow) absorption features. Measuring their abundance would yield useful limits on the strength of feedback processes in the IGM as well as their effect on reionization.  相似文献   

10.
Many models of early structure formation predict a period of heating immediately preceding reionization, when X-rays raise the gas temperature above that of the cosmic microwave background. These X-rays are often assumed to heat the intergalactic medium (IGM) uniformly, but in reality will heat the gas more strongly closer to the sources. We develop a framework for calculating fluctuations in the 21-cm brightness temperature that originate from this spatial variation in the heating rate. High-redshift sources are highly clustered, leading to significant gas temperature fluctuations (with fractional variations ∼40 per cent, peaking on   k ∼ 0.1 Mpc−1  scales). This induces a distinctive peak-trough structure in the angle-averaged 21-cm power spectrum, which may be accessible to the proposed Square Kilometre Array. This signal reaches the ∼10 mK level, and is stronger than that induced by Lyα flux fluctuations. As well as probing the thermal evolution of the IGM before reionization, this 21-cm signal contains information about the spectra of the first X-ray sources. Finally, we consider disentangling temperature, density and Lyα flux fluctuations as functions of redshift.  相似文献   

11.
We present the results of a continuing survey to detect Lyα emitting galaxies at redshifts   z ∼ 9  : the ' z equals nine' (ZEN) survey. We have obtained deep VLT Infrared Spectrometer and Array Camera observations in the narrow J -band filter NB119 directed towards three massive lensing clusters: Abell clusters 1689, 1835 and 114. The foreground clusters provide a magnified view of the distant Universe and permit a sensitive test for the presence of very high redshift galaxies. We search for   z ∼ 9 Lyα  emitting galaxies displaying a significant narrow-band excess relative to accompanying J -band observations that remain undetected in Hubble Space Telescope ( HST )/Advanced Camera for Surveys (ACS) optical images of each field. No sources consistent with this criterion are detected above the unlensed 90 per cent point-source flux limit of the narrow-band image,   F NB= 3.7 × 10−18 erg s−1 cm−2  . To date, the total coverage of the ZEN survey has sampled a volume at   z ∼ 9  of approximately 1700 comoving Mpc3 to a Lyα emission luminosity of  1043 erg s−1  . We conclude by considering the prospects for detecting   z ∼ 9 Lyα  emitting galaxies in light of both observed galaxy properties at   z < 7  and simulated populations at   z > 7  .  相似文献   

12.
We show that spatial correlations in a stochastic large-scale velocity field in an otherwise smooth intergalactic medium (homogeneous comoving density) superposed on the general Hubble flow may cause a 'line-like' structure in QSO spectra similar to the population of unsaturated Lyα forest lines which usually are attributed to individual clouds with 1011 ≲ N H i  5 × 1013 cm−2. Therefore there is no clear observational distinction between a diffuse intergalactic medium and discrete intergalactic clouds. It follows that the H  i density in the diffuse intergalactic medium might be substantially underestimated if it is determined from the observed intensity distribution near the apparent continuum in high-resolution spectra of QSOs. Our tentative estimate implies a diffuse neutral hydrogen opacity τGP ∼ 0.3 at z  ∼ 3 and a current baryon density ΩIGM ≃ 0.08, assuming a Hubble constant H 0 = 70 km s−1 Mpc−1.  相似文献   

13.
We present optical and near-infrared spectroscopic observations of the optical Einstein ring 0047 – 2808. We detect both [O III ] lines λλ4959, 5007 near ∼ 2.3 μm, confirming the redshift of the lensed source as z  = 3.595. The Lyα line is redshifted relative to the [O III ] line by 140 ± 20 km s−1. Similar velocity shifts have been seen in nearby starburst galaxies. The [O III ] line is very narrow, 130 km s−1 FWHM. If the ring is the image of the centre of a galaxy, the one-dimensional stellar velocity dispersion σ = 55 km s−1 is considerably smaller than the value predicted by Baugh et al. for the somewhat brighter Lyman-break galaxies. The Lyα line is significantly broader than the [O III ] line, probably due to resonant scattering. The stellar central velocity dispersion of the early-type deflector galaxy at z  = 0.485 is 250 ± 30 km s−1. This value is in good agreement both with the value predicted from the radius of the Einstein ring (and a singular isothermal sphere model for the deflector), and with the value estimated from the D n −σ relation.  相似文献   

14.
The galaxy population at   z ≈ 6  has been the subject of intense study in recent years, culminating in the Hubble Ultra Deep Field (HUDF) – the deepest imaging survey yet. A large number of high-redshift galaxy candidates have been identified within the HUDF, but until now analysis of their properties has been hampered by the difficulty of obtaining spectroscopic redshifts for these faint galaxies. Our 'Gemini Lyman-Alpha at Reionization Era' (GLARE) project has been designed to undertake spectroscopic follow-up of faint  ( z ' < 28.5)  i '-drop galaxies at   z ≈ 6  in the HUDF. In a previous paper we presented preliminary results from the first 7.5 h of data from GLARE. In this paper we detail the complete survey. We have now obtained 36 h of spectroscopy on a single GMOS slitmask from Gemini-South, with a spectral resolution of  λ/ΔλFWHM≈ 1000  . We identify five strong Lyα emitters at   z > 5.5  , and a further nine possible line emitters with detections at lower significance. We also place tight constraints on the equivalent width of Lyα emission for a further ten i '-drop galaxies and examine the equivalent width distribution of this faint spectroscopic sample of   z ≈ 6  galaxies. We find that the fraction of galaxies with little or no emission is similar to that at   z ≈ 3  , but that the   z ≈ 6  population has a tail of sources with high rest-frame equivalent widths. Possible explanations for this effect include a tendency towards stronger line emission in faint sources, which may arise from extreme youth or low metallicity in the Lyman-break population at high redshift, or possibly a top-heavy initial mass function.  相似文献   

15.
We study the reionization of He  ii by quasars using a numerical approach that combines 3D radiative transfer calculations with cosmological hydrodynamical simulations. Sources producing the ionizing radiation are selected according to an empirical quasar luminosity function, and are assigned luminosities according to their intrinsic masses. The free parameters associated with this procedure are (1) a universal source lifetime, (2) a minimum mass cut-off, (3) a minimum luminosity cut-off, (4) a solid angle specifying the extent to which radiation is beamed, and (5) a tail-end spectral index for the radiative energy distribution of the sources. We present models in which these parameters are varied, and examine characteristics of the resultant reionization process that distinguish the various cases. In addition, we extract artificial spectra from the simulations and quantify statistical properties of the spectral features in each model.
We find that the most important factor affecting the evolution of He  ii reionization is the cumulative number of ionizing photons that are produced by the sources. Comparisons between He  ii opacities measured observationally and those obtained by our analysis reveal that the available ranges in plausible values for the parameters provide enough leeway to provide a satisfactory match. However, one property common to all our calculations is that the epoch of He  ii reionization must have occurred at a redshift in the range  3≲ z ≲4  . If so, future observational programmes will be able to directly trace the details of the ionization history of helium and to probe the low-density phase of the intergalactic medium during this phase of the evolution of the Universe.  相似文献   

16.
We study cosmic microwave background (CMB) secondary anisotropies produced by inhomogeneous reionization by means of cosmological simulations coupled with the radiative transfer code crash . The reionization history is consistent with the Wilkinson Microwave Anisotropy Probe Thomson optical depth determination. We find that the signal arising from this process dominates over the primary CMB component for   l ≳ 4000  and reaches a maximum amplitude of   l ( l + 1) Cl /2π≃ 1.6 × 10−13  on arcmin scales (i.e. l as large as several thousands). We then cross-correlate secondary CMB anisotropy maps with neutral hydrogen 21-cm line emission fluctuations obtained from the same simulations. The two signals are highly anticorrelated on angular scales corresponding to the typical size of H  ii regions (including overlapping) at the 21-cm map redshift. We show how the CMB/21-cm cross-correlation can be used: (i) to study the nature of the reionization sources; (ii) to reconstruct the cosmic reionization history; (iii) to infer the mean cosmic ionization level at any redshift. We discuss the feasibility of the proposed experiment with forthcoming facilities.  相似文献   

17.
The z  = 2.286  IRAS galaxy F10214 + 4724 remains one of the most luminous galaxies in the Universe, despite its gravitational lens magnification. We present optical and near-infrared spectra of F10214 + 4724, with clear evidence for three distinct components: lines of width ∼ 1000 km s−1 from a Seyfert 2 nucleus; ≲ 200 km s−1 lines which are likely to be associated with star formation; and a broad (∼ 4000 km s−1) C  III ] 1909-Å emission line which is blueshifted by ∼ 1000 km s−1 with respect to the Seyfert 2 lines. Our study of the Seyfert 2 component leads to several new results. (i) From the double-peaked structure in the Lyα line, and the lack of Lyβ, we argue that the Lyα photons have emerged through a neutral column of N H ∼ 2.5 × 1025 m−2, possibly located within the AGN narrow-line region, as proposed for several high-redshift radio galaxies. (ii) The resonant O  VI 1032, 1036-Å doublet (previously identified as Lyβ) is in an optically thick (1:1) ratio. At face value this implies an extreme density ( n e ∼ 1017 m−3) more typical of broad-line region clouds. However, we attribute this instead to the damping wings of Lyβ from the resonant absorption. (iii) A tentative detection of He  II 1086 suggests little extinction in the rest frame ultraviolet.  相似文献   

18.
The introduction of low-frequency radio arrays is expected to revolutionize the study of the reionization epoch. Observation of the contrast in redshifted 21-cm emission between a large H  ii region and the surrounding neutral intergalactic medium (IGM) will be the simplest and most easily interpreted signature. However, the highest redshift quasars known are thought to reside in an ionized IGM. Using a semi-analytic model we describe the redshifted 21-cm signal from the IGM surrounding quasars discovered using the i -drop-out technique (i.e. quasars at   z ∼ 6  ). We argue that while quasars at   z < 6.5  seem to reside in the post-overlap IGM, they will still provide valuable probes of the late stages of the overlap era because the light-travel time across a quasar proximity zone should be comparable to the duration of overlap. For redshifted 21-cm observations within a 32-MHz bandpass, we find that the subtraction of a spectrally smooth foreground will not remove spectral features due to the proximity zone. These features could be used to measure the neutral hydrogen content of the IGM during the late stages of reionization. The density of quasars at   z ∼ 6  is now well constrained. We use the measured quasar luminosity function to estimate the prospects for discovery of high-redshift quasars in fields that will be observed by the Murchison Widefield Array.  相似文献   

19.
Motivated by recent observational studies of the environment of   z ∼ 6  QSOs, we have used the Millennium Run (MR) simulations to construct a very large  (∼4°× 4°)  mock redshift survey of star-forming galaxies at   z ∼ 6  . We use this simulated survey to study the relation between density enhancements in the distribution of i 775-dropouts and Lyα emitters, and their relation to the most massive haloes and protocluster regions at   z ∼ 6  . Our simulation predicts significant variations in surface density across the sky with some voids and filaments extending over scales of 1°, much larger than probed by current surveys. Approximately one-third of all   z ∼ 6  haloes hosting i -dropouts brighter than   z = 26.5  mag  (≈ M *UV, z =6)  become part of   z = 0  galaxy clusters. i -dropouts associated with protocluster regions are found in regions where the surface density is enhanced on scales ranging from a few to several tens of arcminutes on the sky. We analyse two structures of i -dropouts and Lyα emitters observed with the Subaru Telescope and show that these structures must be the seeds of massive clusters in formation. In striking contrast, six   z ∼ 6  QSO fields observed with Hubble Space Telescope show no significant enhancements in their i 775-dropout number counts. With the present data, we cannot rule out the QSOs being hosted by the most massive haloes. However, neither can we confirm this widely used assumption. We conclude by giving detailed recommendations for the interpretation and planning of observations by current and future ground- and space-based instruments that will shed new light on questions related to the large-scale structure at   z ∼ 6  .  相似文献   

20.
One of the most sought-after signatures of reionization is a rapid increase in the ionizing background (usually measured through the Lyα optical depth towards distant quasars). Conventional wisdom associates this with the 'overlap' phase when ionized bubbles merge, allowing each source to affect a much larger volume. We argue that this picture fails to describe the transition to the post-overlap Universe, where Lyman-limit systems (LLSs) absorb ionizing photons over moderate length-scales  (≲20–100   Mpc)  . Using an analytic model, we compute the probability distribution of the amplitude of the ionizing background throughout reionization, including both discrete ionized bubbles and LLSs (parametrized by an attenuation length, which we impose rather than attempt to model self-consistently). We show that the overlap does not by itself cause a rapid increase in the ionizing background or a rapid decrease in the mean Lyα transmission towards distant quasars. More detailed seminumeric models support these conclusions. We argue that the rapid changes should instead be interpreted as evolution in the attenuation length itself, which may or may not be directly related to overlap.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号