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1.
Three independent observational studies have now detected a narrow  (Δ z ≃ 0.5)  dip centred at   z = 3.2  in the otherwise smooth redshift evolution of the Lyα forest effective optical depth. This feature has previously been interpreted as an indirect signature of rapid photoheating in the intergalactic medium (IGM) during the epoch of He  ii reionization. We examine this interpretation using a semi-analytic model of inhomogeneous He  ii reionization and high-resolution hydrodynamical simulations of the Lyα forest. We instead find that a rapid  (Δ z ≃ 0.2)  boost to the IGM temperature  (Δ T ≃ 104 K)  beginning at   z = 3.4  produces a well understood and generic evolution in the Lyα effective optical depth, where a sudden reduction in the opacity is followed by a gradual, monotonic recovery driven largely by adiabatic cooling in the low-density IGM. This behaviour is inconsistent with the narrow feature in the observational data. If photoheating during He  ii reionization is instead extended over several redshift units, as recent theoretical studies suggest, then the Lyα opacity will evolve smoothly with redshift. We conclude that the sharp dip observed in the Lyα forest effective optical depth is instead most likely due to a narrow peak in the hydrogen photoionization rate around   z = 3.2  , and suggest that it may arise from the modulation of either reprocessed radiation during He  ii reionization, or the opacity of Lyman limit systems.  相似文献   

2.
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   

3.
Assuming simple dynamics for the growth of density fluctuations, we implement six-dimensional (6D) radiative transfer calculations to elucidate the effects of photon propagation during the reionization of an inhomogeneous universe. The ionizing sources are postulated to be AGN-like in this paper. The present simulations reveal that radiative transfer effects are still prominent considerably after the percolation epoch, in which patchy ionized regions connect with each other. In other words, owing to the collective opacity, the Universe does not become perfectly transparent against ionizing radiation even though strongly self-shielded regions disappear. It turns out that the inhomogeneity of the medium enhances the opacity effects and delays the end of reionization. Owing to such radiative transfer effects, the reionization in an inhomogeneous universe proceeds fairly slowly, in contrast to the prompt reionization in a homogeneous universe, and as a result the surface of reionization is not so sharply edged, but highly uneven. As a signature of the uneven surface of reionization, the cosmic IR background (CIB) radiation, which is produced by Ly photons resulting from radiative recombination, could exhibit strong anisotropies, reflecting the amplitude of density fluctuations at the reionization era. The predicted CIB intensity lies on a level of possible detection by forthcoming IR space telescope facilities.  相似文献   

4.
We use long-slit spectroscopic optical data to derive the properties of the extended emitting gas and the nuclear luminosity of a sample of 18 Seyfert 2 galaxies. From the emission-line luminosities and ratios we derive the density, reddening and mass of the ionized gas as a function of distance up to 2–4 kpc from the nucleus. Taking into account the geometric dilution of the nuclear radiation, we derive the radial distribution of covering factors and the minimum rate of ionizing photons emitted by the nuclear source. This number is an order of magnitude larger than that obtained from the rate of ionizing photons 'intercepted' by the gas and measured from the Hα luminosity. A calibration is proposed to recover this number from the observed luminosity. The He  ii λ4686/Hβ line ratio was used to calculate the slope of the ionizing spectral energy distribution (SED), which in combination with the number of ionizing photons allows the calculation of the hard X-ray luminosities. These luminosities are consistent with those derived from X-ray spectra in the eight cases for which such data are available and recover the intrinsic X-ray emission in Compton-thick cases. Our method can thus provide reliable estimates of the X-ray fluxes in Seyfert 2 galaxies for the cases where it is not readily available. We also use the ionizing SED and luminosity to predict the infrared luminosity under the assumption that it is dominated by reprocessed radiation from a dusty torus, and find a good agreement with the observed IRAS luminosities.  相似文献   

5.
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   

6.
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   

7.
Large-scale polarization of the cosmic microwave background measured by the WMAP satellite requires a mean optical depth to Thomson scattering,  τe∼ 0.17  . The reionization of the Universe must therefore have begun at relatively high redshift. We have studied the reionization process using supercomputer simulations of a large and representative region of a universe which has cosmological parameters consistent with the WMAP results (  Ωm= 0.3, ΩΛ= 0.7, h = 0.7, Ωb= 0.04, n = 1  and  σ8= 0.9  ). Our simulations follow both the radiative transfer of ionizing photons and the formation and evolution of the galaxy population which produces them. A previously published model with ionizing photon production as expected for zero-metallicity stars distributed according to a standard stellar initial mass function (IMF) (1061 photons per unit solar mass of formed stars) and with a moderate photon escape fraction from galaxies (5 per cent), produces  τe= 0.104  , which is within 1.0 to  1.5σ  of the 'best' WMAP value. Values of up to 0.16 can be produced by taking larger escape fractions or a top-heavy IMF. The data do not require a separate populations of 'miniquasars' or of stars forming in objects with total masses below  109 M⊙  . Reconciling such early reionization with the observed Gunn–Peterson troughs in   z > 6  quasars may be challenging. Possible resolutions of this problem are discussed.  相似文献   

8.
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   

9.
Observations made with the Coronal Diagnostic Spectrometer (CDS) onboard the Solar and Heliospheric Observatory ( SOHO ) are used to investigate the behaviour of the intensities of the emission lines of He  i , He  ii and O  iii at the quiet Sun-centre and at  θ= 60°  towards the equatorial limb. The aim is to examine the possible effects of photon scattering on the spatial variation of the optically thick helium lines. At the quiet Sun-centre, we find that, in agreement with previous work, the ratios of the intensities of the He  i 584-Å and He  ii 304-Å lines to those of the O  iii 600-Å line decrease systematically as the intensity of the O  iii line increases. However, we find that the dependence of these ratios on the O  iii intensity is not unique, but differs between the individual regions studied. Similar results are found at  θ= 60°  . We have also used line intensities and intensity ratios to investigate limb-to-disc effects and variations across a sample of supergranulation cell boundaries and adjacent cell interiors at both locations. The results do not exclude photon scattering as the cause of the larger observed ratios in cell interiors. The differences between the apparent widths of boundaries in O  iii at Sun-centre and 60° show that the emitting material is extended in height, which will aid the process of scattering into cell interiors. Photon scattering could also account for the lack of oscillations in the He  i intensities in a cell interior studied by Pietarila & Judge. Three-dimensional radiative transfer calculations in chosen geometries are now needed to account for the observations in detail.  相似文献   

10.
We develop a coupled model for the evolution of the global properties of the intergalactic medium (IGM) and the formation of galaxies, in the presence of a photoionizing background due to stars and quasars. We use this model to predict the thermodynamic history of the IGM when photoionized by galaxies forming in a cold dark matter (CDM) universe. The evolution of the galaxies is calculated using a semi-analytical model, including a detailed treatment of the effects of tidal stripping and dynamical friction on satellite galaxies orbiting inside larger dark matter haloes. We include in the model the negative feedback on galaxy formation from the photoionizing background. Photoionization inhibits galaxy formation in low-mass dark matter haloes in two ways: (i) heating of the IGM and inhibition of the collapse of gas into dark haloes by the IGM pressure, and (ii) reduction in the rate of radiative cooling of gas within haloes. The result of our method is a self-consistent model of galaxy formation and the IGM. The IGM is reheated twice (during reionization of H  i and He  ii ), and we find that the star formation rate per unit volume is slightly suppressed after each episode of reheating. We find that galaxies brighter than L ★ are mostly unaffected by reionization, while the abundance of faint galaxies is significantly reduced, leading to present-day galaxy luminosity functions with shallow faint-end slopes, in good agreement with recent observational data. Reionization also affects other properties of these faint galaxies, in a readily understandable way.  相似文献   

11.
This work presents the first integral field spectroscopy of the Homunculus nebula around η Carinae in the near-infrared spectral region ( J band). We confirmed the presence of a hole on the polar region of each lobe, as indicated by previous near-IR long-slit spectra and mid-IR images. The holes can be described as a cylinder of height (i.e. the thickness of the lobe) and diameter of 6.5 and  6.0 × 1016  cm, respectively. We also mapped the blue-shifted component of He  i  λ10830 seen towards the NW lobe. Contrary to previous works, we suggested that this blue-shifted component is not related to the Paddle but it is indeed in the equatorial disc.
We confirmed the claim of N. Smith and showed that the spatial extent of the Little Homunculus matches remarkably well the radio continuum emission at 3 cm, indicating that the Little Homunculus can be regarded as a small H  ii region. Therefore, we used the optically thin 1.3 mm radio flux to derive a lower limit for the number of Lyman-continuum photons of the central source in η Car. In the context of a binary system, and assuming that the ionizing flux comes entirely from the hot companion star, the lower limit for its spectral type and luminosity class ranges from O5.5  iii to O7  i . Moreover, we showed that the radio peak at 1.7 arcsec NW from the central star is in the same line-of-sight of the 'Sr-filament' but they are obviously spatially separated, while the blue-shifted component of He  i λ10830 may be related to the radio peak and can be explained by the ultraviolet radiation from the companion star.  相似文献   

12.
We present time-resolved spectroscopy of the soft X-ray transient XTE J2123–058 in outburst. A useful spectral coverage of 3700–6700 Å was achieved spanning two orbits of the binary, with single-epoch coverage extending to ∼9000 Å. The optical spectrum approximates a steep blue power law, consistent with emission on the Rayleigh–Jeans tail of a hot blackbody spectrum. The strongest spectral lines are He  ii 4686 Å and C  iii /N  iii 4640 Å (Bowen blend) in emission. Their relative strengths suggest that XTE J2123–058 was formed in the Galactic plane, not in the halo. Other weak emission lines of He  ii and C  iv are present, and Balmer lines show a complex structure, blended with He  ii . He  ii 4686-Å profiles show a complex multiple S-wave structure, with the strongest component appearing at low velocities in the lower-left quadrant of a Doppler tomogram. H α shows transient absorption between phases 0.35 and 0.55. Both of these effects appear to be analogous to similar behaviour in SW Sex type cataclysmic variables. We therefore consider whether the spectral line behaviour of XTE J2123–058 can be explained by the same models invoked for those systems.  相似文献   

13.
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14.
We study the inhomogeneous reionization in a critical density CDM universe resulting from stellar sources, including Population III objects. The spatial distribution of the sources is obtained from high-resolution numerical N -body simulations. We calculate the source properties, taking into account a self-consistent treatment of both radiative (i.e. ionizing and H2-photodissociating photons) and stellar (i.e. SN explosions) feedbacks regulated by massive stars. This allows us to describe the topology of the ionized and dissociated regions at various cosmic epochs, and to derive the evolution of H, He and H2 filling factors, soft UV background, cosmic star formation rate and the final fate of ionizing objects. The main results are: (i) galaxies reionize the intergalactic medium by z ≈10 (with some uncertainty related to the gas clumping factor), whereas H2 is completely dissociated already by z ≈25; (ii) reionization is mostly caused by the relatively massive objects which collapse via H line cooling, while objects the formation of which relies on H2 cooling alone are insufficient for this purpose; (iii) the diffuse soft UV background is the major source of radiative feedback effects for z ≤15; at higher z direct flux from neighbouring objects dominates; (iv) the match of the calculated cosmic star formation history with that observed at lower redshifts suggests that the conversion efficiency of baryons into stars is ≈1 per cent; (v) we find that a very large population of dark objects which failed to form stars is present by z ≈8. We discuss and compare our results with similar previous studies.  相似文献   

15.
A survey towards a selection of 35 methanol maser and/or ultracompact (UC) H  ii regions, reported in Papers I and II and by Norris et al., has been conducted in the near-infrared (NIR). Out of 25 methanol maser sites surveyed, 12 are associated with a NIR counterpart. Out of 18 UC H  ii regions (8 of which overlap with maser emission), 12 are associated with a NIR counterpart. Counterparts can be confidently identified not only by the positional agreements, but also by their unusually red colours. Spectral types for the embedded stars can be unambiguously determined for six sources, all of which imply massive, ionizing stars. One of these infrared sources has methanol maser emission, but no UC H  ii region. It is possible that the maser emission associated with this source arises from a pre-UC H  ii phase of massive stellar evolution or it could be that nearly all the ultraviolet photons are absorbed by dust within the UC H  ii region. We have modelled the spectral energy distributions (SEDs) for some sources and find that a single blackbody can be used to estimate the stellar luminosity, but cannot represent the whole infrared SED. A two-component blackbody model and a radiative transfer model were also used to derive essential parameters of the infrared sources. The radiative transfer model also indicates which infrared sources are relatively young and which are older. Both models show that silicate absorption at 9.7 μm must be a dominant feature of these SEDs.  相似文献   

16.
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17.
In this talk I will present a model for primordial galaxy formation. In particular, I will review the feedback effects that regulate the process: (i) radiative (i.e. ionizing and H2-photodissociating photons) and (ii) stellar (i.e. SN explosions) feedback produced by massive stars. I will also address how the IGM reionization can be influenced by this population of primordial galaxies and describe a Monte Carlo method for the radiative transfer of ionizing photons through the IGM. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

18.
It has recently been suggested that the power spectrum of redshifted 21 cm fluctuations could be used to measure the scale of baryonic acoustic oscillations (BAOs) during the reionization era. The resulting measurements are potentially as precise as those offered by the next generation of galaxy redshift surveys at lower redshift. However, unlike galaxy redshift surveys, which in the linear regime are subject to a scale-independent galaxy bias, the growth of ionized regions during reionization is thought to introduce a strongly scale-dependent relationship between the 21 cm and mass power spectra. We use a seminumerical model for reionization to assess the impact of ionized regions on the precision and accuracy with which the BAO scale could be measured using redshifted 21 cm observations. For a model in which reionization is completed at   z ∼ 6  , we find that the constraints on the BAO scale are not systematically biased at   z ≳ 6.5  . In this scenario, and assuming the sensitivity attainable with a low-frequency array comprising 10 times the collecting area of the Murchison Widefield Array, the BAO scale could be measured to within 1.5 per cent in the range  6.5 ≲ z ≲ 7.5  .  相似文献   

19.
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   

20.
We explore the ability of measurements of the 21-cm power spectrum during reionization to enable the simultaneous reconstruction of the reionization history and the properties of the ionizing sources. For various sets of simulated 21-cm observations, we perform maximum likelihood fits in order to constrain the reionization and galaxy formation histories. We employ a flexible six-parameter model that parametrizes the uncertainties in the properties of high-redshift galaxies. The computational speed needed is attained through the use of an analytical model that is in reasonable agreement with numerical simulations of reionization. We find that one-year observations, with the Murchison Widefield Array, should measure the cosmic ionized fraction to  ∼1 per cent  accuracy at the very end of reionization, and a few per cent accuracy around the mid-point of reionization. The mean halo mass of the ionizing sources should be measurable to 10 per cent accuracy when reionization is 2/3 of the way through, and to 20 per cent accuracy throughout the central stage of reionization, if this mass is anywhere in the range 1/3 to 100 billion solar masses.  相似文献   

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