1,720,999 research outputs found
Probing reionization with the cosmological proximity effect and high-redshift supernovae rates
No full textWe develop and assess the potential of several powerful techniques, designed to investigate the details of reionization. First, we present a procedure to probe the neutral fraction, xHI, using the Lyman α transmission statistics of high-redshift (z ≳ 6) sources. We find that only tens of bright quasar spectra could distinguish between xHI ∼ 1 and xHI ≲ 10-2. A rudimentary application of such a technique on quasar SDSS J1030+0524 has yielded compelling evidence of a large neutral fraction (xHI ≳ 0.2) at z ∼ 6. We also generate the observable, high-z supernovae (SNe) rates and quantify the prospects of detecting the suppression of star-formation in low-mass galaxies at reionization from such SNe rates, specifically from those obtainable from the James Webb Space Telescope (JWST). Our analysis suggests that searches for SNe could yield thousands of SNe per unit redshift at z ∼ 6, and be a valuable tool at studying reionization features and feedback effects out to z ≲ 13. © 2005 Elsevier B.V. All rights reserved
Lyα damping wing constraints on inhomogeneous reionization
No full textOne 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. © 2008 RAS
Lyα emitters during the early stages of reionization
No full textWe investigate the potential of exploiting Lyα emitters (LAEs) to constrain the volume-weighted mean neutral hydrogen fraction of the intergalactic medium, at high redshifts (specifically z ∼ 9). We use 'seminumerical' simulations to efficiently generate density, velocity and halo fields at z = 9 in a 250-Mpc box, resolving haloes with masses M ≥ 2.2 × 108 M⊙. We construct ionization fields corresponding to various values of. With these, we generate LAE luminosity functions and 'counts-in-cell' statistics. As in previous studies, we find that LAEs begin to disappear rapidly when. Constraining with luminosity functions is difficult due to the many uncertainties inherent in the host halo mass ↔ Lyα luminosity mapping. However, using a very conservative mapping, we show that the number densities derived using the six z ∼ 9 LAEs recently discovered by Stark et al. (2007a) imply. On a more fundamental level, these LAE number densities, if genuine, require substantial star formation in haloes with M ≲ 109 M⊙, making them unique among the current sample of observed high-z objects. Furthermore, reionization increases the apparent clustering of the observed LAEs. We show that a 'counts-in-cell' statistic is a powerful probe of this effect, especially in the early stages of reionization. Specifically, we show that a field of view (typical of upcoming infrared instruments) containing LAEs has ≳10 per cent higher probability of containing more than one LAE in a universe than a universe with the same overall number density. With this statistic, an ionized universe can be robustly distinguished from one with using a survey containing only ∼20-100 galaxies. © 2008 RAS
Evidence of a Cosmological Strömgren Surface and of Significant Neutral Hydrogen Surrounding the Quasar SDSS J1030+0524
No full textA bright quasar residing in a dense and largely neutral intergalactic medium at high redshifts (z ≳ 6) will be surrounded by a large cosmological Strömgren sphere. The quasar's spectrum will then show a sharp increase in resonant Lyman line absorption at wavelengths approaching and shorter than that corresponding to the Strömgren sphere's boundary along the line of sight. We show here that simultaneously considering the measured absorption in two or more hydrogen Lyman lines can provide the dynamical range required to detect this feature. We model broad and robust features of the Lyα and Lyβ regions of the spectrum of the z = 6.28 quasar SDSS J1030+0524, using a hydrodynamical simulation. From the steep wavelength dependence of the inferred absorption opacity, we detect the boundary of the Strömgren sphere at a proper distance of 6.0 ± 0.2 Mpc away from the source redshift. From the spectrum alone, we also find that beyond this distance, cosmic hydrogen turns nearly neutral, with a neutral fraction of xH1 ≳ 0.2, and that the ionizing luminosity of this quasar is in the range of (5.2 ± 2.5) × 1056 photons s-1. The method presented here, when applied to future quasars, can probe the complex topology of overlapping ionized regions and can be used to study the details of the reionization process
The inhomogeneous ionizing background following reionization
No full textWe study the spatial fluctuations in the hydrogen-ionizing background in the epoch following reionization (z ∼ 5-6). The rapid decrease with redshift in the photon mean free path (mfp), combined with the clustering of increasingly rare ionizing sources, can result in a very inhomogenous ionizing background during this epoch. We systematically investigate the probability density functions (PDFs) and power spectra of ionizing flux, by varying several parameters such as the mfp, minimum halo mass capable of hosting stars and halo duty cycle. In order to be versatile, we make use of analytic, seminumeric and numeric approaches. Our models show that the ionizing background indeed has sizable fluctuations during this epoch, with the PDFs being a factor of few wide at half of the maximum likelihood. The clustering of sources dominates the width of the PDFs, so analytic models must take large-scale clustering into account. The distributions also show marked asymmetries, with a high-value tail set by clustering on small scales, and a shorter low-value tail which is set by the mfp. The power spectrum of the ionizing background is much more sensitive to source properties than the PDF and can be well understood analytically with a framework similar to the halo model (usually used to describe dark matter clustering). Nevertheless, we find that Lyα forest spectra are extremely insensitive to the details of the ultraviolet background, despite marked differences in the PDFs and power spectra of our various ionizing backgrounds. Assuming a uniform ionizing background only underestimates the value of the mean ionization rate, 〈Γ12〉, inferred from the Lyα forest by a few per cent. Instead, analysis of the Lyα forest is dominated by the uncertainties in the density field. Thus, our results justify the common assumption of a uniform ionizing background in Lyα forest analysis even during this epoch. © 2009 RAS
How does radiative feedback from an ultraviolet background impact reionization?
No full textAn ionizing ultraviolet background (UVB) inhibits gas accretion and photoevaporates gas from the shallow potential wells of small, dwarf galaxies. During cosmological reionization, this effect can result in negative feedback: suppressing star formation inside HII regions, thus impeding their continued growth. It is difficult to model this process, given the enormous range of scales involved. We tackle this problem using a tiered approach: combining parametrized results from single-halo collapse simulations with large-scale models of reionization. In the resulting reionization models, the ionizing emissivity of galaxies depends on the local values of the reionization redshift and the UVB intensity.We present a physically motivated analytic expression for the average minimummass of star-forming galaxies, M̄min, which can be readily used in modelling galaxy formation. We findthat UVB feedback: (i) delays the end stages of reionization by δz ≲ 0.5; (ii) results in a more uniform distribution of HII regions, peaked on smaller scales (with large-scaleionization power suppressed by 10s of per cent) and (iii) suppresses the global photoionization rate per baryon by a factor of ≲2 towards the end of reionization. However, the impact is modest, since the hydrodynamic response of the gas to the UVB occurs on a time-scale comparable to reionization. In particular, the popular approach of modelling UVB feedback with an instantaneous transition in Mmin, dramatically overestimates its importance.UVB feedback on galaxies does not significantly affect reionization unless: (i) molecularly cooled galaxies contribute significantly to reionization; or (ii) internal feedback processes strongly couple with UVB feedback in the early Universe. Since both are considered unlikely, we conclude that there is no significant self-regulation of reionization by UVB feedback. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
Constraints on Reionization and Source Properties from the Absorption Spectra of z > 6.2 Quasars
No full textWe make use of hydrodynamical simulations of the intergalactic medium (IGM) to create model quasar absorption spectra. We compare these model spectra with the observed Keck spectra of three z > 6.2 quasars with full GunnPeterson troughs: SDSS J1148+5251 (z = 6.42), SDSSJ1030+0524 (z = 6.28), and SDSS J1623+3112 (z = 6.22). We fit the probability density distributions (PDFs) of the observed Lyct optical depths (τα) with those generated from the simulation by exploring a range of values for the size of the quasar's surrounding H n region, RS; the volumeweighted mean neutral hydrogen fraction in the ambient IGM, x̄HI; and the quasar's ionizing photon emissivity, ṄQ. In order to avoid averaging over possibly large sight line-to-sight line fluctuations in IGM properties, we analyze each observed quasar independently. We find the following results for Jl 148+5251, J1030+0524, and J1623+3112: the best-fit sizes Rs are 40, 41, and 29 (comoving) Mpc, respectively. For the later two quasars, the value is significantly larger than the radius corresponding to the wavelength at which the quasar's flux vanishes. These constraints are tight, with only ∼10% uncertainties, comparable to those caused by redshift determination errors. The best-fit values of ṄQ are 2.1, 1.3, and 0.9 × 1057 s -1, respectively, with a factor of ∼2 uncertainty in each case. Finally, the best-fit values of JCH , are 0.16,1.0, and 1.0, respectively. The uncertainty in the case of J1148+5251 is large, and x̄H I i is not well constrained. However, for both J1030+0524 and J1623+3112, we find a significant lower limit of x̄H I ∼ 0.033. Our method is different from previous analyses of the GP absorption spectra of these quasars, and our results strengthen the evidence that the rapid end stage of reionization is occurring near z ∼ 6. © 2007. The American Astronomical Society. All rights reserved
Constraints on the Small-Scale Power Spectrum of Density Fluctuations from High-Redshift Gamma-Ray Bursts
No full textCosmological models that include suppression of the power spectrum of density fluctuations on small scales exhibit an exponential reduction of high-redshift, nonlinear structures, including a reduction in the rate of gamma-ray bursts (GRBs). Here we quantify the constraints that the detection of distant GRBs would place on structure formation models with reduced small-scale power. We compute the number of GRBs that could be detectable by the Swift satellite at high redshifts (z ≳ 6), assuming that the GRBs trace the cosmic star formation history, which itself traces the formation of nonlinear structures. We calibrate simple models of the intrinsic luminosity function of the bursts to the number and flux distribution of GRBs observed by the Burst and Transient Source Experiment. We find that a discovery of high-z GRBs would imply strong constraints on models with reduced small-scale power. For example, a single GRB at z ≳ 10 or 10 GRBs at z ≳ 5 discovered by Swift during its scheduled 2 year mission would rule out an exponential suppression of the power spectrum on scales below Rc = 0.09 Mpc (exemplified by warm dark matter models with a particle mass of mx = 2 keV). Models with a less sharp suppression of small-scale power, such as those with a red tilt or a running scalar index, ns, are more difficult to constrain, because they are more degenerate with an increase in the power-spectrum normalization, σ8, and with models in which star formation is allowed in low-mass minihalos. We find that a tilt of δns ≈ 0.1 is difficult to detect; however, an observed rate of one GRB yr-1 at z ≳ 12 would yield an upper limit on the running of the spectral index, α ≡ dns/d ln k > -0.05. © 2005. The American Astronomical Society. All rights reserved
Relic HII regions and radiative feedback at high redshifts
Full text linkUltraviolet (UV) radiation from early astrophysical sources could have a large impact on subsequent star formation in nearby protogalaxies, and in general on the progress of cosmological reionization. Theoretical arguments based on the absence of metals in the early Universe suggest that the first stars were likely massive, bright, yet short-lived, with lifetimes of a few million years. Here we study the radiative feedback arising from such stars using hydrodynamical simulations with transient UV backgrounds (UVBs) and persistent Lyman-Werner backgrounds (LWBs) of varying intensity. We extend our prior work in Mesinger et al., by studying a more typical region whose protogalaxies form at lower redshifts, z ∼ 13-20, in the epoch likely preceding the bulk of reionization. We confirm our previous results that feedback in the relic H ii regions resulting from such transient radiation is itself transient. Feedback effects dwindle away after ∼30 per cent of the Hubble time, and the same critical specific intensity of JUV ∼ 0.1 × 10-21 erg s-1 cm-2 Hz-1 sr-1 separates positive and negative feedback regimes. This suggests that overall feedback is fairly insensitive to the large-scale environment, overdensity and redshift-dependent halo parameters, and can accurately be modelled in this regime with just the intensity of the impinging UVB. Additionally, we discover a second episode of eventual positive feedback in haloes which have not yet collapsed when their progenitor regions were exposed to the transient UVB. When exposed to the transient UVB, this gas suffers relatively little density depletion but a significant enhancement of the molecular hydrogen abundance, thus resulting in net positive feedback. This eventual positive feedback appears in all runs, regardless of the strength of the UVB. However, this feedback regime is very sensitive to the presence of Lyman-Werner radiation, and notable effects disappear under fairly modest background intensities of JLW ≳ 10-3 × 10 -21 erg s-1 cm-2 Hz-1 sr -1, assuming the region is optically thin for LW photons. Nevertheless, when exposed to the same LWB, haloes inside relic H ii regions always have a higher H2 abundance and shorter cooling times than haloes outside relic H ii regions, allowing gas to cool faster once it finally begins to collapse on to the halo. We conclude that UV radiative feedback in relic H ii regions, although a complicated process, seems unlikely to have a major impact on the progress of cosmological reionization, provided that present estimates of the lifetime and luminosity of a Population III star are accurate. More likely is that the build-up of the LWB ultimately governs the feedback strength until a persistent UV background can be established. © 2009 RAS
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