1,721,093 research outputs found
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
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
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
Probing the Reionization History Using the Spectra of High-Redshift Sources
No full textWe quantify and discuss the footprints of neutral hydrogen in the intergalactic medium (IGM) on the spectra of high-redshift (z ∼ 6) sources, using mock spectra generated from hydrodynamical simulations of the IGM. We show that it should be possible to extract relevant parameters, including the mean neutral fraction in the IGM and the radius of the local cosmological Strömgren region, from the flux distribution in the observed spectra of distant sources. We focus on quasars, but a similar analysis is applicable to galaxies and gamma-ray burst (GRB) afterglows. We explicitly include uncertainties in the spectral shape of the assumed source template near the Lyα line. Our results suggest that a mean neutral hydrogen fraction, xH, of unity can be statistically distinguished from xH ≈ 10-2 by combining the spectra of tens of bright (M ≈ -27) quasars. Alternatively, the same distinction can be achieved using the spectra of several hundred sources that are ∼100 times fainter. Furthermore, if the radius of the Strömgren sphere can be independently constrained to within ∼10%, this distinction can be achieved using a single source. The information derived from such spectra will help in settling the current debate as to what extent the universe was reionized at redshifts near z ∼ 6
Focusing on warm dark matter with lensed high-redshift galaxies
No full textWe propose a novel use of high-redshift galaxies, discovered in deep Hubble Space Telescope (HST) fields around strong lensing clusters. These fields probe small comoving volumes (~103 Mpc3) at high magnification (μ ≳ 10) and can detect otherwise inaccessible ultrafaint galaxies. Even a few galaxies found in such small volumes require a very high number density of collapsed dark matter (DM) haloes. This implies significant primordial power on small scales, allowing these observations to rule out popular alternatives to standard cold dark matter (CDM) models, such as warm dark matter (WDM). In this work, we analytically compute WDM halo mass functions at z = 10, including the effects of both particle freestreaming and residual velocity dispersion. We show that the two z ≈ 10 galaxies already detected by the Cluster Lensing And Supernova survey with Hubble (CLASH) survey are sufficient to constrain the WDM particle mass to mx < 1 (0.9) keV at 68 per cent (95 per cent) confidence limit (for a thermal relic relativistic at decoupling). This limit depends only on the WDM halo mass function and, unlike previous constraints on mx, is independent of any astrophysical modelling. The forthcoming HST Frontier Fields can significantly tighten these constraints. © 2013 The Authors
Implications of the Lyα Emission Line from a Candidate z=10 Galaxy
No full textThe z = 10 galaxy recently discovered by Pello and coworkers has a strong Lyα emission line that does not appear to have the expected asymmetry with more transmission on the red side. The blue wing of a Lyα line originating at high redshift should be strongly suppressed by resonant hydrogen absorption along the line of sight, an expectation borne out by the observed asymmetric shapes of the existing sample of Lyα-emitting sources at lower redshifts (3 < z < 6.7). The observed line is inconsistent with the galaxy being embedded in a fully neutral intergalactic medium (IGM) and having no receding peculiar velocity relative to the surrounding absorbing gas at the 95.0%-98.8% confidence level. Absorption on the blue side of the line of the Pello et al. source could be reduced if the IGM in the vicinity of the galaxy is highly ionized, but we show that this requires an unrealistically high ionizing emissivity. We suggest instead that the Lyα-emitting gas is receding relative to the surrounding gas with a velocity of ≳35 km s-1, which reduces the inconsistency confidence level to less than 76.0%-94.5%. We find that with this velocity shift, the observed strength and shape of the line is more consistent with the galaxy being surrounded by its own Strömgren sphere embedded in a fully neutral IGM. More generally, we predict that at any given redshift, the bright Lyα emitters with broader lines would exhibit stronger asymmetry than fainter ones. Bright galaxies with symmetric Lyα lines may be signposts for groups and clusters of galaxies, within which they can acquire random velocities comparable to or larger than their line widths. © 2005. 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
Evidence of Gunn-Peterson damping wings in high-z quasar spectra: strengthening the case for incomplete reionization
No full textUltraviolet Radiative Feedback on High-Redshift Protogalaxies
No full textWe use three-dimensional hydrodynamic simulations to investigate the effects of a transient photoionizing UV flux on the collapse and cooling of pregalactic clouds. These clouds have masses in the range 105-10 7 M⊙, form at high redshifts (z ≳18), and are assumed to lie within the short-lived cosmological H II regions around the first generation of stars. In addition, we study the combined effects of this transient UV flux and a persistent Lyman-Werner (LW) background (at photon energies below 13.6 eV) from distant sources. In the absence of a LW background, we find that a critical specific intensity of JUV ∼ 0.1 × 10-21 ergs s-1 cm-2 Hz-1 sr -1 demarcates a transition from net negative to positive feedback for the halo population. A weaker UV flux stimulates subsequent star formation inside the fossil H II regions, by enhancing the H2 molecule abundance. A stronger UV flux significantly delays star formation by reducing the gas density, and increasing the cooling time, at the centers of collapsing halos. At a fixed JUV the sign of the feedback also depends strongly on the density of the gas at the time of UV illumination. Regardless of whether the feedback is positive or negative, we find that once the UV flux is turned off, its impact starts to diminish after ∼30% of the Hubble time. In the more realistic case when a LW background is present, with JLW ≳ 0.01 × 10-21 ergs s-1 cm-2 Hz -1 sr-1, strong suppression persists down to the lowest redshift (z = 18) in our simulations. Finally, we find evidence that heating and photoevaporation by the transient UV flux render the ∼ 106 M⊙ halos inside fossil H II regions more vulnerable to subsequent H2 photodissociation by a LW background. © 2006. The American Astronomical Society. All rights reserved
- …
