3,215 research outputs found

    Cosmic reionization in a dynamic quintessence cosmology

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    In this paper we investigate the effects that a dynamic dark energy component dominant in the universe at late epochs has on reionization. We follow the evolution of HII regions with the analytic approach of Furlanetto and Oh (2005) in two different universes for which we assume the Peebles and Ratra (2003) and Brax and Martin (2000) quintessence models and we compare our results to the LCDM scenario. We show that, for a fixed ionization efficiency, at the same cosmological epoch the topology of bubbles is dominated by high-mass objects and the characteristic size of the ionized regions is slightly smaller than in the LCDM model, especially at the latest stages of reionization, due to the higher recombination efficiency. As a consequence, the bubbles' `epoch of overlap' happens earlier than in LCDM. Finally, we show how the different evolution of the HII regions affects the transmission of the high-z QSO spectra, reducing the Lyman flux absorption at small optical depths.Comment: 10 pages, minor changes to match the version accepted for publication by MNRA

    High resolution spectroscopy of the three dimensional cosmic web with close QSO groups

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    We study the three-dimensional distribution of matter at z~2 using high resolution spectra of QSO pairs and simulated spectra drawn from cosmological hydro-dynamical simulations. We present a sample of 15 QSOs, corresponding to 21 baselines of angular separations evenly distributed between ~1 and 14 arcmin, observed with the Ultraviolet and Visual Echelle Spectrograph (UVES) at the European Southern Observatory-Very Large Telescope (ESO-VLT). The observed correlation functions of the transmitted flux in the HI Lya forest transverse to and along the line of sight are in agreement, implying that the distortions in redshift space due to peculiar velocities are relatively small and - within the relatively large error bars - not significant. The clustering signal is significant up to velocity separations of ~300 km/s, corresponding to about 5 h^{-1} comoving Mpc. Compatibility at the 2 sigma level has been found both for the Auto- and Cross-correlation functions and for the set of the Cross correlation coefficients. The analysis focuses in particular on two QSO groups of the sample. Searching for alignments in the redshift space between Lya absorption lines belonging to different lines of sight, it has been possible to discover the presence of a wide HI structures extending over about ten Mpc in comoving space, and give constraints on the sizes of two cosmic under-dense regions in the intergalactic medium.Comment: Accepted by MNRAS, version matching the published on

    On the formation of dwarf galaxies and stellar halos

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    Using analytic arguments and a suite of very high resolution (10^3 Msun per particle) cosmological hydro-dynamical simulations, we argue that high redshift, z ~ 10, M ~ 10^8 Msun halos, form the smallest `baryonic building block' (BBB) for galaxy formation. These halos are just massive enough to efficiently form stars through atomic line cooling and to hold onto their gas in the presence of supernovae winds and reionisation. These combined effects, in particular that of the supernovae feedback, create a sharp transition: over the mass range 3-10x10^7 Msun, the BBBs drop two orders ofmagnitude in stellar mass. Below ~2x10^7 Msun, galaxies will be dark with almost no stars and no gas. Above this scale is the smallest unit of galaxy formation: the BBB. A small fraction (~100) of these gas rich BBBs fall in to a galaxy the size of the Milky Way. Ten percent of these survive to become the observed LG dwarf galaxies at the present epoch. Those in-falling halos on benign orbits which keep them far away from the Milky Way or Andromeda manage to retain their gas and slowly form stars - these become the smallest dwarf irregular galax ies; those on more severe orbits lose their gas faster than they can form stars and become the dwarf spheroidals. The remaining 90% of the BBBs will be accreted. We show that this gives a metallicity and total stellar mass consistent with the Milky Way old stellar halo (abridged).Comment: 15 pages, 7 figures, one figure added to match accepted version. Some typos fixed. MNRAS in pres

    The power spectrum of the flux distribution in the Lyman-alpha forest of a Large sample of UVES QSO Absorption Spectra (LUQAS)

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    The flux power spectra of the Lyman-alpha forest from a sample of 27 QSOs taken with the high resolution echelle spectrograph UVES on VLT are presented. We find a similar fluctuation amplitude at the peak of the ``3D'' flux power spectrum at k ~ 0.03 (km/sec)^(-1) as the study by Croft et al. (2002), in the same redshift range. The amplitude of the flux power spectrum increases with decreasing redshift if corrected for the increase in the mean flux level as expected if the evolution of the flux power spectrum is sensitive to the gravitational growth of matter density fluctuations. This is in agreement with the findings of McDonald et al. (2000) at larger redshift. The logarithmic slope of the "3D" flux power spectrum, P_F(k), at large scales k < 0.03 (km/sec)^(-1), is 1.4 +- 0.3, i.e. 0.3 shallower than that found by Croft et al. (2002) but consistent within the errors.Comment: 18 pages, 9 PS figures, 6 tables. Note that the k-values of the 1D flux power spectrum had been erroneously shifted by half a bin size (in log k) in the previous version. All the other results are unaffected. New tables can be found at http://www.ast.cam.ac.uk/~rtnigm/luqas.ht

    The impact of feedback from galaxy formation on the Lyman-alpha transmitted flux

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    The forest of Lyman-alpha absorption lines seen in the spectra of distant quasars has become an important probe of the distribution of matter in the Universe. We use large, hydrodynamical simulations from the OWLS project to investigate the effect of feedback from galaxy formation on the probability distribution function and the power spectrum of the Lyman-alpha transmitted flux. While metal-line cooling is unimportant, both galactic outflows from massive galaxies driven by active galactic nuclei and winds from low-mass galaxies driven by supernovae have a substantial impact on the flux statistics. At redshift z=2.25, the effects on the flux statistics are of a similar magnitude as the statistical uncertainties of published data sets. The changes in the flux statistics are not due to differences in the temperature-density relation of the photo-ionised gas. Instead, they are caused by changes in the density distribution and in the fraction of hot, collisionally ionised gas. It may be possible to disentangle astrophysical and cosmological effects by taking advantage of the fact that they induce different redshift dependencies. In particular, the magnitude of the feedback effects appears to decrease rapidly with increasing redshift. Analyses of Lyman-alpha forest data from surveys that are currently in process, such as BOSS/SDSS-III and X-Shooter/VLT, must take galactic winds into account.Comment: 15 pages, 11 figures. MNRAS in pres

    How cold is cold dark matter? Small scales constraints from the flux power spectrum of the high-redshift Lyman-alpha forest

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    We present constraints on the mass of warm dark matter (WDM) particles derived from the Lyman-alpha flux power spectrum of 55 high- resolution HIRES spectra at 2.0 < z < 6.4. From the HIRES spectra, we obtain a lower limit of mwdm > 1.2 keV 2 sigma if the WDM consists of early decoupled thermal relics and mwdm > 5.6 keV (2 sigma) for sterile neutrinos. Adding the Sloan Digital Sky Survey Lyman-alpha flux power spectrum, we get mwdm > 4 keV and mwdm > 28 keV (2 sigma) for thermal relics and sterile neutrinos. These results improve previous constraints by a factor two.Comment: Some issues clarified (especially resolution related). Conclusions unchanged. Accepted version by PR

    The impact of coupled dark energy cosmologies on the high-redshift intergalactic medium

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    We present an analysis of high-resolution hydrodynamical N-body simulations of coupled dark energy cosmologies which focuses on the statistical properties of the transmitted Lyman alpha flux in the high-redshift intergalactic medium (IGM). In these models the growth of the diffuse cosmic web differs from the standard Lambda CDM case: the density distribution is skewed towards underdense regions and the matter power spectra are typically larger (in a scale-dependent way). These differences are also appreciable in the Lyman alpha flux and are larger than 5 per cent (10 per cent) at z = 2-4 in the flux probability distribution function (pdf) for high-transmissivity regions and for values of the coupling parameter beta = 0.08 (beta = 0.2). The flux power spectrum is also affected at the similar to 2 per cent (similar to 5-10 per cent) level for beta = 0.08 (beta = 0.2) in a redshift-dependent way. We infer the behaviour of flux pdf and flux power for a reasonable range of couplings and present constraints using present high-and low-resolution data sets. We find an upper limit beta less than or similar to 0.15 (at 2 sigma confidence level), which is obtained using only IGM data and is competitive with those inferred from other large-scale structure probes

    Dark energy records in lensed cosmic microwave background

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    We consider the weak lensing effect induced by linear cosmological perturbations on the cosmic microwave background (CMB) polarization anisotropies. We find that the amplitude of the lensing peak in the BB mode power spectrum is a faithful tracer of the dark energy dynamics at the onset of cosmic acceleration. This is due to two reasons. First, the lensing power is non-zero only at intermediate redshifts between the observer and the source, keeping record of the linear perturbation growth rate at the corresponding epoch. Second, the BB lensing signal is expected to dominate over the other sources. The lensing distortion on the TT and EE spectra do exhibit a similar dependence on the dark energy dynamics, although those are dominated by primary anisotropies. We investigate and quantify the effect by means of exact tracking quintessence models, as well as parameterizing the dark energy equation of state in terms of the present value (w0w_{0}) and its asymptotic value in the past (w‚ąěw_{\infty}); in the interval allowed by the present constraints on dark energy, the variation of w‚ąěw_{\infty} induces a significant change in the BB mode lensing amplitude. A Fisher matrix analysis, under conservative assumptions concerning the increase of the sample variance due to the lensing non-Gaussian statistics, shows that a precision of order 10% on both w0w_{0} and w‚ąěw_{\infty} is achievable by the future experiments probing a large sky area with angular resolution and sensitivity appropriate to detect the lensing effect on the CMB angular power spectrum. These results show that the CMB can probe the differential redshift behavior of the dark energy equation of state, beyond its average.Comment: New version including substantial text change, three more figures and two more table

    Detecting X-ray filaments in the low redshift Universe with XEUS and Constellation-X

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    We propose a possible way to detect baryons at low redshifts from the analysis of X-ray absorption spectra of bright AGN pairs. A simple semi-analytical model to simulate the spectra is presented. We model the diffuse warm-hot intergalactic medium (WHIM) component, responsible for the X-ray absorption, using inputs from high-resolution hydro-dynamical simulations and analytical prescriptions. We show that the number of OVII absorbers per unit redshift with column density larger than 1013.510^{13.5} cm‚ąí2^{-2} - corresponding to an equivalent width of ‚ąľ\sim 1 km/s - which will be possibly detectable by {\it XEUS}, is \magcir 30 per unit redshift. {\it Constellation-X} will detect ‚ąľ6\sim 6 OVII absorptions per unit redshift with an equivalent width of 10 km/s. Our results show that, in a őõ\LambdaCDM Universe, the characteristic size of these absorbers at z‚ąľ0.1z\sim 0.1 is ‚ąľ1\sim 1 h‚ąí1h^{-1} Mpc. The filamentary structure of WHIM can be probed by finding coincident absorption lines in the spectra of background AGN pairs. We estimate that at least 20 AGN pairs at separation \mincir 20 arcmin are needed to detect this filamentary structure at a 3ŌÉ\sigma level. Assuming observations of distant sources using {\it XEUS} for exposure times of 500 ksec, we find that the minimum source flux to probe the filamentary structure is ‚ąľ2√ó10‚ąí12\sim 2\times 10^{-12} erg cm‚ąí2^{-2} s‚ąí1^{-1}, in the 0.1-2.4 keV energy band. Thus, most pairs of these extragalactic X-ray bright sources have already been identified in the {\it ROSAT} All-Sky Survey. Re-observation of these objects by future missions could be a powerful way to search for baryons in the low redshift Universe.Comment: 18 pages, 10 Figures. Two figures added, Sections 2 and 3 expanded. More optimistic results for Constellation-X. Accepted by MNRA

    Possible evidence for an inverted temperature-density relation in the intergalactic medium from the flux distribution of the Lyman-alpha forest

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    We compare the improved measurement of the Lya forest flux probability distribution at 1.7<z<3.2 presented by Kim et al. (2007) to a large set of hydrodynamical simulations of the Lya forest with different cosmological parameters and thermal histories. The simulations are in good agreement with the observational data if the temperature-density relation for the low density intergalactic medium (IGM), T=T_0 Delta^{gamma-1}, is either close to isothermal or inverted (gamma<1). Our results suggest that the voids in the IGM may be significantly hotter and the thermal state of the low density IGM may be substantially more complex than is usually assumed at these redshifts. We discuss radiative transfer effects which alter the spectral shape of ionising radiation during the epoch of HeII reionisation as a possible physical mechanism for achieving an inverted temperature-density relation at z~3.Comment: 16 pages, 6 figures, accepted for publication in MNRAS following minor revision. The accepted version includes an expanded discussion of the flux power spectru
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