Résultats de recherche pour « Intergalactic medium »

I study the effect of the galaxies of the Milky Way subgroup on the chemical enrichment of the diffuse gas within the Local Group. The preferred model predicts that the present-day intergalactic medium mass is 1/22 of the initial baryonic mass and its metallicity is 1/20 $Z_\odot$. Nearly 80 % of intergalactic metals were produced in the Galactic bulge.

Intergalactic space is filled with a pervasive medium of ionized gas, the intergalactic medium (IGM). A residual neutral fraction is detected in the spectra of quasistellar objects at both low and high redshifts, revealing a highly fluctuating medium with temperatures characteristic of photoionized gas. The statistics of the fluctuations are well reproduced by numerical gravity-hydrodynamics simulations within the context of standard cosmological structure formation scenarios. Thus, the study of the IGM offers an opportunity to probe the nature of the primordial density fluctuations on scales unavailable to other methods. The simulations also suggest that the IGM is the dominant reservoir of baryons produced by the Big Bang, and so the principal source of the matter from which galaxies formed. The detection of metal systems within the IGM shows that it was enriched by evolved stars early in its history, demonstrating an intimate connection between galaxy formation and the IGM. A comprehensive review of the current understanding of the structure and physical properties of the IGM and its relation to galaxies is presented, concluding with comments on prospects for furthering the study of the IGM using future ground-based facilities and space-based experiments.

Very recently a new inversion method has been developped to analyze the intergalactic medium seen in absorption in quasar spectra (the so-called Lyman-alpha forest). This method is applied to recover the temperature of the gas and the underlying density field. Using constraints from the Lyman-beta forest, it is possible to recover this field up to over-densities delta=10. By inverting the HI and CIV absorptions together it has been shown that the CIV/HI ratio varies through the profile of strong lines, beeing larger in the wings. The method can be applied to reconstruct the 3D density field from multiple lines of sight and is shown to give good results up to mean separations of 3 arcmin. Results from a survey of QSO pairs performed with HST/STIS and VLT/UVES-FORS are summarized.

The bulk of cosmic matter resides in a dilute reservoir that fills the space between galaxies, the intergalactic medium (IGM). The history of this reservoir is intimately tied to the cosmic histories of structure formation, star formation, and supermassive black hole accretion. Our models for the IGM at intermediate redshifts (2≲z≲5) are a tremendous success, quantitatively explaining the statistics of Lyα absorption of intergalactic hydrogen. However, at both lower and higher redshifts (and around galaxies) much is still unknown about the IGM. We review the theoretical models and measurements that form the basis for the modern understanding of the IGM, and we discuss unsolved puzzles (ranging from the largely unconstrained process of reionization at high z to the missing baryon problem at low z), highlighting the efforts that have the potential to solve them.

Several popular cosmological models predict that most of the baryonic mass in the local universe is located in filamentary and sheet-like structures associated with groups and clusters of galaxies. This gas is expected to be gravitationally heated to ~10^6 K and therefore emitting in the soft X-rays. We have investigated three fields with large scale structures of galaxies at redshifts 0.1, 0.45, 0.79 and found signatures of warm-hot thermal emission (kT< 1 keV) correlated with the distribution of galaxies for the first two. The correlation and the properties of both X-ray and galaxy distribution strongly suggest that the diffuse X-ray flux is due to extragalactic emission by the Warm-Hot Intergalactic Medium (WHIM) predicted by cosmological models.

At redshifts z ≳ 2, most of the baryons reside in the smooth intergalactic medium which is responsible for the low column density Lyα forest. This photoheated gas follows a tight temperature-density relation which introduces a cut-off in the distribution of widths of the Lyα 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.

We present our proposal for a small X-ray mission DIOS (Diffuse Intergalactic Oxygen Surveyor) to perform survey observations of warm-hot intergalactic medium using OVII and OVIII emission lines. This will be proposed to a small satellite program planned by ISAS/JAXA in Japan for a launch in 2008. The instrument consists of an array of TES microcalorimeters with an energy resolution 2 eV, cooled by mechanical coolers. The X-ray telescope will employ 4-stage reflection mirrors with a focal length as short as 70 cm and an angular resolution 2'. In addition to DIOS, we briefly describe the NeXT (New X-ray Telescope) mission, which is a larger Japanese X-ray observatory to be launched in 2010 and plans to explore non-thermal processes in the universe.

We develop an efficient method of studying the effects of reionization history on the temperature-density relation of the intergalactic medium in the low-density limit (overdensity δ≾5). It is applied to the study of photo-reionization models in which the amplitude, spectrum and onset epoch of the ionizing flux, as well as the cosmology, are systematically varied. We find that the mean temperature-density relation at z = 2−4 is well approximated by a power-law equation of state for uniform reionization models. We derive analytical expressions for its evolution and exhibit its asymptotic behaviour: it is found that for sufficiently early reionization, imprints of reionization history prior to z∼10 on the temperature-density relation are washed out. In this limit the temperature at the cosmic mean density is proportional to . While the amplitude of the radiation flux at the ionizing frequency of H i is found to have a negligible effect on the temperature-density relation as long as the universe reionizes before z∼5, the spectrum can change the overall temperature by about 20 per cent, through variations in the abundances of helium species. However, the slope of the mean equation of state is found to lie within a narrow range for all reionization models we study, where reionization takes place before z∼5. We discuss the implications of these findings for the observational properties of the Lyα forest. In particular, uncertainties in the temperature of the intergalactic medium, arising from the uncertain reionization history of our Universe, introduce a 30 per cent scaling in the amplitude of the column density distribution while the slope of the distribution is only affected by about 7 per cent. Finally, we discuss how a fluctuating ioiuzing field affects the above results. We argue that under certain conditions, the loss of memory of reionization history implies that at late times, the temperature-density relation of a gas in a fluctuating ionizing background can be approximated by one that results from a uniform radiation field, provided the universe reionizes sufficiently early.

The temperature of the low-density intergalactic medium is set by the balance between adiabatic cooling resulting from the expansion of the universe, and photo-heating by the UV-background. A sudden injection of entropy from the reionization will increase the temperature of the gas, leading to a broadening of the hydrogen Lyman-alpha absorption lines produced in the IGM, and observed in the spectra of background quasars. We present a method based on wavelets to characterise objectively the line widths of such absorption lines. We use high resolution hydrodynamical simulations to demonstrate that the algorithm can detect changes in temperature of order of 50 per cent on scales > 5000 km/s. We apply the method to a UVES/VLT spectrum of quasar 0055--269 (emission redshift=3.7) and detect at the 99 per cent confidence level a sudden increase in temperature below redshift z=3.3, which we interpret as evidence for HeliumII reionization.

We use high spectral resolution () and high signal-to-noise ratio ( per pixel) spectra of 19 high-redshift () quasars to investigate the metal content of the low-density intergalactic medium using pixel-by-pixel procedures. This high quality homogeneous survey gives the possibility to statistically search for metals at H i optical depths smaller than unity. We find that the gas is enriched in carbon and oxygen for neutral hydrogen optical depths > 1. Our observations strongly suggest that the C iv/H i ratio decreases with decreasing with . We do not detect C iv absorption statistically associated with gas of < 1. However, we observe that a small fraction of the low density gas is associated with strong metal lines as a probable consequence of the IGM enrichment being highly inhomogeneous. We detect the presence of O vi down to ~ 0.2 with log / ~ -2.0. We show that O vi absorption in the lowest density gas is located within ~300 km s-1 of strong H i lines. This suggests that this O vi phase may be part of winds flowing away from overdense regions. This effect is more important at the largest redshifts (z > 2.4). Therefore, at the limit of present surveys, the presence of metals in the underdense regions of the IGM is still to be demonstrated.