We revisit Lyman-alpha bounds on the dark matter mass in Lambda Warm Dark Matter (Lambda-WDM) models, and derive new bounds in the case of mixed Cold plus Warm models (Lambda-CWDM), using a set up which is a good approximation for several theoretically well-motivated dark matter models. We combine WMAP5 results with two different Lyman-alpha data sets, including observations from the Sloan Digital Sky Survey. We pay a special attention to systematics, test various possible sources of error, and compare the results of different statistical approaches. Expressed in terms of the mass of a non-resonantly produced sterile neutrino, our bounds read m_NRP > 8 keV (frequentist 99.7% confidence limit) or m_NRP > 12.1 keV (Bayesian 95% credible interval) in the pure Lambda-WDM limit. For the mixed model, we obtain limits on the mass as a function of the warm dark matter fraction F_WDM. Within the mass range studied here (5 keV < m_NRP < infinity), we find that any mass value is allowed when F_WDM < 0.6 (frequentist 99.7% confidence limit); similarly, the Bayesian joint probability on (F_WDM, 1/m_NRP) allows any value of the mass at the 95% confidence level, provided that F_WDM < 0.35.
We investigate a new hybrid-model universe containing two types of dark matter, one ``warm'' and the other ``hot''. The hot component is an ordinary light neutrino with mass $\sim 25h^2$~eV while the warm component is a sterile neutrino with mass $\sim 700h^2$~eV. The two types of dark matter arise entirely within the neutrino sector and do not require separate physical origins. We calculate the linear transfer functions for a representative sample of warm-plus-hot models. The transfer functions, and results from several observational tests of structure formation, are compared with those for t…
High resolution N-body simulations of cold dark matter (CDM) models predict that galaxies and clusters have cuspy halos with excessive substructure. Observations reveal smooth halos with central density cores. One possible resolution of this conflict is that the dark matter is warm (WDM); this will suppress the power spectrum on small scales. The Lyman-alpha forest is a powerful probe of the linear power spectrum on these scales. We use collisionless N-body simulations to follow the evolution of structure in WDM models, and analyze artificial Lyman-alpha forest spectra extracted from them. By …
We discuss the effects of gravitational collapse on the shape of absorption line profiles for low column density (log N(HI) < 14) Lyman Alpha forest clouds and argue by comparison with cosmological simulations that Lyman alpha forest observations show the signs of ongoing gravitational structure formation at high redshift. The departures of observed line profiles from thermal Voigt profiles (caused by bulk motion of infalling gas and compressional heating) are evident from the results of profile fitting as a correlation in velocity space among pairs of components with discrepant Doppler parame…
We present an analysis of the Lyman alpha forests of five quasar spectra in the near UV. Properties of the intergalactic medium (IGM) at an intermediate redshift interval (0.9 < z < 1.9) are studied. The amount of baryons in the diffuse photoionised IGM and the warm-hot intergalactic medium (WHIM) are traced to get constraints on the redshift evolution of the different phases of the intergalactic gas. The baryon density of the diffuse IGM is determined with photoionisation calculations under the assumption of local hydrostatic equilibrium. We assume that the gas is ionised by a metagalactic ba…
In light of recent findings which seem to disfavor a scenario with (warm) dark matter entirely constituted of sterile neutrinos produced via the Dodelson-Widrow (DW) mechanism, we investigate the constraints attainable for this mechanism by relaxing the usual hypothesis that the relic neutrino abundance must necessarily account for all of the dark matter. We first study how to reinterpret the limits attainable from X-ray non-detection and Lyman-alpha forest measurements in the case that sterile neutrinos constitute only a fraction fs of the total amount of dark matter. Then, assuming that ster…
The matter power spectrum at comoving scales of (1-40) h^{-1} Mpc is very sensitive to the presence of Warm Dark Matter (WDM) particles with large free streaming lengths. We present constraints on the mass of WDM particles from a combined analysis of the matter power spectrum inferred from the large samples of high resolution high signal-to-noise Lyman-alpha forest data of Kim et al. (2004) and Croft et al. (2002) and the cosmic microwave background data of WMAP. We obtain a lower limit of m_wdm > 550 eV (2 sigma for early decoupled thermal relics and m_wdm > 2.0 keV (2 sigma) for sterile neut…
Molecular hydrogen is quite underabundant in damped Lyman-alpha systems at high redshift, when compared to the interstellar medium near the Sun. This has been interpreted as implying that the gas in damped Lyman-alpha systems is warm. like the nearby neutral intercloud medium, rather than cool, as in the clouds which give rise to most H I absorption in the Milky Way. Other lines of evidence suggest that the gas in damped Lyman-alpha systems -- in whole or part -- is actually cool; spectroscopy of neutral and ionized carbon, discussed here, shows that the damped Lyman-alpha systems observed at …
We have collected data for 69 Damped Lyman-alpha (DLA) systems, to investigate the chemical evolution of galaxies in the redshift interval 0.0 < z < 4.4. In doing that, we have adopted the most general approach used so far to correct for dust depletion. The best solution, obtained through chi^2 minimization, gives as output parameters the global DLA metallicity and the dust-to-metals ratio. Clear evolution of the metallicity vs. redshift is found (99.99% significance level), with average values going from ~1/30 solar at z~4.1 to ~3/5 solar at z~0.5. We also find that the majority of DLAs (~60%…
We analyse the ROSAT PSPC hardness ratio and the 0.5-2 keV to 2-10 keV flux ratio of 65 Active Galactic Nuclei (AGN) for which there are both ROSAT archival observations available and 2-10 keV fluxes, mostly from the HEAO-1 MC-LASS survey. We conclude that the simplest spectral model for the AGN that can accommodate the variety of X-ray colours obtained is a standard power law (with energy spectral index $α\sim 0.9$) plus a $\sim 0.1$ keV black body both partially absorbed. In our sample, type 1 AGN require an absorbing column around $10^{22}\, {\rm cm}^{-2}$ with covering fractions between 20…
(ABRIDGED) We present a study of the highly ionized gas (plasma) associated with damped Lyman-alpha (DLA) systems at z=2.1-3.1. We search for O VI absorption and corresponding Si IV, C IV, and N V in a Very Large Telescope/Ultraviolet-Visible Echelle Spectrograph (VLT/UVES) sample of 35 DLA systems with data covering O VI at S/N>10. We report twelve DLAs (nine intervening and three at <5000 km/s from the QSO redshift) with detections of O VI absorption. There are no clear O VI non-detections, so the incidence of O VI in DLAs is between 34% (12/35) and 100%. Analysis of the line widths together…
Stable particle dark matter may well originate during the decay of long-lived relic particles, as recently extensively examined in the cases of the axino, gravitino, and higher-dimensional Kaluza-Klein (KK) graviton. It is shown that in much of the viable parameter space such dark matter emerges naturally warm/hot or mixed. In particular, decay produced gravitinos (KK-gravitons) may only be considered cold for the mass of the decaying particle in the several TeV range, unless the decaying particle and the dark matter particle are almost degenerate. Such dark matter candidates are thus subject …
We study the distribution of low-redshift Ly$α$ clouds in a CDM model using numerical simulations including photoionization and cooling of the baryonic component. The ionizing background is found to be efficient enough to keep most of the gas warm ($T$~=~1--5~10$^4$~K) and to prevent most of it from collapsing. In this scenario, the numerous low redshift ($z$~$<$~0.5) neutral hydrogen Ly$α$ lines recently revealed by HST observations of QSOs originate in the warm gas that traces the potential wells of filamentary structures defined by the dark matter. The predicted line number density is consi…
We investigated abundance ratios along the profiles of six high-redshift Damped Lyman-alpha systems, three of them associated with H2 absorption, and derived optical depths in each velocity pixel. The variations of the pixel abundance ratios were found to be remarkably small and usually smaller than a factor of two within a profile. This result holds even when considering independent sub-clumps in the same system. The depletion factor is significantly enhanced only in those components where H2 is detected. There is a strong correlation between [Fe/S] and [Si/S] abundances ratios, showing that …
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.
I consider constraints from observations on a cutoff scale in clustering due to free streaming of the dark matter in a warm dark matter cosmological model with a cosmological constant. The limits are derived in the framework of a sterile neutrino warm dark matter universe, but can be applied to gravitinos and other models with small scale suppression in the linear matter power spectrum. With freedom in all cosmological parameters including the free streaming scale of the sterile neutrino dark matter, limits are derived using observations of the fluctuations in the cosmic microwave background, …
We use observations of the cosmological large-scale structure to derive limits on two-component hot dark matter consisting of mass-degenerate neutrinos and hadronic axions, both components having velocity dispersions corresponding to their respective decoupling temperatures. We restrict the data samples to the safely linear regime, in particular excluding the Lyman-alpha forest. Using standard Bayesian inference techniques we derive credible regions in the two-parameter space of m_a and sum(m_nu). Marginalising over sum(m_nu) provides m_a < 1.2 eV (95% C.L.). In the absence of axions the same …
Quantum fluctuations created during inflation can account for the observed matter distribution in the linear regime if the universe has two components of dark matter, one which is cold and collisionless, and the other which is hot and free streams on small scales. However, this free streaming property of the hot component prevents early structure formation, and since objects, such as damped Lyman-$α$ systems, have been observed at high redshift, it is necessary to produce more power on small scales. Here, we show that the situation can be improved substantially in models where cosmic strings a…
We analyze the flux power spectrum and its covariance using simulated Lyman alpha forests. We find that pseudo-hydro techniques are good approximations of hydrodynamical simulations at high redshift. However, the pseudo-hydro techniques fail at low redshift because they are insufficient for characterizing some components of the low-redshift intergalactic medium, notably the warm-hot intergalactic medium. Hence, to use the low-redshift Lyman alpha flux power spectrum to constrain cosmology, one would need realistic hydrodynamical simulations. By comparing one-dimensional mass statistics with fl…
Numerical hydrodynamical simulations have proven a successful means of reproducing many of the statistical properties of the Lyman-Alpha forest as measured in high redshift quasar spectra. Pseudo-hydrodynamical methods based only on simulating the dark matter component have been claimed to yield a comparable level of success. We investigate the degree to which two pseudo-methods, with and without allowing for a pseudo-gas pressure, are able to match the predictions of fully hydrodynamical plus dark matter simulations. We also address the requirements for convergence to the statistics of the sp…
Analysis of the absorption lines in the afterglow spectrum of the gamma-ray burst GRB010222 indicates that its host galaxy (at a redshift of z=1.476) is the strongest damped Lyman-alpha (DLA) system known, having a very low metallicity and modest dust content. This conclusion is based on the detection of the red wing of Lyman-alpha plus a comparison of the equivalent widths of ultraviolet Mg I, Mg II, and Fe II lines with those in other DLAs. The column density of H I, deduced from a fit to the wing of Lyman-alpha, is (5 +/- 2) 10^22 cm^-2. The ratio of the column densities of Zn and Cr lines …
We have detected two high ionization phases of gas in the z ~ 2.77 partial Lyman limit system (LLS) towards QSO 1157-3143. We detect the first phase by CIV and SiIV absorption, and the second phase -- which is either warmer or undergoing larger random bulk motions than the first -- via OVI absorption. Both phases of gas are present in similar column density ratios in each of the 5 velocity components, making it appear that this LLS is constructed of 5 very similar building blocks. We find that this system displays some of the properties expected of a hierarchical merging event, although publis…
We use LUQAS, a sample of 27 high resolution high signal-to-noise UVES quasar (QSO) spectra (Kim et al. 2004), and the Croft et al. (2002) sample together with a set of high resolution large box size hydro-dynamical simulations run with the code GADGET-II to recover the linear dark matter power spectrum at z>2 and at scales of 1-40 Mpc/h. These scales cannot be probed by any other observable. We address some of the uncertainties in the theoretical modelling of the Lyman-alpha forest structures such as the difference between full hydro-dynamical simulations and a more simplified scheme based on…
We present a VLT/UVES spectrum of a proximate sub-damped Lyman-alpha (sub-DLA) system at z=2.65618 toward the quasar Q0331-4505 (z_qso=2.6785+/-0.0030). Absorption lines of O I, Si II, Si III, Si IV, C II, C III, C IV, Fe II, Al II, and O VI are seen in the sub-DLA, which has a neutral hydrogen column density log N(H I)=19.82+/-0.05. The absorber is at a velocity of 1820+/-250 km/s from the quasar; however, its low metallicity [O/H]=-1.64+/-0.07, lack of partial coverage, lack of temporal variations between observations taken in 2003 and 2006, and non-detection of N V imply the absorber is not…
Warm Dark Matter (WDM) has been invoked to resolve apparent conflicts of Cold Dark Matter (CDM) models with observations on subgalactic scales. In this work we provide a new and independent lower limit for the WDM particle mass (e.g. sterile neutrino) through the analysis of image fluxes in gravitationally lensed QSOs. Starting from a theoretical unperturbed cusp configuration we analyze the effects of intergalactic haloes in modifying the fluxes of QSO multiple images, giving rise to the so-called anomalous flux ratio. We found that the global effect of such haloes strongly depends on their m…
We have reanalyzed the existing data on Zinc abundances in damped Ly-alpha (DLA) absorbers to investigate whether their mean metallicity evolves with time. Most models of cosmic chemical evolution predict that the mass- weighted mean interstellar metallicity of galaxies should rise with time from a low value ~ 1/30 solar at z ~ 3 to a nearly solar value at z ~ 0. However, several previous analyses have suggested that there is little or no evolution in the global metallicity of DLAs. We have used a variety of statistical techniques to quantify the global metallicity-redshift relation and its un…
We present a quantitative analysis of the abundances of heavy elements in damped Ly-alpha galaxies in the sample of Lu et al. (1996). In particular, we compare the observed gas-phase abundances with those expected when the intrinsic (i.e., nucleosynthetic) pattern is the same as that in either the Sun or in Galactic halo stars and when the depletion pattern is the same as that in the warm Galactic interstellar medium, but with various values of the dust-to-metals ratio. We find that the observations are equally consistent with the solar and halo-star intrinsic patterns and that they favor some…
We apply phase-space density considerations to obtain lower bounds on the mass of sterile neutrino as dark matter candidate. The bounds are different for non-resonant production, resonant production in the presence of lepton asymmetry and production in decays of heavier particles. In the former case our bound is comparable to, but independent of the Lyman-alpha bound, and together with X-ray upper limit it disfavors non-resonantly produced sterile neutrino dark matter. An interesting feature of the latter case is that warm dark matter may be composed of heavy particles.
The Warm Ionized Medium (WIM), also referred to as Diffuse Ionized Gas, contains most of the mass of interstellar medium in ionized form, contributing as much as 30% of the total atomic gas mass in the solar neighborhood. The advent of CCDs has enabled unprecedented study of this medium in external galaxies, probing a variety of environments. In particular, we can derive the morphology of the WIM, its distribution across disks, and the correlation with other Population I material. Spectroscopy of the WIM makes it possible to test various ionization models. I will review here our current unders…
We use high-resolution UV spectra taken by the Space Telescope Imaging Spectrograph instrument on board the Hubble Space Telescope to study the 5.5 pc line of sight to the K0 V star 36 Oph A. The one interstellar component detected for this line of sight has a velocity inconsistent with the local interstellar cloud (LIC) flow vector, but consistent with the flow vector of the so-called G cloud, a very nearby warm cloud in the Galactic Center direction. From our data, we measure the following values for the interstellar temperature, nonthermal velocity, H I column density, and D/H value: T=5900…
We have searched for molecular hydrogen in damped Lyman-alpha (DLA) and sub-DLA systems at z>1.8 using UVES at the VLT. Out of the 33 systems in our sample, 8 have firm and 2 have tentative detections of associated H2 absorption lines. Considering that 3 detections were already known from past searches, H2 is detected in 13 to 20 percent of the newly-surveyed systems. We report new detections of molecular hydrogen at z=2.087 and 2.595 toward, respectively, Q 1444+014 and Q 0405-443, and also reanalyse the system at z=3.025 toward Q 0347-383. We find that there is a correlation between metallic…
We report results from our mini-survey for molecular hydrogen in eight high redshift damped Lyman-alpha (DLA) systems using the ESO Ultra-violet and Visible Spectrograph on the VLT. In addition, we investigate two systems using ESO public data. We include in the sample the only system where H2 was previously detected and studied at high-spectral resolution. Altogether our sample consists of eleven absorbers with 1.85<z_abs<3.4. We confirm the presence of H2 in the z_abs = 2.3377, metal-poor ([Si/H] = -1.20) system toward PKS 1232+082. The derived molecular fraction, f = 2N(H2)/(2N(H2)+N(HI)) =…
Damped Lyman-alpha absorbers (DLAs), seen in absorption against a background quasar, provide the most detailed probes available of element abundances in the Universe over > 90 % of its age. DLAs can be used to observationally measure the global mean metallicity in the Universe and its evolution with time. Paradoxically, these observations are more difficult at lower redshifts, where the absorber rest-frame UV spectra are cut-off due to the atmospheric absorption. We present here high-resolution VLT/UVES observations of several elements contained in three DLAs and one sub-DLA with 0.6<z_abs<0.9…
We present constraints on the amplitude and shape of the matter power spectrum and the density of dark matter within the framework of a standard LambdaCDM model. We use a Markov Chain Monte Carlo approach to combine independent measurements of the three dimensional weak gravitational lensing shear field by the COSMOS survey, of low and high resolution Ly-alpha forest flux power spectrum by SDSS and LUQAS, and of Cosmic Microwave Background temperature and polarization anisotropies by WMAP. We note good agreement between the amplitude of the matter power spectrum on intermediate and small scale…
We present a study of C IV absorption in a sample of 63 damped Lyman-alpha (DLA) systems and 11 sub-DLAs in the redshift range 1.75<z_abs<3.61, using a dataset of high-resolution (6.6 km/s FWHM), high signal-to-noise VLT/UVES spectra. Narrow and broad C IV absorption line components indicate the presence of both warm, photoionized and hot, collisionally ionized gas. We report new correlations between the metallicity (measured in the neutral-phase) and each of the C IV column density, the C IV total line width, and the maximum C IV velocity. We explore the effect on these correlations of the su…
We study the clustering of matter in the intergalactic medium from the Lyman-alpha forests seen in the spectra of pairs or groups of z ~ 2 quasars observed with FORS2 and UVES at the VLT-UT2 Kueyen ESO telescope. The sample consists of five pairs with separations 0.6, 1, 2.1, 2.6 and 4.4 arcmin and a group of four quasars with separations from 2 up to 10 arcmin. This unprecedented data set allows us to measure the transverse flux correlation function for a range of angular scales. Correlations are clearly detectable at separations smaller than 3 arcmin. The shape and correlation length of the …
We describe a technique that for the first time measures star formation rates (SFRs) in damped Lyman alpha systems(DLAs) directly. We assume that massive stars form in DLAs, and that the FUV radiation they emit heats the gas by the grain photoelectric mechanism. We infer the heating rate from the cooling rate measured by the strength of CII* 1335.7 absorption. Since the heating rate is proportional to the dust-to-gas ratio and the SFR per unit area, we deduce the SFR per unit area for DLAs in which both quantities have been measured. We consider models in which the the dust comprises carbonace…
We report coincident damped Lyman alpha (DLA) and sub-DLA absorption at z = 2.66 and z = 2.94 towards the z ~ 3 13.8 arcsecond separation binary quasar SDSS 1116+4118 AB. At the redshifts of the absorbers, this angular separation corresponds to a proper transverse separation of ~ 110 kpc. A third absorber, a sub-DLA at z = 2.47, is detected towards SDSS 1116+4118 B, but no corresponding high column density absorber is present towards SDSS 1116+4118 A. We use high resolution galaxy simulations and a clustering analysis to interpret the coincident absorption and its implications for galaxy struc…
We are living in a dusty universe: dust is ubiquitously seen in a wide variety of astrophysical environments, ranging from circumstellar envelopes around cool red giants to supernova ejecta, from diffuse and dense interstellar clouds and star-forming regions to debris disks around main-sequence stars, from comets to interplanetary space to distant galaxies and quasars. In this review I focus on interstellar grains, with particular emphasis on the extinction (absorption plus scattering) and emission properties of cold submicron-sized ``classical'' grains which, in thermal equilibrium with the a…
We examine the Ly-alpha absorber population at z<0.3 detected in spectra of the QSOs PG0953+415 and H1821+643 taken with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. We compare their statistical properties to those in carefully-constructed mock quasar spectra drawn from a cosmological hydrodynamic simulation of a Lambda-CDM universe. We find very good agreement in the column density and b-parameter distributions, down to the smallest observable absorbers with N_HI\approx 10^{12.3} cm^-2. The observed absorber population is complete for N_HI\ga 10^{13} cm^-2, with…
Intergalactic absorbers along lines of sight to distant quasars are a powerful diagnostic for the evolution and content of the intergalactic medium (IGM). In this study, we use the FUSE satellite to search 129 known lya absorption systems at z<0.15 toward 31 AGN for corresponding absorption from higher Lyman lines and the important metal ions OVI. We detect OVI in 40 systems over a smaller range of column density than seen in HI. The co-existence of OVI and HI suggests a multiphase IGM, with warm photoionized and hot ionized components. With improved OVI detection statistics, we find a steep d…
