Lyα absorption in the nearby Universe: the sightline to Q1821+643

We present the first results of a survey designed to understand the origin of Lyα-forest absorption systems at low redshift. Using the WYFFOS and HYDRA multifibre spectrographs on the William Herschel and Wisconsin Indiana Yale NOAO (WIYN) telescopes, we have identified 51 galaxies brighter than b _j := :18.5 within 30 arcmin of the sightline of the QSO 1821+643. We find three galaxies within 500 h ⁻¹ kpc of the QSO sightline; the nearest galaxy is 104 h ⁻¹ kpc away from the line of sight, and is at the same redshift as a strong ( W _r :=:0.63Å) Lyα absorption line. The remaining two galaxies have no corresponding absorption to extremely low equivalent width limits (< 0.05 Å). Beyond 500 h ⁻¹ kpc, Lyα absorption lines are found at redshifts similar to those of several galaxies, but we show that these coincidences are likely to be accidental.   Half of the Lyα systems for which we could have found at least an L ^* galaxy have no galaxies at the redshifts of the absorbers. For the majority of the remainder, we show that any apparent association with galaxies is probably coincidental. These Lyα systems are characterized by their weak equivalent widths ( W _r :<:0.2Å), and we conclude that this population of absorbers is uncorrelated, or at best weakly correlated, with galaxies.     

The low-redshift evolution of the Lyman-β forest

The low-redshift evolution of the intergalactic medium is investigated using hydrodynamic cosmological simulations. The assumed cosmological model is a critical density cold dark matter universe. The imposed uniform background of ionizing radiation has the amplitude, shape and redshift evolution as computed from the observed quasar luminosity function by Haardt &38; Madau. We have analysed simulated Lyman-α spectra using Voigt-profile fitting, mimicking the procedure with which quasar spectra are analysed. Our simulations reproduce the observed evolution of the number of Lyman-α absorption lines over the whole observed interval of z  = 0.5 to 4. In particular, our simulations show that the decrease in the rate of evolution of Lyman-α absorption lines at z  ≤ 2, as observed by the Hubble Space Telescope , can be explained by the steep decline in the photoionizing background resulting from the rapid decline in quasar numbers at low redshift.     

Magnetic spiral arms in galaxies

The Doppler parameter distribution of Lyα absorption lines is calculated for a set of numerical simulations with different re-ionization histories. The differences in temperature between different re-ionization histories are as large as a factor of 3–4 depending on the spectrum of the ionizing sources and the redshift of helium re-ionization. These temperature differences result in observable differences in the Doppler parameter distribution. The best agreement with the observed Doppler parameter distribution between redshifts of 2 and 4 is found if hydrogen and helium are re-ionized simultaneously at or before a redshift of 5 with a quasar-like spectrum.     

Probing the Universe with the Ly α forest — II. The column density distribution

I apply the well controlled hydro-PM (HPM) approximation of Gnedin &38; Hui to model the column density distribution of the Ly α forest for 25 different flat cosmological scenarios, including variants of the standard cold dark matter (CDM), tilted CDM, CDM with a cosmological constant, and cold + hot dark matter (CHDM) models. I show that, within the accuracy of the HPM approximation, the slope of the column density distribution reflects the degree of non-linearity of the cosmic gas distribution and is a function of the rms linear density fluctuation at the characteristic filtering scale only. The amplitude of the column density distribution, expressed as the value for the ionizing intensity, is derived as a function of the cosmological parameters (to about 40 per cent accuracy). The observational data are currently consistent with the value for the ionizing intensity being constant in the redshift interval z  ∼ −4.     

Application of infrared spectroscopy to studies of silicate glass structure: Examples from the melilite glasses and the systems Na2O-SiO2 and Na2O-Al2O3-SiO2

Infrared (IR) and Raman spectroscopic methods are important complementary techniques in structural studies of aluminosilicate glasses. Both techniques are sensitive to small-scale (<15 Å) structural features that amount to units of several SiO₄ tetrahedra. Application of IR spectroscopy has, however, been limited by the more complex nature of the IR spectrum compared with the Raman spectrum, particularly at higher frequencies (1200–800 cm^?1) where strong antisymmetric Si-O and Si-O-Si absorptions predominate in the former. At lower frequencies, IR spectra contain bands that have substantial contributions from ‘cage-like’ motions of cations in their oxygen co-ordination polyhedra. In aluminosilicates these bands can provide information on the structural environment of Al that is not obtainable directly from Raman studies. A middle frequency envelope centred near 700 cm^?1 is indicative of network-substituted AlO₄ polyhedra in glasses with Al/(Al+Si)>0·25 and a band at 520–620cm^?1 is shown to be associated with AlO₆ polyhedra in both crystals and glasses. The IR spectra of melilite and melilite-analogue glasses and crystals show various degrees of band localization that correlate with the extent of Al, Si tetrahedral site ordering. An important conclusion is that differences in Al, Si ordering may lead to very different vibrational spectra in crystals and glasses of otherwise gross chemical similarity.     

Researches on the stability of porphyra-334 solution and its influence factors

1　Introduction　Ultravioletradiationisahighlyactivecomponentofsolarspectrum .Ultravioletradiationhasdetrimentaleffectsonlivingthingsexposedtosunlight ,includingthedestructionofDNA ,proteinsandothermolecules(Harm ,1980 ) ,inhibitionofphotosynthesisandgrow…     

X-ray photoelectron study of oxygen bonding in crystalline C–S–H phases

We recorded the photoelectron spectra of various crystalline calcium silicate hydrates (C–S–Hs) and have examined their O 1 s photoelectron spectra. The spectra are asymmetric, with contributions assigned primarily to bridging and non-bridging oxygen species. There is an increased contribution due to the presence of non-bonding oxygen atoms with increasing calcium:silicon ratio. Additionally, there are slight changes in theO 1s-binding energies with changes in calcium:silicon ratio. These changes are explained in terms of bonding and silicate structure.     

Insights into oxygen-cation bonding in fresnoite-type structures from O <Emphasis Type="Italic">K</Emphasis>- and Ti <Emphasis Type="Italic">L</Emphasis><Subscript>23</Subscript>-electron energy-loss spectra and ab initio calculations of the electronic structure

O K- and Ti L₂₃-core-loss spectra of fresnoite Ba₂TiSi₂O₈ (BTS) and Sr₂TiSi₂O₈ (STS), which is isotypic to BTS, have been measured by electron energy-loss spectroscopy (EELS). The energy-loss near-edge structures (ELNES) of the O K edge have been identified on the basis of theoretical simulations and interpretations of the X-ray absorption near-edge structures (XANES), which have been modelled in the framework of self-consistent full multiple-scattering (FMS) theory using FEFF8. Herewith, the K-absorption spectra of oxygen (E) and the local partial electron density of states (DOS) of all atoms have been calculated. For BTS, the observed spectral features in the O K-edge spectra are interpreted in terms of mixing between the central O p and neighbouring Ba 5d and 4f, Si 3p and 3d, and Ti 3d orbitals. The observed differences in the O K-edge spectra for STS and BTS can mainly be attributed to three properties: (1) The lack of high local partial Sr unoccupied DOS with 4f symmetry near the Fermi level compared to the high Ba 4f unoccupied DOS results in differences of overlapping O 2p – cation orbitals. (2) The differences in the ionic radii of Sr and Ba result in a larger unit cell for BTS and, thus, in larger oxygen-cation bonding distances. (3) In comparison to STS, the strength of the incommensurate 2-D structural modulation is significantly weaker in BTS, i.e. distortions of coordination polyhedra occur to a much lesser extent. All these effects alter the oxygen-cation hybridization and, hence, result in a variation of the O 1s p transition and consequently of the O K-edge spectral shape. The observed peak broadening in Ti L₂₃ ELNES of STS compared to BTS is correlated with strong displacive modulations hosted in STS.     

海洋顶层铅垂温差分析(英文)

本文依据海洋顶层水温的铅垂分布特征,利用Prandtl相似假设,提出描述海洋顶层铅垂温度廓线的一种模式。在该模式中考虑到风浪和漂流的效应,且对有关热通量、浪与流的特征量作参量化表示。最后,应用该模式对海洋顶层的典型情况作验算,研究表明本模式与现实相符合。