We report observations of four sub-damped Lyman-alpha (sub-DLA) quasar absorbers at z<0.5 obtained with the Hubble Space Telescope Cosmic Origins Spectrograph. We measure the available neutrals or ions of C, N, O, Si, P, S, Ar, Mn, Fe, and/or Ni. Our data have doubled the sub-DLA metallicity samples at z<0.5 and improved constraints on sub-DLA chemical evolution. All four of our sub-DLAs are consistent with near-solar or super-solar metallicities and relatively modest ionization corrections; observations of more lines and detailed modeling will help to verify this. Combining our data with measurements from the literature, we confirm previous suggestions that the N(HI)-weighted mean metallicity of sub-DLAs exceeds that of DLAs at all redshifts studied, even after making ionization corrections for sub-DLAs. The absorber toward PHL 1598 shows significant dust depletion. The absorbers toward PHL 1226 and PKS 0439-433 show the S/P ratio consistent with solar, i.e., they lack a profound odd-even effect. The absorber toward Q0439-433 shows super-solar Mn/Fe. For several sub-DLAs at z<0.5, [N/S] is below the level expected for secondary N production, suggesting a delay in the release of the secondary N or a tertiary N production mechanism. We constrain the electron density using Si II* and C II* absorption. We also report different metallicity vs. Delta V_90 relations for sub-DLAs and DLAs. For two sub-DLAs with detections of emission lines from the underlying galaxies, our measurements of the absorption-line metallicities are consistent with the emission-line metallicities, suggesting that metallicity gradients are not significant in these galaxies.
We investigate the spatial distribution of Lyman-$α$ (Ly $α$) absorbers within cosmic voids. We create a catalogue of cosmic voids in Sloan Digital Sky Survey Data Release 7 (SDSS DR7) with the VoidFinder algorithm of the Void Analysis Software Toolkit (VAST). Using the largest catalogue of low-redshift (z $\leq$ 0.75) IGM absorbers to date, we identify 392 Ly $α$ absorbers inside voids. The fraction of Ly $α$ absorbers inside voids (65 per cent) is comparable to the volume filling fraction of voids (68 per cent), and significantly greater than the fraction of galaxies inside voids (21 per cen…
We estimate the abundance of dust in damped Lyman-α absorbers (DLAs) by statistically measuring the excess reddening they induce on their background quasars. We detect systematic reddening behind DLA consistent with the SMC type reddening curve, but it is inconsistent with the Milky Way type reddening. We find that the derived dust-to-gas ratio is, on average, inversely proportional to the column density of neutral hydrogen, implying that the amount of dust is constant, irrespective of the column density of hydrogen. It means that the average metallicity is inversely proportional to the column…
We study the H2 molecular content in high redshift damped Lyman-alpha systems (DLAs) as a function of the HI column density. We find a significant increase of the H2 molecular content around log N(HI) (cm^-2)~21.5-22, a regime unprobed until now in intervening DLAs, beyond which the majority of systems have log N(H2) > 17. This is in contrast with lines of sight towards nearby stars, where such H2 column densities are always detected as soon as log N(HI)>20.7. This can qualitatively be explained by the lower average metallicity and possibly higher surrounding UV radiation in DLAs. However, unl…
We examine the kinematic structure of Damped Lyman-alpha Systems (DLAs) in a series of cosmological hydrodynamic simulations using the AREPO code. We are able to match the distribution of velocity widths of associated low ionisation metal absorbers substantially better than earlier work. Our simulations produce a population of DLAs dominated by halos with virial velocities around 70 km/s, consistent with a picture of relatively small, faint objects. In addition, we reproduce the observed correlation between velocity width and metallicity and the equivalent width distribution of SiII. Some disc…
We present a comprehensive analysis of the spin temperature/covering factor degeneracy, T/f, in damped Lyman-alpha absorption systems. By normalising the upper limits and including these via a survival analysis, there is, as previously claimed, an apparent increase in T/f with redshift at z > 1. However, when we account for the geometry effects of an expanding Universe, neglected by the previous studies, this increase in T/f at z > 1 is preceded by a decrease at z < 1. Using high resolution radio images of the background continuum sources, we can transform the T/f degeneracy to T/d^2, where d …
We present new estimates for the statistical properties of damped Lyman-$α$ absorbers (DLAs). We compute the column density distribution function at $z>2$, the line density, $\mathrm{d}N/\mathrm{d}X$, and the neutral hydrogen density, $Ω_\mathrm{DLA}$. Our estimates are derived from the DLA catalogue of Garnett 2016, which uses the SDSS-III DR12 quasar spectroscopic survey. This catalogue provides a probability that a given spectrum contains a DLA, allowing us to use even the noisiest data without biasing our results and thus substantially increase our sample size. We measure a non-zero column…
The explosion energy of early stellar populations: The Fe-peak element ratios in low metallicity damped Lyman-alpha systems
铁峰元素（钛锌）在最低金属丰度的相对丰度与核心坍缩超新星爆炸的物理学密切相关。以25个贫金属阻尼莱曼α体系为样本，研究了铁峰元素比值随金属丰度的变化趋势。对于25种DLA中的9种，可以从检测到的吸收线确定Ti、Cr、Co、Ni、Zn/Fe丰度比中的一种或多种的直接测量值（或有用的上限）。对于其余的系统（没有检测），我们设计了一种新的形式的光谱叠加估计典型的铁峰元素比的DLA人口在这个金属丰度制度。我们比较这些数据类似的测量贫金属恒星的星系晕和详细的计算爆炸核合成在无金属恒星。我们的结论是，我们样本中的大多数DLA都是由那些在核心坍缩超新星爆炸时释放能量小于1.2 x 10^51尔格的恒星富集的。最后，我们讨论了当30米级望远镜出现时，测量Fe/H < 1/1000太阳的阻尼莱曼α系统中Fe峰元素比的令人兴奋的前景。只有到那时，我们才能确定第一批恒星的超新星释放的能量。
We measure the effect of high column density absorbing systems of neutral hydrogen (HI) on the one-dimensional (1D) Lyman-alpha forest flux power spectrum using cosmological hydrodynamical simulations from the Illustris project. High column density absorbers (which we define to be those with HI column densities $N(\mathrm{HI}) > 1.6 \times 10^{17}\,\mathrm{atoms}\,\mathrm{cm}^{-2}$) cause broadened absorption lines with characteristic damping wings. These damping wings bias the 1D Lyman-alpha forest flux power spectrum by causing absorption in quasar spectra away from the location of the absor…
Correlations measured in three dimensions in the Lyman-alpha forest are contaminated by the presence of the damping wings of high column density (HCD) absorbing systems of neutral hydrogen (HI; having column densities $N(\mathrm{HI}) > 1.6\times10^{17}\,\mathrm{atoms}\,\mathrm{cm}^{-2}$), which extend significantly beyond the redshift-space location of the absorber. We measure this effect as a function of the column density of the HCD absorbers and redshift by measuring 3D flux power spectra in cosmological hydrodynamical simulations from the Illustris project. Survey pipelines exclude regions…
We develop an automated technique for detecting damped Lyman-$α$ absorbers (DLAs) along spectroscopic lines of sight to quasi-stellar objects (QSOs or quasars). The detection of DLAs in large-scale spectroscopic surveys such as SDSS-III sheds light on galaxy formation at high redshift, showing the nucleation of galaxies from diffuse gas. We use nearly 50 000 QSO spectra to learn a novel tailored Gaussian process model for quasar emission spectra, which we apply to the DLA detection problem via Bayesian model selection. We propose models for identifying an arbitrary number of DLAs along a given…
We present the characteristics of the Damped Lyman-$α$ (DLA) systems found in the data release DR16 of the extended Baryon Oscillation Spectroscopic Survey (eBOSS) of the Sloan Digital Sky Survey (SDSS). DLAs were identified using the convolutional neural network (CNN) of~\cite{Parks2018}. A total of 117,458 absorber candidates were found with $2 \leq \zdla \leq 5.5$ and $19.7 \leq \lognhi \leq 22$, including 57,136 DLA candidates with $\lognhi \geq 20.3$. Mock quasar spectra were used to estimate DLA detection efficiency and the purity of the resulting catalog. Restricting the quasar sample t…
We present a semi-analytic model for the Lyman-$α$ forest that is inspired by the Halo Model. This model is built on the absorption line decomposition of the forest. Flux correlations are decomposed into those within each absorption line (the 1-absorber term) and those between separate lines (the 2-absorber term), treating the lines as biased tracers of the underlying matter fluctuations. While the nonlinear exponential mapping between optical depth and flux requires an infinite series of moments to calculate any statistic, we show that this series can be resumed (truncating at the desired ord…
We cross-correlate the positions of damped Lyman-$α$ systems (DLAs) and their parent quasar catalog with a convergence map derived from the Planck cosmic microwave background (CMB) temperature data. We make consistent measurements of the lensing signal of both samples in both Fourier and configuration space. By interpreting the excess signal present in the DLA catalog with respect to the parent quasar catalog as caused by the large scale structure traced by DLAs, we are able to infer the bias of these objects: $b_{\rm DLA}=2.6\pm0.9$. These results are consistent with previous measurements mad…
The distribution of the absorption line broadening observed in the Ly$α$ forest carries information about the temperature and widths of the filaments in the intergalactic medium (IGM). In this work, we present and test a new method for inferring the temperature of the IGM, the size of the absorbing filaments and the photo-ionization rate of hydrogen from the properties of absorption line broadening in the Ly$α$ forest. We apply our method to mock spectra from the reference model of the EAGLE cosmological simulation, and we demonstrate that we are able to reconstruct the IGM properties. Our met…
For around 100 years, hydrogen spectral modelling has been based on Voigt profile fitting. The semi-classical Voigt profile is based on a 2-level atom approximation. Whilst the Voigt profile is excellent for many circumstances, the accuracy is insufficient for very high column density damped Lyman-$α$ absorption systems. We have adapted the quantum-mechanical Kramers-Heisenberg model to include thermal broadening, producing a new profile, the KHT profile. Interactions involving multiple discrete atomic levels and continuum terms, not accounted for in the Voigt model, generate asymmetries in th…
HI content, metallicities and spin temperatures of damped and sub-damped Lyman alpha systems in the redshift desert (0.6 < z < 1.7)
Hl 21 cm吸收光学深度和由Lya吸收导出的N（HI）可以组合以产生DLAs的自旋温度（Ts）。虽然Ts测量存在的样品的DLAs与z <0.6 and z >1.7，中间红移制度目前只包含2 HI 21厘米的检测，导致一个“红移沙漠”，跨越4个陀螺的宇宙时间。为了连接低和高z区域，我们给出了六个0.6<z<1.7 HI 21 cm吸收体的Lya线的观测结果。该数据集是补充VLBA观测（推导出吸收覆盖因子，f），并从中确定金属丰度的光学阶梯光栅光谱。因此，我们的数据集不仅提供了迄今为止最大的HI 21 cm吸收体统计研究，并弥合了红移沙漠，而且也是第一个使用完全f校正的数据集来寻找基于金属性的趋势。与银河系视线中发现的趋势一致，我们发现最低的N（HI）吸收剂往往由温暖的气体主导。在DLA制度，自旋温度显示了更广泛的价值观比银河系的数据，可以预期在一个异质星系人口。然而，我们发现，低金属含量的DLAs占主导地位的小冷气体馏分，只有吸收剂具有相对较高的金属含量表现出显着的冷气体馏分。使用汇编HI 21厘米吸收剂，选择具有f-校正的自旋温度，我们确认金属丰度和Ts之间的反相关性在3.4西格玛的意义。最后，我们的样品中的一个DLA是新发现的HI 21 cm吸收体（z=0.602，朝向J1431+3952），我们发现它具有文献中报道的最低的f-校正自旋温度：Ts=90+-23 …
