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首页> 外文期刊>The Astrophysical journal >THE IUE SURVEY FOR DAMPED LYMAN-a AND LYMAN-LIMIT ABSORPTION SYSTEMS: EVOLUTION OF THE GASEOUS CONTENT OF THE UNIVERSE
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THE IUE SURVEY FOR DAMPED LYMAN-a AND LYMAN-LIMIT ABSORPTION SYSTEMS: EVOLUTION OF THE GASEOUS CONTENT OF THE UNIVERSE

机译：阻尼lyman-a和lyman-limit吸收系统的IUE调查：宇宙气态含量的演变

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摘要

We present a spectroscopic survey for moderate-redshift absorption features from damped Lyα and Lyman-limit absorption systems. The survey is based on a set of optimally extracted and co-added International Ultraviolet Explorer spectra of QSOs, BL Lac objects, and Seyfert galaxies. Results of this survey are combined with those of previous surveys carried out at optical wavelengths in order to examine evolution of the gaseous content of the universe over the redshift interval 0.008 approx< z approx< 3.5. The main results of the survey are that (1) the product of the absorption cross section and the comoving spatial number density of damped Lyα absorption systems does not appear to evolve strongly over the redshift interval 0.008 approx< z approx< 3.5, (2) the cosmological mass density Ω_g(z) of neutral gas associated with damped Lyα absorption systems appears to decrease significantly from z ≈ 3.5 to ≈ 0.008, and (3) the observed evolution of Ω_g(z) results from a steady decrease in the incidence of high column density absorption systems with decreasing redshift. Together these results suggest that the average mass of neutral gas per absorption system decreases with time, which is qualitatively consistent with the hypothesis that the observed evolution of Ω_g(z) results from the consumption of gas into stars at z approx< 3.5. We present additional arguments supporting this hypothesis and assess the implications of the observations of Ω_g(z) for scenarios of galaxy formation and evolution. Under a "closed box assumption" we determine a characteristic epoch z_s of star formation in galaxies of z_s = 2-3 and a characteristic timescale τ_s of star formation in galaxies of τ_S = 0.5-1.0 x 10~9 h~(-1) yr at z approx > 1.5. Considering recent chemical abundance measurements by Pettini et al. and others we conclude that the cosmic metallicity at z ≈ 2.5 is given by log Z(z ≈ 2.5)/Z_☉ = —1.0 ± 0.5. Comparing the inferred cosmological mass density of metals in gas and stars at the epoch z_s with the nominal cosmological mass density of metals in nearby galaxies, we conclude that the characteristic epoch of metal production in galaxies occurred after the characteristic epoch of star formation in galaxies. The implication of this result is that the bulk of stars in nearby galaxies should be metal poor, whereas only a small fraction of the disk stars in the solar neighborhood are actually metal poor. This inconsistency represents a "cosmic G-dwarf problem" in that a straightforward interpretation of our observations predicts many more metal-poor stars than are found in the solar neighborhood. We propose that the cosmic G-dwarf problem may be resolved if damped Lyα absorption systems trace not only galactic disk evolution, as suggested previously by Wolfe, but also galactic spheroid evolution.
机译：我们对来自阻尼的Lyα和Lyman极限吸收系统的中等红移吸收特征进行了光谱调查。该调查基于一组最佳提取和共添加的QSO，BL Lac物体和Seyfert星系的国际紫外线探测器光谱。该调查的结果与先前在光波长下进行的调查的结果相结合，以检查在0.008近似 1.5。考虑到Pettini等人最近的化学丰度测量。其他人得出的结论是，z≈2.5时的宇宙金属性由log Z（z≈2.5）/Z_☉= -1.0±0.5给出。将推定的z_s时代气体和恒星中金属的宇宙质量密度与附近星系中金属的名义宇宙质量密度进行比较，我们得出结论，银河系中金属生产的特征性时期发生在星系中恒星形成的特征性时期之后。该结果的含义是，附近星系中的大部分恒星应该是金属贫乏的，而在太阳附近的盘状恒星中只有一小部分实际上是金属贫乏的。这种不一致表示“宇宙G矮问题”，因为对我们观测结果的简单解释预测，与太阳附近相比，金属贫乏的恒星要多得多。我们建议，如果阻尼的Lyα吸收系统不仅像沃尔夫先前所暗示的那样跟踪银河系盘的演化，而且还跟踪银河系球体的演化，那么可以解决宇宙G矮问题。

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