标题： 强吸收莱曼α星系的化学演化：红移 z > 2 时的 [S/Zn] 丰度比 显示英文标题

标题： Chemical Evolution of Damped Ly alpha galaxies: The [S/Zn] abundance ratio at redshift z > 2

摘要： 相对元素丰度，特别是 α/Fe 比率，是研究强吸收线 (damped Lyα 系统, DLAs) 化学演化的重要诊断工具。S/Zn 比不受尘埃选择性消光的影响，是 α/Fe 比率的一个极佳估算值。我们报告了 6 个新的强吸收线系统（zabs ≥ 2）的硫丰度测定，这些系统的锌丰度已知。结合现有文献中的数据，可以测量总共 11 个高红移 DLA 系统的 S/Zn 丰度比。观测到的[S/Zn] 比未显示出银河系贫金属星（相同金属丰度 [Zn/H] ~ -1）中观测到的典型[α/Fe] 增强现象。DLA 数据的行为与随金属丰度[Zn/H] 增加 [S/Zn] 比降低的一般趋势一致。这可能是强吸收线系统化学演化过程中预期的 α/Fe 比下降的第一个证据。然而，与我们在银河系中的观察相反，α/铁峰元素比在金属丰度较低时（[Zn/H] < -1）似乎达到太阳值，并在更高金属丰度下低于太阳值。α/Fe 比的变化挑战了高红移 DLA 系统是旋涡星系前身这一常见假设，反而支持它们起源于低恒星形成率的星系，这与低红移 DLA 的成像研究结果一致，在这些研究中候选 DLA 星系显示出各种形态类型，包括矮星系和低表面亮度星系，并且只有少数为旋涡星系。 显示英文摘要

摘要： Relative elemental abundances, and in particular the alpha/Fe ratio, are an important diagnostic tool of the chemical evolution of damped Ly alpha systems (DLAs). The S/Zn ratio is not affected by differential dust depletion and is an excellent estimator of the alpha/Fe ratio. We report 6 new determinations of sulphur abundance in DLAs at zabs greater than or equal to 2 with already known zinc abundances. The combination with extant data from the literature provides a measure of the S/Zn abundance ratio for a total of 11 high redshift DLA systems. The observed [S/Zn] ratios do not show the characteristic [alpha/Fe] enhancement observed in metal-poor stars of the Milky Way at comparable level of metallicity ([Zn/H] ~ -1). The behaviour of DLAs data is consistent with a general trend of decreasing [S/Zn] ratio with increasing metallicity [Zn/H]. This would be the first evidence of the expected decrease of the alpha/Fe ratio in the course of chemical evolution of DLA systems. However, in contrast to what observed in our Galaxy, the alpha/iron-peak ratio seems to attain solar values when the metallicity is still low ([Zn/H] < -1) and to decrease below solar values at higher metallicities. The behaviour of the alpha/Fe ratio challenges the frequently adopted hypothesis that high redshift DLAs are progenitors of spiral galaxies and favours instead an origin in galaxies characterized by low star formation rates, in agreement with the results from imaging studies of low redshift DLAs, where the candidate DLA galaxies show a variety of morphological types including dwarfs and LSBs and only a minority of spirals.