标题： 低红移MgII宽吸收线类星体来自斯隆数字天空调查 显示英文标题

标题： Low-z MgII Broad Absorption-Line Quasars from the Sloan Digital Sky Survey

摘要： 我们展示了一个包含68个低红移MgII低电离宽吸收线(loBAL)类星体的样本。 该样本根据以下标准从SDSS5中统一选取：(1) 0.4<z<0.8，(2) 中值信噪比>7，(3) MgII吸收线宽度>1600\kms 。 最后一个标准是在完整性和与经典BAL类星体定义的一致性之间的权衡，经典BAL类星体在CIV BAL中有`balnicity index' BI>0。 我们采用这样的标准是为了确保约90%的样本是经典的BAL类星体，完整性约为80%，这是基于对高红移类星体样本的广泛测试，这些样本同时测量了CIV和MgII BAL。 我们发现(1) MgII BAL在具有较窄Hbeta发射线、较弱的[OIII]发射线、较强的光学FeII多重态和较高光度的类星体中更频繁地被检测到。 在黑洞吸积系统的基本物理参数方面，具有较高Eddington比率的类星体中loBAL比例显著高于具有较低Eddington比率的类星体。 在所涉及的黑洞质量范围内，该比例不依赖于黑洞质量。 整体比例分布较广，表明吸收物质的覆盖因子范围较大。 (2) [OIII]-弱的loBAL类星体平均显示未检测到[NeV]发射线，且[NeV]与[OIII]的线比非常小。 然而，非BAL类星体中的线比远大于[OIII]-弱的loBAL类星体中的线比，并且与[OIII]线的强度无关。 (3) loBAL和非-loBAL类星体在近红外到光学波段的颜色相似，但在紫外波段的颜色不同。 (4) 具有中间宽度MgII吸收线的类星体在光学发射线特性上与非-loBAL类星体无法区分，但它们的颜色与loBAL类星体相似，比非BAL类星体更红。 我们还讨论了这些结果的含义。 显示英文摘要

摘要： We present a sample of 68 low-z MgII low-ionization broad absorption-line (loBAL) quasars. The sample is uniformly selected from the SDSS5 according to the following criteria: (1) 0.4<z<0.8, (2) median S/N>7, and (3) MgII absorption-line width > 1600 \kms. The last criterion is a trade-off between the completeness and consistency with respect to the canonical definition of BAL quasars that have the `balnicity index' BI>0 in CIV BAL. We adopted such a criterion to ensure that ~90% of our sample are classical BAL quasars and the completeness is ~80%, based on extensive tests using high-z quasar samples with measurements of both CIV and MgII BALs. We found (1) MgII BAL is more frequently detected in quasars with narrower Hbeta emission-line, weaker [OIII] emission-line, stronger optical FeII multiplets and higher luminosity. In term of fundamental physical parameters of a black hole accretion system, loBAL fraction is significantly higher in quasars with a higher Eddington ratio than those with a lower Eddington ratio. The fraction is not dependent on the black hole mass in the range concerned. The overall fraction distribution is broad, suggesting a large range of covering factor of the absorption material. (2) [OIII]-weak loBAL quasars averagely show undetected [NeV] emission line and a very small line ratio of [NeV] to [OIII]. However, the line ratio in non-BAL quasars, which is much larger than that in [OIII]-weak loBAL quasars, is independent of the strength of the [OIII] line. (3) loBAL and non-loBAL quasars have similar colors in near-infrared to optical band but different colors in ultraviolet. (4) Quasars with MgII absorption lines of intermediate width are indistinguishable from the non-loBAL quasars in optical emission line properties but their colors are similar to loBAL quasars, redder than non-BAL quasars. We also discuss the implication of these results.