In an effort to study Damped Lyman Alpha galaxies at low redshift, we have been using the Sloan Digital Sky Survey to identify galaxies projected onto QSO sightlines and to characterize their optical properties. For low redshift galaxies, the HI 21cm emission line can be used as an alternate tool for identifying possible DLA galaxies, since HI emitting galaxies typically exhibit HI columns that are larger than the classical DLA limit. Here we report on follow-up HI 21 cm emission line observations of two DLA candidates that are both low-redshift spiral galaxies, Mrk 1456 and SDSS J211701.26-002633.7. The observations were made using the Green Bank and Arecibo Telescopes, respectively. Analysis of their HI properties reveal the galaxies to be about one and two M_HI* galaxies, respectively, and to have average HI mass, gas-richness, and gas mass fraction for their morphological types. We consider Mrk 1456 and SDSS J211701.26-002633.7 to be candidate DLA systems based upon the strength of the CaII absorption lines they cause in their QSO's spectra, and impact parameters to the QSO that are smaller than the stellar disk. Compared to the small numbers of other HI-detected DLA and candidate DLA galaxies, Mrk 1456 and SDSS J211701.26-002633.7 have high HI masses. When compared with the expected properties of low-z DLAs from an HI-detected sample of galaxies, Mrk 1456 and SDSS J211701.26-002633.7 fall within the ranges for impact parameter and B-band absolute magnitude; and the HI mass distribution for the HI-detected DLAs agrees with that of the expected HI mass distribution for low-z DLAs. Our observations support galaxy-evolution models in which high mass galaxies make up an increasing contribution to the DLA cross-section at lower redshifts. [abridged]
Disappearance of a narrow Mg II absorption system with a measured velocity up to $\rm 166,000 km s^{-1}$
类星体J152645.61+193006.7（$z_{\rm e}=2.5771$）于2006年5月31日被斯隆数字巡天（SDSS）观测到，并于2011年4月9日再次被观测到。在类星体静止框架下，SDSS两次观测的时间间隔为497.4天。从这个类星体的光谱中，我们探测到一个窄的$\rm Mg II λ\lambda2796,2803$吸收系统的消失现象，该吸收系统相对于类星体的速度高达$\rm 166,129 km s^{-1}$。这种消失事件可能是由吸收气体电离状态的变化或吸收气体穿过类星体视线的整体运动引起的。覆盖率分析表明，这种吸收剂部分覆盖的有效覆盖率为$C_{\rm f}=0.40\pm0.06$的背景发射源。时间变化分析和覆盖率分析表明，这种吸收体可能是类星体的本征吸收体。然而，根据目前的可用数据，不能排除位于$z=0.9170$的宇宙学分离的前景物体的情况。
