Identification of MgII Absorption Line Systems from SDSS Quasar Catalogue

The quasar absorption lines are crucial to our understanding of the Universe since the absorption lines provide a wealth of information on the gaseous Universe from high redshift to present day. The absorption lines can also allow us to probe the metallicity and ionization state of the gas (Wild et al. 2008). Owing to the advent of large spectroscopic surveys such as the Sloan Digital Sky Survey (SDSS; York et al. 2000), tens of thousands of quasar absorption lines can be identified. However, it is very difficult to identify absorption line systems, especially for the weak absorption line system and the spectrum of low signal-to-noise ratio. In the most previous works, the identification of absorption line systems were mainly completed by two steps: estimating the continuum using the principal component analysis (PCA) and autoidentifying the absorption line systems by computer programs (see, e.g., Lundgren et al. 2009). In this work, we provide our method for identifying the MgII absorption line systems.