Being the most luminous member of the Local Group and the nearest archetypical spiral galaxy, M31 serves as the best astrophysical laboratory for the studies of the physical and astrophysical processes that govern the structure, kinematics and the formation and evolution of distant galaxies. Recent deep optical surveys have revealed complex substructures within hundreds of kpc of M31, with some of them stretching from M31 all the way to M33, which is about 200 kpc from M31, suggesting a possible recent close encounter of the two galaxies (Ibata et al. 2007; McConnachie et al. 2009). Detailed chemical and kinematic investigations of M31 and associated substructures are vital for our understanding of M31, and also for the theory of galaxy formation and evolution in general. However, at the distance of M31, even a luminous red giant branch star has an I magnitude fainter than 20, hence high resolution spectroscopic determinations of their chemical and kinematic information are no easy tasks even for a 10m class telescope (Ibata et al. 2005; Gilbert et al. 2009).

  　Finding background quasars in the vicinity of M31 have the following two potential important applications. Firstly, the background quasars can serve as an ideal reference frame which would allow highly accurate astrometric measurement of the minute proper motion (PM) of M31 and also the PMs of its associated coherent substructures. Secondly, absorption-line spectroscopy of bright background quasars can be used to probe the distribution, chemical composition and kinematics of the interstellar medium (ISM) of M31, the Milky Way and the intergalactic medium (IGM) of the Local Group of galaxies. This technique can be used to probe structures further out from the center of the galaxy than would be possible with traditional stellar spectroscopy, or using HI 21cm observations.

  　We search for background quasars in the vicinity of M31 and M33 based on the Guoshoujing Telescope (Large Sky Area Multi-Object Fiber Spectroscopic Telescope; LAMOST), the National Major Scientific Project. More than 500 new quasars are discovered in a stripe of ~135 deg2 from M31 to M33 along the Giant Stellar Stream during the 2010-2011 commissioning observations and the pilot survey (Fig. 1; Huo et al. 2013). For comparison, 75 quasars in the outer halo of M31 presented by SDSS, 155 previously known quasars with redshift estimates reported in the NED archive within a 10 deg. radius of M31 and of M33, these newly-discovered quasars represent a significant increase of the number of identified quasars in the vicinity of M31 and M33. A total of 93 quasars are now known with locations within 2.5 deg (about 35 kpc) of M31, of which 73 are newly discovered. The much enlarged sample of known quasars behind the Giant Stellar Stream, the extended halo and its associated substructures of M31, can potentially be utilized to construct a perfect astrometric reference frame to measure the minute PMs of M31 and M33, along with the PMs of substructures associated with the Local Group of galaxies. There are now 26, 62 and 139 known quasars in this region of the sky with i magnitudes brighter than 17.0, 17.5 and 18.0 respectively, of which 5, 20 and 75 are newly discovered. These bright quasars provide an invaluable collection with which to probe the chemistry and kinematics of the ISM/IGM in the Local Group of galaxies. These 500 quasars in the vicinity of M31 and M33 discovered with the LAMOST, are the largest quasar sample in this region.

 Fig 1: Spatial distributions of background quasars in the vicinity of M31 and M33. Orange, cyan and blue filled circles represent quasars identified in the LAMOST 2009, 2010 and 2011 datasets, respectively. Crosses and open squares represent SDSS quasars and previously known quasars with redshifts archived in the NED, respectively. The magenta stars mark the central positions of M31 and M33, while the magenta ellipse represents the optical disk of M31 of radius R25 = 95.3 arcmin. The insert panel displays the deep photometric image of M31/M33 area obtained with CFHT (McConnachie et al. 2009).