It has been widely accepted that large galaxies grow in mass through continual accretion of smaller ones.  These galaxy mergers result in a smooth, extended halo with loosely bound stars surrounding the central galaxy.  Remnants of the cannibalized galaxies exist in the halo in the form of stellar substructures (such as streams).  Previous wide-field photometric surveys have revealed numerous stellar streams in the outer halo of the Andromeda Galaxy (M31), a nearby large spiral system that resembles our Milky Way in morphology.  In order to study the properties of substructures and backtrace the merger history of M31, we have been obtaining deep spectroscopy of planetary nebulae (PNe) that are associated with the stellar streams in M31’s halo, using the 10.4-meter Gran Telescopio Canarias (GTC, Observatory of Roque de los Muchachos, La Palma, Spain). In this talk, I will introduce new findings based on our GTC observations and discuss their significance in the context of the current grand picture, as inferred from previous wide-field photometric studies, that M31’s giant halo is a result of

complex galactic interactions and merger processes.

 

  I will also briefly introduce the GTC instruments that are available to the Chinese astronomers for 2018B.