Peering into the Halo of the Andromeda Galaxy Using Planetary Nebulae

Abstract

It has been widely accepted that in hierarchical cosmology, a large galaxy evolves through accretion/merging of smaller galaxies. The effects of such galaxy interaction are imprinted in the outer disk or halo of a galaxy in the form of stellar condensations (i.e., substructures) like tidal streams. As the nearest large spiral system, the Andromeda Galaxy (M31) is an excellent candidate to study galaxy interaction and evolution. In order to study the halo and substructures of M31 using planetary nebulae (PNe) as tracers of chemistry and kinematics, we carried out very deep spectroscopic observations of a carefully selected sample of PNe using the 10.4m Gran Telescopio Canarias (GTC, La Palma). The target PNe are located in different regions: the substructures (the Northern Spur and the Giant Stream), the outer halo (or the exodisk) of M31, and M32. Our chemical study reveals: 1) the halo PNe, as far as ~180 kpc from galactic center, have oxygen abundances close to the solar value, supporting the current view that the external regions of M31 are the result of complex interaction and merger process; and 2) the substructure PNe have lower oxygen abundances, indicating their different origin.