Recent studies of planetary nebulae and deep color-magnitude diagrams in M 31 have unveiled some unusual star-formation events beyond its spiral arms into its outer regions extending to 100 kpc in projected distance. In this paper I summarize these observations. I shall also highlight some special interpretive surprises suggested by observations of PNe related to their O/H abundances, ages, and the luminosities on PNe well beyond the inner disk.

Recent studies of planetary nebulae and deep color-magnitude diagrams in M 31 have unveiled some unusual star-formation events beyond its spiral arms into its outer regions extending to 100 kpc in projected distance. In this paper I summarize these observations. I shall also highlight some special interpretive surprises suggested by observations of PNe related to their O/H abundances, ages, and the luminosities on PNe well beyond the inner disk.

Globular clusters (GCs) are excellent tracers for the formation, assembly and evolutionary history of their host galaxies. However, their origin and their role in galaxy evolution are still unclear. There are accumulating evidences that a significant fraction of GCs in massive galaxies (e.g., M 31) are accreted during their assembly history (e.g., Mackey et al. 2010). In this contribution, we report the discovery of a possible stream of accreted GCs in M 31 using data from the literature. Unlike previous substructures of GCs identified as clumps in the phase and metallicity space (Ashman, Keith & Christina 1993), the members of this stream are widely spread but tightly correlated in the position-velocity space (see Fig. 1). The tight correlation suggests that they possibly follow very similar orbits. A number of stellar streams have been discovered in the outer halo of M 31 (e.g., Ibata et al. 2001; McConnachie et al. 2009), one of which may be physically associated with the GC stream. If the association is established, it will not only provide a key evidence for accretion origins of some GCs in M 31, but also place a strong constraint on the mass distribution of M 31.

Due to its proximity, size, complex structure and high inclination angle, M31 offers an excellent opportunity for studying galactic structures outside the Milky Way and for drawing implications for their cosmological origin. We have studied the stellar populations of M 31 using the Sloan Digital Sky Survey (SDSS) photometry and the Spitzer far-infrared (FIR) mappings of dust. Combining these data, we have constructed a 3-dimensional model of the galaxy, laying constraints on the intrinsic (dust-free) properties of the galaxy and its stellar populations: their apparent and intrinsic luminosities, luminosity distributions, colours, shapes and sizes. We have interpreted the derived spectral energy distributions with synthetic stellar populations created with the Starburst99 software, in order to constrain the ages and masses of the stellar components.