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Abstract:
In this talk on the Andromeda (M31) galaxy, I will present the results
from a homogenous, extended survey of Planetary Nebulae covering the
entire disc and inner halo of M31 out to 50 kpc radius. Taken jointly
with observational results from previous extensive investigations,
these findings support a recent (~2.5 Gyr), massive (1:4 mass ratio)
accretion event from a satellite infalling along the Giant stellar
stream, in a nearly radial orbit. This tumultuous recent past of
Andromeda is very different from our own Milky Way galaxy, that
experienced a rather quiescent evolution in the last 8 Gyrs. I will
present the evidence for a younger thin and older thicker disc
component in M31 and present the age-velocity dispersion relation at
the corresponding radial distance of the solar neighbourhood in
M31. Then I explore the chemical composition of these discs using
Oxygen and Argon element abundance. The two kinematically distinct
discs in Andromeda are also chemically distinct, 1) with the thin disc
reaching higher Argon abundances than the thicker disc, 2) the thicker
disc having a positive Oxygen and null Argon radial gradient, 3) while
the thin disc presents a negative radial gradient in both Oxygen and
Argon. The latter gradient is consistent with the negative Oxygen
gradient previously measured for the HII regions. I then use the
log(O/Ar) vs. (12 + log(Ar/H) ) distribution of stars with different
ages to constraint the chemical evolution of the parent stellar
populations in the thin and thicker M31 discs. The distributions in
this plane show that the chemical and also the structural properties
of the thin and thicker discs in M31 are remarkably different from
those determined for the Milky Way. I then investigate the stellar
populations and kinematics of the inner halo substructures - NE and W
shelves, Giant stellar stream and G1 clump - and compare them with the
detailed predictions from simulations of a major merger event in
Andromeda. The goal is to understand whether it is a wet or a dry
merger, and to further constrain the pre-merger M31 disc population
and that of the satellite as well. As part of the forward look, I
shall present the potential information from the combined use of
Oxygen and Argon to constrain the early phases of star formation in
z>4.0 galaxies.
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