MR. SOURADEEP BHATTACHARYA

Andromeda (M31), the nearest giant spiral to the Milky Way, is rich in inner-halo substructures observed as stellar overdensities in photometric surveys. Planetary Nebulae (PNe) are excellent tracers of light, chemistry and motion in galaxies and can be firmly associated with M31 with negligible contamination from the Milky Way halo stars and background galaxies. With the MegaCam@CFHT, we survey the central 54 sq. degrees of M31, covering the disc and inner halo, using a narrow-band filter centered on the [OIII] 5007Å emission line and a broad-band g filter. We identify ~5000 PNe in M31 using continuum subtracted narrow-band images, of which ~4000 are newly discovered. This is the largest sample of PNe in any galaxy. We obtain the PN Luminosity Function ~6 mag below the bright cut-off, showing an unprecedented steep rise in the number of PNe ~5 mag below the bright cut-off. This has important implications for understanding post-AGB stellar evolution. From spectroscopic observations of PNe in the M31 disc, with Hectospec@MMT, we obtain kinematics of the thin and thicker disc of M31 from high- and low- extinction PNe respectively. We are able to assign ages of ~2.5 Gyr and ~4.5 Gyr to the high- and low- extinction PNe populations respectively. We thus obtain the age-velocity dispersion relation (AVR) in the M31 disc. This is the first identification of the thin and thicker disc of M31, and its AVR with clear age separation. The thin and thicker disc of M31 is found to be 2 and 3 times dynamically hotter than the corresponding MW disc populations of comparable age, indicating a recent major merger event between 2.5 and 4.5 Gyr ago. Further spectroscopic follow up will provide information on the kinematics and oxygen abundance of the inner halo substructures of M31.