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Survey of planetary nebulae in Andromeda (M31). discrete tracers in the disc and inner halo
Survey of planetary nebulae in Andromeda (M31). discrete tracers in the disc and inner halo
Andromeda (M31) is the nearest giant spiral galaxy to our Milky Way (MW) and the most massive member of our Local Group, with its diffuse halo spanning over 100 sq. deg. on the sky. Given its proximity, its structure has been widely studied with various tracers. The observations of red-giant-branch stars in M31 resulted in the discovery of a plethora of substructures in its inner halo, revealing its tumultuous recent formation history. M31 thus stands in sharp contrast to the MW which has had no major galaxy mergers over the past ~10 Gyr. Uniform measurements of the kinematics and chemical abundances over the entire major axis of the M31 disc and its inner halo substructures is essential to understand the structure and evolutionary history of M31. Such measurements based on absorption features on the stellar continuum light, via integral field spectroscopy or spectra of individual stars, are beyond what is currently possible even with the most advanced facilities. This is because of the very large angular size of M31, the low surface brightness of the outer regions and the further complication of the contamination from the MW halo stars. Thus, discrete tracers firmly in the M31 system are required to unravel its recent formation history. Planetary Nebulae (PNe), bright emission-line nebulae of the late-phase of stellar evolution, are excellent discrete tracers of light, chemistry and motion in galaxies. In this thesis, we carry out a complete and homogeneous deep narrow band OIII survey with the MegaCam@CFHT to identify PNe in M31, covering its disc and inner halo substructures. We obtain deep PN luminosity functions (PNLFs) corresponding to distinct substructures in the M31 inner halo and disc. We observe a rise in the faint-end of the PNLF that is driven by the fraction of older stars in the parent stellar population. Comparing the PNLFs of the different regions in M31, we find that the Giant Stream, NE and W Shelves are consistent with having formed from debris of a single satellite while the G1 clump is formed from the perturbed M31 disc. Stream-D is composed of stellar remnants from a distinct satellite. Spectroscopic follow-up of a subsample of the PNe in the M31 disc was carried out with Hectospec@MMT. The identified PNe were separated into high- and low-extinction samples having average ages of ~2.5 Gyr and ~4.5 Gyr, forming the dynamically colder thin and dynamically hotter thick discs of M31 respectively beyond a galactocentric radius, RGC=$14 kpc. The two discs are also found to be chemically distinct from the argon abundance distributions of their PNe and thus have distinct origins. This is the first identification of the kinematically and chemically distinct thin and thick discs in M31, which at the equivalent distance of the Sun in M31 have a velocity dispersion twice and thrice that of the stars of the same age in the MW respectively. Such a steep age-velocity dispersion relation in M31 is consistent with a single major merger that occurred 2.5 - 4.5 Gyr ago with a merger mass ratio ~1:5. The radial abundance gradients of the M31 thin and thick disc are also much flatter than expected from secular processes and independently support the massive accretion event with significant impact on the M31 disc structure.
Planetary Nebulae, galaxy evolution, galaxies individual (M31), stellar population
Bhattacharya, Souradeep
2020
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Bhattacharya, Souradeep (2020): Survey of planetary nebulae in Andromeda (M31): discrete tracers in the disc and inner halo. Dissertation, LMU München: Faculty of Physics
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Abstract

Andromeda (M31) is the nearest giant spiral galaxy to our Milky Way (MW) and the most massive member of our Local Group, with its diffuse halo spanning over 100 sq. deg. on the sky. Given its proximity, its structure has been widely studied with various tracers. The observations of red-giant-branch stars in M31 resulted in the discovery of a plethora of substructures in its inner halo, revealing its tumultuous recent formation history. M31 thus stands in sharp contrast to the MW which has had no major galaxy mergers over the past ~10 Gyr. Uniform measurements of the kinematics and chemical abundances over the entire major axis of the M31 disc and its inner halo substructures is essential to understand the structure and evolutionary history of M31. Such measurements based on absorption features on the stellar continuum light, via integral field spectroscopy or spectra of individual stars, are beyond what is currently possible even with the most advanced facilities. This is because of the very large angular size of M31, the low surface brightness of the outer regions and the further complication of the contamination from the MW halo stars. Thus, discrete tracers firmly in the M31 system are required to unravel its recent formation history. Planetary Nebulae (PNe), bright emission-line nebulae of the late-phase of stellar evolution, are excellent discrete tracers of light, chemistry and motion in galaxies. In this thesis, we carry out a complete and homogeneous deep narrow band OIII survey with the MegaCam@CFHT to identify PNe in M31, covering its disc and inner halo substructures. We obtain deep PN luminosity functions (PNLFs) corresponding to distinct substructures in the M31 inner halo and disc. We observe a rise in the faint-end of the PNLF that is driven by the fraction of older stars in the parent stellar population. Comparing the PNLFs of the different regions in M31, we find that the Giant Stream, NE and W Shelves are consistent with having formed from debris of a single satellite while the G1 clump is formed from the perturbed M31 disc. Stream-D is composed of stellar remnants from a distinct satellite. Spectroscopic follow-up of a subsample of the PNe in the M31 disc was carried out with Hectospec@MMT. The identified PNe were separated into high- and low-extinction samples having average ages of ~2.5 Gyr and ~4.5 Gyr, forming the dynamically colder thin and dynamically hotter thick discs of M31 respectively beyond a galactocentric radius, RGC=$14 kpc. The two discs are also found to be chemically distinct from the argon abundance distributions of their PNe and thus have distinct origins. This is the first identification of the kinematically and chemically distinct thin and thick discs in M31, which at the equivalent distance of the Sun in M31 have a velocity dispersion twice and thrice that of the stars of the same age in the MW respectively. Such a steep age-velocity dispersion relation in M31 is consistent with a single major merger that occurred 2.5 - 4.5 Gyr ago with a merger mass ratio ~1:5. The radial abundance gradients of the M31 thin and thick disc are also much flatter than expected from secular processes and independently support the massive accretion event with significant impact on the M31 disc structure.