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Stellar halos of massive galaxies: morphology, kinematics, and cosmological origin
Stellar halos of massive galaxies: morphology, kinematics, and cosmological origin
This thesis studies the kinematics, the photometry, and the intrinsic shapes of massive early type galaxies (ETGs) out to large radii, using both observations and cosmological simulations. The goal is the characterisation of the structural properties of these galaxies, their variation with radius, and their dependence on the merger history. The bright central regions (~1 Re) of ETGs have long been known to display a bimodal distribution of physical properties, so that they are distinguished in fast (FRs) and slow rotators (SRs). On the other hand, much less is known about the dynamical structure of ETGs at larger radii. Stellar kinematic measurements in the ETG faint outskirts are observationally challenging as they rely on absorption line spectroscopy, which is limited to the central ~2 Re. In this study, this issue is overcome by using planetary nebulae (PNe) as tracers of the stellar halo kinematics. As part of the ePN.S survey, I performed a kinematic analysis of 33 nearby ETG halos out to typically 6 Re. This work revealed that ETGs have a larger diversity of kinematic behaviors in the halos than they do in their central regions: a considerable fraction of the ePN.S FRs shows reduced rotational support at large radii, and almost half of the FR sample shows indications for a variation of their intrinsic shape, from oblate in the center to triaxial in the halo. SRs instead are found to have increased but still modest rotation at large radii. These results were compared and interpreted using simulated galaxies from the IllustrisTNG cosmological magneto-hydrodynamical simulations. Kinematics and intrinsic shapes are found to be deeply connected: transitions to lower rotational support in the halos of FRs are accompanied by changes from flattened and oblate to more spheroidal shapes, with a higher degree of triaxiality. SRs have more homogeneous structural properties with radius, with overall high triaxiality and modest rotational support. The properties of simulated ETG stellar halos are largely determined by the balance between the in-situ component and the stars accreted through mergers, which strongly depends on stellar mass. In low mass systems, the in-situ stars determine peaked rotation profiles and near-oblate shapes with flattening decreasing with radius. In higher mass systems, mergers modify both rotation and shape profiles, generating local correlations between rotational support, shapes, and ex-situ fractions, and dynamically couple the stellar component to the dark matter halo. These results suggest that the large variety of kinematic and photometric properties of stellar halos is the direct consequence of the evolution of ETGs in a cosmological context: at large radii the FR/SR dichotomy of the cores partially breaks and is substituted by a smooth continuity of halo properties. In this picture, ETG halos would represent the connection between the bimodal core regions and the accretion dominated stochastic regime of large scale structure formation.
extragalactic astronomy, elliptical and lenticular galaxies, cD, evolution, halos, kinematics and dynamics, photometry, structure
Pulsoni, Claudia
2021
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Pulsoni, Claudia (2021): Stellar halos of massive galaxies: morphology, kinematics, and cosmological origin. Dissertation, LMU München: Faculty of Physics
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Abstract

This thesis studies the kinematics, the photometry, and the intrinsic shapes of massive early type galaxies (ETGs) out to large radii, using both observations and cosmological simulations. The goal is the characterisation of the structural properties of these galaxies, their variation with radius, and their dependence on the merger history. The bright central regions (~1 Re) of ETGs have long been known to display a bimodal distribution of physical properties, so that they are distinguished in fast (FRs) and slow rotators (SRs). On the other hand, much less is known about the dynamical structure of ETGs at larger radii. Stellar kinematic measurements in the ETG faint outskirts are observationally challenging as they rely on absorption line spectroscopy, which is limited to the central ~2 Re. In this study, this issue is overcome by using planetary nebulae (PNe) as tracers of the stellar halo kinematics. As part of the ePN.S survey, I performed a kinematic analysis of 33 nearby ETG halos out to typically 6 Re. This work revealed that ETGs have a larger diversity of kinematic behaviors in the halos than they do in their central regions: a considerable fraction of the ePN.S FRs shows reduced rotational support at large radii, and almost half of the FR sample shows indications for a variation of their intrinsic shape, from oblate in the center to triaxial in the halo. SRs instead are found to have increased but still modest rotation at large radii. These results were compared and interpreted using simulated galaxies from the IllustrisTNG cosmological magneto-hydrodynamical simulations. Kinematics and intrinsic shapes are found to be deeply connected: transitions to lower rotational support in the halos of FRs are accompanied by changes from flattened and oblate to more spheroidal shapes, with a higher degree of triaxiality. SRs have more homogeneous structural properties with radius, with overall high triaxiality and modest rotational support. The properties of simulated ETG stellar halos are largely determined by the balance between the in-situ component and the stars accreted through mergers, which strongly depends on stellar mass. In low mass systems, the in-situ stars determine peaked rotation profiles and near-oblate shapes with flattening decreasing with radius. In higher mass systems, mergers modify both rotation and shape profiles, generating local correlations between rotational support, shapes, and ex-situ fractions, and dynamically couple the stellar component to the dark matter halo. These results suggest that the large variety of kinematic and photometric properties of stellar halos is the direct consequence of the evolution of ETGs in a cosmological context: at large radii the FR/SR dichotomy of the cores partially breaks and is substituted by a smooth continuity of halo properties. In this picture, ETG halos would represent the connection between the bimodal core regions and the accretion dominated stochastic regime of large scale structure formation.