Logo Logo
Help
Contact
Switch language to German
Formation and physical properties of central structures in barred galaxies
Formation and physical properties of central structures in barred galaxies
The evolution of disc galaxies is governed not only by violent and external processes, but also by slow and continuous, internal evolution. One of the main drivers of this secular evolution are bars: these strongly non-axisymmetric stellar structures are present in 2/3 of all massive disc galaxies in the local Universe and have a substantial impact on their evolution. Amongst many other effects, they efficiently facilitate the inflow of gas from the main galaxy disc to their central regions, where this gas settles and eventually new stellar structures, such as nuclear discs, nuclear rings, and inner bars, are built. In this thesis, we aim to better characterise the properties of these central stellar structures and constrain their formation and evolutionary history. To this end, we employ integral-field spectroscopic observations from MUSE, obtained as part of the TIMER survey. The sample consists of 21 barred disc galaxies in the local Universe exhibiting a variety of central structures, and about 100000 science-ready spectra are available for each galaxy. In order to facilitate the exploitation of this massive data set, we develop the sophisticated software framework GIST for the analysis and visualisation of spectroscopic data. Using this tool, we derive stellar kinematics and mean population properties in the central regions of these galaxies at an unprecedented spatial resolution. We find that nuclear discs are characterised by high rotational velocities, low velocity dispersions, and near-circular orbits, while consisting of stellar populations that are significantly younger, more metal-rich, and less [α/Fe] enhanced, as compared to their direct surroundings. These properties of nuclear discs are consistent with the bar-driven formation scenario. Based on our radial population profiles in the nuclear discs, in particular stellar ages decreasing from the galaxy centre to their outer edge, and a correlation between the radii of (gaseous) nuclear rings and the bar length, we propose a new inside-out formation scenario for nuclear discs: in this picture, (stellar) nuclear discs are formed from a series of star-forming nuclear rings that grow in radius as the bar evolves. Combining measurements of both kinematics and stellar population properties, we find only little evidence for the presence of large, classical bulges in these galaxies. Although the galaxy sample is biased towards barred galaxies, the absence of classical bulges in such massive galaxies is surprising. Investigating the stellar population content of the three inner bars in TIMER, we find that these structures are characterised by high metallicities and low [α/Fe] abundances, similar to main bars. Moreover, inner bars exhibit slightly younger stellar ages at their outer ends, an effect known from studies on main bars as orbital age separation. In addition, radial profiles of metallicities and [α/Fe] enhancements are flat along the inner bar major axis, but show significantly steeper slopes along their minor axis, again analogous to previous findings in the context of main bars. These results suggest that inner and main bars are dynamically similar structures that differ only in the spatial scale on which they exist.
Not available
Bittner, Adrian
2021
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Bittner, Adrian (2021): Formation and physical properties of central structures in barred galaxies = Entstehung und Eigenschaften von zentralen Strukturen in Balkengalaxien. Dissertation, LMU München: Faculty of Physics
[img]
Preview
PDF
Bittner_Adrian.pdf

71MB

Abstract

The evolution of disc galaxies is governed not only by violent and external processes, but also by slow and continuous, internal evolution. One of the main drivers of this secular evolution are bars: these strongly non-axisymmetric stellar structures are present in 2/3 of all massive disc galaxies in the local Universe and have a substantial impact on their evolution. Amongst many other effects, they efficiently facilitate the inflow of gas from the main galaxy disc to their central regions, where this gas settles and eventually new stellar structures, such as nuclear discs, nuclear rings, and inner bars, are built. In this thesis, we aim to better characterise the properties of these central stellar structures and constrain their formation and evolutionary history. To this end, we employ integral-field spectroscopic observations from MUSE, obtained as part of the TIMER survey. The sample consists of 21 barred disc galaxies in the local Universe exhibiting a variety of central structures, and about 100000 science-ready spectra are available for each galaxy. In order to facilitate the exploitation of this massive data set, we develop the sophisticated software framework GIST for the analysis and visualisation of spectroscopic data. Using this tool, we derive stellar kinematics and mean population properties in the central regions of these galaxies at an unprecedented spatial resolution. We find that nuclear discs are characterised by high rotational velocities, low velocity dispersions, and near-circular orbits, while consisting of stellar populations that are significantly younger, more metal-rich, and less [α/Fe] enhanced, as compared to their direct surroundings. These properties of nuclear discs are consistent with the bar-driven formation scenario. Based on our radial population profiles in the nuclear discs, in particular stellar ages decreasing from the galaxy centre to their outer edge, and a correlation between the radii of (gaseous) nuclear rings and the bar length, we propose a new inside-out formation scenario for nuclear discs: in this picture, (stellar) nuclear discs are formed from a series of star-forming nuclear rings that grow in radius as the bar evolves. Combining measurements of both kinematics and stellar population properties, we find only little evidence for the presence of large, classical bulges in these galaxies. Although the galaxy sample is biased towards barred galaxies, the absence of classical bulges in such massive galaxies is surprising. Investigating the stellar population content of the three inner bars in TIMER, we find that these structures are characterised by high metallicities and low [α/Fe] abundances, similar to main bars. Moreover, inner bars exhibit slightly younger stellar ages at their outer ends, an effect known from studies on main bars as orbital age separation. In addition, radial profiles of metallicities and [α/Fe] enhancements are flat along the inner bar major axis, but show significantly steeper slopes along their minor axis, again analogous to previous findings in the context of main bars. These results suggest that inner and main bars are dynamically similar structures that differ only in the spatial scale on which they exist.