Yttergren, Lenita Mari Madeleine
(2020).
Evolutionary Group Dynamics in Stephan's Quintet. Optical spectroscopy & radio observations with the LBT & IRAM 30m.
PhD thesis, Universität zu Köln.
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Abstract
One of the most fundamental questions in astronomy is that of the evolution of galaxies. Ever since the quantum fluctuations present at the era of recombination, structures have evolved in the Universe. Dark matter halos facilitate congregation of baryonic matter and the gravitational attraction creates groups and clusters of galaxies. Galaxies evolve through both external and internal processes. Internal processes are driven by instabilities in the galactic disk, spiral arms, bars and oval distortions, while external processes are driven by outside forces, such as galaxy mergers and harassment. External processes can have an immense impact on the galaxies involved, by altering the galaxies' morphology and content by moving large amounts of gas and inciting starbursts and active galactic nuclei (AGN). Due to the abundance of gas and the proximity of the galaxies in the early Universe, it is accepted that galaxy mergers/interactions occurred often and were vital in driving galaxy evolution. The environment in which a galaxy resides plays an important role in its formation and evolution, and more than half of the galaxies in the Universe reside in galaxy groups. Compact galaxy groups are perfect laboratories for studying galaxy evolution through extreme galaxy interactions due to their galaxy density and activity. These groups can also reveal key information regarding the connections between galaxies and their environment, as well as details regarding galaxy evolution at high redshift. Low-redshift compact galaxy groups allow us to study the impact of galaxy interactions and mergers on galaxy evolution at high-resolution, thereby providing an insight into the conditions of the galaxies in the early Universe when such interactions were more common. Stephan's Quintet is a nearby compact galaxy group of 5 galaxies with a rich and intriguing history of interactions. The past interactions can be traced via the tidal tails coursing through the group, while the current interaction is causing a galaxy-wide ridge of shock-driven star formation. This shocked star-forming ridge is enabled by the large amount of intergalactic gas present in the group, deposited in the intergalactic medium (IGM) during the previous galaxy interactions. Stephan's Quintet is one of the most well-studied groups in our Universe and every time the group has been observed in a new wavelength window or with higher resolution and sensitivity, new fascinating features have been revealed and our understanding of the processes and structures increased. Multi-wavelength analyses of galaxies are essential, since it is only through comparison and combination of the tracers of galaxy dynamics (i.e., stars, atomic gas and molecular gas) that we can truly study the evolution of galaxies. To fill in blanks in the wavelength ranges covering Stephan's Quintet and contribute to increased understanding of our Universe, I have carried out optical spectroscopy and radio observations. The optical wavelength regime provides information regarding the stellar population as well as the atomic gas, while spectroscopy enables spatial and spectral information to be gathered simultaneously. This facilitates studies of the abundances and kinematics as well as the excitation mechanisms across the group. Covering a part of Stephan's Quintet in multiple slits with the Multi-Object Double Spectrograph at the Large Binocular Telescope in Tucson, Arizona, USA, I am able to achieve a pseudo-Integral Field Spectroscopy observation of the main part of, and the most intense, interactions and activity in group. Focusing on the nucleus of the galaxies and the large-scale dynamics, I detail the kinematics of the IGM and the galaxies. I present an extensive analysis of the mapped area, including fluxes, velocity dispersions, line-of-sight velocities and excitation mechanisms in NGC7319, NGC7318A and B, NGC7317, the bridge, the west ridge and the star-forming ridge. NGC7319 shows a disturbed galaxy, where the gaseous disk is decoupled from the stellar disk. I find a broad line region component in the nucleus, revealing for the first time the Seyfert 1 nature of this galaxy, and I confirm the presence of a blue outflow to the south-west of the nucleus at an average of 476±13.8 km/s. The stellar and gaseous disks are approximately perpendicular to each other and the gas is excited by AGN radiation, indicating that the gas is present in a large-scale nuclear wind. The data further reveal extensive gas emission in the shocked star-forming ridge as well as in the west ridge (south-west of the NGC7318 pair) and the bridge connecting the NGC7318 pair and NGC7319. I confirm dual velocity components (as suggested by Duarte Puertas et al. (2019)) in several parts of the IGM and note that the shock increases the ionisation to LINER-like emission-line ratios in several regions along the star-forming ridge and the west ridge. Furthermore, the multiple velocity components present in many parts of the IGM and galaxies, spanning 5600-7000 km/s, coincide with that of other galaxies, revealing the potential origin. Cold molecular gas, best traced by CO emission detected in radio wavelengths, is a key ingredient in star formation and one of the main ingredients in galaxies. Analysing the behaviour of molecular gas is vital in determining the morphology and understanding the evolution of galaxies. I have, therefore, observed Stephan's Quintet using the IRAM 30m telescope in Sierra Nevada, Spain. Adopting an on-the-fly mapping technique I observed the 12CO(1-0), (2-1) and the 13CO(1-0) emission in a 5.67 arcmin^2 area covering the group. I present maps and spectra of the emission, including abundances and velocities of the respective three CO lines, as well as molecular hydrogen gas masses. I further discuss the line ratios together with the excitation temperatures and the optical depth. I find that the brunt of the CO emission is in/near NGC7319, extending towards and into the bridge and the star-forming ridge. 52-56% of the molecular hydrogen gas mass in Stephan's Quintet is in/near NGC7319, while 38-40% is in the star-forming ridge, the final 4-10% is spread out across the NGC7318 pair and their surroundings. The distribution of 12CO(2-1), however, favours NGC7317, the NGC7318 pair, SQ-A and the star-forming ridge, which retain approximately half of the 12CO(2-1) emission, while NGC7319 contains less than 20%. This highlights the increased temperatures present in the shocked star-forming ridge. The data confirms the presence of multiple velocity components in the group, spanning 5600-7200 km/s. Up to 4 clearly distinguishable velocity components can be found, with NGC7319 and the star-forming ridge showing the highest number of components. Again the velocity components often coincide with that of the other galaxies, tracing the complex history of the group. Furthermore, the CO line ratios indicate optically thick gas at low temperature in NGC7319 and the bridge. While in the star-forming ridge and SQ-A, the gas is found to be dense, optically thick and warm, as expected considering the current interaction of the IGM and NGC7318B. The gas surrounding NGC7318B at a line-of-sight velocity of ~5800 km/s shows an inclination towards being warm, dense and optically thin. This work favours a group evolution scenario of Stephan's Quintet that includes previous interactions of both NGC7319-NGC7317 and NGC7319-NGC7320C, and a scenario in which NGC7318B has passed through and is currently located in front of the group, supported by the multi-component IGM and the tidal streams that connect NGC7318B to the IGM and the galaxies. In addition, NGC7318B increases the group's energy and adds to the IGM gas content which stalls the gas depletion required for aging the group further. The enhancements of the passages of NGC7320C and NGC7318B are expected to be vital in hindering the group's imminent merger into a final fossil state. NGC7318B shows us the impact of diffuse IGM, while NGC7319 reveals a fascinating case of AGN feeding and feedback in a decoupled stellar/gas disk. NGC7319 shows lack of ongoing star formation while still appearing to contain molecular gas, although likely off-nuclear, with an outflow impacting the surrounding gas and structures - raising questions regarding the feeding mechanisms and lifetime of this AGN. Stephan's Quintet differs from other groups due to the prominent extended tidal features and the currently occurring collision with NGC7318B. It is possible that these structures are short-lived and that all compact groups exhibit this kind of variety of galaxy interaction indicators, stellar and gaseous tidal features and galaxy-wide shock structures at some point during their evolution. Understanding these processes in Stephan's Quintet sheds light on the evolution of galaxies at a time in the history of the Universe when gas was abundant and interactions were common.
| Item Type: | Thesis (PhD thesis) | ||||||||
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| URN: | urn:nbn:de:hbz:38-205574 | ||||||||
| Date: | 16 October 2020 | ||||||||
| Language: | English | ||||||||
| Faculty: | Faculty of Mathematics and Natural Sciences | ||||||||
| Divisions: | Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I | ||||||||
| Subjects: | Natural sciences and mathematics Physics |
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| Date of oral exam: | 11 September 2020 | ||||||||
| Referee: |
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| Funders: | Max Planck Institute for Radio Astronomy (MPIfR), The International Max Planck Research School for Astronomy and Astrophysics at the Universities of Bonn and Cologne (IMPRS), SFB956.Conditions and Impact of Star Formation | ||||||||
| Refereed: | Yes | ||||||||
| URI: | http://kups.ub.uni-koeln.de/id/eprint/20557 |
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