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Tracing galaxy evolution through internal structures

Student thesis: Doctoral Thesis

Galactic bars are a structure made from stars, gas and dust in elongated orbits
found in the central region of many disc galaxies; depending on the exact definition, and wavelength considered, they can be found in ∼30%-70% of galaxies, and the Milky Way is known to have a bar. Through the radial flows it drives, a bar plays an important part of its host galaxy’s internal evolution. Strong bars are more likely to be found in more massive, redder disc galaxies suggesting a link between bars and processes which end star-formation in discs. Galactic evolution of this kind has been somewhat overlooked in the past, instead concentrating on external processes like mergers. But after z∼1 major mergers, are rare, so the majority of disc galaxies evolve in relative isolation. By studying the bar and the effects that cause and are caused by it, we can get an in depth view of the slow natural evolution of galaxies as the universe ages. With the continued expansion of the universe, the merger rate of galaxies will decrease and secular processes like bar driven flows will become even more important.

In this thesis, I present research into the role of bars in galaxy evolution, specifically the effect of a strong bar on the star-formation rates and gas morphology of the host galaxies. I observe the neutral Hydrogen (HI) morphology in barred galaxies to reveal the effects a bar has on the gas morphology. Hydrogen is the most abundant element in the universe and is the raw material for the formation of new stars. By measuring the HI mass found in a galaxy we can see the potential the galaxy has for future star-formation.

It is unusual to find galaxies that appear to have a well defined, strong bar and also have large reserves of HI left, since bars are more common in more massive, redder and gas poor disc galaxies (Masters et al., 2012). Yet, these galaxies exist. This thesis aims to find out what has made these galaxies defy the norm in their own evolution, what has happened in their past to make them
age differently to their peers? To do this I collected a small sample of HI Rich Barred (HIRB) galaxies to study individually. Their bar strength is found using the visible morphological classifications from the Galaxy Zoo project (Willett et al., 2013). Optical data is available from the Sloan Digital Sky Survey, which reveals they have a variety of star-formation properties, from relatively quiescent to star-forming, despite being HI rich. Total HI content was measured with ALFALFA(Haynes et al., 2011). Then I collected, reduced and imaged resolved HI data for six of the identified HIRB galaxies, using the Karl G. Jansky Very Large Array (VLA) and the Giant Metrewave Radio Telescope (GMRT). The detail of these processes are outlined in this thesis.

I find that the HIRB’s individual HI morphologies differ from one another, and agree with simulations of gas morphology in barred galaxies to varying degrees. Two of the HIRBs have central HI holes, as predicted, two have holes offset from the centre, and two have no holes at all. The HI is also not evenly distributed in most of the HIRB galaxies. These distributions can be explained by the bar, and some by local companion galaxies to the HIRB sample. Four of the HIRBs had at least one companion that was also detected in the resolved HI observations and all six however have companions within 1Mpc, which suggests tidal interaction as an alternative mechanism to cause distortion of the HI. Tidal interactions may also trigger bar formation. These close companions can also accrete HI onto the HIRB galaxy, making a galaxy that was previously gas-poor, and perhaps already had a bar, become gas rich.

To find out the likely evolutionary path, we study the typical environments of these HIRB galaxies, as well as the the average environment of galaxies that were just barred and just gas-rich. The HIRB galaxies followed the trends of a gas-rich galaxy control sample, but have quite different environments to strongly barred galaxy control sample. This similarity suggests that the HIRBs are most likely to be gas-rich galaxies that have had a bar form due to tidal interactions with a nearby companion, rather than barred galaxies which have accreted HI. I discuss my findings with regards to the slow, secular evolution of galaxies, and the impact of close companions. I also propose future work into the study of CO in HIRB galaxies, as well as a study into the direct link between star-formation and spiral arms in a galaxy. It has only been possible to identify large samples of HI or CO rich galaxies in the last 10 years, making this type of research ideal for studying galaxy evolution in the present day.
Original languageEnglish
Awarding Institution
Supervisors/Advisors
  • Karen Masters (Supervisor)
  • Kelley Hess (External person) (Supervisor)
  • Professor Bob Nichol (Supervisor)
Award dateSep 2019

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