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- Ejdetjärn, Timmy
(author)
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Exploring the nature of ISM turbulencein disc galaxies
- 2024
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Licentiate thesis (other academic/artistic)abstract
- Galaxy formation is a continuous process that started only a few hundred million yearsafter the Big Bang. The first galaxies were very volatile, with bursts of star formationand disorganised gas motions. However, even as these galaxies evolved to have orderlyrotating gas discs, the gas within the disc, referred to as the interstellar medium (ISM),still remained highly turbulent. In fact, the ISM is supersonically turbulent, meaning thatthe disorganised gas motion exceeds the speed of sound in the medium. This supersonicturbulence has been connected to several crucial properties related to galaxy evolution; forexample, increasing (and decreasing in some regions) the ISM gas density, star formation,and gas mixing.Many observation have shown that all of the gas phases in the ISM experience su-personic levels of turbulence, with line widths (an observational method to quantify theamount of turbulence) as high as σg ≲ 100 km s−1 in high-redshift (younger) disc galaxies,while local quiescent discs have σg ≲ 40 km s−1 . However, the ISM contains a variety ofgas phases that cover a wide range of temperatures and densities, which exhibit differentlevels of turbulence. For example, the warm ionised gas phase represents the upper limitsquoted above, while colder denser gas only reaches σg ≲ 40 km s−1 and σg ≲ 15 km s−1 inhigh-redshift and local galaxies, respectively.The physical processes driving this turbulence are not fully understood, but a combi-nation of stellar feedback (e.g. supernova) and gravitational instability (e.g. during cloudcollapse) have been suggested to provide a majority of the turbulent energy. In particular,stellar feedback is crucial in the formation of warm ionised gas and may therefore have asignificant contribution on the turbulence within ionised gas. Furthermore, heterogeneousdata of widely different galaxies (in terms of e.g. mass and size) at different resolutions(which causes artificial line broadening) complicates understanding the underlying cause.A commonly used tracer of ionised gas is the Hα emission line and has been usedextensively in high-redshift surveys. However, the contribution of the Hα signal comesfrom two primary sources: the radiatively ionised regions around massive newborn starsembedded in molecular gas (called H II regions) and diffuse ionised gas (DIG) filling theentire galactic disc. Observations have found that these two sources contribute, on average,roughly the same amount to the Hα signal (although with a large spread), but the levelsof turbulence is starkly different; with the DIG being roughly 2-3 times more turbulethan the gas in H II regions.Numerical simulations have come a long way and are now able to simulate entire discgalaxies at parsec-scale resolution (in regions of interest). Furthermore, galaxy simulationshave been able to reproduce the level of turbulence observed in local and high-redshiftgalaxies. Direct comparisons between numerical and observational studies are crucial tounderstand the relevant physics driving observed correlations. However, numerical andobservational work have different data available and the reduction/analysis varies betweenauthors, and so diligence is required to perform qualitative comparisons.In this work, I perform numerical simulations to investigate ISM turbulence in differentgas phases. My simulations model a Milky Way-like galaxy at two different redshifts(using gas fraction as a proxy for redshift) and with/without stellar feedback physics, toevaluate its impact. I perform mock observations to explore the relation between the starformation rate and turbulence, and investigate what is driving this relation. Additionally, Ianalyse the Hα emission line and compare the contribution in intensity and line broadening(turbulence) from H II regions and DIG.
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