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Do variations in the initial mass function affect galaxy evolution?

Prgomet, Mateo LU (2020) In Lund Observatory Examensarbeten ASTM31 20201
Lund Observatory
Department of Astronomy and Theoretical Physics
Abstract
The initial mass function (IMF) describes the stellar mass distribution for a population of stars at birth. This parameter represents a topic of long standing discussion within astronomy due to its apparent insensitive nature to environmental conditions. However, in the last decade, significant evidence suggesting the contrary has been presented. Due to the role of the IMF as an input variable for a myriad of astronomical studies, its functional form is expected to influence the understanding within many topics of astronomy. In this thesis, we expand the research within the field of IMF variations as well as galaxy formation, adopting a heuristic approach in order to investigate the impact potential IMF variations might have on the... (More)
The initial mass function (IMF) describes the stellar mass distribution for a population of stars at birth. This parameter represents a topic of long standing discussion within astronomy due to its apparent insensitive nature to environmental conditions. However, in the last decade, significant evidence suggesting the contrary has been presented. Due to the role of the IMF as an input variable for a myriad of astronomical studies, its functional form is expected to influence the understanding within many topics of astronomy. In this thesis, we expand the research within the field of IMF variations as well as galaxy formation, adopting a heuristic approach in order to investigate the impact potential IMF variations might have on the formation and evolution of dwarf galaxies. We consider two distinct models for variation, motivated by either empirical results or theoretical arguments. Both the models result in the over-production of massive stars for low-metallicity environments. It is important to probe the most extreme environments for such a study, since it is there star formation could be very different. To this end, we consider the ultra faint dwarf galaxies in this work, which represent the extreme lower limit of the galaxy formation process, thus representing prime laboratories for studies of IMF variations. Their evolution over a period of 13.8 Gyr is modelled using state-of-the-art cosmological N-body + hydrodynamical simulations that feature zoomed in galaxy formation as well as a sophisticated recipe for feedback. It is found that IMF variations can dramatically impact the stellar mass growth history of dwarf galaxies, allowing for up to a 2 dex reduction in final stellar mass. In addition, all of the simulations considered are fully compatible with observational data of galaxy sizes, velocity dispersions, V-band magnitudes and metallicity. Thus, it is demonstrated in this work, for the first time, that IMF variations can represent a channel for the formation of the least massive galaxies to ever exist with a realistic enrichment. (Less)
Popular Abstract
On the largest scales of the universe, there exists enormous collections of stars and gas, which are bound together by the attractive forces of gravity. These galaxies act as light sources for astronomers, allowing for the exploration of the universe. How do these galaxies form? What processes affect their evolution? Why are they so incredibly diverse? These are questions that are readily asked within the active and steadily growing research field of extragalactic astronomy,
which deals with objects outside our own galaxy. The challenge behind finding an answer to these questions lies in the nature of galaxies themselves. Associated with the evolution of galaxies are a plethora of physical processes, where each affect their surrounding... (More)
On the largest scales of the universe, there exists enormous collections of stars and gas, which are bound together by the attractive forces of gravity. These galaxies act as light sources for astronomers, allowing for the exploration of the universe. How do these galaxies form? What processes affect their evolution? Why are they so incredibly diverse? These are questions that are readily asked within the active and steadily growing research field of extragalactic astronomy,
which deals with objects outside our own galaxy. The challenge behind finding an answer to these questions lies in the nature of galaxies themselves. Associated with the evolution of galaxies are a plethora of physical processes, where each affect their surrounding environments. How large of an impact such a physical process has depends on, among other things, how far it reaches within the galaxy and how energetic it is. These two parameters, in combination, can provide clues on how efficient the process is at removing gas from its environment. This is of considerable importance for galaxies since stars are ultimately formed from this gas. If there is no gas, there can be no star formation, and the formation of the galaxy stops!

Stars are extremely bright objects, as can be verified by a hot summer day. Due to stars being so luminous and energetic, they strongly influence their surroundings. Astronomers call this
energetic interaction stellar feedback. For extragalactic astronomers, this feedback is important because it is very efficient at expelling gas. This is an important mechanism in the context of galaxies since they contain a large number of stars, which cumulatively act to remove the gas from the galaxy. The energetic properties of a star is strongly mass dependent, so the efficiency of stellar feedback from a cluster depend on its distribution of stellar masses. Furthermore,
this initial mass function (IMF) for a population of stars often enters into many astronomical studies. However, this distribution appears to be universal. Within our own galaxy, it appears that the fraction of low, medium and high mass stars for a young stellar population is constant, independently of environmental properties. An analogy to this seemingly abstract statement can be found in something as simple as apple trees. Picture three apple trees, one planted on
the icy planes of Antarctica, one in the sunny fields of Spain and the last in the dry Saharan desert. Ignoring some basic principles of agriculture, naturally, one would expect these trees to spawn significantly different apple populations. So why do clusters of stars that form in different environments still yield the same IMF? Although many theories have been suggested, there is no definitive answer to this question. However, if it is shown that the IMF does depend on environment, it could be detrimental for the understanding of astronomical topics, due to its role as an necessary ingredient in many astronomical studies. Worryingly, the observational evidence for IMF variation has become considerable in the last decade, cementing the origin of the IMF as a central problem in astronomy.

In this thesis, we perform a study that considers the impact of these alleged IMF variations. More specifically, we investigate how a varying IMF for stellar populations will affect the evolution of small and faint dwarf galaxies. This is done through large-scale computer simulations of a small patch of the universe that contains galaxies. Will there be an impact if variations of the mass distribution of stars within these galaxies are implemented? If so, is it noticeable? What would happen to a universe where such variations are present? Can we rule out the scenario of a varying mass function? These are some of the questions we wish to shed light upon in this study, in hopes of contributing to the understanding regarding the impact the initial mass function plays in shaping the universe we live in. (Less)
Please use this url to cite or link to this publication:
author
Prgomet, Mateo LU
supervisor
organization
course
ASTM31 20201
year
type
H2 - Master's Degree (Two Years)
subject
keywords
galaxy, galaxy formation, galaxy evolution, feedback, star formation, ultra faint dwarf galaxy, initial mass function, variations, universe, redshift, hydrodynamics, cosmology, zoomed-in simulations Galaxy formation and evolution, galaxy feedback, stellar feedback, IMF variations, cosmological zoomed-in simulations
publication/series
Lund Observatory Examensarbeten
report number
2020-EXA161
language
English
id
9014859
date added to LUP
2020-06-10 16:24:01
date last changed
2020-06-10 16:24:01
@misc{9014859,
  abstract     = {The initial mass function (IMF) describes the stellar mass distribution for a population of stars at birth. This parameter represents a topic of long standing discussion within astronomy due to its apparent insensitive nature to environmental conditions. However, in the last decade, significant evidence suggesting the contrary has been presented. Due to the role of the IMF as an input variable for a myriad of astronomical studies, its functional form is expected to influence the understanding within many topics of astronomy. In this thesis, we expand the research within the field of IMF variations as well as galaxy formation, adopting a heuristic approach in order to investigate the impact potential IMF variations might have on the formation and evolution of dwarf galaxies. We consider two distinct models for variation, motivated by either empirical results or theoretical arguments. Both the models result in the over-production of massive stars for low-metallicity environments. It is important to probe the most extreme environments for such a study, since it is there star formation could be very different. To this end, we consider the ultra faint dwarf galaxies in this work, which represent the extreme lower limit of the galaxy formation process, thus representing prime laboratories for studies of IMF variations. Their evolution over a period of 13.8 Gyr is modelled using state-of-the-art cosmological N-body + hydrodynamical simulations that feature zoomed in galaxy formation as well as a sophisticated recipe for feedback. It is found that IMF variations can dramatically impact the stellar mass growth history of dwarf galaxies, allowing for up to a 2 dex reduction in final stellar mass. In addition, all of the simulations considered are fully compatible with observational data of galaxy sizes, velocity dispersions, V-band magnitudes and metallicity. Thus, it is demonstrated in this work, for the first time, that IMF variations can represent a channel for the formation of the least massive galaxies to ever exist with a realistic enrichment.},
  author       = {Prgomet, Mateo},
  keyword      = {galaxy,galaxy formation,galaxy evolution,feedback,star formation,ultra faint dwarf galaxy,initial mass function,variations,universe,redshift,hydrodynamics,cosmology,zoomed-in simulations Galaxy formation and evolution,galaxy feedback,stellar feedback,IMF variations,cosmological zoomed-in simulations},
  language     = {eng},
  note         = {Student Paper},
  series       = {Lund Observatory Examensarbeten},
  title        = {Do variations in the initial mass function affect galaxy evolution?},
  year         = {2020},
}