Dust and gas in local galaxies in the equatorial H-ATLAS fields

Abstract

One of the main challenges for extragalactic astronomy is to understand how the baryonic components of galaxies evolve from simple clouds of unenriched atomic gas into complex systems consisting of stars, dust, heavy elements and the different gas phases we observe today. This transformation is driven by the ongoing star formation in galaxies, yet there are many other poorly understood interactions between the different constituents that strongly affect the evolution. The physical properties of the galaxy population have been observed to change over time. For example, the stellar mass is built up monotonically, and the star formation rate within galaxies has changed drastically over the past ∼ 10 billion years. The main challenge in galaxy evolution is to procure a more detailed understanding of the various physical and chemical processes responsible for the observed changes in the physical properties of galaxies. Galaxies evolve over cosmic time, and thus much too slow to observe any changes directly. In order to study how the physical properties of a galaxy change, they need to be compared to the physical properties of galaxies at different evolutionary stages. There are two approaches to achieve this. The first one is to study the average change in the galaxy properties with redshift. Many studies (see next sections) have used this approach to study the redshift-evolution of various galaxy properties and these have dramatically changed our understanding of galaxy evolution. However, because of the difficulties of observing very distant objects, this method can only be used to study the evolution out to a given limiting redshift, which is determined by the used wavelength and telescope. Especially for atomic hydrogen (HI) gas, which has a hyperfine emission line at 21 cm, the current generation of telescopes strongly restricts the observations to the relatively nearby Universe. And since the HI gas is a key component in galaxy evolution, it is hard to get a detailed understanding of galaxy evolution beyond the local Universe. One way to extract information for HI beyond the most nearby sources is to use a ‘stacking’ analysis technique. Stacking is the process of combining many low signal-to-noise observations of different individual objects in order to retrieve a high-significance statistical detection (e.g. Delhaize et al., 2013). This technique enables studies of the changes in average galaxy properties out to larger distances (and thus larger lookback times). Part III of this thesis describes the ‘HI-stacking’ analysis of dust-selected sources. The second approach to study galaxy evolution is to investigate the differences in galaxy properties between galaxies at different evolutionary stages, rather than between galaxies at different times. In this context, the evolutionary stage of a galaxy can be defined by its gas fraction, i.e. by how much gas has been converted into stars. So as galaxies evolve, they move from high to low gas fractions and the changes in the physical galaxy properties are studied as a function of gas fraction rather than as a function of time. The rate at which galaxies evolve is determined by their star formation rate, which is in turn dependent on the galaxy’s halo mass and environment. Galaxies span a wide range of halo masses and environments, and hence a correspondingly large range in star formation rates. By the current epoch, some galaxies have converted most of their gas into stars, whereas others still mainly consist of gas. Galaxies in the local Universe span a range of different evolutionary stages due to the differences in their star formation histories. Depending on how the sources are selected, a sample can consist of more evolved or more unevolved sources. In Part I of this thesis, we present a local HI-selected sample and compare to a local stellar mass selected and local dust-selected sample in order to study the changes in galaxy properties over as much of the evolutionary track as possible. Galaxy evolution entails much more than the formation of stars from the available gas. As stars evolve, they synthesise metals (i.e. all elements except hydrogen and helium) in their cores, and subsequently expel them into the interstellar medium (ISM) at the end of the stars’ lives. About half of these metals are locked up in dust grains (Whittet, 1992). This dust absorbs about 30 to 50% of the optical light emitted by galaxies (e.g. Driver et al., 2016; Viaene et al., 2016) and enshrouds some of the most interesting environments in these galaxies. It is therefore difficult to develop a thorough understanding of galaxy evolution without also understanding how dust affects the observations. In Part I of this thesis we will put additional focus on the dust content of galaxies selected by their HI, dust and stellar content. In Part II, we determine the metal content of the same galaxies and study how dust is formed and destroyed by comparing dust and chemical evolution models (that predict the build-up of dust, gas and metals) with observed galaxy properties. This thesis describes three distinct, but closely related, research projects I conducted during the course of my PhD, all dealing with cosmic dust and HI gas in the context of galaxy evolution. This introductory chapter briefly summarises the current theoretical and observational framework of galaxy evolution, with a focus on cosmic dust and HI gas. Chapter 2 explains how we have selected the HIGH sample (HI-selected Galaxies in H-ATLAS) and dealt with observational issues. Chapter 3 describes the pipeline that was developed to perform the photometry and the SED fitting code that was used to determine the galaxy properties. Chapter 4 details the scaling relations between the different galaxy properties and how the galaxy properties of HIGH compare to a stellar selected and dust-selected sample. Chapter 5 explains how metallicities have been derived using fibre optical spectroscopy. In Chapter 6, we study dust sources and sinks by comparing models of the build-up of dust, gas and metals with the observed properties of galaxies. Chapter 7 presents the HI-stacking methods and preliminary results. Finally, in Chapter 8 we summarise our conclusions and describe potential future work.