I have studied the hot, diffuse gas around and between galaxies. Specifically, I have used the EAGLE numerical simulations of galaxy formation to predict the properties of this gas, and I have used... Show moreI have studied the hot, diffuse gas around and between galaxies. Specifically, I have used the EAGLE numerical simulations of galaxy formation to predict the properties of this gas, and I have used those properties to predict specific observables: soft X-ray absorption and emission lines. Measuring this gas is challenging, but if we can observe and characterise it, we can learn much about the gas flows in and out of galaxies that regulate their formation and evolution. Observations of soft X-ray lines with future X-ray telescopes, such as Athena and XRISM, will enable us to do so. For these future X-ray telescopes, the strongest X-ray absorption lines and essentially all detectable line emission will come from the gaseous haloes surrounding galaxies. Some weaker, but still detectable absorption lines will come from the more diffuse gas outside these haloes. Photo-ionisation by the intergalactic ultraviolet/X-ray radiation background affects the absorption and emission lines of the very diffuse gas between galaxies, and the diffuse edges of the galaxy haloes. Emission from this photo-ionised gas is not expected to be detectable, but some absorption should be. Show less
When observing star-forming galaxies, we are not only seeing stellar light, but we also see how this interacts with galactic gas and dust. This thesis contains studies of the stellar, nebular and... Show moreWhen observing star-forming galaxies, we are not only seeing stellar light, but we also see how this interacts with galactic gas and dust. This thesis contains studies of the stellar, nebular and dust properties of low mass star-forming galaxies. We analyse data from the first statistical sample of low mass galaxies with stellar masses down to 10^7 solar masses, and investigate the emission from the stellar populations and their impact on and properties of the ionised surrounding gaseous nebulae, in order to understand the amount of ionising photons that are produced and the fraction of these that can escape from the galaxies. Since the stellar masses in low-mass systems may be stochastically sampled, we predict the influence of this on their nebular emission lies, and in particular on the derivation of their gas metallicity (the abundances of elements heavier than hydrogen and helium in the gas). To interpret observations of galaxies correctly, it is important to know the amount of interstellar dust grains, and its impact on the stellar and nebular light. We therefore finalise this thesis with a new method to derive dust properties in star-forming galaxies. Show less
Galaxies are environments where gas coalesces, cools, and is converted into stars. However, it remains unclear the exact mechanisms through which galaxies acquire, redistribute and lose their... Show moreGalaxies are environments where gas coalesces, cools, and is converted into stars. However, it remains unclear the exact mechanisms through which galaxies acquire, redistribute and lose their gas. The fresh gas that flows into galaxies is primarily composed of Hydrogen and Helium. But because a galaxy’s stars synthesize metals (elements heavier than Hydrogen and Helium), the diffuse gas in galaxies also becomes polluted with these new elements. As a result, by measuring the metal content (metallicity) of gas in galaxies, we can study the history of the gas flows. It is generally thought that the amount of gas in galaxies sets the rate at which stars are formed. Consequently, as the gas is exhausted, star formation rate will slow. Previously thought to be true for galaxies as a whole, our work suggests that this equilibrium may exist on much smaller scales within galaxies today. In this thesis we develop and apply a method that accounts for the blurring caused by the Earth’s atmosphere, which allows us to infer the true metallicity of more distant galaxies. With this we are able to show that, with respect to metallicity, the largest galaxies in the past were not dissimilar to those today. Show less
From its origin at the center of a star to the edge, through the surrounding gas and dust in the distant galaxy, through the intergalactic medium, traveling billions of light years only to be... Show moreFrom its origin at the center of a star to the edge, through the surrounding gas and dust in the distant galaxy, through the intergalactic medium, traveling billions of light years only to be reflected by a mirror and captured by a detector; the little amount of light observed from galaxies in the early universe contains a wealth of historic information. This thesis concentrates on translating the luminosities and colors of distant galaxies to physical properties such as distance, mass, age of the stellar population, and dust content. Analyzing deep optical and infrared observations of distant (redshift z~2.5) galaxies, we learn that massive galaxies have on average redder colors than less massive galaxies, although simulations show that mass underestimates of star-forming galaxies are possible. Roughly half of the red galaxies at high redshift owe their color to an old, quiescent stellar population. The other half is still actively star-forming, but obscured by large columns of dust. The abundance of star-forming and quiescent galaxies is consistent with a model that assigns a key role to collisions between gas-rich disk galaxies involving quasar activity, and leaving red spheroids as remnants. However, this model does not reproduce the colors of dusty red galaxies. Show less
A key issue in astronomy today is understanding the star-formation and assembly history of massive galaxies. Stellar population studies show that the bulk of the stars in low-redshift massive... Show moreA key issue in astronomy today is understanding the star-formation and assembly history of massive galaxies. Stellar population studies show that the bulk of the stars in low-redshift massive galaxies is formed at z~2 or even higher. Furthermore, there are strong indications that about 50% of the massive, quiescent galaxies in the local universe was already in a quiescent phase when the universe was only half its current age. This raises the question of whether quiescent galaxies can be found at even earlier epochs, and when massive galaxies formed and assembled their stellar mass. In order to address these questions we have pushed current studies to higher redshift and conducted a near-infrared spectroscopic survey of massive galaxies at z~2.5. In total we obtained deep spectra with a wavelength coverage of 1-2.5 micron for a sample of 36 galaxies. A surprising result of our survey is that a significant fraction of the massive galaxies at z~2.5 is already in a quiescent phase. Furthermore, several massive galaxies host active galactic nuclei, which may be responsible for the quenching of star formation. This thesis introduces the survey, presents these main results, and discusses the resulting constraints on the formation history of massive galaxies. Show less
