No smoke without fire : cosmic dust emission as a tracer of star formation in galaxies

Abstract

Studies of the history of the Universe are for a large part concerned with mapping the evolution of galaxies over cosmic time. Beginning from the seeds of density perturbations in the early Universe, and building up through gravitational and astrophysical interactions to form the wide diversity seen in the present day, galaxies allow us to observe the distribution of luminous (and dark) matter over a wide range of look-back times. A key process in galaxy evolution is the formation of stars, an activity which is readily observed by indirect means, although the detailed mechanism is not fully understood. One of the most successful methods for tracing star formation is to observe the emission from dust in galaxies. These tiny particles of carbon- and silicon-based solids resemble smoke, pervading the interstellar medium in many (if not all) galaxies, and blocking the short-wavelength radiation from hot, newly-formed stars. They re-radiate this energy as far-infrared radiation (wavelengths ~10-1000 microns), which can be detected from sources throughout the Universe by telescopes such as the Spitzer and Herschel space observatories. The spectral form of this radiation varies from one galaxy to another, depending on many factors such as the activity within the galaxy, the amount of dust, and the sources heating the dust. Hence, with careful interpretation, we can use these observations to trace the star-forming activity and dust mass in different types of galaxies from early times through to the present day. In this thesis I describe three projects, each of which utilises multi-wavelength datasets from large surveys to probe the dust emission from samples of galaxies at different cosmic epochs, and explore the relationship between dust emission and other galaxy properties. The first project samples the most massive galaxies at a range of redshifts spanning the peak era of star formation, and investigates the correlation between far-infrared and radio emission. I use a `stacking' methodology to avoid bias towards the brightest star-forming galaxies, and show that the far-infrared and radio tracers of star formation agree up to high redshifts in typical massive galaxies. In the second project I apply the stacking method to a large sample of low-redshift galaxies selected from a major optical survey spanning the last four billion years of evolution. I make use of the largest ever sub-millimetre imaging survey to produce a detailed and unbiased census of the dust mass in ordinary galaxies as a function of optical brightness, colour and look-back time. I show that the luminosity and temperature of dust is a strong function of galaxy mass and colour, while the dust masses of all galaxy types have decreased rapidly over the time span probed. The final project focuses on a small sample of nearby galaxies and utilises data obtained and reduced by myself to probe the molecular-gas content of galaxies selected to have large dust masses. This study addresses questions about how well the cold dust, traced by the sub-millimetre wavebands of Herschel, is correlated with the cold gas, which provides the fuel for ongoing star formation. The thesis demonstrates the utility of statistical techniques for large surveys, and also contains aspects of data reduction and extensive discussion of the astrophysical interpretation of results. Through these various analyses I show that dust emission can provide a valuable window on the growth of galaxies through star formation. The work contained herein represents significant progress in the field of observational extragalactic astronomy, including work recently published in the scientific literature in two collaborative research papers led by myself, in addition to a third paper that I am currently preparing