Nathan Bourne

I am interested in extragalactic astrophysics, and in trying to answer questions such as: how do galaxies evolve and interact? How and when did galaxies form their stars? When and why did they finish doing so? As an observational astronomer, I try to answer these questions by studying the multi-wavelength energy output of galaxies, and understanding what different types of observations can tell us about the activity within galaxies, the physical components that make them up, and how these properties vary between galaxies in different environments in the universe and at different epochs in the history of the Universe.

The light from stars in galaxies near and far allows us to see them in optical, ultraviolet and infrared telescopes, but there are other components making up galaxies, which are hidden from these observations. Dark matter famously makes up around 80% of the mass of the Universe, although we cannot see it directly and do not yet know what it is composed of. However, the other important components of galaxies can be observed using telescopes observing in different wavelength regimes. Gas is a vital component since it is needed to form stars, while dust (microscopic grains of carbonates, silicates and soot) is just a small part of the mass but absorbs about half of the starlight and so changes the spectrum of galaxy light significantly. Formation of dust is also the first step towards forming planets. Our understanding of the history of stars and planets in the Universe therefore relies upon measuring the dust and gas contents of galaxies across cosmic time.

My work aims to solve these problems through understanding observational tracers of dust (through far-infrared and sub-millimetre emission) and gas (through emission lines in the millimetre/microwave part of the spectrum). I am involved in several projects using state-of-the-art observatories such as ALMA, NOEMA and the IRAM 30m telescope.

The Atacama Large Millimetre/submillimetre Array - Credit: ALMA (ESO/NAOJ/NRAO)

My research focuses on methods for combining imaging data at a broad range of wavelengths, with varying angular resolution. I am a collaborator in the ASTRODEEP project, which aims to do just this for a number of deep survey fields imaged by large telescopes such as Hubble, Spitzer, Herschel, JCMT and ALMA. My work concentrates on the particular problems encountered in far-infrared/sub-millimetre wavebands due to low angular resolution, large numbers of faint background sources and extreme levels of confusion or blending between sources. For example, the images below show the same galaxies in the optical (left) and sub-mm (right) and demonstrate some of the problems we encounter. The sub-mm sources are much less sharply resolved; multiple sources are blended into one; objects which are bright in the optical are faint in the sub-mm, while some sub-mm sources are very faint or even undetected in the optical. In a recently published paper, I applied new techniques to overcome these issues in sub-millimetre imaging from the JCMT SCUBA-2 Cosmology Legacy Survey and demonstrated how this allows us to uncover faint emission from the most distant galaxies selected by stellar mass, in order to measure the obscured star formation density of the early universe.

As a core member of the H-ATLAS survey, I have been involved in the preparation of data products, identifying optical counterparts to sources detected in the sub-millimetre, and solving the problem of measuring consistent fluxes in multiple wavebands with differing resolutions (in collaboration with GAMA). This is necessary to understand the full spectral energy distribution of light emitted from galaxies, and to investigate properties such as the star formation rate, dust mass and temperature in different types of galaxies. My latest work in this area has focused on the delicate issues of matching galaxies between the H-ATLAS, GAMA and SDSS surveys, and I published a paper on how weak lensing can be a problem for matching sub-millimetre and optical surveys. I also produced the public catalogues of optical counterparts, multi-wavelength photometry and redshifts of sub-millimetre sources in the H-ATLAS Phase 1 Data Release, described in a paper that I led.

I also completed a study of molecular gas in some of the dustiest galaxies in the local Universe, selected from H-ATLAS. These galaxies contain large masses of interstellar dust, the presence of which is thought to be linked to prolific star formation in the past. By observing microwave line emission from CO molecules, we can estimate the amount of molecular hydrogen in a galaxy, which is important because this gas is essential for a galaxy to continue forming stars. By understanding the links between cold dust and cold gas we can understand what phase of evolution these dusty galaxies are in, and we can use these links to understand the evolution of galaxies in the more distant Universe which are too far away for us to observe the CO emission.

Much of my work has made use of "stacking" techniques to determine the statistical properties of large samples of galaxies in an unbiased way. In this way I can collect the light emitted from galaxies at far-infrared and sub-millimetre wavelengths, from which I can infer the properties of interstellar dust in galaxies. By stacking I can investigate how the dust properties depend on the stellar properties of galaxies, inferred from optical and near-infrared observations, and how they evolve over cosmic time.

I have applied these techniques to large samples of galaxies selected at optical wavelengths at high and low redshifts, studying the tight correlation between far-infrared and radio emission in one paper, and the evolution of dust content in ordinary galaxies in another.

For a detailed record of my research and experience follow this link to my CV

Click here for a list of my published and unpublished work.