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I have just completed my PhD working with Prof. Scott Chapman tracing the evolution of Sub-Millimeter Galaxies (SMGs).
The Origin of Extreme Star Formation in the Universe
Rapid Star Formation in the Distant Universe: Some of the most extreme star formation in the Universe occurred 3 billion years after the Big Bang, in a population of galaxies enshrouded in dust. The ultra-violet (UV) radiation emitted from the young massive stars in these systems is reprocessed by the dust and emitted at far-infrared wavelengths. Since the radiation we observe from these galaxies was emitted 10 billion years ago it is redshifted to longer wavelengths. This results in the emission from these galaxies being detectable with sub-millimeter telescopes and therefore giving rise to their name: Sub-Millimeter Galaxies (SMGs). The intense star formation (creating approximately 1000 solar masses per year) and large reservoirs of gas available to fuel the star formation in SMGs means that they can grow rapidly, creating a massive galaxy in only 100 million years. SMGs are therefore interpreted to be an immensely active phase in galaxy evolution, which could seed the growth of the massive galaxies observed in the present day Universe.
Triggering the Extreme Star Formation: After 15 years of intense study, our detailed understanding of SMGs is still limited to only a handful of objects with spatially resolved images and spectra. In particular, the trigger of the extreme star formation in SMGs has not been fully understood, although galaxy simulations predict that two galaxies merging together could cause such an ultra-luminous burst of activity. One of the themes of my thesis has been the investigation of the potential triggers. This has only been made possible in recent years with the development of integral field units, which provide spectra as a function of position within a galaxy. Spatially resolving the properties of the gas allows for studies of the morphologies and dynamics of the systems, enabling a test of the hypothesis that SMGs are made up of multiple colliding galaxies.
Are Merging Galaxies the Trigger: The hydrogen Halpha spectral line is emitted from regions where hydrogen is ionized by hot young stars and therefore traces the star-forming gas. I mapped this emission line within the SMGs to gain spatially resolved information about the star formation intensity and the velocity and dispersion of the gas. I have established that the gas in the SMGs is disturbed and turbulent, tracing multiple interacting components. This provides evidence that the SMGs are merging systems and therefore the merging process could trigger the intense star formation.
Fueling the Star Formation:The advent of telescopes capable of millimeter interferometry has enabled transitions of the carbon monoxide molecule to be observed in SMGs, providing tracers of the molecular gas - the fuel for the rapid star formation – in the same detail as the ionized gas. The atomic carbon fine structure lines, which originate from the cooling gas and therefore trace the SMGs’ cold gas reservoirs, can also be observed in this way. The luminosities of the carbon monoxide and atomic carbon lines can be used to constrain the physical conditions of the gas in the interstellar medium of the SMGs by comparison to the outputs of molecular cloud models. Through this comparison it is possible to constrain the gas densities, temperatures, elemental abundances and radiation fields. I have found, through this analysis, that the SMGs are most similar to local ultra-luminous galaxies however there is a tail towards lower densities and radiation fields, more similar to local starburst galaxies, providing evidence for extended star formation in some of these systems. This implies that 10 billion years ago the processes causing the extreme luminosities may have been very different from today.
MPhys in Physics and Astronomy (2005-2009), 1st Class (Honours), Durham University