Far-infrared-to-centimeter-wave spectroscopicobservations of <em>Planck</em>-selected starburst galaxies

Abstract

Star-formation processes in the early Universe have had a dramatic influence on the further development of galaxies. The peak era for stellar production occurred in early cosmic times, and most of the stars that could have formed in the Universe had already taken place during these times. The physical properties of the cold molecular gas are still poorly investigated, yet this gas serves as a primary fuel to sustain such on-going star-formation. This thesis presents detailed analyses of the turbulent molecular gas and dust in some of the most rare and extreme star-forming galaxies in the early Universe. These systems will likely become massive galaxies with more than 100 billion solar masses, and may reside in centers of local galaxy cluster environments. The Milky Way galaxy has been evolving for more than 10 billion years, with a current star-formation rate of about two solar masses per year. Dusty star-forming galaxies have extreme star formation rates, forming 100 to 1000 times the amount of stars, and existed primarily between two to four billion years after the initial conditions of the Universe. Studying such galaxies during these early cosmic times is an important way to better understand the star formation history of the Universe. It is well known that such star-forming galaxies have molecular gas mass to stellar mass fractions up to 50-80% or more. This is 5 - 10 times the amount of molecular gas available to form stars for most local star-forming galaxies. The largely unknown nature of the gas excitation conditions are due to observational limitations and the distant nature of these objects, yet radio/(sub)mm technology has been rapidly advancing the past twenty years. Some of these observational limitations were avoided in this thesis work by exploiting the natural magnification which occurs due to strong gravitational lensing. The galaxies examined in this thesis are along the line of sight with an intervening foreground galaxy, which results in this lensing effect. This effect amplifies the apparent flux density of the background, lensed galaxy, reducing telescope integration times. This thesis focuses on 24 lensed, dusty star-forming galaxies identified by the <em>Planck</em> satellite telescope, each with an inferred star formation rate of about 1000 times the Milky Way value. <br /> Radio-to-millimeter spectroscopic observations and analyses of carbon and carbon monoxide (CO) emission lines were used to address the nature of the gas supply. The core of this thesis work includes the modelling of the gas excitation conditions using 162 CO, and 37 atomic carbon, emission lines in this sample of 24 galaxies. This is the largest CO and carbon line study of any sample of distant, star-forming galaxies, and contributes to roughly 15-20% of all such line detections published to-date. The modelling procedure follows a novel approach to simultaneously model all emission lines and the dust continuum radiation field. Overall, these <em>Planck</em>-selected galaxies are some of the most gas-rich, infrared luminous galaxies ever observed. The high values for the surface density of molecular gas mass and IR luminosity suggest that both mechanical feedback from stars, combined with the accretion of intergalactic gas, are involved in the total gas excitation. Our results are consistent with theoretical models of turbulent motions regulating the molecular gas conditions within star-forming galaxies. This thesis work therefore helps to better understand the average gas conditions for the most massive star-forming galaxies in the early Universe as they were rapidly forming.