Accurso, GE; (2016) Measuring molecular gas in galaxies with applications to star formation efficiency. Doctoral thesis , UCL (University College London).

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Abstract

The recently established equilibrium model for galaxy evolution gives major importance to the role of gas in regulating galaxy growth; the efficiency of star formation as gas is cycled in and out of galaxies is a central theme in this emergent picture. Low mass galaxies in the local universe, with their low metallicity environments, high gas mass fractions, and low LCO ratios, make them ideal and interesting candidates to test this SFR model. Several questions remain: How can we accurately trace the total molecular gas in sub-solar metallicity environments where photodissociation of CO occurs? Is there a fundamentally different star formation efficiency in these systems compared to their high mass counterparts? This Thesis answers these questions using a new multiwavelength data set based on the IRAM xCOLD GASS survey, alongside auxiliary [CII] 158um line spectroscopy observations, using PACS on-board Herschel, with a newly built multi-phase ISM Radiative Transfer model and robust statistical methods. To accurately trace molecular gas in sub-solar metallicity environments we wish to use the [CII] emission line to quantify the CO-dark H2 gas however, as the [CII] 158μm emission line can arise in all phases of the ISM, being able to disentangle the different ISM contributions is an important yet unresolved problem when undertaking galaxy-wide, integrated [CII] observations. We first present a new multi-phase 3D Radiative Transfer interface that couples starburst99, a stellar spectrophotometric code, with the photoionisation and astrochemistry codes Mocassin and 3d-pdr. We model entire star forming regions, including the ionised, atomic and molecular phases of the ISM, and apply a Bayesian inference methodology to parametrise how the fraction of the [CII] emission originating from molecular regions, f[CII],mol, varies as a function of typical integrated properties of galaxies in the local Universe.  Once this has been achieved we present the first results using xCOLD GASS, a legacy survey of molecular gas in nearby galaxies, now extending down to stellar masses of 109M⊙ from its precursor survey COLD GASS. Using the IRAM 30m telescope we measure the CO(1−0) line and, with Herschel PACS observations, measure the [CII] 158μm emission line in a sub-sample of 24 intermediate mass (9 < log M∗/M⊙ < 10) and low metallicity (0.4 < Z/Z⊙ < 1.0) galaxies. We provide the first scaling relations for the integrated and molecular region L[CII]/LCO(1-0) ratio as a function of galaxy properties. We connect the L[CII]/LCO(1-0) ratio to the CO-to-H2 conversion factor and find a multivariate con- version function, which can be used up to z∼2.5. We then apply this to the full xCOLD GASS and PHIBSS1 surveys and investigate molecular gas scaling relations across red- shifts, aiming to test the equilibrium model. Finally we discuss ongoing survey planning and the latest software development for the ongoing JINGLE survey, which aims to simultaneously quantify gas and dust content for a large sample of galaxies. By conducting such a survey we shall be able to investigate how quantities such as the dust temperature, emissivity and dust-to-gas ratio vary systematically across the galaxy population; refining an alternative method to quantify gas masses of galaxies. The first results from JINGLE are presented, demonstrating the potential of the survey and highlighting the work carried out by this author.

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