MAJOR MERGERS BETWEEN GALAXIES: Their role in the high-redshift universe & two nearby case studies

One of the fundamental questions astrophysicists attempt to answer is how galaxies evolve over time. A key process in this context is the gravitational attraction and eventual merging of two near-equal mass galaxies, a so-called major merger. Profoundly transformative events, major mergers substantially affect virtually all properties of a galaxy on a timescale of several hundred million years. Mergers between two gas-rich disk galaxies in particular are believed to be responsible for triggering galaxy-wide starbursts, quasar activity, and the creation of elliptical galaxies. However, we still lack a good understanding both of the details of the merger process, and the role of major mergers at high redshifts, when the universe was roughly half its current age. The aim of this thesis is to address these shortcomings. To this end, we utilise adaptive optics assisted near-infrared imaging and integral field spectroscopy data and millimetre interferometry data of two nearby prototypical major mergers and of a population of submillimetre-bright high-redshift galaxies. We first give an overview of the field (Chapter 1) and techniques used (Chapter 2). In Chapter 3 we then investigate the mechanisms that lead to the exceptional luminosities of a class of high redshift galaxies known as ‘Submillimetre-luminous Galaxies’ (SMGs hereafter). Subarcsecond interferometric CO line emission data of 12 SMGs (for four of which we present new data) are employed to show that morphologically and kinematically, at least eight of them show strong indications of being mergers. For those systems consisting of two spatially separated galaxies, we find that the mass ratios of the merger partners are 1:3 or closer to unity, i.e. that these systems are major mergers. We then discuss the extent, to which local LIR ≥ 10L⊙and high-redshift S850μm-selected major mergers sample the same merger stages, and compare their respective ratios of binary to coalesced systems. From this we infer that most, if not all, of those SMGs in our sample which morphologically and kinematically could be either isolated disk galaxies or late-stage, coalesced major mergers, must indeed be major mergers. We also compare our dynamical and gas mass measurements of our SMG sample to the stellar masses derived by other authors, lending strong support to the lower end of the range of published values. This in turn supports cosmological models in which SMGs arise through major mergers. We can thus convincingly show that most, if not all, of these systems – representing the brightest galaxies in the early universe – are mergers between near-equal mass, gas rich galaxies. We then turn to two nearby prototypical merging galaxies; NGC6240 and Arp 220. Due to their proximity (96 and 77Mpc, respectively), they afford a rare opportunity to study on-going mergers in detail. To this end, we utilise adaptive optics assisted near-infrared integral field spectroscopic and interferometric millimetre CO line data of their central regions. From archival HST data, we show that the Calzetti et al. (2000) reddening law is the appropriate formalism to use when correcting for the effects of extinction in these gas-rich major mergers. For NGC6240 (Chapter 4), we concentrate primarily on the stellar kinematics and stellar populations, the merger stage and geometry, and the nature and origin of the nuclei. We discuss constraints on the merger phase and geometry, and use these to interpret the observables in light of typical merger star formation histories. We are able to show that less than a third of NGC6240’s prodigious luminosity is due to the merger-induced starburst. We furthermore derive the mass of the two nuclei via Jeans modelling, which, in combination with the presence of a prominent old stellar population, indicate that the nuclei are remnants of the progenitor galaxies’ bulges. Arp 220 (Chapter 5) we find to be severely affected by extinction in the K-band. We are able to quantify that we are only seeing less than 10% of the emitted stellar light. A young starburst contributes at least ∼ 50% of the K-band luminosity, with the remainder most likely due to a population & 1Gyr old. The nuclei are most likely the remainders of the progenitor’s bulges – in the western nucleus, the kinematic centre of the stellar kinematics is coincident with the hot dust emission posited to be the signature of an accreting central black hole, and in the eastern nucleus, which in the K-band tracers displays a less clear-cut picture due to strong extinction, we find mm-wavelength CO(2-1) emission to be best fitted by two compact, unresolved sources rotating around localised hot dust emission, akin to the western nucleus. We thus have obtained several key results, which significantly advance our understanding of the mechanisms and importance of major mergers; in particular their role as triggers of the highest galactic luminosities in the high-redshift universe, and the star formation history, mass and luminosity budget of advanced, pre-coalescence present-day major mergers.