Probing the Multiphase Interstellar Medium in an Extreme Starburst at High Redshift

In this thesis we present a range of observations and results relating to the lensed hyperluminous sub-mm galaxy ‘9io9’ at z = 2.6, where we have used sub-mm/mm spectroscopy targeting the dense and cold interstellar medium to better undertstand the conditions underlying extreme star formation (1000× that of the Milky Way) in galaxies the early Universe. In Chapter 2 we present new observations with the Atacama Large Millimeter/sub-millimeter Array (ALMA) of the 122- and 205-μm fine-structure line emission of singly-ionised nitrogen in 9io9. The 122-/205-μm [N II] line ratio is sensitive to electron density, ne, in the ionised interstellar medium, and we use this to measure ne ≈ 300cm−3 averaged across the galaxy. This is over an order of magnitude higher than the Milky Way average, but comparable to localised Galactic star-forming regions. Combined with observations of the atomic carbon (C I(1–0)) and carbon monoxide (CO J = 4–3) in the same system, we reveal the conditions in this intensely star-forming system. The majority of the molecular interstellar medium has been driven to high density, and the resultant conflagration of star formation produces a correspondingly dense ionised phase, presumably co-located with myriad H II regions that litter the gas-rich disk. In Chapter 3 we present the detection of the ground state rotational emission of ammonia, ortho- NH3 (JK = 10 → 00) in 9io9. The integrated line profile is consistent with other molecular and atomic emission lines which have resolved kinematics well-modelled by a 5 kpc-diameter rotating disc. This implies that the gas responsible for NH3 emission is broadly tracing the global molecular reservoir, but likely distributed in pockets of high density (n ≳ 5×104 cm−3). With a luminosity of 2.8×106 L⊙, the NH3 emission represents 2.5×10−7 of the total infrared luminosity of the galaxy, comparable to the ratio observed in the Kleinmann-Low nebula in Orion and consistent with sites of massive star formation in the Milky Way. If LNH3/LIR serves as a proxy for the ‘mode’ of star formation, this hints that the nature of star formation in extreme starbursts in the early Universe is similar to that of Galactic star-forming regions, with a large fraction of the cold interstellar medium in this state, plausibly driven by a storm of violent disc instabilities in the gas-dominated disc. This supports the ‘full of Orions’ picture of star formation in the most extreme galaxies seen close to the peak epoch of stellar mass assembly. In Chapter 4 we present new ALMA observations of 9io9 detecting COJ=5→4 and its isotopologues 13COJ=5→4 and C18OJ=5→4. Since 13C is mainly produced by intermediatemass stars and 18O is produced by massive stars, 13CO/C18O is sensitive to the shape of the stellar initial mass function (IMF), where the IMF of the Milky Way has a power law slope α2 ≈ 2.3–2.6 for stars of masses above 0.5M⊙. We measure a galaxy-integrated luminosity ratio 13CO/C18O = 1.6±0.1, consistent with the ratio observed in local ultraluminous infrared galaxies and submillimetre-selected galaxies at high redshift, and significantly lower than the 13CO/C18O of the Milky Way. It has been argued that the low 13CO/C18O observed in extreme star-forming galaxies in the early Universe is evidence for a top-heavy IMF in these systems. In this work we use state-of-the-art chemical evolution models to conclude that irrespective of stellar rotation, the observed 13CO/C18O is consistent with a Kroupa IMF with high-mass slope of α2 = 2.3 (as in our Milky Way models) and also the steeper ‘top-heavy’ α2 = 2.1 slope. In Chapter 5 we present a discussion of ongoing work on an ALMA spectral scan of 9io9, alongside some potential future avenues of research, in particular pushing to higher resolution with ALMA, and MIRI observations with JWST probing the mid infrared PAH emission as a AGN diagnostic.