Gas matters: The Molecular and Atomic Gas Budget in Nearby Galaxies from Centers to Outskirts

Abstract

Hydrogen is the most fundamental component of the baryonic universe, and its various phases have played a key role in its evolution from the Big Bang to the present day. To probe and study these phases of hydrogen in the interstellar medium (ISM) we have to use various direct and indirect diagnostic tools. This thesis deals with two environments that harbor the extremes of hydrogen phases: Centres (consisting of almost all molecular gas) and outskirts (all atomic gas) of nearby disk galaxies. Specifically, these are of interest to study because the dynamic processes in the galaxies drive mass flows from the outermost regions replenishing the central regions with fresh material that is converted into molecular gas, fueling ongoing star formation. <br /> Observing multiple molecules in extragalactic space is currently only possible in a small range of environments. This is because species tracing for example dense molecular or shocked gas are faint and thus harder to detect than the bulk molecular gas tracer Carbon Monoxide (CO). Centers of galaxies are one of these rare extragalactic environments and serve as ideal laboratories to study a plethora of molecular line emissions to gain insight into the physical and chemical state of the gas. The first scientific project presents multi-molecular line observations from the IRAM Plateau de Bure Interferometer (PdBI) toward the center of the nearby starburst double-barred galaxy NGC 6946. Our results reveal for the first time that an inner smaller bar affects the molecular gas such that the locations of the densest molecular gas material (that is traced by molecules such as HCN, HNC, N₂H⁺, or HC₃N) are not necessarily the very centers of the galaxies (i.e. their nuclear region). Furthermore, we find that the star formation rate (SFR) varies within the sub-regions along the bar and show that the SFR efficiency of the dense gas exhibits a different behavior than compared to disk observations. We address the question of whether ratios of molecular line emission can be used as a diagnostic of the physical and chemical state of the gas (temperature and density) and the environment (indicating the presence of an active galactic nucleus) in the center of NGC 6946 and additional 8 galaxy centers (taken from the EMPIRE survey and two high angular resolutions observations). This thesis shows that empirical molecular line diagnostics are a useful tool, but reach their limits and/or cannot be unambiguously interpreted at (sub)kpc scales in nearby galaxy centers. <br /> The atomic gas budget in the outskirts of nearby galaxies and its kinematics on small-scale (turbulent nature of the ISM) and large-scale (disk rotation and mass flow rates) is best explored using the 21 cm HI emission. The second scientific project in this thesis presents the largest Karl G. Jansky Very Large Array (VLA) mosaic observations that we combine with <em>Green Bank Telescope</em> (GBT) single dish data targeting the super-extended HI disk of the nearby grand design galaxy M 83. We find along the ~50 kpc radial extending HI disk, HI line widths, also called velocity dispersion, greater than inferred from the warm neutral medium (i.e. >8 km/s) that is expected to be in the outskirts of nearby galaxies. Our analysis reveals that dynamical features (such as rings and spiral arms) have a significant impact on mass flow rate profiles and radial HI velocity dispersion profiles. In particular, this project shows that mass flow rate profiles are highly sensitive to kinematic parameters, in particular inclination thus, such rates should be interpreted with caution. <br /> The third in-progress scientific project addresses the questions of where in nearby galaxies atomic gas transitions to molecular gas (i.e. where their surface densities are approximately equal, &#931;_atom &#8776; &#931;_mol) using new high-quality observations from More of Karoo Array Telescope (MeerKAT) together with Atacama Large Millimeter/submillimeter Array (ALMA), and how the transition <em>R</em>_mol = &#931;_mol / &#931;_atom = 1) depends on local conditions, as probed by a range of multi-wavelength observations, across the galaxy disks. Our results reveal that when normalizing the radial extent of HI in 8 nearby galaxies by their optical radius the transition between an HI-dominated and a predominantly H₂-dominated ISM takes place at 0.4 <em>r></em>_gal / <em>r</em>₂₅. This analysis reveals that the dynamical equilibrium pressure (i.e. the sum of the weight of the ISM due to the self-gravity and due to stellar gravity) tightly correlates with the gas transition, <em>R</em>_mol. This indicates that the balance between HI and H₂ is primarily determined by the dynamical equilibrium pressure. <br /> Overall, we have gained a detailed picture of molecular and atomic gas tracers, and their diagnostics for physical and chemical processes in individual nearby galaxies. These dissertation projects are advancing future investigations toward a statistical characterization (i.e., a larger sample) of a variety of different nearby galaxy centers and outskirts to improve our understanding of the physics of ISM gas and its role in galaxy evolution.