Abstract
Galaxy mergers play a fundamental role in shaping both the structural and dynamical evolution of galaxies and are a natural consequence of hierarchical structure formation in the current cosmological model. During these chaotic interactions, gas is funneled toward the central regions of the merging system, providing fuel for the growth of supermassive black holes (SMBHs). This inflow, where gas accretes onto an SMBH, can trigger the onset of active galactic nuclei (AGNs). Among the most striking outcomes of gas-rich galaxy mergers are ultra-luminous infrared galaxies (ULIRGs), which are powered by intense, short-lived episodes of extremely high star formation rates known as starbursts and often host obscured AGNs. These extreme systems provide key laboratories for studying AGN feedback, the interplay between accreting SMBHs and their surrounding interstellar medium (ISM), thought to regulate star formation and black hole growth. A prototypical example of such a system is NGC 6240,a nearby, gas-rich major merger known for its complex morphology, dual AGN, and powerful outflows.
This thesis aims to study the kinematics of ionized and molecular gas in the nuclear region of NGC 6240, using archival observations from the Near-Infrared Spectrograph of the James Webb Space Telescope (JWST/NIRSpec) complemented with data from the Atacama Large Millimeter Array (ALMA).
Our first result is based on a novel multi-component emission line fitting routine in which one kinematic component is constrained to match the stellar kinematics obtained from stellar continuum fitting with Penalized Pixel-Fitting (pPXF). We find that the near-infrared (NIR) emission lines in NGC 6240 exhibit gas components with kinematics that are strongly decoupled from those of the stars. This decoupling is evident in both molecular (H2) and ionized gas tracers (e.g., Paα,[Fe II] λ1.644μm), with the largest deviation observed in the rotational and ro-vibrational H2 lines.We also identify a biconical outflow dominated by ionized gas, likely launched from the northern AGN and expanding along the minor axis of stellar rotation. Its western cone spatially coincides with a recently identified AGN-ionization region. Our analysis suggests that this outflow is either exposed to intense UV radiation or entrains only a negligible amount of H2 gas. In addition, we identify another, more massive outflow, likely launched from the southern nucleus, and propose that it is connected to the molecular outflow that has been previously mapped in CO emission out to an extent of10 kpc. Moreover, we confirm that the NIR H2 emission is shock-dominated, consistent with previous studies. We find a spatial alignment between the region with the strongest H2/PAH (polycyclic aromatic hydrocarbons)enhancement and the southern massive outflow, which suggests that the increase in H2/PAH ratio can be explained by shocks driven by the outflow. While feedback in the form of powerful galactic-scale outflows remains observationally rare, our study of NGC 6240 provides direct evidence of such a mechanism in action. These outflows appear to play a crucial role in regulating both star formation and black hole growth by injecting energy in to the ISM and altering gas dynamics. Our findings highlight the importance of feedback during the merger phase and offer new insights into how AGN-driven outflows shape galaxy evolution. Future research into the interplay between gas, dust, and magnetic fields in the central regions of NGC 6240 might provide a more complete understanding of the forces shaping the multiphase ISM in merging systems.
Supervisor:
Professor Claudia Cicone, Institute of Theoretical Astrophysics, UiO
Postdoctoral fellow Rubinur Khatun, Institute of Theoretical Astrophysics, UiO
Intern. assessor: Professor Boris Vilhelm Gudiksen, RoCS, Institute of Theoretical Astrophysics, UiO
Extern. assessor: Researcher Filippo Mannucci, INAF Arcetri Astrophysical Observatory