Fuelling the central engine in nearby disc galaxies

Verwilghen, Pierrick (2025): Fuelling the central engine in nearby disc galaxies: simulating the evolution of central gas reservoirs = Zufuhrprozesse des zentralen Antriebs in nahegelegenen Scheibengalaxien : Simulation der Evolution zentraler Gasreservoirs. Dissertation, LMU München: Fakultät für Physik

Abstract

Central regions of nearby disc galaxies display a large variety of structures in their stellar and gas disc that illustrates the outcome of a complex and dynamic evolution. The most visible central structure inside the bar region is the inner molecular disc (the so-called Central Molecular Zone (CMZ) in the Milky Way). Recent observational campaigns have shown that those inner molecular discs have a typical size ranging from a few hundred parsecs to one kiloparsec and tend to appear at the centre of barred discs. The physical phenomena involved in the building, consumption (e.g., star formation) and long-term evolution of those inner gas structures are still strongly debated. It is commonly accepted that the bar plays a role in the fuelling of gas from the large few kiloparsec scale to the inner molecular disc region. However, the exact physical processes involved in the loss of gas angular momentum (e.g., gravitational torques, shear, feedback) and its transport to the centre are not fully understood. Moreover, inner gas discs are intermediate-scale structures which connect the large kiloparsec scale and the subparsec scale physics (e.g., stellar-driven feedback, magnetic torques) involved in the fuelling of the central supermassive black hole (SMBH). Therefore, those inner discs can be considered as ‘gas reservoirs’ and may be efficient suppliers of material for the flickering of the central black hole, the so-called Active Galactic Nuclei (AGN). To investigate the physics and characterise the dynamical processes involved in the formation of those gas reservoirs, we have performed a suite of high-resolution hydrodynamical simulations of isolated galaxies with the Adaptive Mesh Refinement (AMR) code RAMSES. This suite of simulations uses a grid of models based on a set of galactic parameters stemming from the Physics at High Angular resolution in Nearby GalaxieS (PHANGS)-ALMA survey. We have selected four control parameters spanning typical properties from the PHANGS galaxy sample: the stellar mass (M⋆: 10^9.5, 10^10, 10^10.5, and 10^11 M⊙), the gas fraction (α: 10, 20, and 40%), the typical stellar scale length (l∗: 2-5 kpc), and the bulge mass fraction (β: 0, 10, and 30%) to study and analyse their impact on the formation and evolution of central gas reservoirs. All the simulations start with axisymmetric initial conditions generated with the Multi Gaussian Expansion (MGE) method. We have initially built a first sample of 16 simulations, focusing on the four stellar mass bins with gas fractions of 10 and 20% and bulge mass fractions of 0 and 10%. We have used this subset to investigate the impact of the control parameters on the formation and evolution of large-scale dynamical structures (i.e., the stellar bar and spiral arms), and their role in fuelling the central 1 kiloparsec region. We have computed the star formation rate (SFR) of our models and found that they match the star-forming main sequence above M⋆≥ 10^10 M⊙. We have found that the presence of a central bulge delays the typical formation time of the bar (tbar), thus confirming results from previous studies. We have used this bar formation time to introduce a dimensionless parameter τ = t/tbar, revealing a similar evolutionary scheme for all our barred models with M⋆≥ 10^10 M⊙. This scheme describes the evolution of the gas mass inside the central 1 kpc region with four phases. We have observed that gas reservoirs were formed inside all barred models but in models belonging to the lowest-stellar mass bin (10^9.5 M⊙), shedding light on a change of regime for the building of gas reservoirs above a stellar mass threshold around 10^10 M⊙. We have then used an extended subset composed of 35 models to investigate this change of regime, including models with a higher gas fraction (40%) and bulge mass fraction (30%). We have observed that higher-stellar mass models (M⋆≥ 10^10 M⊙) show a characteristic dip in the central region of their gas surface density profile after the formation of the bar, indicating the presence of a gas reservoir. We have compared the evolution between phys- ical tracers such as the inner Lindblad resonance (ILR), probability distribution function (PDF), and distribution of the virial parameter and the Mach number for low- and high- stellar mass regimes and revealed different behaviours before and after the formation of the bar. We have shown that the balance between the feedback due to supernovae and gravity is the key to understanding this difference in the building of the central mass concentration and the emergence or not of the central gas reservoir, leading to two separate stellar mass regimes. We have finally designed an analytical toy model which aims at describing the physics of the change of regime and predicts its theoretical stellar mass threshold around 10^9.75 M⊙. We briefly describe the time evolution of the size of the gas reservoir, and we show that its growth rate is mostly constant in time after a brief abrupt early increase, confirming the inside-out formation scenario already proposed by the community. We show that our reservoirs have an elliptical shape with a constant ellipticity. Their vertical structure is made of an underlying nuclear disc, revealing the thinner structure of the central stellar disc, surrounded by a ‘boxy peanut’ bulge made of newly-formed stars. We show that the gas reservoirs’ orientation with respect to the bar forms an expected angle of around 90 degrees, considering the underlying x2 orbital structure. We complete our analysis by computing the location of the central black hole (BH) over time. We show some oscillations of its position over time along the bar with large amplitudes in lower stellar-mass models (M⋆≤ 10^10 M⊙), increasing with the gas fraction, until the end of the run. In higher-stellar mass models (M⋆≥ 10^10 M⊙), the amplitude of the oscillations is smaller and stops once the gas reservoir forms. The BH is then at the centre of the central stellar disc and the gas reservoir, suggesting a favourable configuration for the subparsec scale fuelling of the black hole. This last result depends on the initial location of the BH and could be mostly due to numerical effects, therefore it should be taken with extreme caution. Overall, this work has confirmed that the bar plays an important role in the fuelling of gas to the centre, and has shown its primordial role in shaping the central regions in nearby disc galaxies. This thesis has confirmed the inside-out scenario, revealed four distinct phases involved in the building of the gas reservoir, and some characteristics of its growth. This work has provided a physical explanation for the differences in the gas and star-formation distribution in low- and high-stellar mass discs, already highlighted in past studies and confirmed in new recent observations (HST, JWST). This work has shown that the physical origin of this change of regime is connected with the balance between the stellar feedback due to supernovae and the strength of the gravitational potential. It has demonstrated that the supernovae feedback may prevent the formation of central gas reser- voirs in low-stellar mass discs (< 10^10 M⊙). This last result could allow us to enhance the interactions and discussions between the observational and computational communities.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: Disc Galaxies, Simulations, Gas reservoir, Gas Transport, Stellar Feedback, ISM structure
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 13. Februar 2025
• 1. Bericht­erstatter:in: Dolag, Klaus