Rhea-RT: Dynamical impact of Central Molecular Zone conditions on ISM properties and stellar feedback coupling

TL;DR

This study uses radiation-MHD zoom simulations with AREPO in a MW-like barred galaxy to compare the CMZ and Solar Circle environments. By resolving high-density gas and incorporating chemistry, radiative feedback, and SNe, the authors demonstrate that the CMZ’s short orbital times and strong shear drive rapid dynamical decoupling of stars and gas, plus frequent re-embedding of young stars, which diminishes direct feedback coupling to their birth clouds and prevents large-scale superbubbles. The CMZ is denser, more molecular, more magnetized, and more turbulent than the Solar Circle, with higher SFR surface densities but similar molecular depletion times; feedback acts mainly as a background source of turbulence rather than a decisive cloud-disrupting agent, suggesting inflow-regulated SF in the CMZ. The work provides a new framework for feedback coupling in extreme environments and highlights the need to consider dynamical decoupling and shear when interpreting SF in galactic centers.

Abstract

The Central Molecular Zone (CMZ) is an extreme star formation environment, characterized by higher density, higher turbulence, stronger orbital shear, and stronger magnetic field strength than the Solar neighborhood. Whether classical theories of star formation hold within this extreme environment is still debated. In order to assess the impact of these different conditions on the interstellar medium (ISM) and on star formation, we present radiation MHD arepo simulations of a Milky Way-type galaxy. We set up a high-resolution ($M_{\rm cell}=20$ M$_\odot$) region in a ring around the Solar radius, as well as in the barred region of the Galaxy to have a coherent comparison between the CMZ and the Solar neighborhood. Although the high densities and strong levels of turbulence influence star formation and feedback, we find that a key difference in the regulation of star formation between the two environments comes from the short orbital times and the strong shear present in the CMZ. In particular, we highlight the role of the quick dynamical decoupling of stars and gas that leads to periodic re-embedding events in the early lifetimes of radiating O stars. Young stellar associations get efficiently sheared apart such that the ISM is deprived of the compounding effect of radiation and supernovae in disrupting molecular clouds. This changes dramatically the evolution of giant molecular clouds and how feedback can regulate star formation in the CMZ. Stellar feedback is no longer directly coupled to the molecular cloud from which they formed and no strong and disruptive superbubbles can develop. Instead, the feedback rather acts as a background source of turbulence.

Figures (25)

• Figure 1: Face-on view of the simulated Milky Way analog. The grayscale shows the total gas column density. The areas highlighted correspond to the enhanced-resolution regions of the simulation.
• Figure 2: Resolution as a function of density. Distribution of the spherical radius (top panel) and mass (bottom panel) of the cells as a function of density. Cells in the high- and low-resolution regions are colored in red and blue, respectively. The contours contain [100, 99, 90, 75, 50, 25]% of the total number of cells. The total number of cells in each region is indicated (bottom left in the bottom panel). We highlight the target mass in the two regions, as well as the maximum and minimum volume limits imposed. In the bottom panel, we show with a gray overlay the region for which gas cells are more massive than 1 and 8 times the Jeans mass. This gas is Jeans-unresolved and potentially star-forming. The dotted lines indicate the demarcation lines for where the Strömgren radius of a 15 and 120 M$_\odot$ star (which is the mass range of O-type stars) is resolved.
• Figure 3: Properties of the star particles in the high resolution regions. We show the distribution of the mass of the star particles (first column), the density of the gas particle that generated the star particle (second column), the ionizing radiation flux assigned to the stellar particle (third column), the total mass of stars drawn from the IMF (fourth column) and the number of stars drawn (last column). Often, no massive star is drawn from the IMF sampling. The mass and parent density distribution of those star particles is shown in the upper right.
• Figure 4: Contribution to the total ionizing photon budget of massive ($>8$ M$_\odot$) stars given their mass weighted with the IMF. The dotted line shows the cumulative fraction of massive stars, the solid line the cumulative fraction of ionizing photons at birth of the star, while the dashed line shows the cumulative fraction of the ionizing photons generated over the entire life of the star as a function of mass.
• Figure 5: Background interstellar UV radiation field (photons with energy $<13.6$ eV) as a function of galactocentric radius. The value is scaled to the Solar neighborhood value $G_0$Draine1978. The cosmic ray ionization rate follows the same profile, scaled to the solar neighborhood value.