SILCC VI - The Thermal and Non-thermal ISM
Tim-Eric Rathjen
Understanding the evolution of the interstellar medium (ISM) within galaxies is crucial to understand the evolution of galaxies. Star formation happens inside the ISM and galactic outflows are launched from it. Constituents of the ISM are the non-stellar and non-relativistic baryonic matter, radiation, magnetic fields and (relativistic) cosmic rays (CRs). The gas component of the ISM is of multi-phase nature, consisting of a hot gas-phase exceeding temperatures of T = 105 K, a warm- and neutral- ionised phase, as well as cold neutral and molecular gas-phase. A plethora of processes governs the shape and evolution of the ISM and the star formation therein. Molecular clouds condense out of the warm gas-phase via gravitational collapse and metal line cooling mechanisms and massive stars and star clusters are born within them. Those massive stars can interact with their environment via so-called early feedback in the form of stellar winds and the formation of HII regions through hydrogen ionising radiation. This early feedback reduces the efficiency of further star formation by hindering the accretion of more stars. The ionising radiation is also a catalyst for the chemical evolution of the ISM. However, the most cataclysmic event is the explosion of Type II supernovae (SNe) at the end of the lifetime of massive stars. Overlapping supernova remnants drive strong shocks into the ISM by generating a hot ionised gas-phase, which is canonically thought to be the main agent of driving galactic outflows out of the galactic mid-plane. Additionally, CRs are accelerated in SN remnants by diffusive shock acceleration. This non-thermal component might play an important role in driving outflows via their long-lived and large-scaled pressure gradients.
Many recent works study the evolution of the ISM via numerical (magneto-) hydrodynamics (MHD) simulations. However, the implemented stellar feedback channels in most works are limited to only accounting for SN feedback and/or stellar winds. Within the SILCC project, we have run a suite of 6 simulations accounting for the first time for all major thermal and non-thermal stellar feedback processes, namely SN, stellar winds, ionising radiation and CR acceleration (Rathjen et al., 2021).
Figure 1 shows an overview of one ISM simulation with solar neighbourhood conditions including all major stellar feedback processes in combination (model SWRC). Our model SWRC predicts a multi-phase ISM with sensible star formation rates and star cluster properties when compared to observations of the local universe. The interaction of the different feedback channels is highly complex and non-linear. Omitting a process, especially the formation of HII regions via ionising radiation, leads to unreasonably high star formation rates, unobserved hot gas volume filling fractions of VFF > 90 per cent, and constant mass loading factors, η, exceeding unity. In Figure 2, we are illustrating this result by showing the star formation rate surface density ΣSFR as a function of the gas surface density ΣH+H2 in the context of observations of local star-forming galaxies. It becomes evident that ionising radiation (simulations labelled with R) is crucial for self-regulating star formation and the evolution of the ISM.
Our models indicate that at low gas surface densities SN-only feedback captures only some characteristics of the star-forming ISM and outflows/inflows relevant for regulating star formation. Instead, star formation is regulated on star cluster scales by radiation and winds from massive stars in clusters, whose peak masses agree with solar neighbourhood estimates. While the picture of regulating star formation via outflows is generally favoured by cosmological galaxy evolution scenarios, it breaks down at the low gas surface density regimes investigated in our work, when including all major feedback processes of massive stars.