Periodic Reporting for period 4 - RadFeedback (The radiative interstellar medium)
Periodo di rendicontazione: 2021-03-01 al 2021-08-31
The pressure, radiation, and ionization from the warm (UV emitting) and hot (X-ray emitting) gas has a significant impact on the cold, star-forming interstellar medium. In the RADFEEDBACK project we carried out a comprehensive 3D study of the turbulent, multi-phase ISM in different environments that includes, for the first time, a proper treatment of UV and X-ray emission from stellar (primary) sources and extended (secondary) sources like cooling shock fronts and evaporating clouds. We did this by means of massively parallel, high-resolution 3D simulations that capture the complex interplay of gravity, magnetic fields, feedback from massive stars (ionizing radiation, radiation pressure, stellar winds, supernovae), heating and cooling including X-rays and cosmic rays, and chemistry. We developed a novel, original and highly efficient method, TreeRay, to accurately treat the transfer of radiation from multiple point and extended sources in the 3D simulations. In particular, based on TreeRay, we developed new methods to treat radiation pressure on dust and gas as well as the radiative transfer of cooling radiation in the Optical, UV, and X-ray bands. Radiation and chemistry were coupled to achieve self-consistent heating, cooling, and ionization rates. This enabled us to determine the efficiency of stellar feedback in different environments and to investigate which feedback process is dominant. Here we found that early feedback from massive stars regulates the star formation efficiency in galaxies, while supernovae initiate galactic outflows which are taken to larger scale heights via the Cosmic Ray pressure gradient that is replenished by the acceleration of Cosmic Rays in supernovae. In particular, we found that UV radiation has a major impact on limiting the star formation efficiency in dense molecular clouds, while stellar winds become dynamically dominant when the massive star is embedded in a lower-density, warm environment. Radiation pressure dominates in high-density, cold environments such as typically prevailing in massive star forming cores and massive giant molecular clouds. Moreover, accurate synthetic observations covering the large dynamic range from X-rays down to radio emission were generated to set the results in the proper observational context. We were also able to show that our 3D simulations can recover a multitude of observed molecular cloud properties, such that the simulations appear to be quite realistic and are used in a number of follow-up collaborative projects, such as how to design a future space mission to observe the signatures of molecular hydrogen formation and dissociation.
The developed, novel radiative transfer scheme TreeRay is working very well and scales perfectly with the number of radiative sources, i.e. the method is independent of the number of sources. Thus, we can use it to study the emission from point sources as well as from extended sources such as cooling bubbles or shocked regions. We are not aware of any other code which can do this in three-dimensional, magneto-hydrodynamical simulations. We have coupled the radiative transfer scheme to the chemical network used in our simulations, such that we are able to follow the impact of the radiation on the molecular gas forming and dissociating in galactic molecular clouds. Further, we implemented the treatment of a time- and energy-dependent X-ray radation field. To our knowledge, our code is also unique in the way both the dust and gas temperature are accurately calculated, e.g. taking into account the heating of dust by UV and infrared radiation (depending on the local optical depth).
The new numerical scheme has been applied to study the evolution of molecular species in clouds which are subject to an X-ray flare, prototypical for the Galactic Center. We found that CO is much more vulnerable than molecular hydrogen and therefore the CO/H2 ratio varies significantly in such environments. Further, we studied the impact of feedback from massive stars that are born in molecular clouds, which are formed from the warm interstellar medium in the SILCC-Zoom project. We find that HII regions flicker on scales of >10 parsec before they are fully developed. This leads to relatively large photo-dissociation regions (defined as regions of intermediate ionization degree) with respect to what is predicted by static or 1D models. We were also able to show that ionizing radiation is the most important feedback mechanism, while stellar winds are only dominating if the massive star is already sitting in a low-density, warm bubble. We have shown that supernovae have a small impact on the dense molecular gas because they quickly cool. We then used our simulations to look at observables, such as molecular line emission and star formation tracers.
We have further made significant progress in comparing simulations and observations in a quantitative manner. The new “synthetic observation pipeline” is operating well and we have implemented the treatment of several molecules, dust, C+ and atomic hydrogen. Using MAPPINGS V, we have refined the high-temperature gas cooling. Optical emission lines result from this gas phase via metal line cooling and we are now able to reproduce the observed emission line diagnostics for all our simulations.
The new numerical scheme has been applied to study the evolution of molecular species in clouds which are subject to an X-ray flare, prototypical for the Galactic Center. We found that CO is much more vulnerable than molecular hydrogen and therefore the CO/H2 ratio varies significantly in such environments. Further, we studied the impact of feedback from massive stars that are born in molecular clouds, which are formed from the warm interstellar medium in the SILCC-Zoom project. We find that HII regions flicker on scales of >10 parsec before they are fully developed. This leads to relatively large photo-dissociation regions (defined as regions of intermediate ionization degree) with respect to what is predicted by static or 1D models. We were also able to show that ionizing radiation is the most important feedback mechanism, while stellar winds are only dominating if the massive star is already sitting in a low-density, warm bubble. We have shown that supernovae have a small impact on the dense molecular gas because they quickly cool. We then used our simulations to look at observables, such as molecular line emission and star formation tracers.
We have further made significant progress in comparing simulations and observations in a quantitative manner. The new “synthetic observation pipeline” is operating well and we have implemented the treatment of several molecules, dust, C+ and atomic hydrogen. Using MAPPINGS V, we have refined the high-temperature gas cooling. Optical emission lines result from this gas phase via metal line cooling and we are now able to reproduce the observed emission line diagnostics for all our simulations.
The main objectives of RADFEEDBACK were the questions “How is the joint energy and momentum output from evolving massive stars processed by the interstellar medium?” and “What is the relative contribution of stellar winds, (ionizing) radiation, radiation pressure, and blast waves to shaping the multi-phase, ISM, driving supersonic turbulence and dispersing molecular clouds?”. We have solved these questions for low- to intermediate mass molecular clouds using the best 3D simulations to date within the SILCC-Zoom project, finding that radiative feedback is the most important process limiting the star formation efficiency in molecular clouds. Supernova remnants have a minor impact on the dense gas as they experience strong radiative cooling as soon as they interact with a dense cloud. For massive and denser clouds which have high escape velocities, the situation is more complex and radiation pressure needs to be taken into account. We have also implemented a new method to treat the radiative transfer and absorption of soft X-rays and their effects on the chemistry of the gas. This has led to new, exciting results, that the carbon-monoxide molecule is more vulnerable to a high X-ray irradiation than molecular hydrogen.This implies that the chemistry of clouds in e.g. the Galactic Center is most likely out of equilibrium and that conclusions based on observational results should be considering this point. Overall, we will be able to explore the star formation in massive molecular clouds in much more detail in the future using the new computational methods we have developed in RADFEEDBACK.
Within RADFEEDBACK we performed the most self-consistent theoretical study of the multi-phase ISM so far, thus building up a leading group for ISM research in Europe. Our results contribute fundamentally to the understanding of the evolution of the interstellar medium in different galactic environments and help to interpret existing and upcoming observations.
Within RADFEEDBACK we performed the most self-consistent theoretical study of the multi-phase ISM so far, thus building up a leading group for ISM research in Europe. Our results contribute fundamentally to the understanding of the evolution of the interstellar medium in different galactic environments and help to interpret existing and upcoming observations.