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Gerhard Hensler
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Supernovae are the most energetic stellar events and influence the interstellar medium by their gasdynamics and energetics. By this, both also affect the star formation positively and negatively. In this paper, we review the development of the complexity of investigations aiming at understanding the interchange between supernovae and their released hot gas with the star-forming molecular clouds. Commencing from analytical studies the paper advances to numerical models of supernova feedback from superbubble scales to galaxy structure. We also discuss parametrizations of star-formation and supernova-energy transfer efficiencies. Since evolutionary models from the interstellar medium to galaxies are numerous and apply multiple recipes of these parameters, only a representative selection of studies can be discussed here.
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By resimulating a region of a global disc simulation at higher resolution, we resolve and study the properties of molecular clouds with a range of masses from a few 100s M$_{odot}$ to $10^6$ M$_{odot}$. The purpose of our paper is twofold, i) to compare the ISM and GMCs at much higher resolution compared to previous global simulations, and ii) to investigate smaller clouds and characteristics such as the internal properties of GMCs which cannot be resolved in galactic simulations. We confirm the robustness of cloud properties seen in previous galactic simulations, and that these properties extend to lower mass clouds, though we caution that velocity dispersions may not be measured correctly in poorly resolved clouds. We find that the properties of the clouds and ISM are only weakly dependent on the details of local stellar feedback, although stellar feedback is important to produce realistic star formation rates and agreement with the Schmidt-Kennicutt relation. We study internal properties of GMCs resolved by $10^4-10^5$ particles. The clouds are highly structured, but we find clouds have a velocity dispersion radius relationship which overall agrees with the Larson relation. The GMCs show evidence of multiple episodes of star formation, with holes corresponding to previous feedback events and dense regions likely to imminently form stars. Our simulations show clearly long filaments, which are seen predominantly in the inter-arm regions, and shells.
Synthetic observations are playing an increasingly important role across astrophysics, both for interpreting real observations and also for making meaningful predictions from models. In this review, we provide an overview of methods and tools used for generating, manipulating and analysing synthetic observations and their application to problems involving star formation and the interstellar medium. We also discuss some possible directions for future research using synthetic observations.
The interstellar medium is a key ingredient that governs star formation in galaxies. We present a detailed study of the infrared (~ 1-500 micron) spectral energy distributions of a large sample of 193 nearby (z ~ 0.088) luminous infrared galaxies (LIRGs) covering a wide range of evolutionary stages along the merger sequence. The entire sample has been observed uniformly by 2MASS, WISE, Spitzer, and Herschel. We perform multi-component decomposition of the spectra to derive physical parameters of the interstellar medium, including the intensity of the interstellar radiation field and the mass and luminosity of the dust. We also constrain the presence and strength of nuclear dust heated by active galactic nuclei. The radiation field of LIRGs tends to have much higher intensity than in quiescent galaxies, and it increases toward advanced merger stages as a result of central concentration of the interstellar medium and star formation. The total gas mass is derived from the dust mass and the galaxy stellar mass. We find that the gas fraction of LIRGs is on average ~ 0.3 dex higher than that of main-sequence star-forming galaxies, rising moderately toward advanced merger stages. All LIRGs have star formation rates that place them above the galaxy star formation main sequence. Consistent with recent observations and numerical simulations, the global star formation efficiency of the sample spans a wide range, filling the gap between normal star-forming galaxies and extreme starburst systems.
The interstellar medium (ISM) is a very complex medium which contains the matter needed to form stars and planets. The ISM is in permanent interaction with radiation, turbulence, magnetic and gravitational fields, and accelerated particles. Everything that happens in this medium has consequences on the dynamics and evolution of the Galaxy, resulting the link that relates the stellar scale with the galactic one. Thus, the study of the ISM is crucial to advance in the knowledge of stellar and galactic astrophysics. In this article I present a summary of what we know about the physics and chemistry of this medium, giving an special emphasis on star formation, and how the processes related to the stars birth and evolution interrelate with the environment that surrounds them.
We address a simple model where the Kennicutt-Schmidt (KS) relation between the macroscopic densities of star-formation rate (SFR, $rho_{rm sfr}$) and gas ($n$) in galactic discs emerges from self-regulation of the SFR via supernova feedback. It arises from the physics of supernova bubbles, insensitive to the microscopic SFR recipe and not explicitly dependent on gravity. The key is that the filling factor of SFR-suppressed supernova bubbles self-regulates to a constant, $fsim 0.5$. Expressing the bubble fading radius and time in terms of $n$, the filling factor is $f propto S,n^{-s}$ with $ssim 1.5$, where $S$ is the supernova rate density. A constant $f$ thus refers to $rho_{rm sfr} propto n^{1.5}$, with a density-independent SFR efficiency per free-fall time $sim 0.01$. The self-regulation to $f sim 0.5$ and the convergence to a KS relation independent of the local SFR recipe are demonstrated in cosmological and isolated-galaxy simulations using different codes and recipes. In parallel, the spherical analysis of bubble evolution is generalized to clustered supernovae, analytically and via simulations, yielding $s simeq 1.5 pm 0.5$. An analysis of photo-ionized bubbles about pre-supernova stars yields a range of KS slopes but the KS relation is dominated by the supernova bubbles. Superbubble blowouts may lead to an alternative self-regulation by outflows and recycling. While the model is over-simplified, its simplicity and validity in the simulations may argue that it captures the origin of the KS relation.
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