The structure of the interstellar medium of star‐forming galaxies

By P. Hopkins, E. Quataert, N. Murray. 2011

237 citations recorded by the source corpus.

We develop and implement numerical methods for including stellar feedback in galaxy-scale numerical simulations. Our models include simplified treatments of heating by Type I and Type II supernovae, gas recycling from young stars and asymptotic giant branch winds, heating from the shocked stellar winds, H ii photoionization heating and radiation pressure from stellar photons. The energetics and time dependence associated with the feedback are taken directly from stellar evolution models. We implement these stellar feedback models in smoothed particle hydrodynamic simulations with pc-scale resolution, modelling galaxies from Small Magellanic Cloud (SMC) like dwarfs and Milky Way (MW) analogues to massive z∼ 2 star-forming discs. In the absence of stellar feedback, gas cools rapidly and collapses without limit into dense sub-units, inconsistent with observations. By contrast, in all cases with feedback, the interstellar medium (ISM) quickly approaches a statistical steady state in which giant molecular clouds (GMCs) continuously form, disperse and re-form, leading to a multiphase ISM. In this paper, we quantify the properties of the ISM and GMCs in this self-regulated state. In a companion paper we study the galactic winds driven by stellar feedback. Our primary results on the structure of the ISM in star-forming galaxies include the following. 1 Star-forming galaxies generically self-regulate so that the cool, dense gas maintains Toomre’s Q∼ 1. Most of the volume is occupied by relatively diffuse hot gas, while most of the mass is in dense GMC complexes created by self-gravity. The phase structure of the gas and the gas mass fraction at high densities are much more sensitive probes of the physics of stellar feedback than integrated quantities such as the Toomre Q or gas velocity dispersion. 2 Different stellar feedback mechanisms act on different spatial (and density) scales. Radiation pressure and H ii gas pressure are critical for preventing runaway collapse of dense gas in GMCs. Shocked supernova ejecta and stellar winds dominate the dynamics of the volume-filling hot gas. However, this gas primarily vents out of the star-forming disc and contributes only modestly to the mid-plane ISM pressure. 3 The galaxy-averaged star formation rate is determined by feedback, with different mechanisms dominating in different galaxy types. For a given feedback efficiency, restricting star formation to molecular gas or modifying the cooling function has little effect on the star formation rate in the galaxies we model (including an SMC-mass dwarf). By contrast, changing the feedback mechanisms or assumed feedback efficiencies directly translates to shifts off of the observed Kennicutt–Schmidt relation. 4 Self-gravity leads to GMCs with an approximately self-similar mass function ∝M−2, with a high-mass cut-off determined by the characteristic Jeans/Toomre mass of the system. In all of our galaxy models, GMCs live for a few dynamical times before they are disrupted by stellar feedback. The net star formation efficiency in GMCs ranges from ∼1 per cent in dwarfs and MW-like spirals to nearly ∼10 per cent in gas-rich rapidly star-forming galaxies. GMCs are approximately virialized, but there is a large dispersion in the virial parameter for a given GMC mass, and lower mass GMCs tend to be preferentially unbound.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/1715c82a706e3a60093364ab841be7ffba4b0ecf

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In Astronomy and Astrophysics

1. P. Hopkins
2. L. Hernquist
3. Volker Springel
4. C. Faucher-Giguére
5. E. Quataert
6. D. Keres̆
7. T. Naab
8. R. Dav'e

Citations in the corpus

• Theoretical Challenges in Galaxy Formation (2016, background): “Alternative models link the accretion rates to gravitational instabilities and torques (Hopkins et al. 2012, Hopkins & Quataert 2011).”
• Dusty Star Forming Galaxies at High Redshift (2014, background): “At the moment when the two galaxy cores and supermassive black holes (SMBHs) merge during final coalescence, an active galactic nucleus (AGN) is formed and is fed by an accretion disk of material, further fueled by the infall from the outer realms of the galaxy merger (Hopkins et al., 2012b,a).”
• TOWARD A COMPLETE ACCOUNTING OF ENERGY AND MOMENTUM FROM STELLAR FEEDBACK IN GALAXY FORMATION SIMULATIONS (2012, background): “Radiation pressure feedback has just recently been considered in numerical work studying isolated galactic disks (Hopkins et al. 2011a, 2012a; Chattopadhyay et al. 2012).”
• The meaning and consequences of star formation criteria in galaxy models with resolved stellar feedback (2013, methodology): “A more quantitative calculation of the dense gas distributions as tracers of the local star formation efficiency is presented in Hopkins et al. (2012a).”
• Neutral hydrogen in galaxy haloes at the peak of the cosmic star formation history (2014, background): “…the structure of the ISM and the properties of giant molecular clouds (GMCs; Hopkins et al. 2012c), galactic winds driven by stellar feedback (Hopkins et al. 2012b), gas inflow in gas rich disks (Hopkins et al. 2012a) and during galaxy mergers (Hopkins et al. 2013a), and a range of other problems.”
• The formation of disc galaxies in high-resolution moving-mesh cosmological simulations (2013, background): “There are now several groups and codes that have made great strides towards forming realistic galaxies, both in cosmological simulations like the ones discussed here, and in simulations of isolated galaxies (e.g. Hopkins et al. 2012).”
• How Stellar Feedback Simultaneously Regulates Star Formation and Drives Outflows (2015, background): “…mass is converted into stars per orbital time and the KennicuttSchmidt relation (Schmidt 1959; Kennicutt 1998) can be reproduced (e.g.Stinson et al. 2006; Shetty & Ostriker 2008, 2012; Kim et al. 2011; Ostriker & Shetty 2011; Hopkins et al. 2012b, 2014; Kim et al. 2013; Agertz & Kravtsov 2015a).”
• (Star)bursts of FIRE: observational signatures of bursty star formation in galaxies (2015, methodology): “We refer the reader to Hopkins et al. (2011, 2012) for details regarding and extensive tests of the stellar feedback models and to Hopkins et al. (2014) for details of the stellar feedback model as implemented in the FIRE simulations.”
• FIRE-2 simulations: physics versus numerics in galaxy formation (2017, methodology): “Before FIRE-1, a series of papers developed the numerical methods, and tested each physical addition individually using higher-resolution ISM-scale simulations and analytic solutions where possible (we refer the interested reader to Hopkins et al. 2011, 2012c,b,a, 2013a,b,c,e,d).”
• On the fraction of star formation occurring in bound stellar clusters (2012, background): “…galaxies in the Local Group, observations show LGGMC ∼ 100 M pc−2 (Solomon et al. 1987; Bolatto et al. 2008; Heyer et al. 2009), but pressure equilibrium implies that for galaxies with g > 100 M pc−2 the GMC surface density is set by the disc surface density GMC ∼ g (also see Hopkins et al. 2012).”
• A general class of Lagrangian smoothed particle hydrodynamics methods and implications for fluid mixing problems (2012, methodology): “We also compare the massloading of galactic winds in each simulation, defined as in Hopkins et al. (2012a) as the ratio of total “wind mass” (material with positive Bernoulli parameter, i.e. which will be unbound in the absence of external pressure forces) to new stellar mass formed since the…”
• Supernova feedback in an inhomogeneous interstellar medium (2014, background): “Galaxy-scale (e.g. Hopkins et al. 2012a; Agertz et al. 2013) and cosmological simulations (e.g. Agertz & Kravtsov 2014; Hopkins et al. 2014) that implement approximations for multiple stellar feedback mechanisms show that they can interact non-linearly and in a non-trivial fashion.”
• Galaxies on FIRE (Feedback In Realistic Environments): stellar feedback explains cosmologically inefficient star formation (2013, methodology): “…galaxy simulations, that these mechanisms produce a quasisteady ISM in which GMCs form and disperse rapidly, with phase structure, turbulence, and disk and GMC properties in good agreement with observations (for various comparisons, see Narayanan & Hopkins 2012; Hopkins et al. 2012b,a, 2013c).”
• A sub-parsec resolution simulation of the Milky Way: global structure of the interstellar medium and properties of molecular clouds (2013, background): “Similarly, Dobbs et al. (2012) emphasized the role of stellar feedback on the formation/destruction cycle of GMCs in the context of spiral galaxies, and Hopkins et al. (2012) focussed on a multi-component stellar feedback in several galaxy types, including models of high redshift discs (z ∼ 2).”
• Feedback-regulated star formation in molecular clouds and galactic discs (2013, background): “However, the subsequent models by Hopkins et al. (2012) include additional feedback processes, including SNe, and are in much better agreement with the data for the KS law used here.”