The meaning and consequences of star formation criteria in galaxy models with resolved stellar feedback

By P. Hopkins, D. Narayanan, N. Murray. 2013

149 citations recorded by the source corpus.

We consider the effects of different criteria for determining where stars will form in gas on galactic scales, in simulations with high (1 pc) resolution, with explicitly resolved physics of Giant Molecular Cloud (GMC) formation and destruction, and stellar feedback from supernovae, radiation pressure, stellar winds and photoheating. We compare: (1) a self-gravity criterion (based on the local virial parameter and the assumption that self-gravitating gas collapses to high density in a single free-fall time), (2) a fixed density threshold, (3) a molecular-gas law, (4) a temperature threshold, (5) a requirement that the gas be Jeans unstable, (6) a criteria that cooling times be shorter than dynamical times and (7) a convergent-flow criterion. We consider all of these in both a Milky Way (MW)-like and high-density (starburst or high-redshift) galaxy. With feedback present, all models produce identical integrated star formation rates (SFRs), in good agreement with the Kennicutt relation; without feedback all produce orders-of-magnitude excessive SFRs. This is totally dependent on feedback and independent of the star formation (SF) law, even if the ‘local’ collapse efficiency is 100 per cent. However, the predicted spatial and density distribution depend strongly on the SF criteria. Because cooling rates are generally fast within galaxy discs, and gas is turbulent, criteria (4)–(7) are very ‘weak’ and spread the SF uniformly over most of the disc (down to densities n ∼ 0.01–0.1 cm^−3). A molecular criterion (3) localizes to slightly higher densities, but still a wide range; for metallicity near solar, it is almost identical to a fixed density threshold at n ∼ 1 cm^−3 (well below the mean density in the central MW or starburst systems). A fixed density threshold (2) can always select the highest resolved densities, but must be adjusted both for simulation resolution and individual galaxy properties – the same threshold that works well in a MW-like simulation will select nearly all gas in a starburst. Binding criteria (1) tend to adaptively select the largest local overdensities, independent of galaxy model or resolution, and automatically predict clustered SF. We argue that this SF model (possible with other secondary criteria) is most physically motivated and presents significant numerical advantages in simulations with a large dynamic range.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/dbd971829afdebb48d8033405e0b580011d8a62f

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Citations in the corpus

A new class of accurate, mesh-free hydrodynamic simulation methods (2014, methodology): “This is essentially the version of SPH in the P-SPH code used for the FIRE project simulations (Hopkins 2013; Hopkins et al. 2013b), and the adapted version of PSPH in SPHGal (Hu et al. 2014).”
FIRE-2 simulations: physics versus numerics in galaxy formation (2017, methodology): “…the global efficiency of star formation (either on galaxy or GMC scales) is necessarily high – we find that it is self-regulated by feedback at ∼ 1−10% per free-fall time (see Hopkins et al. 2011, 2013e, 2014) even in gas with densities & 100cm−3, in agreement with observations (Lee et al. 2016).”
RECONCILING DWARF GALAXIES WITH ΛCDM COSMOLOGY: SIMULATING A REALISTIC POPULATION OF SATELLITES AROUND A MILKY WAY–MASS GALAXY (2016, background): “Stars form only in locally self-gravitating, molecular gas (Hopkins et al. 2013) at densities of nSF>1000 cm −3 with instantaneous efficiency of 100% per free-fall time, and the maximum density reached is ≈107 cm−3 (corresponding to hgas ≈ 1 pc).”
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.”
Stellar and Quasar Feedback in Concert: Effects on AGN Accretion, Obscuration, and Outflows (2015, background): “As shown in Hopkins et al. (2013b) this is especially important for small scales around black holes, where any simple constant-density threshold for star formation fails to account for the radially-dependent tidal forces.”
The difficulty of getting high escape fractions of ionizing photons from high-redshift galaxies: A view from the FIRE cosmological simulations (2015, methodology): “This SF prescription adaptively selects the largest over-densities and automatically predicts clustered SF (Hopkins et al. 2013).”
The cosmic baryon cycle and galaxy mass assembly in the FIRE simulations (2016, background): “…efficiency on larger scales is regulated by the rate of formation and disruption of giant molecular clouds by feedback processes (e.g. Faucher-Giguère et al. 2013), and it is fairly insensitive to the density threshold and other details of the small scale star formation law (Hopkins et al. 2013).”
The structure and dynamical evolution of the stellar disc of a simulated Milky Way-mass galaxy (2016, unknown): “We follow the star formation criteria in Hopkins et al. (2013) and allow star formation to take place only in locally selfgravitating, self-shielding/molecular gas which also exceeds a hydrogen number density threshold nth = 5cm−3.”
Understanding the shape and diversity of dwarf galaxy rotation curves in ΛCDM (2016, background): “…capture the momentum injection into the interstellar medium (ISM) generated by expanding bubbles of shock heated gas (e.g. Kimm et al. 2015); and (ii) star formation becomes self-regulated by stellar feedback (e.g. Kravtsov 2003; Saitoh et al. 2008; Hopkins et al. 2011, 2013; Agertz et al. 2013).”
Not so lumpy after all: modelling the depletion of dark matter subhaloes by Milky Way-like galaxies (2017, background): “Star formation occurs in self-gravitating gas (according to the criterion in Hopkins et al. 2013) that is also molecular and self-shielding (following Krumholz & Gnedin 2011), Jeans unstable, and exceeds a minimum density threshold nsf > 1000 cm −3.”
The lifecycle of molecular clouds in nearby star-forming disc galaxies (2019, background): “…the parsec-scale physics of star formation and feedback within GMCs and their impact on galaxy evolution is to resolve the scales of individual clouds within galaxies and empirically constrain their life- cycles as a function of the galactic environment (e.g. Lada et al. 2010; Hopkins et al. 2013).”
FIRE in the field: Simulating the threshold of galaxy formation (2016, background): “Star formation occurs only in locally self-gravitating (following Hopkins et al. 2013), selfshielding and molecular (following Krumholz & Gnedin 2011), Jeans-unstable regions with densities > 1000 cm−3; gas that meets these criteria is turned into stars on its free-fall time.”
Galaxies on FIRE (Feedback In Realistic Environments): stellar feedback explains cosmologically inefficient star formation (2013, methodology): “…only for particles with density above n > ncrit (generally ncrit = 100cm−3), which are also locally self-gravitating using the criteria developed in Hopkins et al. (2013d) (α ≡ δv2 δr/Gmgas(< δr) ≈ β (|∇ × v|2 + |∇·v|2)/Gρ < 1, with β ≈ 0.25), and which have a non-zero molecular fraction fmol > 0.”
The origin and evolution of the galaxy mass–metallicity relation (2015, background): “Star formation is allowed only in dense, molecular, and selfgravitating regions with hydrogen number density above some threshold nth = 10–100 cm−3 (Hopkins et al. 2013b).”
Dusty Star Forming Galaxies at High Redshift (2014, background): “The increase in turbulence driven by the accreted gas reduces the star formation efficiency, and allows gas fractions to rise accordingly (though see Hopkins et al., 2013b, for counter arguments).”
Dark matter cores all the way down (2015, background): “…us insensitive to the details of our ‘sub-grid’ numerical parameters (for a detailed discussion on this, see e.g. Kimm et al. 2015; for earlier work on the utility of resolving the interstellar medium (ISM) see e.g. Kravtsov 2003; Saitoh et al. 2008; Hopkins et al. 2011, 2013; Agertz et al. 2013).”
Theoretical Challenges in Galaxy Formation (2016, background): “…infrared galaxies (ULIRGS) and explain observations of ’extra light’ in the centers of low mass early-type galaxies (Hayward et al. 2014; Hopkins et al. 2009a, 2013, 2009c; Kormendy 1999; Kormendy et al. 2009; Mihos & Hernquist 1994a,c, 1996; Springel 2000; Teyssier, Chapon & Bournaud…”