Molecular Hydrogen and Global Star Formation Relations in Galaxies

By Brant Robertson, Andrey V. Kravtsov. 2007

249 citations recorded by the source corpus.

We use hydrodynamical simulations of disk galaxies to study relations between star formation and properties of the molecular ISM. We implement a model for the ISM that includes low-temperature (T < 104 K ) cooling, directly ties the SFR to the molecular gas density, and accounts for the destruction of H2 by an interstellar radiation field from young stars. We demonstrate that the ISM and star formation model simultaneously produces a spatially resolved molecular gas surface density Schmidt-Kennicutt relation of the form ΣSFR ∝ ΣnmolH2, with nmol ≈ 1.4 independent of galaxy mass, and a total gas surface density-SFR relation ΣSFR ∝ Σntotgas with a power-law index that steepens from ntot ∼ 2 for large galaxies to ntot≳ 4 for small dwarf galaxies. We show that deviations from the disk-averaged ΣSFR ∝ Σ1.4gas correlation determined by Kennicutt owe primarily to spatial trends in the molecular fraction fH2 and may explain observed deviations from the global Schmidt-Kennicutt relation. In our model, such deviations occur in regions of the ISM where the fraction of gas mass in molecular form is declining or significantly less than unity. Long gas consumption timescales in low-mass and low surface brightness galaxies may owe to their small fractions of molecular gas rather than mediation by strong supernova-driven winds. Our simulations also reproduce the observed relations between ISM pressure and molecular fraction and between SFR, gas surface density, and disk angular frequency. We show that the Toomre criterion that accounts for both gas and stellar densities correctly predicts the onset of star formation in our simulated disks. We examine the density and temperature distributions of the ISM in simulated galaxies and show that the density pdf generally exhibits a complicated structure with multiple peaks corresponding to different temperature phases of the gas. The overall density pdf can be well modeled as a sum of lognormal pdf's corresponding to individual, approximately isothermal phases. We also present a simple method to mitigate numerical Jeans fragmentation of dense, cold gas in SPH codes through the adoption of a density-dependent pressure floor.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/697d9bfcb643e066f9bd48cc4f0afdfaf05309bb

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

In Nuclear and High Energy Physics

Citations in the corpus

Thin disc, thick disc and halo in a simulated galaxy (2012, background): “A pressure floor is added (Robertson & Kravtsov 2008), and a maximum density limit is imposed by setting a minimum SPH smoothing length, hsm, to 14 of the gravitational softening to ensure that gas resolves the Jeans mass and does not artificially fragment.”
A MOLECULAR STAR FORMATION LAW IN THE ATOMIC-GAS-DOMINATED REGIME IN NEARBY GALAXIES (2011, background): “, the dissociating radiation field (e.g., Robertson & Kravtsov 2008), variations in interstellar gas pressure and density (e.g., Elmegreen 1994; Wong & Blitz 2002; Blitz & Rosolowsky 2006), and external perturbations that drive gas to higher densities (e.g., Blitz & Rosolowsky 2006; Bournaud et al.…”
THE STAR FORMATION EFFICIENCY IN NEARBY GALAXIES: MEASURING WHERE GAS FORMS STARS EFFECTIVELY (2008, unknown): “Robertson & Kravtsov (2008) recently found support from simulations for α ∼ 0.9.”
KECK SPECTROSCOPY OF 3 < z < 7 FAINT LYMAN BREAK GALAXIES: THE IMPORTANCE OF NEBULAR EMISSION IN UNDERSTANDING THE SPECIFIC STAR FORMATION RATE AND STELLAR MASS DENSITY (2012, background): “…processes might be invoked to impede star formation, such as the inefficient formation of molecular hydrogen in low metallicity galaxies (e.g., Robertson & Kravtsov 2008; Gnedin et al. 2009; Krumholz & Dekel 2012) or an increase in the efficiency with which cold gas is removed via large-scale…”
FORMING REALISTIC LATE-TYPE SPIRALS IN A ΛCDM UNIVERSE: THE ERIS SIMULATION (2011, background): “…where star formation is more diffuse and the energy injection from SNe is more evenly deposited in the ISM: cold gas accumulates in the inner regions already at very high redshifts and continues to turn into stars unabated by feedback (see also Ceverino & Klypin 2009; Robertson & Kravtsov 2008).”
ENVIRONMENTAL DEPENDENCE OF THE KENNICUTT–SCHMIDT RELATION IN GALAXIES (2010, methodology): “The density distribution itself depends on thermodynamics of gas (see, e.g., Robertson & Kravtsov 2008) and metallicity, as more metal rich gas may be more efficient in building regions of higher densities via radiative shocks arising in the highly turbulent medium of gaseous disks.”
Hierarchical formation of bulgeless galaxies: why outflows have low angular momentum (2010, background): “Star formation occurs when cold gas reaches a given threshold density (Stinson et al. 2006), which is dependent on the volume of the star forming regions that can be resolved (Saitoh et al. 2008; Robertson & Kravtsov 2008; Tasker & Bryan 2008).”
METALLICITY-DEPENDENT QUENCHING OF STAR FORMATION AT HIGH REDSHIFT IN SMALL GALAXIES (2011, background): “On the simulation side, Robertson & Kravtsov (2008), Gnedin et al. (2009), Gnedin & Kravtsov (2010), and Kuhlen et al. (2011) present simulations showing that low metallicity systems, either dwarfs in the local universe or starforming galaxies at high redshift, form stars at a much lower rate than…”
The CO-H2 Conversion Factor in Disc Galaxies and Mergers (2011, background): “2006; Dobbs et al. 2008; Robertson & Kravtsov 2008; Pelupessy & Papadopoulos 2009; Gnedin et al. 2009; Gnedin & Kravtsov 2010).”
The SAMI Galaxy Survey: Instrument specification and target selection (2014, background): “…increase in reflectivity of up to 17% in addition to the cleaning of the optics which is estimated to have contributed of order 10% in the blue and a little less in the red, and the upgrade of the blue CCD (∼ 5%). ies (Robertson & Kravtsov 2008; Gnedin & Kravtsov 2010; Krumholz & Dekel 2012).”
The meaning and consequences of star formation criteria in galaxy models with resolved stellar feedback (2013, background): “Recently, various studies have considered molecular criteria: using some combination of density and metallicity to estimate a sub-grid molecular gas fraction and restricting star formation to the “molecular” gas (Robertson & Kravtsov 2008; Kuhlen et al. 2012).”
Evolution of the atomic and molecular gas content of galaxies (2013, background): “…and cosmological numerical hydrodynamic simulations, tracking the multi-phase gas content and implementing H2-based star formation prescriptions (e.g. Pelupessy, Papadopoulos & van der Werf 2006; Robertson & Kravtsov 2008; Gnedin & Kravtsov 2011; Christensen et al. 2012; Kuhlen et al. 2012).”
Implementing molecular hydrogen in hydrodynamic simulations of galaxy formation (2012, methodology): “We chose not to include an artificial pressure floor to support the gas against gravitational collapses other authors have done (e.g. Robertson & Kravtsov 2008).”
How Stellar Feedback Simultaneously Regulates Star Formation and Drives Outflows (2015, background): “The cooling rate per unit area is Ėcool A = ΛneniV A ≈ Λµ−1〈Σg〉Zng, (A2) wherene, ni, andng are the electron, ion, and total gas number densities, respectively,V is the volume,Z is the metallicity, andΛ ∼ 10−22 ergs s−1 cm3 is the net cooling rate (e.g.Robertson & Kravtsov 2008).”
Simulations of the grand design galaxy M51: a case study for analysing tidally induced spiral structure (2009, background): “Several simulations show the formation of giant molecular clouds in this manner (Wada & Norman 1999; Wada, Meurer & Norman 2002; Li, Mac Low & Klessen 2005; Tasker & Bryan 2006; Robertson & Kravtsov 2008), although collisions between clouds may also contribute (Tasker & Tan 2009).”
Theoretical Challenges in Galaxy Formation (2016, background): “…local H2 fraction (e.g. Christensen et al. 2012; Feldmann, Gnedin & Kravtsov 2012; Gnedin, Tassis & Kravtsov 2009; Kuhlen et al. 2012; Monaco et al. 2012; Pelupessy, Papadopoulos & van der Werf 2006; Robertson & Kravtsov 2008) which is, however, itself connected to the H2 formation time-scale.”
MaGICC discs: matching observed galaxy relationships over a wide stellar mass range (2012, background): “…to ensure that gas resolves the Jeans mass and does not artificially fragment, pressure is added to the gas in high density star-forming regions (Robertson & Kravtsov 2008), (ii) a maximum density limit is imposed by setting a minimum SPH smoothing length of 0.25 times that of the gravitational…”