The Role of Pressure in GMC Formation II: The H2-Pressure Relation

By L. Blitz, Erik Rosolowsky. 2006

474 citations recorded by the source corpus.

We show that the ratio of molecular to atomic gas in galaxies is determined by hydrostatic pressure and that the relation between the two is nearly linear. The pressure relation is shown to be good over 3 orders of magnitude for 14 galaxies, including dwarfs, H I-rich, and H2-rich galaxies, as well as the Milky Way. The sample spans a factor of 5 in mean metallicity. The rms scatter of individual points of the relation is only about a factor of 2 for all the galaxies, although some show much more scatter than others. Using these results, we propose a modified star formation prescription based on pressure determining the degree to which the ISM is molecular. The formulation is different in high- and low-pressure regimes, defined by whether the gas is primarily atomic or primarily molecular. This formulation can be implemented in simulations and provides a more appropriate treatment of the outer regions of spiral galaxies and molecule-poor systems, such as dwarf irregulars and damped Lyα systems.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/eaaf193b858fe34b49bf31680c39464c7e45766b

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In Astronomy and Astrophysics

1. P. Hopkins
2. L. Hernquist
3. R. Somerville
4. R. Dav'e
5. Volker Springel
6. A. Dekel
7. T. Naab
8. Mark R. Krumholz

Citations in the corpus

• THE INTERSTELLAR MEDIUM IN DISTANT STAR-FORMING GALAXIES: TURBULENT PRESSURE, FRAGMENTATION, AND CLOUD SCALING RELATIONS IN A DENSE GAS DISK AT z = 2.3 (2011, background): “…π 2 GΣgas [ Σgas + ( σgas σ∗ ) Σ∗ ] (1) where Σgas and Σ⋆ are the surface density of the gas and stars, and σgas and σ∗ are the vertical velocity dispersion of the gas and the stars, respectively (with no assumption that the gas scale length is smaller than the stars; c.f. Blitz & Rosolowsky 2006).”
• The SILCC (SImulating the LifeCycle of molecular Clouds) project - II. Dynamical evolution of the supernova-driven ISM and the launching of outflows (2015, methodology): “Since the observations by Blitz & Rosolowsky (2006) are only resolved down to ∼ 100 pc it seems reasonable to also compare their pressures to the simulation values based on averaged quantities from ±100 pc.”
• METALLICITY EVOLUTION OF DAMPED Lyα SYSTEMS OUT TO z ∼ 5 (2012, methodology): “The red diamonds are mean metallicities of DLAs extracted from the 48 Mpc/h momentum-driven wind simulation presented in Davé et al. (2011), using the self-shielding criterion in Popping et al. (2009), including an molecular fraction calculation based on Blitz & Rosolowsky (2006).”
• The KMOS Redshift One Spectroscopic Survey (KROSS):dynamical properties, gas and dark matter fractions of typical z~1 star-forming galaxies (2016, background): “…column densities and interstellar pressures in star-forming galaxies (with gas 10 M pc−2, for KROSS gas = 54 ± 5 M pc−2) mean that most of the cold interstellar gas is likely in molecular form and the contribution of atomic hydrogen can be neglected (Blitz & Rosolowsky 2006; Tacconi et al. 2010).”
• A unified multiwavelength model of galaxy formation (2015, methodology): “We calculate the star formation rate (SFR) in the disk using the empirical Blitz & Rosolowsky (2006) law (as implemented inGAL - FORM in Lagos et al. 2011b), which is based on observations of nearby star-forming disk galaxies (see also Leroy et al. 2008; Bigiel et al. 2008).”
• Implementing molecular hydrogen in hydrodynamic simulations of galaxy formation (2012, background): “Similarly, Blitz & Rosolowsky (2004, 2006) developed a pressure-based star formation law from this relationship.”
• SIMULATION OF THE COSMIC EVOLUTION OF ATOMIC AND MOLECULAR HYDROGEN IN GALAXIES (2009, methodology): “Observations in the local Universe show that spiral galaxies with metallicities differing by a factor 5 fall on the same Rmol −P relation (Blitz & Rosolowsky 2006).”
• The Atomic-to-Molecular Transition in Galaxies. I. An Analytic Approximation for Photodissociation Fronts in Finite Clouds (2008, background): “The problem is especially severe in galaxies with low metallicities and interstellar pressures, where the predominantly molecular parts of cloud complexes generally constitute a small part of the total mass and volume (e.g. Blitz & Rosolowsky 2006).”
• A COMPARISON OF METHODS FOR DETERMINING THE MOLECULAR CONTENT OF MODEL GALAXIES (2010, methodology): “…network of hydrogen and helium and non-equilibrium cooling and 5 In contrast, we do not compare to the H2-midplane pressure correlation model of Blitz & Rosolowsky (2006), because recent observations indicate that it breaks down on the sub-kpc scales resolved in the simulations (Fumagalli et…”
• A study of the gas–star formation relation over cosmic time (2010, background): “Throughout this paper, we have discussed the ‘molecular’ gas–star formation relations and have restricted ourselves to molgas > 3 M pc−2 where molecular gas should begin to dominate the interstellar medium (ISM; Blitz & Rosolowsky 2006).”
• Turbulent molecular clouds (2012, background): “They also show that the correlation of the molecular gas fraction with the midplane pressure observed in external galaxies by Blitz & Rosolowsky (2004, 2006) is retrieved only if the epicyclic frequency κ and Σ are kept proportional.”
• Quenching and ram pressure stripping of simulated Milky Way satellite galaxies (2017, methodology): “…two-phase subgrid ISM model employed in the Auriga simulations, where gas in star-forming cells is assumed to be split between a hot phase and a cold phase that are in pressure equilibrium and is empirically calibrated to observations of nearby galaxies (Leroy et al. 2008; Blitz & Rosolowsky 2006).”
• Evolution of the atomic and molecular gas content of galaxies (2013, methodology): “This could provide an explanation for the strong empirical correlation between H2 fraction and disc midplane pressure found by Blitz & Rosolowsky (2006).”
• Theory of Star Formation (2007, methodology): “…from having primarily atomic to primarily molecular gas typically occurs where the total gas surface density Σgal ≈ 12 M⊙ pc−2 (Wong & Blitz, 2002; Blitz & Rosolowsky, 2004) and where the mean midplane pressure is inferred to lie in the range P/k = 104 − 105 K cm−3 (Blitz & Rosolowsky, 2006).”
• VERY HIGH GAS FRACTIONS AND EXTENDED GAS RESERVOIRS IN z = 1.5 DISK GALAXIES (2009, background): “However, we expect the vast majority of the hydrogen to be molecular given that the high observed densities and expected pressures in the interstellar medium of these galaxies (see, e.g., Blitz & Rosolowsky 2006; Leroy et al. 2008; Obreschkow & Rawlings 2009).”
• REGULATION OF STAR FORMATION RATES IN MULTIPHASE GALACTIC DISKS: A THERMAL/DYNAMICAL EQUILIBRIUM MODEL (2010, methodology): “…formation rate in normal galaxies is proportional to the mass in gravitationally bound GMCs, we believe that this self-regulation mechanism is the physical basis for the relationship between molecular surface density and midplane pressure empirically identified by Blitz & Rosolowsky (2004, 2006).”
• THREE-DIMENSIONAL HYDRODYNAMIC SIMULATIONS OF MULTIPHASE GALACTIC DISKS WITH STAR FORMATION FEEDBACK. I. REGULATION OF STAR FORMATION RATES (2013, background): “This explains empirical correlations of molecular gas and star formation with pressure identified by Blitz & Rosolowsky (2004, 2006) and Leroy et al. (2008).”
• On the evolution of the H i column density distribution in cosmological simulations (2012, methodology): “As the thin solid line in the right panel of Figure 2 shows, accounting for H2 using the empirical relation between H2 fraction and pressure, based on z = 0 observations (Blitz & Rosolowsky 2006), does reproduce a sharp cut-off.”
• Molecular and atomic gas in the Local Group galaxy M 33 (2010, background): “As mentioned by Blitz & Rosolowsky (2006) for M33 and Leroy et al. (2006) for IC 10 and discussed more extensively by Gardan et al. (2007), the Star Formation Rate (SFR) per unit H2 mass or Star Formation Efficiency (SFE= SFR/M(H2) in yr−1) was found to be up to an order of magnitude higher in…”
• On the relation between the Schmidt and Kennicutt-Schmidt star formation laws and its implications for numerical simulations (2007, background): “Empirical prescriptions that take into account the molecular fraction may work better in this regime (Blitz & Rosolowsky 2006).”