Regulation of star formation rates in multiphase galactic disks: a thermal/dynamical equilibrium model
Summary (4 min read)
1. INTRODUCTION
- Star formation is regulated by many physical factors, with processes from sub-parsec to super-kiloparsec scales contributing to setting the overall rate (see, e.g., McKee & Ostriker 2007).
- Section 3 then compares to the observed data set previously presented in Leroy et al. (2008).
2.1. Model Concepts and Construction
- The authors construct a local steady-state model for the star formation rate in the disk, with independent variables the total surface density of neutral gas (Σ), the midplane stellar density (ρs), and the dark matter density (ρdm).
- The abundance of gravitationally bound, starforming clouds is nevertheless important for establishing an equilibrium state in the diffuse gas, because the FUV that heats the diffuse ISM originates in young OB associations.
- Thus, an equilibrium state, in which cooling balances heating and pressure balances gravity, can be obtained by a suitable division of the gas mass into star-forming (gravitationally bound) and diffuse components such that their ratio is proportional to the vertical gravitational field.
- 6 Dopita (1985) previously showed that assuming the pressure to be proportional to the star formation rate yields scaling properties similar to observed relationships.
- The authors then discuss, from a physical point of view, how the various feedback processes might act to adjust the system over time, steering it toward the equilibrium they have identified (Section 2.6).
2.2. Gas Components
- The authors divide the neutral ISM into two components.
- One component consists of the gas that is collected into gravitationally bound clouds (GBCs) localized near the galactic midplane, with mean surface density (averaged over ∼ kpc scales) of ΣGBC.
- Here, the authors use the term “diffuse” in the sense of being widely dispersed or scattered throughout the volume; the diffuse component may include both tenuous, volume-filling gas and small, dense cloudlets (see below).
- The authors treat the diffuse gas as a two-phase cloud–intercloud medium in thermal pressure equilibrium, with turbulent vertical velocity dispersion v2z assumed to be the same for warm and cold phases.
- Very recent numerical studies provide support for the quasi-equilibrium assumption—see C.-G. Kim et al. (2010, in preparation).
2.3. Vertical Dynamical Equilibrium of Diffuse Gas
- In the circumstance that the diffuse-gas scale height is much larger than that of the stars, gz ≈ 2πGΣs would be substituted for the gravity of the stellar component, yielding a contribution analogous to that in Equation (5) with ΣGBC → Σs .
- Note that if the Toomre parameter for the stellar disk and the vertical-to-horizontal velocity dispersion ratio are both constant with radius, then ρs ∝ ρdm ∝ (Vc/R)2.
- Numerical simulations in multiphase gas have shown that the magnetic field is amplified by the magnetorotational instability to a level B2/(8π ) = (1 − 2)Pth, independent of the mass fractions of cold and warm gas and the vertical gravitational field strength (Piontek & Ostriker 2005, 2007), while |Bz/Bφ| 1.
2.4. Thermal Equilibrium of Diffuse Gas
- As expressed by Equation (11), the thermal pressure in the diffuse gas must respond to the dynamical constraint imposed by vertical momentum conservation in the disk.
- Thus, the authors expect the midplane thermal pressure in the diffuse gas to be comparable to the two-phase value defined by the thermal equilibrium curve, Pth ≈ Ptwo-phase.
- For simplicity, the authors neglect variations in JFUV associated with the radiative transfer here; they shall simply assume JFUV ∝ ΣFUV ∝ ΣSFR.
- Equivalently, since Wolfire et al. (2003) predict the value of Pth,0/JFUV,0 from theory, their model depends on the measured ratio of local FUV intensity to local star formation rate, JFUV,0/ΣSFR,0.
2.5. The Equilibrium Star Formation Rate
- In normal galaxies, GBCs are identified with GMCs (the outer layers of which are in fact atomic—see below).
- These are mild (no more than a factor of 2–3 over the range Σ = 10–100 M pc−2) and almost all find a consistent normalization, with 2 Gyr being a typical timescale.
- Note that because CO is optically thick in clouds with NH,cloud ∼ 1022 cm−2 and normal metallicity, observed CO emission in unresolved clouds may in fact trace the atomic and “dark gas” portions of GBCs as well as the regions where CO is present, because these contribute to the gravitational potential and therefore the total CO linewidth.
- GBC is calibrated from observations in which star-forming clouds are primarily molecular (and observable in CO lines), their basic approach would remain unchanged for GBCs in different parameter regimes, provided that a well-defined value of tSF,GBC is known (from either observations with appropriate corrections for atomic and dark gas, or from theory).the authors.
2.6. Approach to Equilibrium
- Figure 1(a) shows an extreme case of higher-than-equilibrium fdiff , in which the midplane pressure is higher than Pmax,warm.
- The arrows in Figure 1(a) indicate how the midplane pressure and thermal equilibrium curve would evolve.
- The authors imagine a region with a bound-cloud proportion 1 − fdiff and ΣSFR/Σ above the self-consistent overall equilibrium values, such that heating associated with the high star formation rate makes the thermal equilibrium curve sit at high pressure (i.e., Ptwo-phase is high).
- With self-consistent numerical simulations, it will be possible to assess whether f̃w and Pth/Ptwo-phase secularly depend on Σ and ρsd.
2.7. Sample Solution for an Idealized Galaxy
- If the dark matter density dominates the stellar density in the outer disk, then since ρdm ∝ (Vc/R)2, constant Vc would imply ΣSFR/Σ ∝ R−1 in the outer disk.
- Second, there are two regimes evident for ΣSFR and Σdiff versus Σ: a high-surface-density regime in which the gas is mostly in self-gravitating clouds and the limiting solution ΣSFR = Σ/tSF is approached, and a low-surface-density regime in which ΣSFR has a steeper dependence.
- In adopting G′0 = ΣSFR/ΣSFR,0 for Equation (15), the authors have neglected optical depth effects.
- Thus, the current simple theory overestimates Σdiff in the central parts of galaxies, where τ⊥ becomes large.
2.8. Additional Considerations
- Finally, the authors remark on a few additional points related to assumptions behind and application of the theory presented above.
- The weak dependence on both ρsd and Σ2, and the fact that only their ratio appears so that variations will be partially compensated, implies that f̃w would indeed be expected to vary only modestly, at least in outer disks.
- The star formation rate could therefore decline to ∼ 0.002 times the local value, or 6 × 10−6 M kpc−2 yr−1, before the metagalactic UV becomes important; this occurs only in the far-outer regions of disks.
- An individual GBC is composed of a mixture of molecular gas and cold atomic gas that depends on shielding, and could be primarily atomic at sufficiently low metallicity.
- Thus, the internal dynamics of primarily atomic GBCs—including the processes that determine the internal star formation efficiency—are expected to be similar to those in primarily molecular GBCs, provided that their gravitational potentials and internal velocity dispersions are similar so that vturb vth,cold.
3. COMPARISON TO OBSERVATIONS
- The formulae derived above yield predictions for ΣSFR as a function of galactic gas and stellar properties, and can be compared with observations.
- The authors note that with slight adjustments of f̃w/α, the flaring of the stellar disk, or the dark matter density compared to the standard parameters and prescriptions, even closer agreement between predicted and observed ΣSFR can be obtained.
- These galaxies also have an irregular— and sublinear on average—relationship between ΣSFR and Σmol as inferred from CO.
- Blitz & Rosolowsky (2006) obtained an empirical fit relating the molecular-to-atomic-gas mass fractions to a midplane pressure estimate, Σmol/Σatom = (Ph/Ph,0)γ , for Ph = PBR ≡ Σ(2Gρs)1/2vg. (26) The empirical formula of Blitz & Rosolowsky (2006) produces somewhat more rapid decline in ΣSFR at low Σ (for large radii) than the prediction of their model.
4. SUMMARY AND DISCUSSION
- The authors have developed a theory for self-regulated star formation in multiphase galactic ISM disks in which stellar heating mediates the feedback.
- Here, the authors have not attempted to address these issues, but instead they have simply adopted an empirical solar-neighborhood value for the ratio of JFUV to ΣSFR to calibrate their relationships.
- The authors also discuss how the galactic environment variables (Σ, ρsd, Z) and the ISM model parameters control the transition between the diffuse-dominated and GBC-dominated regimes, and how presence of both regimes in a given annulus (due to spiral structure) affects estimates of the star formation rate.
- The authors start with the diffuse-gas thermal equilibrium Equation (18) relating the thermal pressure to the star formation rate ΣSFR = ΣGBC/tSF, which may be expressed as Pth = 1 φd Pth,0 ΣSFR,0 ΣGBC tSF .
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References
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...While the implied scaling ΣSFR ∝ ΣΩ is roughly satisfied globally (Kennicutt 1998), supporting the notion that galaxies evolve towards states with Q roughly near-critical (e.g. Quirk 1972), for more local observations this does not provide an accurate prediction of star formation (e.g. Leroy et al.…...
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...…whole range of star-forming systems, from entire spiral galaxies to circumnuclear starbursts, the global average of the surface density of star formation, ΣSFR, is observed to be correlated with the global average of the neutral gas surface density Σ as ΣSFR ∝ Σ1+p with 1+p ≈ 1.4 (Kennicutt 1998)....
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...…the expectation based on theory and numerical simulations (Goldreich & Lynden-Bell 1965; Kim & Ostriker 2001) in thin, single-phase gaseous disks is that gravitational instabilities leading to star formation would grow only if the Toomre parameter (Toomre 1964) Q ≡ vthκ/(πGΣ) is sufficiently small....
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...Star formation is regulated by many physical factors, with processes from sub-pc to super-kpc scales contributing to setting the overall rate (see e.g. McKee & Ostriker 2007)....
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...At a fundamental level (see McKee & Ostriker 2007), the star formation timescale within a GBC is expected to depend on the mean density (which sets the mean gravitational free-fall time tff) and on the amplitude of turbulence and strength of the magnetic field (since these properties determine how…...
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