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Register a new userMolecular cloud regulated star formation in galaxies
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C. M. Booth
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We describe a numerical implementation of star formation in disk galaxies, in which the conversion of cooling gas to stars in the multiphase interstellar medium is governed by the rate at which molecular clouds are formed and destroyed. In the model, clouds form from thermally unstable ambient gas and get destroyed by feedback from massive stars and thermal conduction. Feedback in the ambient phase cycles gas into a hot galactic fountain or wind. We model the ambient gas hydrodynamically using smoothed particle hydrodynamics (SPH). However, we cannot resolve the Jeans mass in the cold and dense molecular gas and, therefore, represent the cloud phase with ballistic particles that coagulate when colliding. We show that this naturally produces a multiphase medium with cold clouds, a warm disk, hot supernova bubbles and a hot, tenuous halo. Our implementation of this model is based on the Gadget N-Body code. We illustrate the model by evolving an isolated Milky Way-like galaxy and study the properties of a disk formed in a rotating spherical collapse. Many observed properties of disk galaxies are reproduced well, including the molecular cloud mass spectrum, the molecular fraction as a function of radius, the Schmidt law, the stellar density profile and the appearance of a galactic fountain.
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We develop a four-phase galaxy evolution model in order to study the effect of accretion of extra-galactic gas on the star formation rate (SFR) of a galaxy. Pure self-regulated star formation of isolated galaxies is replaced by an accretion-regulated star formation mode. The SFR settles into an equlibrium determined entirely by the gas accretion rate on a Gyr time scale.
Feedback from massive stars is believed to play a critical role in shaping the galaxy mass function, the structure of the interstellar medium (ISM), and the low efficiency of star formation, but the exact form of the feedback is uncertain. In this paper, the first in a series, we present and test a novel numerical implementation of stellar feedback resulting from momentum imparted to the ISM by radiation, supernovae, and stellar winds. We employ a realistic cooling function, and find that a large fraction of the gas cools to <100K, so that the ISM becomes highly inhomogeneous. Despite this, our simulated galaxies reach an approximate steady state, in which gas gravitationally collapses to form giant molecular clouds (GMCs), dense clumps, and stars; subsequently, stellar feedback disperses the GMCs, repopulating the diffuse ISM. This collapse and dispersal cycle is seen in models of SMC-like dwarfs, the Milky-Way, and z~2 clumpy disk analogues. The simulated global star formation efficiencies are consistent with the observed Kennicutt-Schmidt relation. Moreover, the star formation rates are nearly independent of the numerically imposed high-density star formation efficiency, density threshold, and density scaling. This is a consequence of the fact that, in our simulations, star formation is regulated by stellar feedback limiting the amount of very dense gas available for forming stars. In contrast, in simulations without stellar feedback, i.e. under the action of only gravity and gravitationally-induced turbulence, the ISM experiences runaway collapse to very high densities. In these simulations without feedback, the global star formation rates exceed observed galactic star formation rates by 1-2 orders of magnitude, demonstrating that stellar feedback is crucial to the regulation of star formation in galaxies.
Does star formation proceed in the same way in large spirals such as the Milky Way and in smaller chemically younger galaxies? Earlier work suggests a more rapid transformation of H$_2$ into stars in these objects but (1) a doubt remains about the validity of the H$_2$ mass estimates and (2) there is currently no explanation for why star formation should be more efficient. M~33, a local group spiral with a mass $sim 10$% and a metallicity half that of the Galaxy, represents a first step towards the metal poor Dwarf Galaxies. We have searched for molecular clouds in the outer disk of M~33 and present here a set of detections of both $^{12}$CO and $^{13}$CO, including the only detections (for both lines) beyond the R$_{25}$ radius in a subsolar metallicity galaxy. The spatial resolution enables mass estimates for the clouds and thus a measure of the $N({rm H}_2) / I_{rm CO}$ ratio, which in turn enables a more reliable calculation of the H$_2$ mass. Our estimate for the outer disk of M~33 is $N({rm H}_2) / I_{rm CO(1-0)} sim 5 times 10^{20} ,{rm cm^{-2}/(K{rm km s^{-1}})}$ with an estimated uncertainty of a factor $le 2$. While the $^{12/13}$CO line ratios do not provide a reliable measure of $N({rm H}_2) / I_{rm CO}$, the values we find are slightly greater than Galactic and corroborate a somewhat higher $N({rm H}_2) / I_{rm CO}$ value. Comparing the CO observations with other tracers of the interstellar medium, no reliable means of predicting where CO would be detected was identified. In particular, CO detections were often not directly on local HI or FIR or H$alpha$ peaks, although generally in regions with FIR emission and high HI column density. The results presented here provide support for the quicker transformation of H$_2$ into stars in M~33 than in large local universe spirals.
We derive a physical model for the observed relations between star formation rate (SFR) and molecular line (CO and HCN) emission in galaxies, and show how these observed relations are reflective of the underlying star formation law. We do this by combining 3D non-LTE radiative transfer calculations with hydrodynamic simulations of isolated disk galaxies and galaxy mergers. We demonstrate that the observed SFR-molecular line relations are driven by the relationship between molecular line emission and gas density, and anchored by the index of the underlying Schmidt law controlling the SFR in the galaxy. Lines with low critical densities (e.g. CO J=1-0) are typically thermalized and trace the gas density faithfully. In these cases, the SFR will be related to line luminosity with an index similar to the Schmidt law index. Lines with high critical densities greater than the mean density of most of the emitting clouds in a galaxy (e.g. CO J=3-2, HCN J=1-0) will have only a small amount of thermalized gas, and consequently a superlinear relationship between molecular line luminosity and mean gas density. This results in a SFR-line luminosity index less than the Schmidt index for high critical density tracers. One observational consequence of this is a significant redistribution of light from the small pockets of dense, thermalized gas to diffuse gas along the line of sight, and prodigious emission from subthermally excited gas. At the highest star formation rates, the SFR-Lmol slope tends to the Schmidt index, regardless of the molecular transition. The fundamental relation is the Kennicutt-Schmidt law, rather than the relation between SFR and molecular line luminosity. We use these results to make imminently testable predictions for the SFR-molecular line relations of unobserved transitions.
We describe cosmological galaxy formation simulations with the adaptive mesh refinement code Enzo that incorporate a star formation prescription regulated by the local abundance of molecular hydrogen. We show that this H2-regulated prescription leads to a suppression of star formation in low mass halos (M_h < ~10^10 M_sun) at z>4, alleviating some of the dwarf galaxy problems faced by theoretical galaxy formation models. H2 regulation modifies the efficiency of star formation of cold gas directly, rather than indirectly reducing the cold gas content with supernova feedback. We determine the local H2 abundance in our most refined grid cells (76 proper parsec in size at z=4) by applying the model of Krumholz, McKee, & Tumlinson, which is based on idealized 1D radiative transfer calculations of H2 formation-dissociation balance in ~100 pc atomic--molecular complexes. Our H2-regulated simulations are able to reproduce the empirical (albeit lower z) Kennicutt-Schmidt relation, including the low Sigma_gas cutoff due to the transition from atomic to molecular phase and the metallicity dependence thereof, without the use of an explicit density threshold in our star formation prescription. We compare the evolution of the luminosity function, stellar mass density, and star formation rate density from our simulations to recent observational determinations of the same at z=4-8 and find reasonable agreement between the two.
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