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Register a new userFormation of Molecular Clouds and Global Conditions for Star Formation
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Giant molecular clouds (GMCs) are the primary reservoirs of cold, star-forming molecular gas in the Milky Way and similar galaxies, and thus any understanding of star formation must encompass a model for GMC formation, evolution, and destruction. These models are necessarily constrained by measurements of interstellar molecular and atomic gas, and the emergent, newborn stars. Both observations and theory have undergone great advances in recent years, the latter driven largely by improved numerical simulations, and the former by the advent of large-scale surveys with new telescopes and instruments. This chapter offers a thorough review of the current state of the field.
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We investigate Schmidts conjecture (i.e., that the star formation rate scales in a power-law fashion with the gas density) for four well-studied local molecular clouds (GMCs). Using the Bayesian methodology we show that a local Schmidt scaling relation of the form Sigma*(A_K) = kappa x (A_K)^{beta} (protostars pc^{-2}) exists within (but not between) GMCs. Further we find that the Schmidt scaling law, by itself, does not provide an adequate description of star formation activity in GMCs. Because the total number of protostars produced by a cloud is given by the product of Sigma*(A_K) and S(> A_K), the differential surface area distribution function, integrated over the entire cloud, the clouds structure plays a fundamental role in setting the level of its star formation activity. For clouds with similar functional forms of Sigma*(A_K), observed differences in their total SFRs are primarily due to the differences in S(> A_K) between the clouds. The coupling of Sigma*(A_K) with the measured S(> A_K) in these clouds also produces a steep jump in the SFR and protostellar production above A_K ~ 0.8 magnitudes. Finally, we show that there is no global Schmidt law that relates the star formation rate and gas mass surface densities between GMCs. Consequently, the observed Kennicutt-Schmidt scaling relation for disk galaxies is likely an artifact of unresolved measurements of GMCs and not a result of any underlying physical law of star formation characterizing the molecular gas.
We test some ideas for star formation relations against data on local molecular clouds. On a cloud by cloud basis, the relation between the surface density of star formation rate and surface density of gas divided by a free-fall time, calculated from the mean cloud density, shows no significant correlation. If a crossing time is substituted for the free-fall time, there is even less correlation. Within a cloud, the star formation rate volume and surface densities increase rapidly with the corresponding gas densities, faster than predicted by models using the free-fall time defined from the local density. A model in which the star formation rate depends linearly on the mass of gas above a visual extinction of 8 mag describes the data on these clouds, with very low dispersion. The data on regions of very massive star formation, with improved star formation rates based on free-free emission from ionized gas, also agree with this linear relation.
We present a revised and extended version of the analytic model for cosmic star formation originally given by Hernquist & Springel in 2003. The key assumption of this formalism is that star formation proceeds from cold gas, at a rate that is limited by an internal consumption timescale at early times, or by the rate of generation of gas via cooling at late times. These processes are analysed as a function of the mass of dark matter haloes and integrated over the halo population. We modify this approach in two main ways to make it more general: (1) halo collapse times are included explicitly, so that the behaviour is physically reasonable at late times; (2) allowance is made for a mass-dependent baryon fraction in haloes, which incorporates feedback effects. This model reproduces the main features of the observed baryonic Tully-Fisher relationship, and is consistent with observational estimates of the baryon mass fraction in the intergalactic medium. With minimal adjustment of parameters, our approach reproduces the observed history of cosmic star formation within a factor of two over the redshift range $0 < z < 10$. This level of agreement is comparable to that achieved by state-of-the-art cosmological simulations. Our simplified apparatus has pedagogical value in illuminating the results of such detailed calculations, and also serves as a means for rapid approximate exploration of non-standard cosmological models.
We present the detection of molecular gas using CO(1-0) line emission and follow up Halpha imaging observations of galaxies located in nearby voids. The CO(1-0) observations were done using the 45m telescope of the Nobeyama Radio Observatory (NRO) and the optical observations were done using the Himalayan Chandra Telescope (HCT). Although void galaxies lie in the most under dense parts of our universe, a significant fraction of them are gas rich, spiral galaxies that show signatures of ongoing star formation. Not much is known about their cold gas content or star formation properties. In this study we searched for molecular gas in five void galaxies using the NRO. The galaxies were selected based on their relatively higher IRAS fluxes or Halpha line luminosities. CO(1--0) emission was detected in four galaxies and the derived molecular gas masses lie between (1 - 8)E+9 Msun. The H$alpha$ imaging observations of three galaxies detected in CO emission indicates ongoing star formation and the derived star formation rates vary between from 0.2 - 1.0 Msun/yr, which is similar to that observed in local galaxies. Our study shows that although void galaxies reside in under dense regions, their disks may contain molecular gas and have star formation rates similar to galaxies in denser environments.
A key uncertainty in galaxy evolution is the physics regulating star formation, ranging from small-scale processes related to the life-cycle of molecular clouds within galaxies to large-scale processes such as gas accretion onto galaxies. We study the imprint of such processes on the time-variability of star formation with an analytical approach tracking the gas mass of galaxies (regulator model). Specifically, we quantify the strength of the fluctuation in the star-formation rate (SFR) on different timescales, i.e. the power spectral density (PSD) of the star-formation history, and connect it to gas inflow and the life-cycle of molecular clouds. We show that in the general case the PSD of the SFR has three breaks, corresponding to the correlation time of the inflow rate, the equilibrium timescale of the gas reservoir of the galaxy, and the average lifetime of individual molecular clouds. On long and intermediate timescales (relative to the dynamical timescale of the galaxy), the PSD is typically set by the variability of the inflow rate and the interplay between outflows and gas depletion. On short timescales, the PSD shows an additional component related to the life-cycle of molecular clouds, which can be described by a damped random walk with a power-law slope of $betaapprox2$ at high frequencies with a break near the average cloud lifetime. We discuss star-formation burstiness in a wide range of galaxy regimes, study the evolution of galaxies about the main sequence ridgeline, and explore the applicability of our method for understanding the star-formation process on cloud-scale from galaxy-integrated measurements.
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