Dynamical Studies of Molecular Cloud Formation in Spiral Galaxies

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AST-0205972OstrikerSeveral lines of observational and theoretical evidence suggest that giant molecular clouds (GMCs) have short lifetimes, forming and dissociating within a few tens of millions of years. Prevailing theoretical arguments favor instability mechanisms in spiral arms to form GMCs, but existing work has suffered from technical limitations, and has not previously directly demonstrated that condensations with the properties of GMCs indeed form. Dr. Eve Ostriker, at the University of Maryland, will lead an investigation of a series of linear stability analyses, nonlinear numerical simulations, and supporting diagnostics to address outstanding questions concerning GMC formation in spiral galaxies. The modeling will include several important technical innovations. These researchers will, for the first time:o Use direct numerical simulations to study development of the Parker instability in 3D with realistic rotational shear self-consistent with large-scale density gradients through arm and interarm regions of galaxies, also incorporating self-gravity to study linear and nonlinear coupling of Parker and Jeans modes;o Comprehensively survey, using spectral methods, shearing-wavelet integrations, and magneto-hydrodynamic (MHD) simulations, the potential effects of the magnetorotational instability in galactic disks, focusing on coupling to self-gravitating modes in both secular-growth and saturated-state regimes;o Incorporate a realistic multi-phase gaseous medium for the initial conditions in two and three- dimensional simulations of self-gravitating and magnetically-driven galactic disk instabilities;o Directly confront stochastic coagulation vs. collective instability mechanisms for forming GMC-scale condensations by performing controlled experiments of multiphase evolution with and without self-gravity and magnetic effects.As GMC formation is intimately coupled to star formation, and the cold ISM is the most dynamically-responsive component of a disk galaxy, the results of this project will have broad impacts on both Galactic and extragalactic astronomical research, with implications for understanding the global regulation of star formation and the structure and evolution of spiral galaxies across the Hubble sequence.***