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6.1. Observations

Systematic surveys and detailed case studies will be enriched by the expansion in millimeter-submillimeter capabilities over the next decade. These will contribute in two main modes: through cloud-scale observations in other galaxies, and in expanding the study of clouds in our own Milky Way. The increased sensitivities of ALMA and NOEMA will sample smaller scales at larger distances, resolving GMC complexes and investigating the physical state and formation mechanisms of GMCs in a variety of extragalactic environments. The smaller interferometers like CARMA and SMA will likely focus on systematic mapping of large areas in the Milky Way or even external galaxies. At cm wavelengths, the recently upgraded JVLA brings new powerful capabilities in continuum detection at 7 mm to study cool dust in disks, as well as the study of free-free continuum, molecular emission, and radio recombination lines in our own galaxy and other galaxies. Single-dish mm-wave facilities equipped with array receivers and continuum cameras such as the IRAM 30m, NRO 45m, LMT 50m, and the future CCAT facility will enable fast mapping of large areas in the Milky Way, providing the much needed large scale context to high resolution studies. These facilities, along with the APEX and NANTEN2 telescopes, will pursue multitransition / multiscale surveys sampling star forming cores and the parent cloud material simultaneously and providing valuable diagnostics of gas kinematics and physical conditions.

There is hope that some of the new observational capabilities will break the theoretical logjam described in this review. At present, it is not possible for observations to distinguish between the very different cloud formation mechanisms proposed in Section 3, nor between the mechanisms that might be responsible for controlling cloud density and velocity structure, and cloud disruption (Section 4), nor between various models for how star formation is regulated (Section 5). There is reason to hope that the new data that will become available in the next few years will start to rule some of these models out.

6.2. Simulations and theory

The developments in numerical simulations of GMCs since PPV will continue over the next few years to PPVII. There is a current convergence of simulations towards the scales of GMCs, and GMC scale physics. Galaxy simulations are moving towards ever smaller scales, whilst individual cloud simulations seek to include more realistic initial conditions. At present, galactic models are limited by the resolution required to adequately capture stellar feedback, internal cloud structure, cloud motions and shocks. They also do not currently realise the temperatures or densities of GMCs observed, nor typically include magnetic fields. Smaller scale simulations, on the other hand, miss larger scale dynamics such as spiral shocks, shear and cloud-cloud collisions, which will be present either during the formation or throughout the evolution of a molecular cloud. Future simulations, which capture the main physics on GMC scales, will provide a clearer picture of the evolution and lifetimes of GMCs. Furthermore they will be more suitable for determining the role of different processes in driving turbulence and regulating star formation in galaxies, in conjunction with analytic models and observations.

Continuing improvements in codes, including the use of moving mesh codes in addition to AMR and SPH methods, will also promote progress on GMCs descriptions, and allow more consistency checks between different numerical methods. More widespread, and further development of chemodynamical modelling will enable the study of different tracers in cloud and galaxy simulations. In conjunction with these techniques, synthetic observations, such as HI and CO maps, will become increasingly important for comparing the results of numerical models and simulations, and testing whether the simulations are indeed viable representations of galaxies and clouds.

Acknowledgments. Support for this work was provided by: the European Research Council through the FP7 ERC starting grant project LOCALSTAR (CLD); the NSF through grants AST09-55300 (MRK), AST11-09395 (M-MML), AST09-08185 (ECO), AST09-55836 (ADB), and AST10-09049 (MH); NASA through ATP grant NNX13AB84G (MRK), Chandra award number GO2-13162A (MRK) issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060, and Hubble Award Number 13256 (MRK) issued by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555; a Research Corporation for Science Advancement Cottrell Scholar Award (ADB); an Alfred P. Sloan Fellowship (MRK); the hospitality of the Aspen Center for Physics, which is supported by the National Science Foundation Grant PHY-1066293 (MRK); and CONACYT grant 102488 (EVS).

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