Spitzer Space Telescope - Theoretical Research Proposal #20040


Global Modeling of Spur Formation in Spiral Galaxies

Principal Investigator: Eve Ostriker
           Institution: University of Maryland

Co-Investigators: 
Stuart Vogel, University of Maryland                                  
Misty Lavigne, University of Maryland                                 
Rahul Shetty, University of Maryland                                  

Science Category: nearby galaxies (z<0.05, v_sys<15,000 km/s) 
Dollars Approved: 73552.0 

Abstract:
Spitzer 8 micron Legacy images of M51 and other SINGS galaxies show,
in addition to the classic ``beads on a string'' associated with
localized star formation, striking intermediate-scale filamentary
structures.  These features consist of many trailing ``spurs''
extending from the main arms well into the interarm regions.
Cospatial ``spur-like'' dust lanes are also evident in HST optical
images of M51 and other disk galaxies -- including barred and
flocculent types -- suggesting that a similar dynamical process drives
their development in many systems.  We believe that these spurs are
created by the magneto-Jeans instability (MJI), and that nonlinear
evolution of the MJI leads to fragmentation into GMCs, and
subsequently arm and interarm HII regions.  In previous MHD
simulations using a simplified local model, the PI demonstrated the
formation of gaseous spurs and bound clouds with masses and spacings
similar to observations.  We propose to extend these models into the
global domain, which will allow for realistic effects including
curvature of spiral arms, spatial variation of the background surface
density, and gradients in the background flow velocity relative to the
spiral pattern.  In addition to modeling tuned for specific galaxies,
we also plan to perform a larger survey to explore the parameter
dependence of structural and dynamical development.  Since much of the
arm and interarm 8 micron emission is produced by PAHs in dusty clumps
and filaments, and star formation may be driven by the corresponding
gas enhancements, understanding how the ISM is concentrated into these
structures may be key to interpretation of many existing and future
Spitzer data sets.