Spitzer Space Telescope - General Observer Proposal #30753


Dust and Star Formation in the Extreme Outer Disks of Spiral Galaxies

Principal Investigator: Daniela Calzetti
           Institution: Space Telescope Science Institute

Co-Investigators: 
Luciana Bianchi, Dept. of Physics and Astronomy, The Johns Hopkins    
Rupali Chandar, Dept. of Physics and Astronomy, The Johns Hopkins     
Robert Kennicutt, Institute of Astronomy, Cambridge University        
Gerhardt Meurer, Dept. of Physics and Astronomy, The Johns Hopkins    
Michael Regan, Space Telescope Science Institute                      
David Thilker, Dept. of Physics and Astronomy, The Johns Hopkins      

Science Category: nearby galaxies (z<0.05, v_sys<15,000 km/s) 
 Observing Modes: IracMap 
  Hours Approved: 6.7 

Abstract:
The discovery by GALEX of extended UV emission in the extreme
outskirts of local star-forming galaxies challenges our current
understanding of star formation, of the modality for polluting the
intergalactic medium, and of the long-term impact of interactions.
The nature of the outer disk UV light is unclear with possible origins
including in-situ star formation as well as dust-scattered photons
originating in the galaxy disk.  Spitzer, with its wavelength windows
sensitive to both the low mass stars and the dust emission, has a
unique capability to address some of these fundamental questions.
Building on the success of our Cycle 2 pilot program on the
UV-emitting outer regions of M83, we propose IRAC imaging of regions
in the extreme outer disks (> 1.5 R_{opt}) of five nearby star-forming
spirals.  The five galaxies are a subsample of all the galaxies with
extended UV emission discovered by GALEX, selected to cover the full
range of UV morphologies and properties, including arm-interarm
contrast and extended emission in the presence/absence of H-alpha
edges.  With this varied sample we plan to:  (1) investigate whether a
fraction of the UV light in those regions is due to dust scattering,
and obtain a census of the UV leakage from galaxy disks;  (2) derive
masses for the UV--emitting knots associated with star formation and
compare this mass range with those of disk clusters;  (3) investigate
variations in the properties of those extreme regions as a function of
the parent galaxy's characteristics.  With a modest committment of
time, Spitzer will provide a milestone for unraveling the properties
of these regions at the boundary between galaxies and the
intergalactic medium.