Spitzer Space Telescope - General Observer Proposal #20289


Dust and Star Formation in Extreme Outer Disks:  The Case of M83
(NGC5236)

Principal Investigator: Daniela Calzetti
           Institution: Space Telescope Science Institute

Co-Investigators: 
Michael Regan, Space Telescope Science Institute                      
Luciana Bianchi, Johns Hopkins University, Dept of Phys & Astronomy   
Gerhardt Meurer, Johns Hopkins University, Dept. of Phys. & Astrono   
David Thilker, Johns Hopkins University, Dept. of Phys. & Astrono     

Science Category: nearby galaxies (z<0.05, v_sys<15,000 km/s) 
 Observing Modes: IracMap 
  Hours Approved: 2.6 

Abstract:
The recent discovery by GALEX of extensive UV emission in the extreme
outskirts of local star-forming galaxies may challenge our current
understanding of star formation, of the modality for polluting the
intergalactic medium, and of the long-term impact of interactions.
Further, the nature outer disk UV light is not settled yet, as UV
photons from the galaxy disk scattered by dust in the outskirts may
contribute to the observed radiation.  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.  We propose IRAC mapping of 2 regions in the extreme outer
disk (>2 R_{opt}) of the nearby star-forming spiral M83 to:  (1)
investigate whether a fraction of the UV light in those regions is due
to dust scattering, and, via models, obtain a census of the UV photons
from the galaxy that are scattered and absorbed outside the
optically-bright disk;  (2) derive masses and star formation rates for
the UV-bright knots associated with stellar populations, to obtain a
census of the current to past star formation and compare the observed
mass distribution with that of disk clusters;  (3) constrain the
presence and mass of star formation older than ~100-300  Myr, which
may provide a link to the past interaction of M83 with its companion.
M83 is a prototype testbed for such study because of its relative
proximity, high star formation rate, presence of an H-alpha edge, and
large number of detected outer disk UV-bright knots.  With a modest
committment of time, Spitzer will provide the first milestone for
unraveling the properties of the boundaries between galaxies and the
intergalactic medium.