Spitzer Space Telescope - General Observer Proposal #60072


The 3.6 micron Surface Brightness Profiles of Outer Disks: NGC 4625 and M 83

Principal Investigator: Stephanie Bush
           Institution: Harvard University

     Technical Contact: Stephanie Bush, Harvard University

Co-Investigators: 
Robert Kennicutt, Institute of Astronomy, Cambridge                   
Giovanni Fazio, Harvard-CfA                                           
Matt Ashby, Harvard-CfA                                               
Ben Johnson, Institute of Astronomy, Cambridge                        
Fabio Bresolin, Institute for Astronomy, Hawaii                       

Science Category: nearby galaxies (z<0.05, v_sys<15,000 km/s) 
 Observing Modes: IRAC Post-Cryo Mapping
  Hours Approved: 5.6 

Abstract:
The outer regions of galactic disks have been a topic of increased interest
since ultraviolet observations with the GALEX observatory demonstrated that
nearly 30% of galaxies show extended UV emission and star formation, tracing to
well beyond the optical radius (R25). Although young stellar populations in
these disks are being studied intensively in the UV and H-alpha, very little is
known about the older stellar population of outer disks. We request 16.8 hours
to image the outskirts of two prototype "XUV" (extended UV emission) galaxies,
with sufficient depth and field coverage to measure their 3.6 micron profiles.
Using these profiles we will derive radial stellar mass profiles, UV-IR color
profiles, and gas fraction profiles. These will allow us to address several
critical questions about the formation and evolution of these outer disks and
the host galaxies themselves.