Spitzer Space Telescope - General Observer Proposal #20016


Search for Very Thick Disks Around NGC 891 and NGC 5907 with the IRS
PUI

Principal Investigator: Martin Burgdorf
           Institution: John Moores University

Co-Investigators: 
Matthew Ashby, Harvard/Smithsonian Center for Astrophysics            

Science Category: dark matter 
 Observing Modes: IrsPeakupImage 
  Hours Approved: 2.1 

Abstract:
Flat galaxy rotation curves routinely observed out to large radii in
HI and in optical emission lines are convincing evidence for the
existence of dark matter.  As yet, we know very little about the
nature of this dark matter.  An intriguing possibility is that a
population of very cool objects - extreme brown dwarfs or "Jupiters''
- accounts for a significant part of its mass.  Their intrinsic
faintness at visible wavelengths would explain why they have not yet
been clearly detected.  IRAC GTO observations of two famous edge-on
galaxies (NGC 891 and 5907) have yielded good evidence for such a very
red, thick disk component that could comprise some of this "missing
mass", but the mid-infrared colors as yet offer no clear discriminant
of the thick disks' makeup.  We propose to use the IRS PUI mode for
follow-up observations of the thick disks seen in the two edge-on IRAC
GTO program galaxies.  By measuring the thick disk fluxes at 16
microns we can obtain crucial information for example about the
temperature of the objects in these disks and hence get a better idea
of their nature.  As the expected signal is much smaller than the
foreground emission from the zodiacal light, we propose to execute
raster maps perpendicular to the midplanes of these galaxies without
touching the regions of strong emission.  The radiation from the thick
disks will then appear as a surface brightness gradient measured as a
function of distance on both sides of the galactic midplanes.
Previous attempts to detect halo or thick disk material around edge-on
galaxies concentrated on the near infrared;  the observations proposed
here would be the first ones at longer wavelengths which are aimed
only at regions outside the visible disks.  In addition to the
proposed 16 micron images, we will obtain 22 micron images in
parallel, from the other IRS peak-up array at similar positions
relative to the galaxies.  We request 2.1 hours to carry out this
program.