Spitzer Space Telescope - General Observer Proposal #40183


Probing the IR Cirrus: Small-Scale Structure and Molecular Content

Principal Investigator: Charles Danforth
           Institution: University of Colorado

     Technical Contact: Charles Danforth, University of Colorado

Co-Investigators: 
J. Michael Shull, U. Colorado                                         
Felix J Lockman, NRAO Greenbank                                       

Science Category: ISM 
 Observing Modes: IracMap MipsScan 
  Hours Approved: 16.5 

Abstract:
The infrared cirrus is a network of diffuse interstellar clouds seen over ~50%
of the sky at high Galactic latitudes. The cirrus has not yet been studied by
Spitzer, yet there are several compelling scientific reasons to do so. First,
the cirrus appears to contain a substantial amount of molecular hydrogen. A
correlation is seen to exist between the column density of H2 seen in absorption
toward extragalactic sight lines and the 100um cirrus emission adjacent to those
sight lines. However, the low spatial resolution of existing all-sky IR maps and
likely variation of cirrus structure on angular scales >5' introduces a great
deal of scatter into the correlation. Using cirrus flux as a proxy for H2 column
density will allow us to correct large-scale HI surveys for the "missing"
molecular contribution and get accurate measurements of the total mass in this
diffuse ISM phase. Second, the cirrus may delineate the disk-halo interface and
be a site of enhanced cooling (via H2 rotational excitation) and triggered star
formation. Detailed studies of cirrus morphology at small scales (<1') can tell
us a great deal about the physical structure of the cirrus: thickness, particle
density, and cooling rates. Additionally, small-scale cirrus structure is a
significant Galactic foreground that needs to be understood and characterized
for future cosmology probes such as Planck and ALMA. We propose a set of Spitzer
observations of seven high-latitude fields around AGN to address these two
science drivers. MIPS observations at 70 and 160um will map the cirrus at
unprecedented resolution while 24um and IRAC 8um fields will map the brightest
dust and PAH emission.