Spitzer Space Telescope - General Observer Proposal #20456


Balancing the Cosmic Energy Budget between AGN and Starbursts in the
Great Observatories Origins Deep Survey

Principal Investigator: Ranga-Ram Chary
           Institution: California Institute of Technology

Co-Investigators: 
Mark Dickinson, NOAO                                                  
David Alexander, Institute for Astronomy, University of Cambridge,    
Lee Armus, Spitzer Science Center, Caltech                            
Andrew Blain, Caltech                                                 
David Elbaz, CEA-Saclay, France                                       
David Frayer, Spitzer Science Center, Caltech                         
Delphine Marcillac, CEA-Saclay, France                                
Alexandra Pope, University of British Columbia, Canada                
Douglas Scott, University of British Columbia, Canada                 
Haojing Yan, Spitzer Science Center, Caltech                          

Science Category: high-z galaxies (z>0.5) 
 Observing Modes: IrsStare 
  Hours Approved: 89.0 

Abstract:
We propose to undertake IRS spectroscopy of 22 cosmologically
interesting sources in the GOODS/Hubble Deep Field-North with 24
micron fluxes above 0.3 mJy.  18 of our 22 targets have spectroscopic
redshifts between 0.5<z<2.5;  IRS offers the best chance to measure
redshifts for the others.  The HDF-N has uniquely deep and extensive
data available, particularly the 2Msec Chandra observations which
provide the best measures of X-ray hardness ratios and strongest
non-detection limits for Compton-thick AGN and starbursts.  The
proposed spectroscopy, specifically measuring the 6.2 and 7.7 micron
PAH line/continuum ratio, in conjunction with the X-ray properties
will disentangle the contribution from star-formation and accretion
activity to the energetics of the objects.  By correlating the
mid-infrared SB/AGN classifications for this IRS sample with their
multiwavelength photometric properties, we will extrapolate our
results to the multitude of fainter GOODS 24micron sources which
dominate the far-infrared luminosity density of the Universe at high
redshifts but which are too faint for IRS spectroscopy.  This will
yield the fraction of the comoving bolometric luminosity density at
0.5<z<2.5 arising from accretion and nucleosynthesis, one of the key
goals of galaxy evolution studies.