Spitzer Space Telescope - General Observer Proposal #20081


Mid-IR spectroscopy of high-redshift ultraluminous dusty galaxies

Principal Investigator: Andrew Blain
           Institution: Caltech

Co-Investigators: 
Scott Chapman, Caltech                                                
Ian Smail, University of Durham                                       
Lee Armus, Caltech/SSC                                                
David Frayer, Caltech/SSC                                             
David Alexander, University of Cambridge                              
Rob Ivison, UKATC                                                     
Harry Teplitz, Caltech/SSC                                            

Science Category: high-z galaxies (z>0.5) 
 Observing Modes: IrsPeakupImage IrsStare 
  Hours Approved: 90.0 

Abstract:
We propose deep IRS spectroscopy of a unique sample of 64
ultraluminous submillimeter-detected galaxies (SMGs) with known
spectroscopic redshifts from 0.5 to 3.6.  The median of their redshift
distribution is z=2.4, spanning the peak epoch of massive galaxy
formation activity and AGN fuelling.  The SMG population certainly
includes some of the most massive galaxies in the Universe.  Much less
common low-redshift galaxies with similar IR luminosities show a rich
variety of mid-infrared(IR) gas and solid-phase spectroscopic
features, revealing the physical conditions in the most active regions
of these highly obscured systems, and especially the importance of AGN
for powering their far-IR luminosity.  The known redshifts and
confirmed high luminosities will allow both the unambiguous
interpretation of individual spectra, and a stacking analysis to
reveal population-wide spectral features, as a function of color,
magnitude and evolutionary state, as revealed by extensive
multiwavelength data, including optical and near-IR imaging at HST
resolution, and all a handful have archived or scheduled IRAC and MIPS
imaging.  IRS spectroscopy will contrast their mid-IR spectra with
local analogs immune to the effects of dust extinction.  For the
subset of 21 targets with ultradeep X-ray data, we will be able to
compare the relative power of X-ray and mid-IR diagnostics to reveal
buried AGN activity.  This ambitious IRS project will provide our
first insight into the astrophysics of a large sample of ultraluminous
galaxies spanning 80% of the lifetime of the Universe.