Spitzer Space Telescope - General Observer Proposal #30202


Revealing the Nature of one of the most H2-bright ULIRGs

Principal Investigator: Helmut Dannerbauer
           Institution: MPIA

Co-Investigators: 
Dimitra Rigopoulou, Astrophysics, Oxford                              
Dieter Lutz, MPE                                                      
Reinhard Genzel, MPE                                                  
Eckhard Sturm, MPE                                                    
Alan F. M. Moorwood, ESO                                              
Helene Roussel, MPIA                                                  

Science Category: intermediate-z galaxies (0.05<z<0.5) 
 Observing Modes: IracMap IrsStare MipsPhot 
  Hours Approved: 1.3 

Abstract:
We propose observations with all three instruments on-board Spitzer of
one of the most H2-bright galaxies known so far, the local ULIRG
IRASF23578-5307 at z~0.125.  The H2-brightness of this ULIRG, relative
to bright lines tracing HII-regions, was discovered through
near-infrared follow-up spectroscopic observations of a sample of
ULIRGs previously observed by ISO.  We note that the NIR H2-emission
in IRASF23578-5307 is even stronger than in the well known H2-bright
object NGC 6240  which was observed by Spitzer (Armus et al.  2005).
Due to the wealth of diagnostic tools in the mid-infrared, the
proposed IRS spectroscopic observations both in low and high
resolution mode will give us deep insight into the nature of the
strong NIR H2-emission lines seen.  We aim at observing atomic fine
structure lines, H2 molecular lines, PAHs and possible silicate
absorption features.  And thus we will apply standard MIR-diagnostic
tools in order to reveal the nature of the unusual strong H2 emission
detected in this ULIRG and determine if starburst or AGN activity is
the dominant energy source.  We will test the relevance of possible
excitation mechanisms of H2 (fluorescence, PDR, shocks, Xray-heating).
In addition, the IRAC 4-band and the MIPS 24 micron observations will
provide us a solid calibration of the IRS-spectroscopy.  MIPS
observations at 70  and 160  micron will allow us to set constraints
on the dust temperatures, the geometry and the dust mass.  The
extraordinary properties in the NIR and the tantalizing ISOPHOT-S
spectrum, make it indispensable to obtain Spitzer observations of this
peculiar object.