Spitzer Space Telescope - General Observer Proposal #50512


Mid-IR Spectroscopy of gas-rich disk galaxies at z=1.5

Principal Investigator: Helmut Dannerbauer
           Institution: MPIA

     Technical Contact: Helmut Dannerbauer, MPIA

Co-Investigators: 
Emanuele Daddi, CEA                                                   
David Elbaz, CEA                                                      
Mark Dickinson, NOAO                                                  
Glenn Morrision, UH-IfA Hawaii                                        
Daniel Stern, JPL, Caltech                                            

Science Category: high-z galaxies (z>0.5) 
 Observing Modes: IrsMap 
  Hours Approved: 15.2 

Abstract:
We have recently obtained the first detection of CO in two ordinary massive z=2
galaxies. These massive disk galaxies were observed with the IRAM Plateau de
Bure Interferometer, selected to lie in the mass-star formation rate correlation
at z~1.5, thus being representative massive high-z galaxies. The CO detection
implies that giant molecular gas reservoirs are present in these sources, with
gas fractions reaching 50-70% of the total galaxy masses. With an infrared
luminosity of L(FIR)~10^12 L(sun), these BzK disk-like galaxies are borderline
ULIRGs but with star formation efficiency similar to local spirals, and an order
of magnitude lower than in submm galaxies, unveiling a new formation mode
previously unknown in the distant universe. We propose here IRS spectroscopy of
these two CO detected galaxies at z=1.5 in order to unveil their mid-IR rest
frame spectral properties and measuring the equivalent widths of PAH features at
6.2, 7.7, 8.6 and 11.3 micron and of the 9.7 micron silicate absorption. The
extreme richness in molecular gas in these sources should affect their mid-IR
spectral properties, and we will investigate how the diversity in gas/star
ratios in distant galaxies is important for the interpretation of mid-IR
luminosities of the high-z populations. In particular, the fact that these
ULIRGs behave like scaled up spiral galaxies in their star formation properties
could be reflected in their mid-IR spectral properties. Obtaining the PAH ratios
will be crucial to constrain better the CO to H2 mass conversion ratio Xco.
These observations will represent a legacy from Spitzer for the years to come
for the interpretations of mid-IR properties of distant star forming galaxies.