Spitzer Space Telescope - General Observer Proposal #40599


Unveiling the Galaxy Counterparts of Damped Lyman-alpha Absorbers using GRB-DLAs

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

     Technical Contact: Ranga-Ram Chary, California Institute of Technology

Co-Investigators: 
Edo Berger, Carnegie Institute of Washington                          
Andrew Blain, Caltech                                                 
George Djorgovski, Caltech                                            
Shrinivas Kulkarni, Caltech                                           

Science Category: high-z galaxies (z>0.5) 
 Observing Modes: IracMap MipsPhot 
  Hours Approved: 77.4 

Abstract:
Damped Lyman-alpha systems (DLAs) have primarily been detected along the line of
sight to bright quasars (QSOs) through absorption line spectroscopy. They harbor
the bulk of the neutral gas in the Universe between redshifts of 0<z<5 and are
therefore thought to be the predecessors of star-forming galaxies. It is
impossible to measure the stellar mass, star-formation rate and dust in QSO-DLAs
at z>2 since the QSO overwhelms the light from the DLA by a factor of 10. The
launch of Swift has enabled localization of the optical counterpart of gamma-ray
bursts (GRBs) while they are still bright. Prompt, high resolution spectroscopy
of these afterglows, which briefly outshine the brightest quasars, reveals a
rich forest of absorption features providing an unprecedented window into the
star-forming environments of distant galaxies. GRB hosts show higher
metallicities, higher neutral gas column densities and depletion onto dust
grains compared to QSO-DLAs. This can be attributed to the fact that QSO sight
lines are more likely to cross the extended outer regions of intervening
systems. GRBs on the other hand, due to their association with massive stars,
provide an unbiased tracer of star-forming environments before the molecular
cloud is destroyed by feedback. As a result, GRB hosts are the strongest
candidates for bridging the evolutionary chasm between Lyman-break galaxies
(LBGs) and QSO-DLAs. We propose Spitzer 3.6 and 24 micron imaging of the
complete sample of 35 GRB host galaxies with well-characterized absorption line
properties and spectroscopic redshifts in the range 2<z<5.6. We will accurately
measure the stellar mass, assess the presence of dust which might be responsible
for metal depletion and measure the dust-obscured star-formation rate in DLA
systems. A comparison between the multiwavelength properties of GRB hosts and
LBGs will help define the notion that GRBs can be used as a tracer of the
co-moving star-formation rate density of the Universe out to the epoch of
reionization.