Spitzer Space Telescope - Directors Discretionary Time Proposal #538

GRB090423: A Lighthouse at the Epoch of Reionization

Principal Investigator: Ranga-Ram Chary
	   Institution: SSC

Co-Investigators: 
Edo Berger, Carnegie Observatories
Mark Dickinson, NOAO
Emanuele Daddi, CEA/Saclay, France
Myunghin Im, Seoul National University, Korea
Giovanni Fazio, SAO
Schuyler Van Dyk, SSC/Caltech
Asantha Cooray, UC Irvine


Science Category: GRBs
Observing Modes: IRAC 3.6 microns
Hours Approved: 39.9 hrs

Abstract:
The reionization of the Universe between redshifts 6 and 30 is the most
important astrophysical event since the Big Bang. By virtue of being
extremely luminous end states in the evolution of massive stars, gamma-ray
bursts are a fundamentally important tracer of star-formation out to the
highest redshifts. The measurement of the spectroscopic redshift of GRB090423
at z=8.1, makes it arguably, the most reliable, highest redshift, astrophysical
source detected thus far, and provides our first ever opportunity to characterize
the properties of star-forming galaxies as the Universe was making a transition
from a neutral state to a fully ionized state. We propose deep Spitzer/IRAC 3.6
micron observations of this exciting source over two epochs to: 1) Characterize
the light curve of a z=8.1 GRB afterglow 2) Measure the properties of the
underlying host galaxy and constrain the star-formation rate density at z~8
3) Develop a light curve template which will enable the search for similar,
non-GRB, high-z transients in Spitzer Warm Mission surveys. 4) Provide a deep
field in which deep, future narrow band observations can be undertaken to search
for other galaxies at z=8.1 which might be clustered around the GRB host.