Spitzer Space Telescope - General Observer Proposal #60176


The Spitzer Lyman Alpha Survey

Principal Investigator: James Rhoads
           Institution: Arizona State University

     Technical Contact: James Rhoads, Arizona State University

Co-Investigators: 
Sangeeta Malhotra, Arizona State University                           
Steven Finkelstein, Texas A&M                                         
Norman Grogin, STScI                                                  
Norbert Pirzkal, STScI                                                
Junxian Wang, USTC                                                    

Science Category: high-z galaxies (z>0.5) 
 Observing Modes: IRAC Post-Cryo Mapping
  Hours Approved: 91.1 

Abstract:
Determining the star formation history of high-redshift galaxies is vital for
understanding galaxy formation and reionization. These galaxies are typically
selected using their rest-frame ultraviolet (UV) fluxes, thus their old stellar
populations can be missed. Spitzer Imaging at 3.6 microns is essential to
measure the rest-frame optical fluxes of high redshift galaxies and therefore
estimate the total stellar mass. Lyman-alpha galaxies form fully half of the
known galaxies at z=3-6. The strength of the Lyman-alpha line, at first glance,
indicates a young (~10 million years old) and dust-free population. This picture
of Lyman-alpha galaxies as a class of less massive and young objects is
simultaneously being confirmed and challenged thanks to Spitzer data. While most
of the Lyman-alpha galaxies are young and low-mass, a subset of them are more
massive and/or dusty. That there may be two types of Lyman-alpha emitters, is
based on the only those few studies that analyze individual galaxies, and not
co-addition of a sample of non-detections. In order to robustly investigate the
statistical fraction of older and younger Lyman-alpha galaxies at any given
redshift and to find out the redshift evolution of this fraction, we
simultaneously need a large sample at many redshifts, and we need deep imaging
so we can study individual objects. We propose a systematic IRAC 3.6 imaging
survey of a spectroscopically confirmed sample of about 100 Lyman-alpha galaxies
between redshifts 3.1<z<6.6. Deep broad-band imaging of all the galaxies in
B,V,R,I and Z; and J and H imaging of a subset with NICMOS already exists. By
fitting Spectral energy distributions we will measure accurately (1) The total
stellar mass in these objects, including old stars which may have formed at
redshifts > 8; (2) The dust extinction in the UV, and therefore a correction to
their present star-formation rates; (3) The fraction of galaxies with old
stellar populations as a function of redshift.