Population III (Pop III) stars, the first generation, metal-free stars in the Universe, have been a main focus of the studies of early galaxy formation and reionization. HeII 1640 emission originates from energetic ionizing photons beyond 54.4eV, and is an ideal tracer of massive Pop III star formation with strong far-UV radiation. HeII has not yet been detected directly in individual galaxies at z<6.3, indicating a small contribution of Pop III star formation at those redshifts. We propose to use WFC3 narrow-band imaging to measure the flux of HeII emission in galaxy IOK-1 (z=6.96), the highest redshift spectroscopically confirmed galaxy to date. At this redshift, the HeII line is perfectly located in the WFC3 F130N filter passband. Our deep narrow-band imaging will detect a HeII flux down to 4x10^-18 erg/s/cm^2 at the 5-sigma level, corresponding to a star formation rate from massive Pop III stars of ~1 M_sun per year, a factor of ~5 improvement to the best ground-based results at lower redshift. Strong HeII emission, if detected, will provide the first direct evidence of significant Pop III formation in early galaxies at the end of the reionization epoch. We will also carry out short F125W broad-band observations to measure the rest-frame UV flux of this galaxy in order to constrain its total star formation rate and to provide continuum subtraction for narrow-band imaging.

We will image in two filters with WFC3/IR: (1) 8-orbit exposure in F130N. (2) 2-orbit exposure in F125W. We will divide the program into two visits. In each visit, we will observe: (1) Four orbits in F130N. We will carry out a 4-point dither sequence with orbit-long (~2600) exposures in the narrow F130N band. This will bring the images to being background-limited. (2) One orbit in F125W. We will do the same 4-point dither sequence with ~650sec exposures to populate one orbit. We will do a small POS-TARG offset for the second visit and repeat the same sequence. COSMOLOGY 12 false 0.0 0.0 false false false false false SUPPORTED GALAXY HIGH REDSHIFT GALAXY NB973 = 24.60; z_AB >27.05 J2000 13 23 59.8000 +27 24 55.80 J2000 WFC3/IR Imaging No No F130N WFC3/IR Imaging No No F125W POS-TARG DITHER 0.572 LINE 0.365 false false 74.653 4 18.528 WFC3-IR-DITHER-BOX-MIN 4 false The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. IOK-1 4-point dither in F130N (4 orbits) and F125W (1 orbit) (Tic-Visit :proposal-id 11587 :visit-id "01" :proposal-version -1 :trans-version "20080609" :date "24-Jun-2008 16:46:23.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 2158672 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11587 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11587 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2845 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3267 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11587 :visit-id "01" :orbit-number 2 :pcs-time 195 :si-vis-utilization 2845 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3267 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11587 :visit-id "01" :orbit-number 3 :pcs-time 195 :si-vis-utilization 2845 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3267 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11587 :visit-id "01" :orbit-number 4 :pcs-time 195 :si-vis-utilization 2845 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3267 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11587 :visit-id "01" :orbit-number 5 :pcs-time 195 :si-vis-utilization 2840 :al-overhead-at-end 0 :internal-dur-at-end 516 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3267 :pcs-gap 0 ) false 5 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. IOK-1 4-point dither: F130N (4 orbits) and F125W (1 orbit), shifted by 17.25 pixels in X and Y from visit 1 for better flat-fielding (Tic-Visit :proposal-id 11587 :visit-id "02" :proposal-version -1 :trans-version "20080609" :date "24-Jun-2008 16:38:23.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 2158672 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11587 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11587 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2845 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3267 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11587 :visit-id "02" :orbit-number 2 :pcs-time 195 :si-vis-utilization 2845 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3267 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11587 :visit-id "02" :orbit-number 3 :pcs-time 195 :si-vis-utilization 2845 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3267 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11587 :visit-id "02" :orbit-number 4 :pcs-time 195 :si-vis-utilization 2845 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3267 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11587 :visit-id "02" :orbit-number 5 :pcs-time 195 :si-vis-utilization 2840 :al-overhead-at-end 0 :internal-dur-at-end 516 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3267 :pcs-gap 0 ) false 5 0