The gravitational magnification of distant galaxies by foreground clusters has enabled the first measurement of the size-luminosity relation for HII regions at redshift z~2-5. A significant offset is seen in this relation with respect to that determined locally. This evolutionary trend has been interpreted as possible evidence for a dfferent mode of star formation in the early universe. To test this hypothesis, we propose to image a sample of 7 lensed galaxies at intermediate redshifts, z~1.0-1.5, carefully chosen so that the Halpha emission falls precisely within an appropriate narrow-band fiter with WFC3/IR. As each galaxy is magnified by a significant factor, this will enable us to resolve and measure the luminosities of ~50 individual HII regions as small as 50 pc to a star formation rate limit of 0.015 Mo/year. By tracking the size-luminosity relation over the full redshift range from 0 to 5, we can differentiate between various explanations proposed for the change in this fundamental relationship.

We perform WFC3 imaging of 7 lensed galaxies at z~1.0 and z~1.5. Each of the targets is extended by 5-20'' so entirely fits in a single WFC3 pointing. To achieve our science goal (Halpha imaging of HII regions), we will observe each target using the appropriate narrow-band filter matching Halpha: F126N, F132N, F164N or F167N. In order to differentiate Halpha emission from the contribution from the stellar continuum, we will also observe the same targets in the corresponding broad-band filter (F125W at z~1, F160W at z~1.5) and produce narrow-band excess images to select HII regions. These broad-band images are free of other strong emission lines. Whenever possible we match the orientation of the narrow-band and the broad-band image to minimize the effect of PSF anisotropy when performing the subtraction. We will detect Halpha fluxes of 10^-17 cgs per HII region in two orbits (5sigma, ~40 min exposure time per orbit). Adopting a conservative factor of x5-20 for the total magnification, this limiting flux corresponds to intrinsic star-formation rates of 0.015-0.05 Msol/yr per HII region, at z~1.0-1.5, which will allow us to probe the required parameter space in the size-luminosity diagram. Similarly, our targets have observed magnitudes of 21-22 AB and extend over ~5-15 arcsec^-2. Adopting a (conservative) constant surface brightness AB=25 mag.arcsec^-2, we are able measure the continuum in the broad-band filter (F125W or F160W) at 5 sigma in a single orbit (40 min). For each WFC3 observation, we will split the orbit into 3 individual exposures and apply a dithering pattern to remove hot/bad pixels and cosmic-ray events. UNRESOLVED STELLAR POPULATIONS AND GALAXY STRUCTURE 12 false 0.0 0.0 false false false false false false SUPPORTED GALAXY GRAVITATIONAL LENS HIGH REDSHIFT GALAXY J2000 11 49 35.4800 +22 23 50.07 J2000 GALAXY GRAVITATIONAL LENS HIGH REDSHIFT GALAXY J2000 00 37 4.8800 +09 10 24.71 J2000 GALAXY GRAVITATIONAL LENS HIGH REDSHIFT GALAXY J2000 08 00 57.5500 +36 03 38.77 J2000 GALAXY GRAVITATIONAL LENS HIGH REDSHIFT GALAXY J2000 09 17 58.9000 +51 43 48.81 J2000 GALAXY GRAVITATIONAL LENS HIGH REDSHIFT GALAXY J2000 09 47 15.9500 +76 23 7.30 J2000 GALAXY GRAVITATIONAL LENS HIGH REDSHIFT GALAXY J2000 01 59 4.9100 -34 12 58.35 J2000 GALAXY GRAVITATIONAL LENS HIGH REDSHIFT GALAXY J2000 11 33 14.2900 +50 08 46.90 J2000 WFC3/IR Imaging No No F164N WFC3/IR Imaging No No F132N WFC3/IR Imaging No No F126N WFC3/IR Imaging No No F132N WFC3/IR Imaging No No F132N WFC3/IR Imaging No No F125W WFC3/IR Imaging No No F164N WFC3/IR Imaging No No F160W WFC3/IR Imaging No No F167N WFC3/IR Imaging No No F160W The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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CASM was run in Phase II mode. (Tic-Visit :proposal-id 12197 :visit-id "04" :proposal-version -1 :trans-version "20100505" :date "29-Jul-2010 10:14:47.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 1079328 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12197 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12197 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3110 :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 3445 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12197 :visit-id "04" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3110 :al-overhead-at-end 0 :internal-dur-at-end 730 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3445 :pcs-gap 0 ) false 2 0 The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12197 :visit-id "05" :proposal-version -1 :trans-version "20100505" :date "29-Jul-2010 10:14:51.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 1462320 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12197 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12197 :visit-id "05" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3276 :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 3633 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12197 :visit-id "05" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3276 :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 3633 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12197 :visit-id "05" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3276 :al-overhead-at-end 0 :internal-dur-at-end 673 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3633 :pcs-gap 0 ) false 3 0 The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12197 :visit-id "06" :proposal-version -1 :trans-version "20100505" :date "29-Jul-2010 10:14:56.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 1514560 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12197 :visit-id "06" :obset-id "06" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12197 :visit-id "06" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2869 :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 3275 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12197 :visit-id "06" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2969 :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 3275 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12197 :visit-id "06" :orbit-number 3 :pcs-time 222 :si-vis-utilization 2969 :al-overhead-at-end 0 :internal-dur-at-end 236 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3275 :pcs-gap 0 ) false 3 0 The scheduling data was generated by CASM 18.3.1. 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