The class of objects that reionized intergalactic hydrogen remains an observational and theoretical problem that is in contention for being the most prominent puzzle piece in contemporary astrophysics. The current consensus - determined almost entirely by ruling out bright active galaxies - is that the process was possibly begun and almost certainly finished by faint, lower-mass galaxies forming their early generations of stars. Recent observations of z~3 galaxies may even have identified the analog populations. However understanding how the emitted ionizing power of galaxies is causally related to their (robustly determined) physical properties is not a study that can be performed at high-z: neither the spatial information nor the standard multi-wavelength diagnostics are available. Moreover, on a case-by-case basis, the intervening IGM absorption is impossible to determine. These considerations have spawned a number of detailed studies with UV space telescopes, the synthesis of which however is that a characteristic population of Lyman continuum (LyC) emitting objects has not yet been identified. We show in this proposal that we have identified a characteristic trait in galaxy spectra that is highly indicative of LyC emission, by combining (a) high-z phenomenological studies, (b) new high-resolution UV spectra of local galaxies, and (c) sophisticated models of radiation transport. Believing that we have determined the signature, we propose to test the new hypothesis with deep spectroscopic observations with HST/COS under the Cycle 21 UV initiative. This programme will observe FOUR star-forming galaxies with COS in both NUV imaging and FUV spectroscopic modes. 8 orbits are allocated; 2 each per galaxy, and hence there are four visits. Our main science goal is to obtain deep spectra in the G140L setting, which transmits the restframe far ultraviolet and most importantly the redshifted Lyman continuum. We have already observed these targets with COS (FUV/G160M setting; GO 12269), and hence benefit from much experience. All the targets are too faint in the UV for target acquisition to be done on-source. We therefore (in 12269) selected only objects with nearby stars that were bright enough in the UV for acquisition in the NUV. We subsequently perform a blind offset. In the list of Fixed Targets, <GALEX_ID> refers to the science target, and <GALEX_ID>-OFFSET refers to the coordinates of the stars. G140L observations use all four different FP-POS settings, TIME-TAG mode, and FLASH to improve wavelength calibration. These galaxies are particularly faint, so the BUFFER TIME turns out consistently to be ~29,000 sec. We simply set it to something longer than the visit : I.e. 10,000 sec. For the acquisition stars, the NUV fluxes have been measured in the COS images. These fluxes are then used to as input to the ETC to estimate the countrate. 3 acquisitions are performed with MIRROR A, and one with MIRROR B. After performing the offset, we will take a 100 second NUV image of the target for wavelength zeropoint purposes. The rest of the 2 orbits is used for G140L spectroscopy, and exposure times in the 4 FP-POS settings maximized. ISM IN EXTERNAL GALAXIES Default false true false false false SUPPORTED GALAXY EMISSION LINE NEBULA STARBURST J2000 STAR UNDESIGNATED S33T005015 J2000 J2000 GALAXY EMISSION LINE NEBULA STARBURST J2000 STAR UNDESIGNATED N5MV000310 J2000 J2000 GALAXY EMISSION LINE NEBULA STARBURST J2000 STAR UNDESIGNATED S33T000300 J2000 J2000 GALAXY EMISSION LINE NEBULA STARBURST J2000 STAR UNDESIGNATED N6SV002695 J2000 J2000 COS/NUV Imaging No No MIRRORA COS/FUV Spectroscopic No No G140L 1280 COS/NUV Imaging No No MIRRORA COS/FUV Spectroscopic No No G140L 1280 COS/NUV Imaging No No MIRRORA COS/FUV Spectroscopic No No G140L 1280 COS/NUV Imaging No No MIRRORA COS/FUV Spectroscopic No No G140L 1280 The scheduling data was generated by CASM 21.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 13452 :visit-id "01" :proposal-version -1 :trans-version "20130502" :date "21-Jun-2013 14:58:07.0" :trans-errors '(:WARNINGS 5 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 87216 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 13452 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13452 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2907 :al-overhead-at-end 0 :internal-dur-at-end 38 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3253 :pcs-gap 0 ) (Tic-Orbit :proposal-id 13452 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3031 :al-overhead-at-end 0 :internal-dur-at-end 376 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3253 :pcs-gap 0 ) true 2 0 The scheduling data was generated by CASM 21.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 13452 :visit-id "02" :proposal-version -1 :trans-version "20130502" :date "21-Jun-2013 14:59:06.0" :trans-errors '(:WARNINGS 5 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 104592 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 13452 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13452 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3084 :al-overhead-at-end 0 :internal-dur-at-end 48 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3430 :pcs-gap 0 ) (Tic-Orbit :proposal-id 13452 :visit-id "02" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3207 :al-overhead-at-end 0 :internal-dur-at-end 376 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3430 :pcs-gap 0 ) true 2 0 The scheduling data was generated by CASM 21.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 13452 :visit-id "03" :proposal-version -1 :trans-version "20130502" :date "21-Jun-2013 14:58:11.0" :trans-errors '(:WARNINGS 5 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 87216 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 13452 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13452 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2878 :al-overhead-at-end 0 :internal-dur-at-end 38 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3253 :pcs-gap 0 ) (Tic-Orbit :proposal-id 13452 :visit-id "03" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3031 :al-overhead-at-end 0 :internal-dur-at-end 376 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3253 :pcs-gap 0 ) true 2 0 The scheduling data was generated by CASM 21.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 13452 :visit-id "04" :proposal-version -1 :trans-version "20130502" :date "21-Jun-2013 14:58:13.0" :trans-errors '(:WARNINGS 5 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 87216 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 13452 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13452 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2873 :al-overhead-at-end 0 :internal-dur-at-end 38 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3229 :pcs-gap 0 ) (Tic-Orbit :proposal-id 13452 :visit-id "04" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3007 :al-overhead-at-end 0 :internal-dur-at-end 376 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3229 :pcs-gap 0 ) true 2 0