NASA Hubble Space Telescope Daily Report #5184
HUBBLE SPACE TELESCOPE – Continuing to Collect World Class Science
DAILY REPORT #5184
PERIOD COVERED: 5am September 17 – 5am September 20, 2010 (DOY 260/09:00z-263/09:00z)
FLIGHT OPERATIONS SUMMARY:
Significant Spacecraft Anomalies: (The following are preliminary reports of potential non-nominal performance that will be investigated.)
HSTARS:
12416 – GSAcq(1,2,1) at 263/5:25:24z and REAcq(1,2,1) at 263/06:57:58z and REAcq(1,2,1) at 263/08:33:54z all failed due to search radius limit exceeded.
Observations affected, COS 3-6 Proposal ID#11728; WFC3 5-10 Proposal ID#11700
COMPLETED OPS REQUEST:
18918-0 Uplink new FGS calibration tables @ 262/23:32z
18921-0 R/T OBAD to Correct Attitude Error @ 263/06:32z
COMPLETED OPS NOTES: (None)
SCHEDULED SUCCESSFUL
FGS GSAcq 19 18
FGS REAcq 21 19
OBAD with Maneuver 15 15
SIGNIFICANT EVENTS:
The FGS distortion and Calibration tables were successfully uplinked at 262/23:30z. Table dumps performed showed the expected changes for the FGS-2R2 values being modified. The first guide star acquisition at 263/01:30z performed nominally.
FGS 12320
The Ages of Globular Clusters and the Population II Distance Scale
Globular clusters are the oldest objects in the universe whose age can be accurately determined. The dominant error in globular cluster age determinations is the uncertain Population II distance scale. We propose to use FGS 1r to obtain parallaxes with an accuracy of 0.2 milliarcsecond for 9 main sequence stars with [Fe/H] < -1.5. This will determine the absolute magnitude of these stars with accuracies of 0.04 to 0.06mag. This data will be used to determine the distance to 24 metal-poor globular clusters using main sequence fitting. These distances (with errors of 0.05 mag) will be used to determine the ages of globular clusters using the luminosity of the subgiant branch as an age indicator. This will yield absolute ages with an accuracy 5%, about a factor of two improvement over current estimates. Coupled with existing parallaxes for more metal-rich stars, we will be able to accurately determine the age for globular clusters over a wide range of metallicities in order to study the early formation history of the Milky Way and provide an independent estimate of the age of the universe. The Hipparcos database contains only 1 star with [Fe/H] < -1.4 and an absolute magnitude error less than 0.18 mag which is suitable for use in main sequence fitting. Previous attempts at main sequence fitting to metal-poor globular clusters have had to rely on theoretical calibrations of the color of the main sequence. Our HST parallax program will remove this source of possible systematic error and yield distances to metal-poor globular clusters which are significantly more accurate than possible with the current parallax data. The HST parallax data will have errors which are 10 times smaller than the current parallax data. Using the HST parallaxes, we will obtain main sequence fitting distances to 11 globular clusters which contain over 500 RR Lyrae stars. This will allow us to calibrate the absolute magnitude of RR Lyrae stars, a commonly used Population II distance indicator. WFC3/IR 12307 A public SNAPSHOT Survey of Gamma-ray Burst Host Galaxies We propose to conduct a public infrared survey of the host galaxies of Swift selected gamma-ray bursts (GRBs) at z<3. By obtaining deep, diffraction limited imaging in the IR we will complete detections for the host galaxies, and in concert with our extensive ground based afterglow and host programmes will compile a detailed catalog of the properties of high-z galaxies selected by GRBs. In particular these observations will enable us to study the colours, luminosities and morphologies of the galaxies. This in turn informs studies of the nature of the progenitors and the role of GRBs as probes of star formation across cosmic history. Ultimately it provides a product of legacy value which will greatly complement further studies with next generation facilities such as ALMA and JWST. ACS/WFC 12292 SWELLS: Doubling the Number of Disk-dominated Edge-on Spiral Lens Galaxies The formation of realistic disk galaxies within the LCDM cosmology is still largely an unsolved problem. Theory is now beginning to make predictions for how dark matter halos respond to galaxy formation, and for the properties of disk galaxies. Measuring the density profiles of dark matter halos on galaxy scales is therefore a strong test for the standard paradigm of galaxy formation, offering great potential for discovery. However, the degeneracy between the stellar and dark matter contributions to galaxy rotation curves remains a major obstacle. Strong gravitational lensing, when combined with spatially resolved kinematics and stellar population models, can solve this long-standing problem. Unfortunately, this joint methodology could not be exploited until recently due to the paucity of known edge-on spiral lenses. We have developed and demonstrated an efficient technique to find exactly these systems. During supplemental cycle-16 we discovered five new spiral lens galaxies, suitable for rotation curve measurements. We propose multi-color HST imaging of 16 candidates and 2 partially-imaged confirmed systems, to measure a sample of eight new edge-on spiral lenses. This program will at least double the number of known disk-dominated systems. This is crucial for constraining the relative contribution of the disk, bulge and dark halo to the total density profile. WFC3/IR 12265 Determining the Physical Nature of a Unique Giant Lya Emitter at z=6.595 We propose deep WFC3/IR imaging for a giant Lya emitter (LAE) with a Keck spectroscopic redshift of z=6.595 discovered by extensive narrow-band imaging with Subaru in the SXDS-UKIDSS/UDS field. This remarkable object is unique in many respects including its large stellar mass and luminous nebula which extends over 17 kpc; no equivalent source has been found in other surveys. The nature of this rare object is unclear. Fundamental to progress is determining the origin of star formation in such an early massive object; if the age of the stellar population is short we are likely witnessing a special moment in the formation history of a massive galaxy. The heating source for the nebula is also unclear; options include intense star formation, the infall of cold gas onto a dark halo or shock heating from a merger. We will take deep broad-band (F125W and F160W) images and an intermediate-band (F098M) image which will be analyzed in conjunction with ultra-deep IRAC 3.6 and 4.5 micron data being taken by the Spitzer/SEDS project. These data will enable us to constrain the star formation rate and stellar age. Moreover, the UV continuum morphology and Lya-line distribution will be investigated for evidence of a major merger, cold accretion, or hot bubbles associated with outflows. We will address the physical origin of the remarkable object observed at an epoch where massive galaxies are thought to begin their assembly. WFC3/UV/IR 12234 Differentiation in the Kuiper belt: a Search for Silicates on Icy Bodies. We currently have a large on-going program (Go Program 11644, 120 orbits) to exploit the superb stability and photometric characteristics of HST and the broad range in wavelength coverage of the WFC3 to make broad-band vis/IR spectral observations of a large sample of Kuiper belt objects. Though the survey is currently only ~50% complete, the quality and unprecedented signal-to-noise of these observations has revealed the existence of a previously undiscovered spectral variability not explainable within our current understanding of these objects. A possible explanation for this variability is that with this faint set of Kuiper belt objects, we are beginning to see the difference between larger differentiated objects and smaller non-differentiated objects. Its seems that the small and likely undifferentiated objects are exhibiting silicate features that affect our photometry – features not exhibited by the icy mantles of larger icy bodies. We propose a small add-on survey to dramatically increase the scientific results of our large program. The proposed observations will use the proven capabilities of WFC3 to make broad and narrow-band photometric observations to detect spectral features in the 1.0-1.3 micron range of a small subset of our sources. The 13 targets have been carefully selected to cover the range of spectral variability detected in our large program as well as sample the entire dynamical range and physical sizes of these targets. These observations will allow the identification of undifferentiated Kuiper belt objects by detection of their silicate features. As a probe for differentiation, these observations could constrain the natal locations of different Kuiper belt classes, a constraint currently unavailable to formation models. This small set of observations will allow the calibration of the spectral variability seen in our large program, and drastically enhance the scientific output of our full Cycle 17 sample. ACS/WFC 12210 SLACS for the Masses: Extending Strong Lensing to Lower Masses and Smaller Radii Strong gravitational lensing provides the most accurate possible measurement of mass in the central regions of early-type galaxies (ETGs). We propose to continue the highly productive Sloan Lens ACS (SLACS) Survey for strong gravitational lens galaxies by observing a substantial fraction of 135 new ETG gravitational-lens candidates with HST-ACS WFC F814W Snapshot imaging. The proposed target sample has been selected from the seventh and final data release of the Sloan Digital Sky Survey, and is designed to complement the distribution of previously confirmed SLACS lenses in lens-galaxy mass and in the ratio of Einstein radius to optical half-light radius. The observations we propose will lead to a combined SLACS sample covering nearly two decades in mass, with dense mapping of enclosed mass as a function of radius out to the half-light radius and beyond. With this longer mass baseline, we will extend our lensing and dynamical analysis of the mass structure and scaling relations of ETGs to galaxies of significantly lower mass, and directly test for a transition in structural and dark-matter content trends at intermediate galaxy mass. The broader mass coverage will also enable us to make a direct connection to the structure of well-studied nearby ETGs as deduced from dynamical modeling of their line-of-sight velocity distribution fields. Finally, the combined sample will allow a more conclusive test of the current SLACS result that the intrinsic scatter in ETG mass-density structure is not significantly correlated with any other galaxy observables. The final SLACS sample at the conclusion of this program will comprise approximately 130 lenses with known foreground and background redshifts, and is likely to be the largest confirmed sample of strong-lens galaxies for many years to come. COS/FUV 12169 The Frequency and Chemical Composition of Planetary Debris Discs around Young White Dwarfs Throughout the past few years, it has become increasingly clear that the most plausible scenario to explain the metal-pollution observed in ~20% of all cool white dwarfs is accretion from rocky debris material – suggesting that these white dwarfs may have had, or may still have terrestial planets as well. This hypothesis is corroborated through the infrared detection of circumstellar dust around the most heavily polluted white dwarfs. Traditionally, the detection of metal pollution is done in the optical using the Ca H/K lines, leading to a strong bias against hot/young white dwarfs. Hence, most of our knowledge about the late evolution of planetary systems is based on white dwarfs with cooling ages >0.5Gyr. We propose an HST/COS ultraviolet spectroscopic snapshot survey to carry out the first systematic investigation of the fraction of metal-pollution among young (20-100Myr) white dwarfs, probing the correlation with white dwarf (and hence progenitor) mass, and determining the Si/H, C/H, and potentially N/H and O/H abundance ratios of their circumstellar debris material.
ACS/WFC 12166
A Snapshot Survey of The Most Massive Clusters of Galaxies
We propose the continuation of our highly successful HST/ACS SNAPshot survey of a sample of 123 very X-ray luminous clusters in the redshift range 0.3-0.7, detected and compiled by the MACS cluster survey. As demonstrated by dedicated HST observations of the 12 most distant MACS clusters (GO-09722) as well as by the MACS SNAPshots of an additional 25 obtained with ACS so far in Cycles 14 and 15, these systems frequently exhibit strong gravitational lensing as well as spectacular examples of violent galaxy evolution. A large number of additional MACS SNAPs have since been obtained with WFPC2, leading to the discovery of several more powerful cluster lenses. The dramatic loss, however, of depth, field-of-view, and angular resolution compared to ACS led to significantly reduced scientific returns, underlining the need for ACS for this project. The proposed observations will provide important constraints on the cluster mass distributions, on the physical nature of ! galaxy-galaxy and galaxy-gas interactions in cluster cores, and will yield a set of optically bright, lensed galaxies for further 8-10m spectroscopy. For those of our targets with existing ACS SNAPshot images, we propose SNAPshots in the WFC3 F110W and F140W passbands to obtain colour information that will greatly improve the secure identification of multiple-image systems and may, in the form of F606W or F814W dropouts, lead to the lensing-enabled discovery of very distant galaxies at z>5. Acknowledging the broad community interest in this sample (16 of the 25 targets of the approved MCT cluster program are MACS discoveries) we waive our data rights for these observations.
This proposal is an updated and improved version of our successful Cycle 15 proposal of the same title. Alas, SNAP-10875 collected only six snapshots in the F606W or F814W passbands, due to, first, a clerical error at STScI which caused the program to be barred from execution for four months and, ultimately, the failure of ACS. With ACS restored, and WFC3 providing additional wavelength and redshift leverage, we wish to resume this previously approved project.
COS/NUV/FUV 12084
G140L/1280 Internal to External Wavelength Scale
This program observes NGC330-B37 to determine the offsets between the PSA and WCA wavelength scales (for FP-POS=3) for the new G140L/1280 mode that will be available starting in Cycle 18. The results of the analysis of these data will be used to update the FUV wavelength dispersion reference file.
In addition since it this the first time that this mode is used, both on on-orbit or on the ground, we obtain also data at FP-POS=1 and 4 which inform us of the extremes of the wavelength range that can be seen with G140L/1280.
Note that this program can only be executed after FSW changes occur (current estimate for these FSW changes is ~Aug 2010 timeframe) since this mode in not yet implemented.
COS/NUV/FUV 12080
COS G140L Optics Alignment and Focus
A G140L focus sweep will be performed using the B2Ia star AZV18 in the Small Magellanic Cloud. A sequence of time-tag spectra will be acquired through the PSA aperture (CENWAVE=1105 A), at a range of focus settings. There will be 17 focus settings sampled, ranging from -800 to +800 in 100-step intervals. The optimum focus will be determined by cross-correlating prominent absorption features in the spectra with a template high resolution STIS E140M spectrum, then choosing the focus setting yielding the narrowest cross-correlation profile from the sequence. This is similar to the focusing procedure used for the G140L grating during SMOV (PID 11484, Visit 3), except that the new focus sweep will extend to more extreme focus positions around 0 (+/-800 instead of +/-600). The inclusion of additional focus positions is necessary because the G140L focus curve from SMOV (cross-correlation FWHM vs. focus position) is broad and shallow, making it difficult to measure the minimum in the curve. After obtaining an exposure at the most extreme positive focus position (+800), the focus is returned to its nominal position (0). A final spectrum is then acquired at that position, for repeatability comparison with the earlier FOCUS=0 spectrum. After the data are analyzed, a patchable constant SMS update of OSM1 focus for the G140L grating will be uplinked.
WFC3/UV 12019
After the Fall: Fading AGN in Post-starburst Galaxies
We propose joint Chandra and HST observations of an extraordinary sample of 12 massive post-starburst galaxies at z=0.4-0.8 that are in the short-lived evolution phase a few 100 Myr after the peak of merger-driven star formation and AGN activity. We will use the data to measure X-ray luminosities, black hole masses, and accretion rates; and with the accurate “clocks” provided by post-starburst stellar populations, we will directly test theoretical models that predict a power-law decay in the AGN light curve. We will also test whether star formation and black hole accretion shut down in lock-step, quantify whether the black holes transition to radiatively inefficient accretion states, and constrain the observational signatures of black hole mergers.
ACS/WFC 11996
CCD Daily Monitor (Part 3)
This program comprises basic tests for measuring the read noise and dark current of the ACS WFC and for tracking the growth of hot pixels. The recorded frames are used to create bias and dark reference images for science data reduction and calibration. This program will be executed four days per week (Mon, Wed, Fri, Sun) for the duration of Cycle 17. To facilitate scheduling, this program is split into three proposals. This proposal covers 308 orbits (19.25 weeks) from 21 June 2010 to 1 November 2010.
WFC3/IR/S/C 11929
IR Dark Current Monitor
Analyses of ground test data showed that dark current signals are more reliably removed from science data using darks taken with the same exposure sequences as the science data, than with a single dark current image scaled by desired exposure time. Therefore, dark current images must be collected using all sample sequences that will be used in science observations. These observations will be used to monitor changes in the dark current of the WFC3-IR channel on a day-to-day basis, and to build calibration dark current ramps for each of the sample sequences to be used by Gos in Cycle 17. For each sample sequence/array size combination, a median ramp will be created and delivered to the calibration database system (CDBS).
WFC3/UVIS/IR 11909
UVIS Hot Pixel Anneal
The on-orbit radiation environment of WFC3 will continually generate new hot pixels. This proposal performs the procedure required for repairing those hot pixels in the UVIS CCDs. During an anneal, the two-stage thermo-electric cooler (TEC) is turned off and the four-stage TEC is used as a heater to bring the UVIS CCDs up to ~20 deg. C. As a result of the CCD warmup, a majority of the hot pixels will be fixed; previous instruments such as WFPC2 and ACS have seen repair rates of about 80%. Internal UVIS exposures are taken before and after each anneal, to allow an assessment of the procedure’s effectiveness in WFC3, provide a check of bias, global dark current, and hot pixel levels, as well as support hysteresis (bowtie) monitoring and CDBS reference file generation. One IR dark is taken after each anneal, to provide a check of the IR detector.
WFC3/UVIS 11907
UVIS Cycle 17 Contamination Monitor
The UV throughput of WFC3 during Cycle 17 is monitored via weekly standard star observations in a subset of key filters covering 200-600nm and F606W, F814W as controls on the red end. The data will provide a measure of throughput levels as a function of time and wavelength, allowing for detection of the presence of possible contaminants.
WFC3/UVIS 11905
WFC3 UVIS CCD Daily Monitor
The behavior of the WFC3 UVIS CCD will be monitored daily with a set of full-frame, four-amp bias and dark frames. A smaller set of 2Kx4K subarray biases are acquired at less frequent intervals throughout the cycle to support subarray science observations. The internals from this proposal, along with those from the anneal procedure (Proposal 11909), will be used to generate the necessary superbias and superdark reference files for the calibration pipeline (CDBS).
COS/NUV 11896
NUV Spectroscopic Sensitivity Monitoring
The purpose of this proposal is to monitor sensitivity of each NUV grating mode to detect any changes due to contamination or other causes.
STIS/CCD 11847
CCD Bias Monitor-Part 2
Monitor the bias in the 1×1, 1×2, 2×1, and 2×2 bin settings at gain=1, and 1×1 at gain = 4, to build up high-S/N superbiases and track the evolution of hot columns.
STIS/CCD 11845
CCD Dark Monitor Part 2
Monitor the darks for the STIS CCD.
ACS/WFC3 11734
The Hosts of High Redshift Gamma-Ray Bursts
Gamma-ray bursts are the most luminous explosive events known, acting as beacons to the high redshift universe. Long duration GRBs have their origin in the collapse of massive stars and thus select star forming galaxies across a wide range of redshift. Due to their bright afterglows we can study the details of GRB host galaxies via absorption spectroscopy, providing redshifts, column densities and metallicities for galaxies far too faint to be accessible directly with current technology. We have already obtained deep ground based observations for many hosts and here propose ACS/WFC3 and WFC3 observations of the fields of bursts at z>3 which are undetected in deep ground based images. These observations will study the hosts in emission, providing luminosities and morphologies and will enable the construction of a sample of high-z galaxies with more detailed physical properties than has ever been possible before.
WFC3/IR 11696
Infrared Survey of Star Formation Across Cosmic Time
We propose to use the unique power of WFC3 slitless spectroscopy to measure the evolution of cosmic star formation from the end of the reionization epoch at z>6 to the close of the galaxy-building era at z~0.3.Pure parallel observations with the grisms have proven to be efficient for identifying line emission from galaxies across a broad range of redshifts. The G102 grism on WFC3 was designed to extend this capability to search for Ly-alpha emission from the first galaxies. Using up to 250 orbits of pure parallel WFC3 spectroscopy, we will observe about 40 deep (4-5 orbit) fields with the combination of G102 and G141, and about 20 shallow (2-3 orbit) fields with G141 alone.
Our primary science goals at the highest redshifts are: (1) Detect Lya in ~100 galaxies with z>5.6 and measure the evolution of the Lya luminosity function, independent of of cosmic variance; 2) Determine the connection between emission line selected and continuum-break selected galaxies at these high redshifts, and 3) Search for the proposed signature of neutral hydrogen absorption at re-ionization. At intermediate redshifts we will (4) Detect more than 1000 galaxies in Halpha at 0.5< z<1.8 to measure the evolution of the extinction-corrected star formation density across the peak epoch of star formation. This is over an order-of-magnitude improvement in the current statistics, from the NICMOS Parallel grism survey. (5) Trace ``cosmic downsizing" from 0.5< z<2.2; and (6) Estimate the evolution in reddening and metallicty in star-forming galaxies and measure the evolution of the Seyfert population. For hundreds of spectra we will be able to measure one or even two line pair ratios -- in particular, the Balmer decrement and [OII]/[OIII] are sensitive to gas reddening and metallicity. As a bonus, the G102 grism offers the possibility of detecting Lya emission at z=7-8.8. To identify single-line Lya emitters, we will exploit the wide 0.8–1.9um wavelength coverage of the combined G102+G141 spectra. All [OII] and [OIII] interlopers detected in G102 will be reliably separated from true LAEs by the detection of at least one strong line in the G141 spectrum, without the need for any ancillary data. We waive all proprietary rights to our data and will make high-level data products available through the ST/ECF. WFC3/ACS/IR 11677 Is 47 Tuc Young? Measuring its White Dwarf Cooling Age and Completing a Hubble Legacy With this proposal we will firmly establish the age of 47 Tuc from its cooling white dwarfs. 47 Tuc is the nearest and least reddened of the metal-rich disk globular clusters. It is also the template used for studying the giant branches of nearby resolved galaxies. In addition, the age sensitive magnitude spread between the main sequence turnoff and horizontal branch is identical for 47 Tuc, two bulge globular clusters and the bulge field population. A precise relative age constraint for 47 Tuc, compared to the halo clusters M4 and NGC 6397, both of which we recently dated via white dwarf cooling, would therefore constrain when the bulge formed relative to the old halo globular clusters. Of particular interest is that with the higher quality ACS data on NGC 6397, we are now capable with the technique of white dwarf cooling of determining ages to an accuracy of +/-0.4 Gyrs at the 95% confidence level. Ages derived from the cluster turnoff are not currently capable of reaching this precision. The important role that 47 Tuc plays in galaxy formation studies, and as the metal-rich template for the globular clusters, makes the case for a white dwarf cooling age for this metal-rich cluster compelling. Several recent analyses have suggested that 47 Tuc is more than 2 Gyrs younger than the Galactic halo. Others have suggested an age similar to that of the most metal poor globular clusters. The current situation is clearly uncertain and obviously a new approach to age dating this important cluster is required. With the observations of 47 Tuc, this project will complete a legacy for HST. It will be the third globular cluster observed for white dwarf cooling; the three covering almost the full metallicity range of the cluster system. Unless JWST has its proposed bluer filters (700 and 900 nm) this science will not be possible perhaps for decades until a large optical telescope is again in space. Ages for globular clusters from the main sequence turnoff are less precise than those from white dwarf cooling making the science with the current proposal truly urgent. COS/NUV/FUV 11598 How Galaxies Acquire their Gas: A Map of Multiphase Accretion and Feedback in Gaseous Galaxy Halos We propose to address two of the biggest open questions in galaxy formation – how galaxies acquire their gas and how they return it to the IGM – with a concentrated COS survey of diffuse multiphase gas in the halos of SDSS galaxies at z = 0.15 – 0.35. Our chief science goal is to establish a basic set of observational facts about the physical state, metallicity, and kinematics of halo gas, including the sky covering fraction of hot and cold material, the metallicity of infall and outflow, and correlations with galaxy stellar mass, type, and color – all as a function of impact parameter from 10 – 150 kpc. Theory suggests that the bimodality of galaxy colors, the shape of the luminosity function, and the mass-metallicity relation are all influenced at a fundamental level by accretion and feedback, yet these gas processes are poorly understood and cannot be predicted robustly from first principles. We lack even a basic observational assessment of the multiphase gaseous content of galaxy halos on 100 kpc scales, and we do not know how these processes vary with galaxy properties. This ignorance is presently one of the key impediments to understanding galaxy formation in general. We propose to use the high-resolution gratings G130M and G160M on the Cosmic Origins Spectrograph to obtain sensitive column density measurements of a comprehensive suite of multiphase ions in the spectra of 43 z < 1 QSOs lying behind 43 galaxies selected from the Sloan Digital Sky Survey. In aggregate, these sightlines will constitute a statistically sound map of the physical state and metallicity of gaseous halos, and subsets of the data with cuts on galaxy mass, color, and SFR will seek out predicted variations of gas properties with galaxy properties. Our interpretation of these data will be aided by state-of-the-art hydrodynamic simulations of accretion and feedback, in turn providing information to refine and test such models. We will also use Keck, MMT, and Magellan (as needed) to obtain optical spectra of the QSOs to measure cold gas with Mg II, and optical spectra of the galaxies to measure SFRs and to look for outflows. In addition to our other science goals, these observations will help place the Milky Way's population of multiphase, accreting High Velocity Clouds (HVCs) into a global context by identifying analogous structures around other galaxies. Our program is designed to make optimal use of the unique capabilities of COS to address our science goals and also generate a rich dataset of other absorption-line systems. WFC3/IR 11591 Are Low-Luminosity Galaxies Responsible for Cosmic Reionization? Our group has demonstrated that massive clusters, acting as powerful cosmic lenses, can constrain the abundance and properties of low-luminosity star-forming sources beyond z~6; such sources are thought to be responsible for ending cosmic reionization. The large magnification possible in the critical regions of well-constrained clusters brings sources into view that lie at or beyond the limits of conventional exposures such as the UDF. We have shown that the combination of HST and Spitzer is particularly effective in delivering the physical properties of these distant sources, constraining their mass, age and past star formation history. Indirectly, we therefore gain a valuable glimpse to yet earlier epochs. Recognizing the result (and limitations) of blank field surveys, we propose a systematic search through 10 lensing clusters with ACS/F814W and WFC3/[F110W+F160W] (in conjunction with existing deep IRAC data). Our goal is to measure with great accuracy the luminosity function at z~7 over a range of at least 3 magnitude, based on the identification of about 50 lensed galaxies at 6.5< z<8. Our survey will mitigate cosmic variance and extend the search both to lower luminosities and, by virtue of the WFC3/IRAC combination, to higher redshift. Thanks to the lensing amplification spectroscopic follow-up will be possible and make our findings the most robust prior to the era of JWST and the ELTs.
