9452( 3) - 04/25/02 14:46 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9452 Version: 3 Check-in Date: 25-Apr-2002 14:45:35 1.Proposal Title: Characterizing the Star Formation History of a Highly Magnified z=5.6 Lyman Alpha Source ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Richard Ellis California Institute of Technology CoI: Prof. Jean-Paul Kneib Observatoire Midi-Pyrenees N CoI: Prof. Konrad Kuijken Kapteyn Astronomical Institute N CoI: Mr. Michael Santos California Institute of Technology N ------------------------------------------------------------------------------------ 5. Abstract We have located a remarkably faint pair of compact images arising from a z=5.6 Lyman Alpha emitting source, magnified 33-fold by the lensing foreground cluster Abell 2218. Keck spectra verify the lensing hypothesis but fail to locate any UV stellar continuum to interesting limits, suggesting the object is a 10^6 M_sun source viewed close to its epoch of formation. We argue the source could be representative of an abundant population of low mass systems forming their first stars at z>5, this example becoming visible only by virtue of the strong gravitational magnification. We seek HST imaging to provide much tighter constraints on the nature and distribution of starlight in this intriguing source. ACS will be used to investigate the spatial extent of UV continuum light on <100 pc scales also providing the equivalent width of the Lyman Alpha emission. NIC will be used to measure the slope of the rest- frame continuum in order to break age and mass degeneracies caused by the unknown amount of dust extinction. HST uniquely provides the resolution and sensitivity to gather detailed information on a remarkable source which may be representative of a population seen in future surveys with NGST. ------------------------------------------------------------------------------------ 9452( 3) - 04/25/02 14:46 - [ 2] Observations Description ------------------------ ACS/WFC: The ACS/WFC observations must detect the expected UV continuum at a point closest in wavelength to the Lyman Alpha emission (800 nm) and also provide the best possible opportunity for resolving the established starlight from the line emitting regions. By dithering exposures we can expect to achieve nearly diffraction-limited resolution of our target in the z' band (ACS handbook Ch. 8). Although we need only a small field of view, because ACS provides a filter (F850LP) well matched to the peak of the theoretical stellar SED, with a throughput higher than that of ACS/HRC (and without PSF halos from scattering within the detector), we consider it the better choice. We calculated our requirements using the STARBURST99 code of Leitherer et al. (1999) which generates output spectra for a variety of different star formation histories and physical assumptions. For our baseline hypothesis of no extinction and a star- formation rate of 1 M_sun yr^-1 for 1 Myr, our proposed ACS/WFC observations would achieve a S/N of about 10 for a relatively compact source, and give us good sensitivity to extended emission (c.f. Fig. 3). We assume 2000 sec of science exposure time per orbit, which is a slightly conservative estimate given the visibility of the source (3420 sec) and normal overheads, including reduced exposure time from our LOW sky background request. Figure 3 shows a 5- orbit simulated ACS observation of a model disk which has formed stars according to our fiducial model (1 M_sun yr^-1 for 1 Myr), with a half-light size of 500 pc and inclination of 40 degrees. The disk is off-set from the compact Lyman Alpha emission by 300 pc. This figure clearly demonstrates that our observations are capable of measuring the spatial extent and offset of a stellar disk. NICMOS:The NICMOS observations are motivated by the need to detect the slope in the spectral energy distribution at F110W and F160W, corresponding to rest-wavelengths of 1600 and 2400 Angstrom respectively. The lensed images have not been detected to J=22.5 or K=21.5 (5 sigma) but this is hardly surprising according to our hypothesis. For a 1 Myr population we expect J~H~25, which is beyond the capability of ground-based telescopes with high OH backgrounds even in the very best conditions. For a young population, the slope beta of the stellar continuum (Meurer, Heckman, & Calzetti 1999) is a good probe of the presence of dust and inter alia the stellar age and its relationship to the snapshot star formation rate offered by mboxLyman Alpha. With 4 NIC3 dithered orbits in each of F110W and F160W, a point source of 1 Myr age would be detected at 9 sigma and 5 sigma, respectively. These projections assume the NCS system cools NICMOS to 78 K and the dark current bump is reproduced. If the dark current bump is found to be spurious, our observations will be somewhat more sensitive. We estimate the additional background from the cD envelope to be marginal for image a and negligible for b (Figure 1). Our NIC exposure times are calculated to ensure a good measurement of beta over the range of values observed in U-dropout galaxies at =2.75, using the determinations of beta and the extinction-beta relationship from Meurer, Heckman, & Calzetti (1999). Real Time Justification ----------------------- ACS/LOW Abell 2218 lies within the CVZ. We need sensitivity to faint, extended emission against the sky background (our dominant source of noise), and ETC calculations show that LOW sky observations are a better use of ACS/WFC orbits than taking advantage of the CVZ opportunity. NIC3/CVZ If the ``dark bump'' is a real effect, our NIC3 observations will be dark limited. We request CVZ scheduling to maximize the efficiency of our NIC orbits. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9452( 3) - 04/25/02 14:46 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 ABELL2218- GALAXY, PROTOGALAXY, RA=16H 35M 52.70S +/- 1", J2000 Z = 5.6 F(8500)=2e-20 GAL- HIGH REDSHIFT GALAXY, DEC=66D 12' 44.0" +/- 1", PLATE- J = 25 163553+66124 STAR FORMING REGION, ID=034O H = 25 4 CLUSTER OF GALAXIES, Coordinate Source: HST_IMAGE Comments: The source is undetected longward of 800 nm. The fluxes above are theoretical estimates (see Description of Observation for more details and Exposure Time Calculator results). 2 ABELL2218- GALAXY, PROTOGALAXY, RA=16H 35M 51.80S +/- 1", J2000 Z = 5.6 F(8500)=2e-20 GAL- HIGH REDSHIFT GALAXY, DEC=66D 12' 48.7" +/- 1", PLATE- J = 25 163552+66124 STAR FORMING REGION, ID=034O H = 25 9 CLUSTER OF GALAXIES, Coordinate Source: HST_IMAGE Comments: The source is undetected longward of 800 nm. The fluxes above are theoretical estimates (see Description of Observation for more details and Exposure Time Calculator results). 9452( 3) - 04/25/02 14:46 - [ 4] Visit: 01 Visit Priority: Visit Requirements: ORIENT 40D TO 60D ORIENT 140D TO 160D ORIENT 90D TO 120D On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 ABELL2218-G ACS/WF ACCUM WFC2 F850LP GAIN=1 1 36.5 M LOW-SKY AL-163552+6 C 61249 ------------------------------------------------------------------------------------------------------------------------------------ 20 ABELL2218-G ACS/WF ACCUM WFC2 F850LP GAIN=1 1 38 M LOW-SKY POS TARG -0.0496, AL-163552+6 C -0.12805 61249 ------------------------------------------------------------------------------------------------------------------------------------ 30 ABELL2218-G ACS/WF ACCUM WFC2 F850LP GAIN=1 1 38 M LOW-SKY POS TARG 0.1984, AL-163552+6 C -0.0345 61249 ------------------------------------------------------------------------------------------------------------------------------------ 40 ABELL2218-G ACS/WF ACCUM WFC2 F850LP GAIN=1 1 38 M LOW-SKY POS TARG 0.0744, AL-163552+6 C 0.1051 61249 ------------------------------------------------------------------------------------------------------------------------------------ 50 ABELL2218-G ACS/WF ACCUM WFC2 F850LP GAIN=1 1 38 M LOW-SKY POS TARG -0.1736, AL-163552+6 C 0.01155 61249 ------------------------------------------------------------------------------------------------------------------------------------ 9452( 3) - 04/25/02 14:46 - [ 5] Visit: 02 Visit Priority: Visit Requirements: CVZ On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 ABELL2218-G NIC3 MULTIAC NIC3 F110W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 20 ABELL2218-G NIC3 MULTIAC NIC3 F110W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG 1.1333, 0.4 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 30 ABELL2218-G NIC3 MULTIAC NIC3 F110W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG 1.5333, 1.5333 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 40 ABELL2218-G NIC3 MULTIAC NIC3 F110W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG 0.4, 1.1333 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 50 ABELL2218-G NIC3 MULTIAC NIC3 F110W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG -0.7333, 0.7333 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 60 ABELL2218-G NIC3 MULTIAC NIC3 F110W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG -1.1333,-0.4 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 70 ABELL2218-G NIC3 MULTIAC NIC3 F110W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG -1.5333,-1.5333 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 80 ABELL2218-G NIC3 MULTIAC NIC3 F110W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG -0.4,-1.1333 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 90 ABELL2218-G NIC3 MULTIAC NIC3 F110W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG 0.7333,-0.7333 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 100 ABELL2218-G NIC3 MULTIAC NIC3 F110W NSAMP=25,SAMP-SEQ=SPARS64 1 DEF AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 9452( 3) - 04/25/02 14:46 - [ 6] Visit: 03 Visit Priority: Visit Requirements: CVZ On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 ABELL2218-G NIC3 MULTIAC NIC3 F160W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 20 ABELL2218-G NIC3 MULTIAC NIC3 F160W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG 1.1333, 0.4 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 30 ABELL2218-G NIC3 MULTIAC NIC3 F160W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG 1.5333, 1.5333 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 40 ABELL2218-G NIC3 MULTIAC NIC3 F160W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG 0.4, 1.1333 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 50 ABELL2218-G NIC3 MULTIAC NIC3 F160W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG -0.7333, 0.7333 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 60 ABELL2218-G NIC3 MULTIAC NIC3 F160W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG -1.1333,-0.4 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 70 ABELL2218-G NIC3 MULTIAC NIC3 F160W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG -1.5333,-1.5333 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 80 ABELL2218-G NIC3 MULTIAC NIC3 F160W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG -0.4,-1.1333 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 90 ABELL2218-G NIC3 MULTIAC NIC3 F160W NSAMP=11,SAMP-SEQ=SPARS256 1 DEF POS TARG 0.7333,-0.7333 AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 100 ABELL2218-G NIC3 MULTIAC NIC3 F160W NSAMP=25,SAMP-SEQ=SPARS64 1 DEF AL-163553+6 CUM 61244 ------------------------------------------------------------------------------------------------------------------------------------ 9452( 3) - 04/25/02 14:46 - [ 7] Summary Form for Proposal 9452 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC2 NIC3 ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC NIC3 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM MULTIACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters GAIN=1 SAMP-SEQ=SPARS256 NSAMP=11 SAMP-SEQ=SPARS64 NSAMP=25 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 40D TO 60D ORIENT 140D TO 160D ORIENT 90D TO 120D LOW-SKY LOW-SKY POS TARG -0.0496,-0.12805 LOW-SKY POS TARG 0.1984,-0.0345 LOW-SKY POS TARG 0.0744, 0.1051 LOW-SKY POS TARG -0.1736, 0.01155 CVZ POS TARG 1.1333, 0.4 POS TARG 1.5333, 1.5333 POS TARG 0.4, 1.1333 POS TARG -0.7333, 0.7333 POS TARG -1.1333,-0.4 POS TARG -1.5333,-1.5333 POS TARG -0.4,-1.1333 POS TARG 0.7333,-0.7333 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F850LP F110W F160W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ABELL2218-GAL-163552+661249 ABELL2218-GAL-163553+661244 ------------------------------------------------------------------------------------------------------------------------------------