7201( 7) - 08/27/97 18:13 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7201 Version: 7 Check-in Date: 27-Aug-1997 18:12:26 1.Proposal Title: Gravitational Imaging in Well-Constrained Clusters: Absolute Masses and Faint Galaxy Redshifts - Cycle 5 DD Observations ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Galaxies & Clusters GO/DD 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Ian Smail UNIVERSITY OF DURHAM CoI: Richard Ellis Institute for Astronomy N CoI: Jean-Paul Kneib Institute for Astronomy N ------------------------------------------------------------------------------------ 6. Abstract The shear of faint galaxy images arising from gravitational lensing by foreground massive clusters provides important constraints on the distribution and total amount of mass in the clusters. Furthermore, in a few rare clusters, the lensing models may be sufficiently well-constrained by arcs and multiple images to yield robust redshift estimates for faint galaxies well beyond reach of current spectrographs. Ground- based telescopes suffer systematic degradations which reduce the detected shear by factors of ~2-3. Detecting the unadulterated shear pattern with HST will yield an absolute calibration of ground-based results and confirm or otherwise the high cluster mass/light ratios obtained so far. We propose deep WFPC-2 imaging of two of the most well-studied clusters for this purpose. Both have the rare distinction of having lensed features of known redshift which strongly constrain the absolute mass. When calibrated with the HST shear, not only will more reliable mass maps result, but the lensing equations can be inverted to provide statistical distances for large numbers of very faint field galaxies. These will be invaluable in studies of galaxy evolution and for subsequent lensing applications in other clusters. ------------------------------------------------------------------------------------ 7201( 7) - 08/27/97 18:13 - [ 2] Observations Description ------------------------ NOTE: Most of the description below is from that of the original 24-orbit version of the program which was severely affected by scattered background light. This DD program now employs the special LOW SKY requirement, and is only a pilot program for 6 orbits in the center of the original field. Most of the text still describes the basic ideas and requirements behind the project, and thus has been left in the file in order to more fully explain what is being done. Bits of the text below have been updated. This note is just intended to alert the reader that some of the text below may have been changed, while some of it is still in its original state. To fully exploit HST's unique capabilities for our proposed lensing study we require continous coverage of a sizeable area in the centre of AC114. The area coverage must be contiguous and cover a field sufficient to adequately measure the morphology of the cluster mass to allow a robust inversion of the distribution in the redshift analysis. Moreover we require enough arclets in the outer regions to overcome noise in the weak shear pattern arising from the intrinsic orientations and shapes of faint galaxies. To accomplish this we have chosen four overlapping pointings. providing nearly complete coverage to a radius of 1.0-1.5 Mpc (H_o=50), and allowing for overlap, a ~20 arcmin^2 field. From ground-based images in sub- arcsecond seeing the induced shear at the 1.0 Mpc radius is 10--15% which determines the accuracy of the final mass reconstruction is the surface density of background galaxies. We propose to sample the mass map via a grid of 20 cells with 0.5 arcmin spacing. To get a signal/noise>3, we require an rms noise in the shear per cell of better than 3% an unfeasibly high surface density of background galaxies. However, the elimination of atmospheric seeing increases our shear sensitivity so that only 20--30 galaxies are required per cell using HST, corresponding to a surface density of ~50 galaxies arcmin^-2. We choose the F702W filter as it is the most efficient filter to reach a high surface density of background sources in the shortest time. Additionally, a long wavelength passband is the most appropriate for deep redshift surveys (c.f.\\ Lilly 1993). A galaxy surface density of 50 arcmin^-2 is reached at a limiting magnitude of R_702=25.5. A deep limiting isophote is essential to ensure each faint image is sufficiently resolved for its shear to be reliably measured. We calculate that ~30 pixels above the 2 Sigma isophote in an R=25.5 galaxy will yield an orientation and ellipticity to the precision required from the considerations discussed above. Scaling from the deepest MDS blank field observations (available to RSE), this limiting isophote is found to be Mu_R = 30.2 mag pixel^-2. For a 2 Sigma detection of 30 pixels at this isophote, we calculate an exposure time of 17 ksec ( *now 6 DD orbits). With four pointings (* this DD=1 ) in the cluster we are thus requesting a total of 24 (*now 6 DD ) orbits. We will additionally employ offsetting of each exposure by a small shift ( +/-1.0 arcsec in x and y) to further reduce effects of flatfield errors. These deep exposures will provide high signal/noise mass maps in this cluster to ~1.5 Mpc with a resolution of 150-200 kpc in the outskirts and ~50 kpc in the core regions. The frames will also provide a sample of around 1000 field galaxies in the cluster for the redshift inversion once all the fields are done from the original 24-orbit program. Again, as noted above, there are only 6 orbits in this DD program. Real Time Justification ----------------------- None. Calibration Justification ------------------------- None. Extensive ground-based imaging (UBVRIK) and spectroscopic data plus satellite X-ray images for the target have already been taken and analysed. Additional Comments ------------------- ------------------------------------------------------------------------------------ 7201( 7) - 08/27/97 18:13 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_address Ship Via: UPS Email: ------------------------------------------------------------------------------------ 7201( 7) - 08/27/97 18:13 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 AC114-POS5 CLUSTER OF GALAXIES, RA=22H 56M 05.44S +/- 0.1S, DEC= 1950.0 Z=0.31 R = 25.5 GRAVITATIONAL LENS, -35D 04' 14.2" +/- 2" V-R = 1.5 RICH CLUSTER Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 1950.0 0.0 0.0 0.0 7201( 7) - 08/27/97 18:13 - [ 5] Visit: 01 Visit Requirements: ORIENT 0D TO 40D On Hold Comments: Additional Comments: According to the convention for WFPC2 coordinates as given in the PhII instructions, Fixed Target List positions are shifted by POS TARG +10.0,+10.0 to bring the field center to the apex of the pyramid. Exposures for the given field are split into three groups with a shift introduced of +1.0 arcseconds in both X and Y for the second group and of -1.0 arcseconds in both X and Y for the third in order to displace objects situated on CCD defects or on seams of the pyramid. Thus, POS TARG +11.0,+11.0 corresponds to the second group, and POS TARG +9.0,+9.0 corresponds to the third. CR- SPLIT=NO is used here for all observations since in all cases there are at least two exposures in order to perform cosmic ray rejection. ORIENT range is 20 +20/-20 DEG due to required field coverage. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 AC114-POS5 WFPC2 IMAGE WFALL-F F702W CR-SPLIT=NO 1 2300 S EXPAND POS TARG +10.0, IX +10.0 LOW-SKY Comments: POS TARG +10,+10 PLACES TARGET COORDS AT APEX OF PYRAMID ------------------------------------------------------------------------------------------------------------------------------------ 2 AC114-POS5 WFPC2 IMAGE WFALL-F F702W CR-SPLIT=NO 1 2300 S EXPAND POS TARG +10.0, IX +10.0 LOW-SKY Comments: POS TARG +10,+10 PLACES TARGET COORDS AT APEX OF PYRAMID ------------------------------------------------------------------------------------------------------------------------------------ 3 AC114-POS5 WFPC2 IMAGE WFALL-F F702W CR-SPLIT=NO 1 2300 S EXPAND POS TARG +11.0, IX +11.0 LOW-SKY Comments: SHIFTED BY +1.0,+1.0 FROM 1 AND 2 TO ALLOW REMOVAL OF CHIP DEFECTS ------------------------------------------------------------------------------------------------------------------------------------ 4 AC114-POS5 WFPC2 IMAGE WFALL-F F702W CR-SPLIT=NO 1 2300 S EXPAND POS TARG +11.0, IX +11.0 LOW-SKY Comments: SHIFTED BY +1.0,+1.0 FROM 1 AND 2 TO ALLOW REMOVAL OF CHIP DEFECTS ------------------------------------------------------------------------------------------------------------------------------------ 5 AC114-POS5 WFPC2 IMAGE WFALL-F F702W CR-SPLIT=NO 1 2300 S EXPAND POS TARG +9.0,+9.0 IX LOW-SKY Comments: SHIFTED BY -1.0,-1.0 FROM 1 AND 2 TO ALLOW REMOVAL OF CHIP DEFECTS ------------------------------------------------------------------------------------------------------------------------------------ 6 AC114-POS5 WFPC2 IMAGE WFALL-F F702W CR-SPLIT=NO 1 2300 S EXPAND POS TARG +9.0,+9.0 IX LOW-SKY Comments: SHIFTED BY -1.0,-1.0 FROM 1 AND 2 TO ALLOW REMOVAL OF CHIP DEFECTS ------------------------------------------------------------------------------------------------------------------------------------ 7201( 7) - 08/27/97 18:13 - [ 6] Summary Form for Proposal 7201 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO/DD ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Galaxies & Clusters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 0D TO 40D EXPAND POS TARG +10.0,+10.0 LOW-SKY EXPAND POS TARG +11.0,+11.0 LOW-SKY EXPAND POS TARG +9.0,+9.0 LOW-SKY ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F702W ------------------------------------------------------------------------------------------------------------------------------------ Target Names AC114-POS5 ------------------------------------------------------------------------------------------------------------------------------------