9984( 1) - 04/16/03 17:26 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9984 Version: 1 Check-in Date: 16-Apr-2003 17:24:53 1.Proposal Title: Cosmic Shear With ACS Pure Parallels ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO/PAR 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Jason Rhodes Goddard Space Flight Center CoI: Dr. Alexandre Refregier Institute for Astronomy, Cambridge N University CoI: Dr. Jonathan P. Gardner Goddard Space Flight Center N CoI: Mr. Nicholas Collins Goddard Space Flight Center N ------------------------------------------------------------------------------------ 5. Abstract Small distortions in the shapes of background galaxies by foreground mass provide a powerful method of directly measuring the amount and distribution of dark matter. Several groups have recently detected this weak lensing by large-scale structure, also called cosmic shear. The high resolution and sensitivity of HST/ACS provide a unique opportunity to measure cosmic shear accurately on small scales. Using 260 parallel orbits in Sloan textiti (F775W) we will measure for the first time: beginlisto\setlength sep0cm setlengthemsep0cm setlength\topsep0cm em the cosmic shear variance on scales <0.7 arcmin, em the skewness of the shear distribution, and em the magnification effect. endlist Our measurements will determine the amplitude of the mass power spectrum sigma_8Omega_m^0.5, with signal-to-noise (s/n) ~ 20, and the mass density Omega_m with s/n=4. They will be done at small angular scales where non-linear effects dominate the power spectrum, providing a test of the gravitational instability paradigm for structure formation. Measurements on these scales are not possible from the ground, because of the systematic effects induced by PSF smearing from seeing. Having many independent lines of sight reduces the uncertainty due to cosmic variance, making parallel observations ideal. ------------------------------------------------------------------------------------ 9984( 1) - 04/16/03 17:26 - [ 2] Observations Description ------------------------ Filter choice. We have opted to use the F775W (Sloan textiti) filter in order to stay as close as possible to the default ACS parallel program. In doing so, we anticipate that other groups will find this filter choice useful since images will be available in other Sloan filters (as time permits) for visits longer than one orbit. We further anticipate that the Sloan filter set will become the filter set of choice, particularly for surveys with the ACS. Survey images taken as parts of other proposals will be useful for the study of cosmic shear. While it is necessary to have many widely separated fields for the study we outline, we will be able to include large contiguous areas taken by other observers in our study, albeit at lower s/n because of cosmic variance. If the default ACS parallel program for visits longer than one orbit is not followed or if significant amounts of survey data are not taken using the F775W filter, it would be advantageous for us to use the F814W filter in place of the F775W. This would be particularly true if other proposals obtain deep images of field galaxies using the F814W filter. Because it is a wider filter, the F814W filter allows us to probe deeper than the F775W. For one orbit exposures, there are about 20\ filter than in the F775W filter (Gardner 1998). Since the s/n for our measurement of cosmic shear scales linearly as the number of galaxies detected, our measurements could be completed with approximately 20\ the F814W filter. Calibration images. Although the geometric distortion in ACS is quite large (see the left hand panel in Figure reffig:acs), this effect is stable and easily corrected (e.g. RRGI). Astrometric shifts in stellar fields will be used to calculate the geometric distortion in ACS early in the instrument's lifetime (SMOV calibration proposal GO-9028). The ellipticities induced by the PSF are small (see the right hand panel of Figure reffig:acs). However, due to the small size of the galaxies we will be studying, a correction for the PSF, which is expected to vary slightly with time, is necessary. M4 globular cluster images will be used for calibration of the PSF as we have done previously for WFPC2 and STIS. We propose using the same fields we used for the WFPC2 calibrations, which contain roughly 1500-2000 stars with magnitudes 16<=sssim I814W <=sssim 25 (Ibata et al.\ 1999). To achieve the s/n level found in the WFPC2 images (approximately 7 for the faintest stars at I814 ~ 25), an exposure time of 350s is needed. We have found this s/n level represents the best compromise between not saturating bright stars and exposing enough faint stars to examine PSF variations over the WFC chip. We propose taking four 360s exposures (to avoid the buffer dump penalty for ACS exposures less than 6 minutes long). Each exposure should be dithered approximately 1 arcminute from the neighboring one so that the resulting dither pattern is a square. This increases the stellar coverage over the WFC chips. The total time for these exposures including guide star acquisition, exposure overhead, and telescope offsets corresponds to roughly 48 minutes per orbit. One additional 360s exposure can be taken if orbital visibility permits. These observations should be repeated three times during the course of the cycle as described below in the Special Requirements section. textitImages in which the ACS is the primary instrument are needed for this because of the differential velocity aberration present in parallel images (Pirzkal et al.\ 2001). These images will provide a fiducial set of PSF measurements that we can not obtain by using stellar fields taken as part of a parallel observing proposal. Another globular cluster with similar numbers and magnitudes of stars may be substituted for M4 if this facilitates scheduling the observations. Observing strategy. For the parallel 9984( 1) - 04/16/03 17:26 - [ 3] observations we propose to use a slight variation of the ACS default pure parallel program observing strategy. For single orbit opportunities, 4 images will be taken with the F775W filter, for a total of 1800s to 2400s exposure time. Our experience with archival STIS images indicates that 400s to 600s exposures facilitate cosmic ray removal and image combination while minimizing differential velocity aberration found in parallel images without significantly compromising the signal to noise level of the combined image. The differential velocity aberration, in the worst case scenario (when STIS is primary), can be up to 0.1 ACS pixels in a 400s exposure and up to a few tenths of an ACS pixel for an 800s exposure (Pirzkal et al.\ 2001). This velocity aberration introduces a small distortion in object shapes but is not expected to be the leading source of error in our measurements. Because of data volume constraints, it is not possible to download more than 4 ACS parallel images per orbit. If WFPC2 is the primary instrument, the bandwidth available for download of ACS parallel data is further reduced and in that case 3 exposures per orbit is acceptable, as the smaller distance between WFPC2 and ACS reduces the velocity aberration. Exposure times will be slightly adjusted to make maximal use of the available observing time in a given orbit. We recognize the scientific gains of having multiple filters and therefore do not propose to change the default parallel program for visits longer than 1 orbit, which calls for taking images in the F850LP, F475W, and F625W (Sloan textitz, g and textitr) filters. textbfImage Reduction and Analysis Our group is well positioned to combine the parallel images and catalog the galaxies within the images. One of us (NC) has been responsible for the reduction and cataloging of all STIS parallel data for the STIS team at GSFC. The software used for this, written in IDL, automatically combines images of the same field (from single or multiple visits). The parameters of the combined images are made part of an IDL database that can be sorted on various attributes including exposure time, coordinates, number of subimages, proposal ID, and filter (Gardner et al. 1998). SExtractor (Bertin & Arnouts 1996) is then run on the images to create a database of objects in each image. We are using these images and catalogs to study cosmic shear in STIS parallel images (Rhodes et al., in preparation, 2001). This software is available to us and can be made to work with the ACS images with minimal effort. We will make the combined images and galaxy catalogs available to the community no later than 6 months after the images are taken. Based on our experience with STIS parallels, this time represents the maximum length of time between different visits that are allowed to go into the same combined image because of the changing noise characteristics and hot pixels of the CCD. The images and catalogs will be valuable for a variety of studies involving galaxy morphology and evolution, particularly if visits of more than one orbit produce images in multiple filters as outlined in the ACS default parallel program. Real Time Justification ----------------------- Because the observation of background galaxies is hampered by foreground stars, we request only parallel opportunities at galactic latitudes |b|>20degrees. As appropriate for a pure parallel proposal, we waive proprietary rights to the data. Calibration Justification ------------------------- 9984( 1) - 04/16/03 17:26 - [ 4] Additional Comments ------------------- ------------------------------------------------------------------------------------ 9984( 1) - 04/16/03 17:26 - [ 5] Visit: 01 Visit Priority: 30 Visit Requirements: On Hold Comments: Additional Comments: 1/2 orbit case Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 ANY ACS/WF ACCUM WFC F775W CR-SPLIT=NO,GAIN=1 1 400 S MAX DUR 150% C ------------------------------------------------------------------------------------------------------------------------------------ 20 ANY ACS/WF ACCUM WFC F775W CR-SPLIT=NO,GAIN=1 1 400 S MAX DUR 150% C ------------------------------------------------------------------------------------------------------------------------------------ 30 ANY ACS/WF ACCUM WFC F775W GAIN=1,CR-SPLIT=NO 1 400 S MAX DUR 150% C ------------------------------------------------------------------------------------------------------------------------------------ 9984( 1) - 04/16/03 17:26 - [ 6] Visit: 02 Visit Priority: 40 Visit Requirements: On Hold Comments: Additional Comments: 1 orbit case Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 ANY ACS/WF ACCUM WFC F775W CR-SPLIT=NO,GAIN=1 1 500 S MIN DUR 90% MAX DUR 125% C ------------------------------------------------------------------------------------------------------------------------------------ 20 ANY ACS/WF ACCUM WFC F775W CR-SPLIT=NO,GAIN=1 1 500 S MIN DUR 90% MAX DUR 125% C ------------------------------------------------------------------------------------------------------------------------------------ 30 ANY ACS/WF ACCUM WFC F775W CR-SPLIT=NO,GAIN=1 1 500 S MIN DUR 90% MAX DUR 125% C ------------------------------------------------------------------------------------------------------------------------------------ 40 ANY ACS/WF ACCUM WFC F775W CR-SPLIT=NO,GAIN=1 1 500 S MIN DUR 90% MAX DUR 125% C ------------------------------------------------------------------------------------------------------------------------------------ 9984( 1) - 04/16/03 17:26 - [ 7] Visit: 03 Visit Priority: 20 Visit Requirements: On Hold Comments: Additional Comments: 700-1200 seconds case Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 ANY ACS/WF ACCUM WFC F775W CR-SPLIT=NO,GAIN=1 1 400 S MIN DUR 88% MAX DUR 150% C ------------------------------------------------------------------------------------------------------------------------------------ 20 ANY ACS/WF ACCUM WFC F775W CR-SPLIT=NO,GAIN=1 1 400 S MIN DUR 88% MAX DUR 150% C ------------------------------------------------------------------------------------------------------------------------------------ 9984( 1) - 04/16/03 17:26 - [ 8] Visit: 04 Visit Priority: 10 Visit Requirements: On Hold Comments: Additional Comments: less than 700 seconds case Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 ANY ACS/WF ACCUM WFC F775W CR-SPLIT=NO,GAIN=1 1 400 S MAX DUR 175% C ------------------------------------------------------------------------------------------------------------------------------------ 9984( 1) - 04/16/03 17:26 - [ 9] Summary Form for Proposal 9984 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO GAIN=1 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO/PAR ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements MAX DUR 150% MIN DUR 90% MAX DUR 125% MIN DUR 88% MAX DUR 150% MAX DUR 175% ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F775W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ANY ------------------------------------------------------------------------------------------------------------------------------------