Cosmic Shear from the STIS parallel Želds R Fosbury, L Collodel, N Pirzkal, W Freudling: ST-ECF T Erben, S White, MPIfA, Garching P Schneider, H. Haemmerle: Universität Bonn S Seitz: Uni-Sternwarte, München B Jain: Johns Hopkins University Abstract The unfiltered CCD imaging mode on the HST imaging spectrograph (STIS) has high sensitivity and a clean, stable point spread function, making it ideal for measurements of Cosmic Shear on a ~1' scale. By exploiting observations made in parallel with other HST instruments, a large number of ~random sky pointings are being accumulated. These allow us to overcome the problems of 'cosmic variance' inherent in small, contiguous field measurements. In this poster we describe the characteristics of the STIS data, the data combination and measurement technique and indicate the precision which can be achieved. The goal of the programme is to measure the shear on a scale of 50" with an accuracy of 10%. Motivation Cosmic Shear measurements can constrain the power spectrum of the cosmic density fluctuations free of assumptions regarding the relative distribution of dark and luminous matter. Measurements on different angular scales can determine both the distribution of matter in the Universe and elucidate the growth of structure in the non-linear regime. Measurements on small angular scales (~ 1') require: 1. Exquisite imaging quality ‹ to give precise ellipticity measurements of small, faint galaxies. 2. Deep images ‹ to detect enough galaxies per Želd to overcome the statistical effects of the intrinsic ellipticities of individual galaxies (assumed to be randomly oriented in space). 3. Many pointings around the sky ‹ to overcome the effects of cosmic variance. Data The HST Space Telescope Imaging Spectrograph (STIS) 50CCD images have a FOV of 51" x 51" with 50mas pixels. The PSF is essentially diffraction-limited. The high sensitivity of the CCD and the very broad (unŽltered) bandpass results in a s/n = 10 limiting V-magnitude of approximately 28 (for a point source with an A0-star SED) in one hour of integration. Imaging observations carried out in parallel with the use of another HST instrument ‹ originally NICMOS and WFPC 2 but currently with the latter only ‹ give a large dataset of pseudo-random sky pointings. A substantial subset of these, with appropriate high Galactic latitude pointings and sufficient total exposure time, are used for the shear measurements. The current analysis has been carried out with public parallel data obtained in 1997/8. We now have a large Cycle 9 GO parallel programme (#8562). Testing Extensive testing of the effects of geometric and PSF distortions on the measurements of ellipticity have been carried out. Both spatial and temporal variations have been investigated. These tests have been carried out using the low Galacic latitude fields which contain a reasonably high stellar density. The results of these investigations will appear in Collodel et al. (2000, in prep.). We conclude that the STIS PSF is well-behaved and stable and the corrections to the measured ellipticities are small (< 1%) compared with the expected signal. Analysis The galaxy catalogues for each high Galactic latitude Želd have been built from both SExtractor and KSB applications. A combination of automatic selection and visual inspection was used to select measureable galaxies. Measurements were made of for each Želd. A maximum likelihood estimate was then made of and its conŽdence limits by randomising the galaxy orientations in each Želd and building distribution functions many times for each of several trial values. Future work Only a small subset of the available STIS parallel imaging has been used in this phase of testing and development. The preliminary result shown in the lower-right plot gives us now a reliable estimate of the number of Želds required to achieve our goal of a 10% measurement on the angular scale of 50". We believe that this is achievable from our Cycle 9 parallel observations combined with the existing Želds. Eventually, the combination of measurements of the shear variance and skewness (3rd moment) can break the degeneracy between the effects of s8 and Wo when examining the predictions of various cosmological models. Acknowledgment We thank the staff of STScI for their help in developing the parallel observing capabilities of HST after SM2 and making this programme possible. Background picture: one of the deepest of the STIS public parallel images Data characterisation Depth, number counts and galaxy sizes The parallel data reach an AB (approximately V-band) magnitude of 28. After selection from the object catalogues, the number of useable galaxies per Želd is shown in the histogram below. Galaxy sizes as a function of magnitude are shown on the right. Number of Želds The initial analysis presented here uses only those data obtained with CR-split=2. There are 1303 such individual, unbinned 50CCD STIS images from which 497 co-added associations were constructed. Of these, approximately 100 have been selected for testing of the analysis procedures. Tracking precision There are two effects which can damage the symmetry of the ~ diffraction limited PSF delivered by the instrument: 1. Differential velocity aberration due to the small angular offset between the primary and the parallel instrument in the HST focal plane. This can be as large as 5mas in a 400s exposure. 2. Random small displacements of the telescope optical axis or 'jitter'. This can be reconstructed from the records of the HST Fine Guidance Sensors and has a typical rms of 3­4mas. We, conservatively, excluded data with a recorded jitter rms > 5mas (ie, 1/10 of a STIS pixel). Reduction Jitter data Used to select images and define the precise pointing and so to make 'associations' ready for co-addition. Cosmic rays and hot pixels Removed using the CR-split pairs and concurrent calibration Žles. Bad-pixel masks used for other features. Co-addition An iterative cross-correlation and 'drizzle' technique was developed which uses the known STIS geometric distortion. The process was terminated when the subsequent shifts became smaller than 0.05 pizels (~2.5mas) For the 110 selected high and low Galactic latitude associations, this shows the distribution of total exposure times. For the selected high Galactic latitude Želds, this shows the distribution of the number of galaxies per Želd whose ellipticity measurements were used for the Žrst shear estimate.For the selected high Galactic latitude Želds, this shows the distribution of the number of galaxies per Želd whose ellipticity measurements were used for the Žrst shear estimate. Results A summary of results expressed in terms of the parameter ‹ an unbiassed estimator of the squared shear in the Želd ‹ and angular scale on the sky. The current state of groundbased measurements (from: http://www.iap.fr/LaboEtActivites/ThemesRecherche/Lentilles/arcs/cosmicshearstatus.html) is shown as the shaded area representing approximately ± 1s. Our initial estimate is shown in black with the 67% conŽdence limits from the likelihood analysis. Several cosmological model predictions are shown on the diagram: red: tCDM (W = 1.0 ; L = 0.0 : G = 0.21 ; s8 = 1.0) pink: CDM (W = 0.3 ; L = 0.7 : G = 0.5 ; cl. norm.) green: LCDM (W = 0.3 ; L = 0.7 : G = 0.21 ; s8 = 1.0) orange: tCDM (W = 1.0 ; L = 0.0 : G = 0.21 ; s8 = 0.6) blue: OCDM (W = 0.3; L = 0.0 : G = 0.21; s8 = 0.85)