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NOAO Newsletter - NOAO Highlights - June 2000 - Number 62


Cosmic Shear Surely Seen!

Based on a Solicited Contribution from David Wittman

David Wittman, Tony Tyson, and David Kirkman (Lucent Technologies); Ian Dell'Antonio (NOAO); and Gary Bernstein (Michigan) have used the CTIO 4-m telescope to make the first measurement of cosmic shear. Cosmic shear occurs when weak gravitational lensing by large-scale variations in the local matter density causes the ellipticities of background galaxies to be correlated over the sky. The correlation strength as a function of angular scale constrains cosmological parameters such as matter and ; measured accurately enough, it should reveal the mass power spectrum in detail. Attempts to measure this effect have been made since 1967, but only null results were obtained. Now, however, deep imaging over wide fields with cameras of mosaiced CCDs enables careful control of systematic errors.

Wittman et al. used the Big Throughput Camera (BTC), a mosaic of four back-illuminated 2K x 4K CCDs built by Tyson and Bernstein, which was operated as a user instrument at the Blanco 4-m from 1997A through 1999A. They imaged three large (42' square) widely separated "blank" fields (i.e., containing no known mass concentration) in 1997 and 1998. To reduce systematic errors due to point-spread function anisotropy (the dominant source of error), the group convolved the images with position-dependent elliptical kernels to produce exquisitely round PSFs at all positions.

Caption: Autocorrelations for each of the two ellipticity components depend on the redshift of the source galaxies and on the cosmological model. The measured correlations are plotted here with the predictions of three cosmologies for our best estimate of the source redshift distribution: cosmological constant cold dark matter universe (CDM, solid line), standard cold dark matter universe (SCDM, short dash), and open cold dark matter universe (OCDM, dotted line). The long-dash line shows the effect of a 20% error in the mean source redshift for CDM. A cosmological model must match both autocorrelations; SCDM is ruled out at many sigma by <e1e1>. CDM and OCDM match <e1e1>, where e1 = e cos(2), very well and are consistent with <e2e2>, where e2 = e sin(2) at the 3- level. The cross-correlation <e1e2> (not shown) is consistent with zero, as expected in the absence of systematic error. The Deep Lens Survey now in progress will provide a much stricter test of cosmological models, or suggest the need for new models.

The images were stacked and convolved again to eliminate any PSF anisotropy due to slight registration errors. The final images each contained ~150,000 galaxies down to R=26, of which ~45,000 in each field survived a magnitude cut (23<R<26), to eliminate as many foreground galaxies as possible and quality checks on their ellipticity measurements.

Angular correlations of galaxy ellipticity in each field revealed a signature of weak gravitational lensing by large-scale structure. Using the mean and the RMS of the three independent fields, the detection was 4 in one angular separation bin for each of two independent correlation functions (the two functions stem from treating ellipticity as a pseudo-vector with two components e cos(2) and e sin(2), where e is the scalar ellipticity and is the position angle). The measurements agree roughly with a Universe having a cosmological constant and with an open Universe, but rule out a standard cold-dark-matter Universe. Three other groups subsequently submitted cosmic shear papers in agreement with this measurement. The indication of a low matter universe is in agreement with a remarkable array of independent observations, including high-redshift supernovae and the cosmic microwave background. Surveys are now underway to improve the accuracy of cosmic shear measurements. Comparison of improved CMB and cosmic shear measurements will provide stringent tests of the underlying assumptions in cosmology and perhaps suggest new models.

As described in NOAO Newsletter No. 61 ("A Shear Way to Find Dark Matter and Transients Too!"), one such survey is being carried out at NOAO by Tyson, Dell'Antonio, and Wittman. One of the surveys approved in the first year of survey proposals, this project will image seven 2-degree square fields in BVRz' over four years with Mosaic cameras on the CTIO and KPNO 4-m telescopes. With a total exposure time of 18,000 seconds in R and 12,000 seconds in BRz', the group will assign photometric redshifts to source galaxies and study the evolution of structure over time by separating the sources into discrete redshift bins on the order of 0.3 in z.

In addition, in the spirit of making immediate best use of the wide-field survey, the group is trying to maximize the scientific return on the data by searching for transients (supernovae and perhaps new classes of bursters, as well as asteroids and Kuiper Belt objects) as the data come in, and by releasing the data six months after completion of each 40' square subfield. The first observing season is done, with 19 of the total of 86 nights completed. Check http://dls.bell-labs.com/ for progress updates and release schedule. Also, anyone with a desire to follow up on interesting transients should contact the team (whittman@physics.bell-labs.com).


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