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NOAO Newsletter - NOAO Highlights - March 2000 - Number 61


A Shear Way to Find Dark Matter --- and Transients, Too!

Based on a Solicited Contribution from Tony Tyson and Ian Dell'Antonio

Tony Tyson, David Wittman (Lucent), and Ian Dell'Antonio (NOAO) are leading a team of some twenty CoIs worldwide to map the large-scale structure of the mass distribution beyond the local universe, using the Mosaic CCD imagers at the Blanco and Mayall telescopes to conduct an ultra-deep, multi-band optical survey. This "Deep Lens Survey" will run five years to completion and comprises deep multicolor imaging in four bands (B,V,R,z') over seven 2 fields. The shear of distant galaxies induced by the mass of foreground structures will be measured. These weak-lensing observations are sensitive to all forms of clumped mass and will yield unbiased mass maps with resolution of 1 in the plane of the sky (~120 h-1 kpc at z=0.2), in multiple redshift ranges. These maps will measure for the first time the change in large-scale structure from z=1 to the present epoch and test the current theories of structure formation, which predict that mass in the low-redshift universe has a particular filamentary/sheetlike structure. These observations will constrain the clustering properties of matter, most notably matter and , and when compared with the results from microwave background anisotropy missions, will test the basic theory of structure formation via gravitational instability. The deep combined data and catalogs for sub-fields will be released to the community as they are completed.

Although it will take some time before the survey can complete its statistical measures of the mass power spectrum, immediate returns may be provided by transient sources identified in the course of the observations. The survey group is sequencing the imaging to detect sources that vary over time scales from hours to months. This is done by spreading observations of individual subfields over four runs and two years. Optical transient events and supernova candidates will be released in real time (see the NOAO Web Page "Science Programs," "Deep Lens Survey Transient Events"). Moving-object lists (asteroids, KBOs, comets) will be listed separately as they are found.

Fast identification of transient events may allow for spectroscopic follow-up by the community while the events are still bright enough to be captured. March and April dark runs at CTIO will produce transient event listings in the 10h and 14h equatorial fields (see below); KPNO dark runs in November and December will target the two northern fields. Transients shorter than a day or so may be very interesting --- the survey should provide the first glimpse of transient phenomena at the faint 20-25 mag attainable with this 4-meter survey. The results for the on-line transient search during the Nov/Dec KPNO runs may be seen by following the "Deep Lens Survey Transient Source Detections" link from http://www.noao.edu/sciprogs. The individual exposures are 600-900s each, depending on the filter taken in blocks consisting of five pointings dithered by up to 8 to shift all low-surface brightness features off themselves (to provide adequate sky-flatfielding) and cover 42' x 42' in one "subfield" (9 subfields = one 2 x 2 field). This dither is repeated (with about a one arcminute offset) in each filter --- and then off to another subfield within one of two fields. In the first two survey runs, the survey team observed five subfields in the 0053+12 field and four subfields in 0919+30 to roughly half of the final depth in B,R and z. The survey observations go deepest in R (to a surface brightness limit of 29 mag/sq.arcsec) and concentrate on R in good seeing to obtain the best lensing signal possible. The other bands are imaged less deeply (approximately half of the exposure time of R) and mainly provide photometric redshift estimates. In Figure 1, a 5' x 5' section of one of the fields (imaged at half depth) is reproduced, showing the level of detail available in the data.


Caption: A sample 5' x 5' section from the Deep Lens Survey imaging data.

The survey team will integrate deeply in the same subfields on successive runs. Most runs will concentrate on two fields, and the runs come in pairs separated by one month (for SN finding). Two of the seven fields are chosen to lie within the existing NOAO Deep Wide-Field Survey fields to save as much telescope time as possible. Below is a listing of the DLS fields. Star density, extinction, two-field accessibility, and existing redshifts were factors in their choice. Much more information about the survey is available at http://dls.bell-labs.com.

                                      DLS Fields

                      RA(J2000)      DEC         l,b        E(B-V)

North:               00 53 25.3   +12 33 55    125,-50       0.06
                     09 19 32.4   +30 00 00    197, 44       0.02

South:                 05 20 00   -49 00 00    255,-35       0.02
                       10 52 00   -05 00 00     257,47      0.025
                       13 55 00   -10 00 00     328,49       0.05

NOAO DWFS fields:      02 10 00   -04 30 00    166,-61      <0.04
                       14 32 06   +34 16 48      57,63      0.015


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