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NOAO Newsletter - NOAO Highlights - September 2000 - Number 63


Going for the Flow

Based on a Solicited Contribution from Mike Hudson

We are terribly saddened by the tragic accident that claimed the life of Jeff Willick on June 18. Jeff had recently used NOAO facilities as part of the "Shellflow" collaboration, and was the leader of the survey described below. His passion for illuminating the dark corners of the Universe will be sorely missed.

Jeff Willick (Stanford) began a large survey program using NOAO facilities to understand the mass distribution of the Universe on scales of hundreds of megaparsecs. The approach is to use the peculiar velocities of a full-sky sample of galaxy clustersthat is, their detectable deviations from an ideal uniform Hubble expansionas a gravitational signature of the true underlying mass distribution of the Universe. Work of this sort was the central theme of Jeff's research. The "shellflow" survey that Jeff started continues through the team he established, which includes Mike Hudson (Waterloo, present PI); Roger Davies, John Lucey, Stephen Quinney, Steve Moore (Durham); David Schade (CADC/HIA); Russell Smith (Católica de Chile); Nick Suntzeff (NOAO); and Gary Wegner (Dartmouth).

A basic problem of large-scale structure has been understanding the origin of our own drift of 600 km/s with respect to the cosmic microwave back-ground, particularly the spatial scale over which this motion is generated. A number of recent studies seem to indicate that large-scale bulk flows of galaxies are largely generated within 6000 km/s of us (using recession velocity as a proxy for distance), and are not seen on larger scales. At the same time, a few other programs suggest that coherent bulk flows continue to exist at far larger scales. This conflict is difficult to resolve with current data; because the surveys have different sky coverage and depths, there is insufficient overlap of common clusters, and there may be uncorrected systematic effects arising from the wide range of instrumentation used by different groups.

The shellflow survey, which uses observations from both KPNO and CTIO, seeks to measure bulk flows through a deep, homogeneous, all-sky spectroscopic and photometric study of 100 X-ray selected clusters within 200 h-1 Mpc (FP200). The fundamental plane distance indicator will be used to make an independent determination of the large-scale flow of clusters of galaxies with respect to the cosmic microwave background frame. With 4,000 early-type galaxies (a factor of four increase over current fundamental plane surveys), the expected combined random and systematic errors should be < 120 km/s for each component of the bulk flow vector. The long-term time allocation of the NOAO survey program allows use of the same instrumental configuration for all northern and southern runs, thus minimizing a potentially important source of systematic errors.

All calibrated photometric and spectroscopic data will be available via the WWW. In addition to peculiar velocity applications, the resulting photometric and spectroscopic database will be a unique and valuable resource to the community for studies of galaxy morphology, stellar populations, and galaxy evolution in the cluster environment. More information is available at http://astro.uwaterloo.ca/~mjhudson/fp200.

Distribution of shellflow survey clusters.
Caption: The distribution of shellflow survey clusters on the sky in equatorial coordinates. The Galactic Plane (|b| < 15) is indicated by the dashed line. Clusters in the survey are indicated by dots. Clusters with existing data (for 10 galaxies) are indicated by open circles (fundamental plane data) or triangles (Tully-Fisher data).


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