Stephane Courteau (UBC), Jeff Willick (Stanford), Michael Strauss, David Schlegel (Princeton), and Marc Postman (STScI) used a suite of telescopes at both CTIO and KPNO to measure distances to a sample of spiral galaxies contained within a spherical shell of radius 6000 km/s. The project, called Shellflow, was designed to make a precise measure of the galaxy bulk flow at that radius. The preliminary flow measured is only 80 ± 150 km/s, suggesting that most of the mass responsible for the motion of the LG lies within 6000 km/s, a result consistent with the work discussed in the previous article.
Cosmic velocity fields are generated by large-scale distribution of matter in the universe. In standard Cold Dark Matter cosmologies the volume of space bounded by our closest nearby superclusters (the Great Attractor, Pisces-Perseus, Coma) is expected to define an inertial frame at rest with respect to the CMB. The distribution of the matter within this volume should explain the ~600 km/s motion of the Local Group in the CMB frame. Early detection of a large amplitude bulk flow ( > 600 km/s on scales exceeding 6000 km/s) in 1994 by Lauer and Postman, followed by recent similar measurements (in amplitude) by Willick (LP10K) and the SMAC team (Durham collaboration) not only challenged the notion that the bulk flow on large scales is small, but would also push cosmological models to the breaking point if they were indicative of true mass flows.
Even on smaller scales, until recently, there were still contradictory claims for the observed bulk flow within a sphere of 6000 km/s. Various studies would conclude that the motion of the Local Group either is due, or is not due, to the gravitational pull of material within 6000 km/s. With the exception of Lauer and Postman, no other single survey had used a systematic, uniform calibration. This technique removes all uncertainties associated with matching heterogeneous data sets, which could mimic the signature of a bulk flow.
The Shellflow survey of nearly 300 late-type spiral galaxies with H rotation curves and I-band luminosity profiles uses the Tully-Fisher relation to estimate distances. The survey extends from 4000 km/s to 7500 km/s, with an effective depth of 5360 km/s. Repeat measurements between observing runs and observatories of both photometric and spectroscopic observations has provided total magnitudes and rotational line widths that reproduce to within 0.03 mag and 3 km/s (rms deviations) respectively; this high level of accuracy is required to achieve a significant bulk flow result.
A maximum likelihood analysis that includes seven inverse Tully-Fisher parameters yields a distance indicator scatter of 0.37 mag, with a slope and scatter comparable to previous modern I-band investigations (e.g., Giovanelli et al.). Modelling the Shellflow galaxy motions with a Hubble expansion and a bulk flow, one finds a low bulk motion of 80 ± 150 km/s. This result is preliminary, but strongly suggests that there is no significant bulk flow of the Shellflow sample relative to the CMB. This is also in agreement with other newly-presented results, derived from Surface Brightness Fluctuation techniques (Tonry and Dressler), nearby SNIa (Reiss et.al.), and previous TF analyses by Giovanelli and collaborators (see previous Newsletter article).
Taken together these results suggest that most of the mass responsible for the motion of the LG lies within 6000 km/s. Thanks to many new peculiar velocity surveys, such as Shellflow, the picture of cosmological bulk flows seems to imply "convergence" of the flow field within 6000 km/s (to the CMB dipole value), with a mix of low and high-amplitude bulk flow measurements on larger scales. Our ability to compare and establish a clear picture of bulk flows on larger scales is hampered by the size, poor sampling, and discordant geometries of current surveys. Larger, denser, and carefully-designed samples will be needed to determine the contribution from the largest scales.
Based on a solicited contribution from
Stephane Courteau (UBC)