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NOAO Newsletter - NOAO Highlights! - September 1999 - Number 59


The Convergence Depth of the Local Universe

Riccardo Giovanelli, Martha Haynes, and Daniel Dale (Cornell), working with a large team of collaborators from other institutions, have used the CTIO 0.9-m, Blanco, and KPNO 0.9-m telescopes to search for the scale on which the motion of the Local Group originates. The Local group and its neighbors all appear to be moving with respect to the cosmic microwave background radiation (CMB); searching for the convergence length of this flow has been a critical problem in cosmology. Giovanelli and collaborators argue that the flow decreases steadily as larger volumes of the universe are sampled, finding that it ends or "converges" at distances corresponding to an expansion velocity of only 4000 km/s or so.

It is believed that the motion of our galaxy induces the strong CMB dipole. The implied "reflex motion" of the Local Group with respect to the CMB reference frame has amplitude 611 22 km/s, directed toward l = 273 3, b = 27 3. This motion is thought to be generated by the net gravitational influence of fluctuations in the matter density field surrounding us. As one looks out to larger and larger distances, including increasingly larger volumes of the universe, on average the fluctuations in enclosed mass should begin to average out, contributing less and less to any net pull on the Local Group. Because various theories of galaxy formation have differing spatial scales of mass distribution, measuring the reflex motion of the LG with respect to progressively more distant shells of material is a key cosmological probe. The distance by which the bulk of the CMB dipole is recovered by the reflex motion is referred to as the "convergence depth."

image Caption: Dipole amplitude (a) and direction (b) of the "reflex peculiar" motion of the Local Group, with respect to several galaxy samples. Filled symbols correspond to the reflex dipole with respect to separate shells extracted from the SFI field galaxy sample; they are labeled 1 through 6 according to increasing distance and for ease of cross-reference between panels (a) and (b); the unfilled triangle identifies the dipole derived from the SCI clusters farther than cz = 3000 km/s, while the unfilled square is the dipole with respect to the SCII cluster sample. The dashed horizontal line in panel (a) identifies the amplitude of the Local Group motion in the CMB reference frame, and the crossed circle in panel (b) the direction of that dipole motion.

In 1994, T. Lauer and M. Postman obtained a disturbing result: using a sample of cluster galaxies of effective depth near 12,000 km/s, they concluded that the whole volume had a bulk flow of about 700 km/s with respect to the CMB reference frame. Several groups were thus strongly motivated to obtain an independent verification of the result. Bulk flows of amplitude comparable to that of Lauer and Postman have more recently been reported by Willick and the SMAC collaboration (M. Hudson, R. Smith, J. Lucey, and colleagues), although the directions of the reported motions disagree widely from the Lauer-Postman result. Large bulk flows over scales in excess of 100 h-1 kpc pose serious difficulties to the otherwise most favored cosmological models and thus stimulate great attention in the field.

Using the Tully-Fisher (TF) relationship between luminosity and rotational width of spiral galaxies, Giovanelli and collaborators have completed three all-sky observational programs aimed at the determination of galaxy distances. The various samples complement each other in a manner that allows useful insights in the dynamics of the local universe. All samples have relied heavily on access to NOAO facilities, namely the KPNO and CTIO 0.9-m telescopes for I-band photometry and the CTIO Blanco Telescope for high dispersion, emission line spectroscopy. The first of such samples is referred to as SCI. It includes 780 galaxies in 24 clusters within cz ~ 9000 km/s. This sample has been used primarily to provide an accurate template for the TF relation and to investigate the impact of various biases on the analysis process. The SCI collaboration included R. Giovanelli, M. Haynes, T. Herter and N. Vogt (Cornell), J. Salzer (Wesleyan), G. Wegner (Dartmouth), and L. da Costa and W. Freudling (ESO). With roughly the same depth as SCI, a field sample of approximately 2000 galaxies referred to as SFI provides dense and homogeneous sampling of the peculiar velocity field and thus reconstruction of the matter density field. This collaboration involved Giovanelli, Haynes, Salzer, Wegner, da Costa, Freudling and P. Chamaraux (Meudon). Finally, a sample approximately twice as deep as the two preceding ones, referred to as SCII, includes distances to 522 galaxies in 52 clusters to 20,000 km/s. It was observed by D. Dale, Giovanelli and Haynes , E. Hardy (NRAO), and L. Campusano (Universidad de Chile). SCII formed the core of Daniel Dale's PhD thesis.

The results of SFI, SCI and SCII, as presented in recent articles in the Astronomical Journal and the Astrophysical Journal, exhibit a relatively quiet velocity field. SCI and SCII measure significantly lower peculiar velocities for clusters than the values obtained by other groups, yielding a 1-d velocity dispersion of about 300 km/s. More graphically dramatic perhaps, the reflex motion of the LG with respect to SFI subsamples in shells of increasing radius exhibits a convergent trend towards the CMB dipole (see Figure): the motion of the LG with respect to a shell of radius greater than 5000 km/s closely matches the CMB dipole. Each shell's thickness in the Figure is 2000 km/s, each filled symbol corresponding to the average of between 275 and 689 galaxies. Consistency between the CMB dipole and that of the SCI clusters (unfilled triangle) between 3000 km/s and 9000 km/s is also seen; reassuringly, the dipole of the more distant SCII sample also matches that of the CMB. The bulk flow of all galaxies within a sphere of 6000 km/s radius, including the LG, is only 200 65 km/s, while that for a sphere of 12,000 km/s is indistinguishable from the measurement error near 200 km/s. The close match not only in amplitude, but also in direction, between the dipole of the peculiar velocity field of spiral galaxies farther than ~ 5000 km/s and that of the CMB provides a strong statement in favor of a relatively modest convergence depth for the local universe. The characteristics of the peculiar velocity field, as well as its comparison with the distribution of luminous matter as evidenced by redshift surveys, imposes important constraints on the value of cosmological parameters, favoring, for example, values of the Hubble constant near 69 km/s Mpc-1 and relatively low values of the density parameter.

Based on a solicited contribution from
Riccardo Giovanelli (Cornell)


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