Local Large-Scale Structures and Flows

During recent years, new surveys have dramatically improved our understanding of the clusters and superclusters that dominate the local Universe. However much is still not known about the precise interplay of these structures and their effect on local dynamics such as the motion of the Local Group. My PhD research has thus focused on several projects designed to map the local density fields, most notably in the Zone of Avoidance (ZoA), and to examine predicted flow fields.

Probing the local velocity field

To date, the PSCz catalogue offers the most complete redshift survey of the entire sky. With redshifts for some 15,000 IRAS galaxies distributed uniformly over 84% of the sky, this survey provides us with a detailed map of the local density field out to 150 h^-1 Mpc (Branchini et al. 1999 [MNRAS, 308, 1]). In order to scale this IRAS galaxy density field to the true total density field, we have assumed linear biasing and have determined the corresponding redshift distortion parameter Beta. This was achieved by comparing the predicted IRAS peculiar velocity field with probes of the true local velocity field. With distance errors of less than 8%, type Ia supernovae make excellent standard candles. Using a sample of 98 such supernovae we were able to constrain the value of Beta to 0.55±0.06 (Radburn-Smith, Lucey & Hudson 2004 [MNRAS, 355, 1378]). This result was found to be robust with respect to various culls by distance, host galaxy extinction and choice of reference frame. Fig. 1 demonstrates the excellent agreement between the predicted and measured peculiar velocities for our value of ß.

Fig. 1. Comparison of SNIa peculiar velocities to PSCz predicted peculiar velocities in the range 0 - 150 h^-1 Mpc with Beta=0.55. The size of the data point is inversely proportional to the total error on each SNIa. The smallest and largest circles correspond to errors of 1290 km/s and 170 km/s respectively. The lines indicate a 1:1 ratio.

This derivation of Beta also provides a further check of the current cosmological model as it can be predicted through a combination of Omega_m^0.6 sigma₈, derived from WMAP data (Spergel et al. 2003 [ApJS, 148, 175]), and sigma_8,IRAS, calculated by integrating the PSCz power spectrum (Hamilton & Tegmark 2002 [MNRAS, 330 556]). The resulting value of 0.4±0.06 determined through these cosmological parameters is found to be in good agreement with our determination.

Analysing the Great Attractor

Of particular importance in understanding the local velocity field and the source of the Local Group's own motion, is analysis of the relative scale and masses of the two most predominant nearby structures, the Great Attractor (GA) and the more distant Shapley supercluster.

Identified by the Seven Samurai in the late 1980s as the source of the observed outflow flow of nearby elliptical galaxies (Lynden-Bell et al. 1988 [ApJ, 326, 19]), the GA is only now being revealed with the help of multi-wavelength imaging. Lying in the ZoA, foreground extinction and stellar contamination have previously hampered optical searches for over-densities in the region. To provide a more complete picture of these hidden features we have recently used the 2dF on the AAT to target 5,353 galaxies in 25 fields around the GA. By combining the 2,595 newly identified redshifts with recent surveys in H I (Henning, Kraan-Korteweg & Staveley-Smith 2005 [ASP Conf. Ser. Vol. 329, 199]), X-rays (Ebeling, Mullis & Tully 2002 [ApJ, 580, 774]), optical (6dFGS, Jones et al. 2004 [MNRAS, 355, 747]) and the near-infrared, we are able to provide an overview of the clusters, groups and filaments that make up the GA.

Fig. 2. A 4000-7000 km/s redshift slice of our newly observed galaxies together with those taken from the literature. The classical position of the Great Attractor is l=307°, b=+7.

Fig. 2 plots a redshift slice of these measurements centred on the classic location of the GA. Immediately obvious is the Norma Cluster (l, b) = (325°, -7°), which lies at the bottom of the GA potential well (Woudt 1998 [Ph.D. Thesis]). From this structure a broad sheet of galaxies is seen edge on, extending down to the Pavo II cluster (332°, -24°) and on towards (30°, -60°). Analysis of the redshifts reveals that this connection also runs in the opposite direction, through the ZoA, where it meets two distinct clusters that we have now resolved: CIZA J1324.7-5736 (307.4°, +5.0°) and the Cen-Crux cluster (305.5°, +4.8°). From here we identify the continuation and eventual termination of the filament at Abell S0639 (280.5°, +10.9°). The extent of this filament leads to an intrinsic mass in the connecting structures alone of at least 2.5x10¹⁵ M_solar.

With our new redshift measurements, we infer masses for several key clusters in the GA and assess the nature and extent of the connecting structures. The combined mass of all these features helps explain the observed peculiar motions in the local velocity field. However, in order to fully understand this relation in detail, more accurate modelling of all local over-densities is required.

Modelling the flow field

The PSCz velocity field traces the over-densities of the IRAS galaxy survey. Unfortunately this sample tends to trace late type galaxies that undersample the cores of clusters. Hence the velocity field predicted from the survey lacks power in assessing the contribution from large clusters.

A complementary mapping of the local density field can be derived from X-ray surveys. Recently Kocevski et al. (2005) [astro-ph/0510106] have compiled the first all sky X-ray cluster survey by combining the REFLEX, eBCS and CIZA X-ray samples. Together with the authors of this catalogue we are reconstructing the real-space positions of the clusters using an iterative approach based on the linear biasing model. With the corrected observed fluxes and positions from this new sample, a peculiar velocity field will be determined by modelling the clusters as simple attractors. This can again be calibrated with local type Ia supernovae as well as peculiar velocities inferred from Fundamental Plane, Tully-Fisher and Surface Brightness Fluctuation distance estimates. The resulting field will then be compared with the PSCz predictions.

The next step is to combine the two velocity field predictions to produce a much more realistic picture of the local universe than either of the separate surveys can realise alone. This will allow us to accurately assess the contributions to the Local Group's motion from various nearby over-densities as well as study the recently proposed flow of galaxies towards clusters intermediate to the GA and the Shapley supercluster (Kocevski et al. [astro-ph/0512321]). Additionally, the construction of such a model is of vital importance in correcting the distances to nearby galaxies used in various studies of local cosmology. Most notably it can be used to correct the low end of mass and luminosity functions (Masters, Haynes & Giovanelli 2004 [ApJ, 607, 115]).

Stellar Populations

For the last part of my PhD, I will be using near-UV (NUV) and far-UV (FUV) colours to examine the stellar populations of early-type galaxies in the cluster environment. NUV colours are far more effective than optical colours in breaking the classic age-metallicity degeneracy that have previously hampered stellar population studies (see Dorman, O'Connell & Rood 2003 [ApJ, 591, 878]). Hence the recently completed NUV and FUV GALEX observations of approximately 900 galaxies in 18 clusters from the NOAO Fundamental Plane Survey (Smith et al. 2004 [AJ, 128, 1558]) provide an excellent opportunity to examine the benefits of UV colours in more detail. The study will also reveal the cause of the observed rise in the SEDs of many early-types shortwards of ~2000 Angstroms (see O'Connell 1999 [ARA&A, 37, 603]). It is this UV excess that causes the huge scatter in FUV emissions from E/S0s of a given optical luminosity; a trait not seen in the optical/infrared properties of such galaxies.