Gemini, Subaru & Keck Discover large-scale funneling of matter onto a massive distant galaxy cluster

June 30, 2004

Large galaxy clusters represent the largest gravitationally stable assemblies of matter in the universe. Massive clusters of galaxies consist of thousands of galaxies and vast quantities of extremely hot intergalactic gas all held together by gravity. It is believed that clusters of galaxies grow in size and in number through time and we see many more galaxy clusters in the nearby (recent) universe than in the distant past. The assembly and growth of massive clusters involve complex interactions between dark matter, diffuse gas, and thousands of individual galaxies. The physical details of these interactions are currently not fully understood. Both computer simulations and theoretical work predict that the growth process of massive clusters is highly non-isotropic with the in-fall of matter and galaxy mergers occurring along preferred directions. The coalescence and funneling of matter should produce highly correlated structures in space, enhancing visibility in these preferred directions. This is referred to the “cosmic web” (Figure 1). Such structures look like highways converging onto a large city, but with the important difference that clusters feeding filaments are organized in three-dimensional space.

Using the entire battery of large telescopes on Mauna Kea, Harald Ebelling, Elizabeth Barrett and David Donovan of the Institute for Astronomy (IfA) at the University of Hawaii (UH), have obtained a unique data set of the X-ray cluster MACS J0717.5+3745 and its surroundings. MACS J0717.5+3745 is one of the most massive clusters of galaxies known at a redshift greater than z=0.5. The UH team obtained wide-field images in the V, R, I and z’ photometric bands with SuprimeCam on the Subaru Telescope, and multi-object spectroscopy with LRIS & Deimos on Keck and GMOS on Gemini North. From these data, they present evidence of a spectacular large-scale filament along which matter and galaxies is being funneled into the cluster. This appears to be a generic filament as opposed to a merger event.

The image of the region (Figure 2) reveals an apparently coherent over-density of red galaxies greater than 6.3 Megaparsec (Mpc) in length, of which about 4.3 Mpc can be attributed to a long filament structure. The red color of the galaxies tracing the filament means that they are old systems very similar to the ones found in the cluster core. This has important implications for models trying to discern between different physical mechanisms that might be responsible for the transformation from blue spiral galaxies in the field to red elliptical galaxies in the cores of galaxy clusters.

The filament extends well beyond the virial radius (~2.3 Mpc) of the cluster, (the virial radius defines the physical size of the gravitationally stable part of the cluster). The observed elongated morphology is indicative of significant dynamical activity at scales greater than 5 Mpc from the cluster, well beyond the virial radius. The observations imply an in-fall of matter along this preferred axis direction. The funneling will persist for roughly the next 4 Gyr, assuming an in-fall speed of ~1000 km/s.

This is the first convincing candidate for the type of filament channeling of matter onto massive clusters predicted by numerical simulations. Colors of the individual galaxies and velocities derived from the GMOS/LRIS/DEIMOS spectra do confirm that the over-density contours (Figure 2) maps galaxies at the cluster redshift (Figure 3). The entire filament is located at z =0.55.

The observed galaxy distribution in and around MACS J07017.5+3745 supports a picture in which massive clusters of galaxies grow via discrete as well as continuous in-fall of matter along large-scale filaments.

More details can be found in the paper “Discovery of a Large-Scale Filament Connected to the Massive Galaxy Cluster MACS J0717.5+3745 at z = 0.55” by H. Ebeling, E. Barrett, and D. Donovan, in The Astrophysical Journal Letters, L49-L52, Vol. 609, 10 July 2004. (Figures 2 and 3 are from this article)