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Abell 1763 and Abell 1770
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Astronomers Probe 'Sandbar' Between Islands of Galaxies

Astronomers have caught sight of an unusual galaxy that has illuminated new details about a celestial "sandbar" connecting two massive islands of galaxies. The research was conducted in part with NASA's Spitzer Space Telescope.
These "sandbars," or filaments, are known to span vast distances between galaxy clusters and form a lattice-like structure known as the cosmic web. Though immense, these filaments are difficult to see and study in detail. Two years ago, Spitzer's infrared eyes revealed that one such intergalactic filament containing star-forming galaxies ran between the galaxy clusters called Abell 1763 and Abell 1770.
Now these observations have been bolstered by the discovery, inside this same filament, of a galaxy that has a rare boomerang shape and unusual light emissions. Hot gas is sweeping the wandering galaxy into this shape as it passes through the filament, presenting a new way to gauge the filament's particle density. Researchers hope that other such galaxies with oddly curved profiles could serve as signposts for the faint threads, which in turn signify regions ripe for forming stars.

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Galaxies tend to give birth to their stars on the road, while travelling down intergalactic highways towards cosmic cities called galaxy clusters, new Spitzer Space Telescope observations reveal.
Galaxies in relatively empty regions of the universe flock towards densely populated galaxy clusters, attracted there by the clusters' gravity. The galaxies tend to move along cosmic highways called filaments, which are concentrations of gas and invisible material called dark matter.
Because galaxy clusters are so densely packed, galaxies that live inside have more opportunities to brush up against one another, sometimes triggering star formation. Astronomers once thought this would make galaxy clusters the most fertile places for stars to form

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New observations from NASA's Spitzer Space Telescope suggest that galaxies prefer to raise stars in cosmic suburbia rather than in "big cities."
Galaxies across the universe reside in cosmic communities, big and small. Large, densely populated galactic communities are called galaxy clusters. Like big cities on Earth, galaxy clusters are scattered throughout the universe, connected by a web of dusty "highways" called filaments. While thousands of galaxies live within the limits of a cluster, smaller galactic communities are sprinkled along filaments, creating celestial suburbs. Over time, astronomers suspect that all galactic suburbanites will make their way to a cluster by way of filaments.
For the first time, Spitzer's supersensitive eyes have caught an infrared glimpse of several galaxies travelling along two filamentary roads into a galaxy cluster called Abell 1763.

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Title: STARBURST GALAXIES IN CLUSTER-FEEDING FILAMENTS UNVEILED BY SPITZER
Authors: Dario Fadda, Andrea Biviano, Francine R. Marleau, Lisa J, Storrie-Lombardi, Florence Durret

We report the first direct detection with Spitzer of galaxy filaments. Using Spitzer and ancillary optical data, we have discovered two filamentary structures in the outskirts of the cluster Abell 1763. Both filaments point toward Abell 1770 which lies at the same redshift as Abell 1763 (z = 0.23), at a projected distance of ~13 Mpc. The X-ray cluster emission is elongated along the same direction. Most of the far-infrared emission is powered by star formation.
According to the optical spectra, only one of the cluster members is classified as an active galactic nucleus. Star formation is clearly enhanced in galaxies along the filaments: the fraction of starburst galaxies in the filaments is more than twice than that in other cluster regions. We speculate that these filaments are feeding the cluster Abell 1763 by the infall of galaxies and galaxy groups. Evidence for one of these groups is provided by the analysis of galaxy kinematics in the central cluster region.

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Abell 1763
Spatial distribution of the 181 spectroscopically selected members of the cluster Abell 1763. Target selection was done exclusively with 24m sources in the region outside the 1.5 Mpc radius central region. In the inner region, early-type galaxies were targeted also to better describe the velocity distribution of the cluster. The dotted rectangle corresponds to the 24 m MIPS field. Starburst galaxies and AGN are marked with filled dots and triangles, respectively. Circles of 1.5 Mpc radius indicate the centres of Abell 1763 and Abell 1770, connected by a dashed line. The shaded regions have been traced by eye to highlight two filamentary structures. In these regions there is a clear excess of starburst galaxies.

Abell 1763 RA 13 35 20.213 Dec 41 00 04.05

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