The Milky Way Galaxy in which we live can seem a little lonely. While we are surrounded by dwarf galaxies, our only peer within what is known as the Local Group is Andromeda. It feels a little like being part of couple stuck in a remote location with only pets for company. Now however, this picture has been put in perspective with the first map of all the bright galaxies within 20 million light years. And it seems we really are part of a galactic village which has raised us.
Galaxies are usually classified as being part of either clusters or groups. Groups such as our own can include anything up to 50 galaxies, while clusters and superclusters can contain thousands of galaxies. Professor Marshall McCall of New York University refers to a lesser known category, a galactic sheet.
"All bright galaxies within 20 million light years, including us, are organized in a 'Local Sheet' 34-million light years across and only 1.5-million light years thick," says McCall. Within this sheet there are 14 large galaxies, 18 small galaxies significant enough for McCall to show, and dozens of dwarf galaxies like our closest neighbors the Magellanic Clouds. The local group sits near the center, with the other galaxies around us in a ring.
"The Milky Way and Andromeda are encircled by twelve large galaxies arranged in a ring about 24-million light years across - this 'Council of Giants' stands in gravitational judgment of the Local Group by restricting its range of influence," says McCall.
Twelve of the 14 large galaxies are spirals like our own, reflecting the low density of galaxies in the sheet. Elliptical galaxies dominate in large clusters. McCall believes it is no coincidence that the two giant ellipticals are at opposite sides of the sheet and thinks their winds may have shaped the other galaxies by pushing gas towards them.
Even at distances of 10 million light years or more away, the Milky Way and Andromeda appear to have exerted a powerful influence, including on galaxies larger than themselves. McCall proposes in the Monthly Notices of the Royal Astronomical Society. “Thinking of a galaxy as a screw in a piece of wood, the direction of spin can be described as the direction the screw would move (in or out) if it were turned the same way as the galaxy rotates. Unexpectedly, the spin directions of Council giants are arranged around a small circle on the sky. This unusual alignment might have been set up by gravitational torques imposed by the Milky Way and Andromeda when the universe was smaller.”
McCall thinks the layout of the galaxies, offers hints of the distribution of mass in our region soon after the Big Bang. “The Local Sheet formed out of a density perturbation of very low amplitude (∼10%),” he writes, “But that normal matter was incorporated into galaxies with relatively high efficiency (∼40%).”
“Recent surveys of the more distant universe have revealed that galaxies lie in sheets and filaments with large regions of empty space called voids in between,” says McCall. “The geometry is like that of a sponge. What the new map reveals is that structure akin to that seen on large scales extends down to the smallest.”
1