12th March 2014

A new map of our galactic neighbourhood

We live in a galaxy known as the Milky Way – a vast collection of 300 billion stars, planets whizzing round them, and clouds of gas and dust floating in between. It has long been known that the Milky Way and its companion, Andromeda, are the dominant members of the "Local Group", a family of galaxies 3 million light years across. However, much less was known about our immediate neighbourhood slightly further out.

A new paper by Professor Marshall McCall of York University, Canada, maps out bright galaxies within 20-million light years – offering an expanded picture of what lies beyond our galactic doorstep. The work is published this week in the journal Monthly Notices of the Royal Astronomical Society.

"All bright galaxies within 20 million light years, including us, are organised in a 'Local Sheet' 34-million light years across and only 1.5-million light years thick", says McCall. "The Milky Way and Andromeda are encircled by 12 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."

McCall says 12 of the 14 giants in the Local Sheet, including the Milky Way and Andromeda, are spiral galaxies that have highly flattened disks in which stars are forming. The remaining two are more puffy "elliptical" galaxies, whose stellar bulks were laid down long ago. Intriguingly, the two ellipticals sit on opposite sides of the Council. Winds expelled in the earliest phases of their development might have shepherded gas towards the Local Group, thereby helping to build the disks of the Milky Way and Andromeda.

McCall also examined how galaxies in the Council are spinning: "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."

The boundary defined by the Council has led to insights about the conditions which led to the formation of the Milky Way. Most importantly, only a very small enhancement in the density of matter in the universe appears to have been required to produce the Local Group. To arrive at such an orderly arrangement as the Local Sheet and its Council, it seems that nearby galaxies must have developed within a pre-existing sheet-like foundation comprised primarily of dark matter.

"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."

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