We live in a galaxy known as the Milky Way – a vast conglomeration of 300 billion stars, planets whizzing around them, and clouds of gas and dust floating in between.
Though it has long been known that the Milky Way and its orbiting companion Andromeda are the dominant members of a small group of galaxies, the Local Group, which is about 3 million light years across, much less was known about our immediate neighbourhood in the universe.
Now, a new paper by York University Physics & Astronomy Professor Marshall McCall, published today in the journal Monthly Notices of the Royal Astronomical Society, maps out bright galaxies within 35-million light years of the Earth, offering up an expanded picture of what lies beyond our doorstep.
“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. “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.”
McCall says twelve of the fourteen giants in the Local Sheet,
including the Milky Way and Andromeda, are "spiral galaxies" which 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. He
comments: “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 important,
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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Robin Heron
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Robert Massey
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rm@ras.org.uk
Animation
This
movie illustrates the positions of the nearby galaxies, including those
in the ‘Council of Giants’, in three dimensions. Credit: Marshall
McCall / York University
Further information
The new work appears in “A Council of Giants”, M. L. McCall, Monthly Notices of the Royal Astronomical Society, Oxford University Press, in press.
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