Milky Way's Place Among 'Council of Giants' Mapped

Astronomers have mapped the Milky Way galaxy's place among the ring of surrounding local galaxies known as "The Council of Giants."

Although the position of the Milky Way and its orbiting companion galaxy Andromeda is well understood amid the 3-million-light-year-wide cluster of regional galaxies called the Local Group, astronomers know much less about other nearby galaxies.

Now, a team astronomers from York University has mapped out bright galaxies within 35-million light years of Earth, presenting a greatly expanded picture of what lies at our galactic 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," said Marshall McCall, a professor of physics and astronomy at York. "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."

When the universe was forming, stellar winds may have shaped the current configuration of the local group, which has two elliptical galaxies sandwiching the 12 spiral galaxies within the Local Sheet.

These galaxies all spin around each other.

"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," McCall said. "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."

Through identifying the boundary of the Council of Giants, astronomers can formulate theories on the conditions that led to the formation of the Milky Way. It appears that only a small amount of the universe's matter was used to create the Local Group, which is puzzling given its orderly arrangement within the Local Sheet and the Council of Giants. To explain this, astronomers suggest the nearby galaxies must have formed within a pre-existing sheet-like foundation comprised mainly 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," McCall said. "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."

McCall's research appears in the Monthly Notices of the Royal Astronomical Society.