March 13, 2014
A new map of our galaxy's broader neighbourhood shows we are surrounded by twelve big galaxies called the Council of Giants that, together with the Milky Way and Andromeda in the middle, form the Local Sheet. This Local Sheet is ordered in such a way, it points to underlying foundation that existed before the galaxies were formed.
How well do you know the map of galaxies around the Milky Way? Chances are, you know about big, spiral Andromeda galaxy and a couple of small satellite galaxies close to both Andromeda and the Milky Way. These two, together with these satellite galaxies that we call dwarf galaxies, form the Local Group, which you also probably heard of. The Local Group is aproximately 3 million light-years across. What we didn't know for certain is how the neighbourhood beyond the Local Group looks like.
A new paper by Marshall McCall, physics and Astronomy professor at York University, Toronto, Canada, comes with the map of galaxies within 35 million light-years. It turns out, the Milky Way and Andromeda are part of a group organised better than you might have expected.
The Local Sheet, viewed from "above". The Council of Giants is made of twelve big galaxies, here in yellow. The Milky Way and Andromeda are in the centre, coloured red. Credit: Marshall McCall/York University
"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."
Here is the Local Sheet as seen from the side. It is only 1.5 million light-years thick, compared to being 34 million light-years across. Credit: Marshall McCall/York University
Interestingly, twelve of these fourteen galaxies, the Milky Way and Andromeda included, are spiral galaxies in which new stars are still being born. The other two, lying on opposite sides of the Council of Giants, are red and dead elliptical galaxies. They lost their cold gas needed for star creation long ago. It is possible that some of this gas ended up in the Local Group.
The Milky Way and Andromeda are at the centre of the Local Sheet and other galaxies form a circle. When viewed from the side, the Local Sheet is thin and not a sphere or some irregular shape like you might have expected. This indicates some underlying blueprint that existed even before the Local Sheet was created.
It would seem that before the galaxies have formed, dark matter – mysterious matter that we still can't detect but which allegedly makes up about 27% of the universe – was already arranged in a sheet-like foundation. We think that dark matter holds galaxies together and it is possible that it does so by having an in some way organised structure as opposed to randomly being scattered across the universe.
"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."
The study further shows that galaxies in the Council of Giants spin around a small circle in the sky. "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," says McCall. "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."