Astronomers know that the Milky Way is parked within the Local Group of galaxies, an interstellar expanse extending 3 million light years across. But we know very little about the area just outside of this. A new map corrects this oversight, showing that we live among the "Council of Giants."
Top image: This is what the Andromeda Galaxy would look like in the night sky if it was bright enough to be seen with the naked eye. Along with the Milky Way, these two galaxies dominate the Council of Giants. Photo credit: Tom Buckley-Houston & Stephen Rahn.
Our planet is located within the Milky Way, a 100,000 lightyear-wide spiral galaxy containing about 300 billion stars and a peanut-shaped core. Our solar system is situated in a branch or "spur" of the Local Arm. Looking outwards, however, we're part of a huge clump of galaxies consisting of two major ones and 52 minor ones — the so-called Local Group.
Back in 1936, Edwin Hubble introduced this term, which he described as "a typical small group of nebulae which is isolated in the general field" (nebulae is what astronomers used to call galaxies). Today, we know it as a group of about 54 galaxies, of which the Milky Way and Andromeda Galaxy are the primary members. Indeed, the Local Group's gravitational center is located somewhere between those two galaxies, with each of them hosting their own system of satellite galaxies. There's also the Triangulum Galaxy to consider (a.k.a. Messier 33), the third largest galaxy in the Local Group, which may be independent of Andromeda.
As noted, the Local Group stretches about 3 million lightyears across. But now, thanks to the work of York University physics and astronomy professor Marshall McCall, we can zoom out and catch a glimpse of how things appear a bit further out from the Milky Way.
The Council of Giants
McCall re-evaluated the distances and near-infrared luminosities of the brightest galaxies in the Local Volume to better describe the expanse of all bright galaxies within 20 million light years. It's called the "Local Sheet" — an area 34 million lightyears across and only 1.5 million lightyears thick.
Video showing a 3D view of the Council:
Within it, the Milky Way and Andromeda are encircled by two large galaxies arranged in a ring about 24 million lightyears across. McCall dubs this the Council of Giants, which in his words, "stands in gravitational judgment of the Local Group by restricting its range of influence." His work shows that the Local Sheet is both geometrically and dynamically distinct from the Local Supercluster (a large group of galaxies and galactic groups even further out).
Of the 14 giants in the Local Sheet, 12 are spiral galaxies. The remaining two, which sit on opposite sides of the Council, are elliptical galaxies. McCall believes that winds expelled in the earliest phases of their formation may have shepherded gas towards the Local Group, contributing to the disks of the Milky Way and Andromeda.
Quite unexpectedly, the spin of the Council giants are arranged around a small circle — an unusual alignment likely caused by gravitational torques imposed by our galaxy and Andromeda when the universe was smaller.
Into the Void
Interestingly, the outer boundary of the Council is helping astronomers understand the conditions which led to the formation of the Milky Way. To create such an orderly configuration as the Local Sheet and its Council, nearby galaxies likely developed within a pre-existing sheet-like foundation made primarily from 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," noted McCall in a statement. "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."