Where are we among the galaxies beyond the Local Group?
For several generations, astronomers have known that our Milky Way Galaxy and its companion galaxy, Andromeda, are the dominant members of a small group of galaxies called the Local Group, which is about three million light-years across. Less well known are the structure and composition of the next step beyond the Local Group—the galaxies deeper into our immediate neighbourhood in the universe.
Now, a paper by astronomer Marshall McCall of York University in Toronto, published in the March Monthly Notices of the Royal Astronomical Society (U.K.), maps out bright galaxies within 35 million light-years of Earth, offering 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 wide 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 discs in which stars are forming. Backyard astronomers are likely familiar with four prominent spirals in the Council: M81, M83, M64 and NGC253.
The Council contains two puffy elliptical galaxies whose stellar bulks were laid down long ago. Intriguingly, the two ellipticals sit on opposite sides of the Council. Stellar winds expelled in the earliest phases of their development might have shepherded gas toward the Local Group, thereby helping to build the discs of the Milky Way and Andromeda.
McCall also examined how galaxies in the Council are spinning. He notes that unexpectedly, the spin axes of the 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 that 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 preexisting sheetlike foundation composed 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.”