Merging Spiral Galaxies Create Ellipticals
by Robert Joseph
galaxy (right) and an elliptical galaxy (left). Images by B. Rothberg
Galaxies are the building blocks of the large-scale Universe. They are
collections of 100 million to 100 billion stars. A typical galaxy
is about 100,000 light-years in diameter, and the distances between galaxies
are on average perhaps 10 million light-years.
Galaxies tend to come in two shapes: spirals and ellipticals. In
spiral galaxies, the stars orbit in a very thin disk, and all the stars
move in the same direction. Two or more spiral arms wind around the center
like a pinwheel. In ellipticals, the orbital planes of stars are oriented
randomly in all directions, so these galaxies look like a ball. One of
the fundamental problems of modern astrophysics is to understand how galaxies
form, and in particular, why they tend to form only these two shapes.
In the 1970s, my friend Alar Toomre, at MIT, suggested that elliptical
galaxies could be formed by the collision and merging of two spiral galaxies. This
hypothesis has been debated, sometimes furiously, for over two decades.
A few years ago, IfA graduate student Barry Rothberg and I decided to embark
on a major project to examine a large sample of merging spirals and to
see if they are evolving into the characteristic properties of elliptical
Four examples of merging galaxies. Images by C. Ishida.
First, we selected a sample of about 50 spiral galaxies that have collided
and are in the process of merging into a single object. Then we identified
two kinds of telescope observations needed to investigate the extent to
which these mergers are evolving toward the properties of elliptical galaxies. We
took infrared images using the UH 88-inch (2.2-meter) telescope, and we
obtained spectra of these mergers using one of the Keck telescopes.
We first used the infrared images to see if the starlight in these objects
decreases with distance from the center of the galaxy the way it does in
spirals or the way it does in ellipticals. The light intensity decreases
very abruptly (exponentially) as one moves outward from the centers of
spirals, but it declines much more slowly with distance from the center
for ellipticals. We found that at least 45 of the 51 mergers have
light profiles characteristic of ellipticals. So far, so good. It
began to look as if most of these mergers really are evolving into elliptical
However, there is a more constraining test. The stars in each elliptical
galaxy move at random speeds in random directions. The speed and direction
of each star, as seen from Earth, is its radial velocity. We can measure
the average of these random radial velocities in an elliptical galaxy and
compare this average with the radius from the center of the galaxy that
contains half the galaxy's total light. The proportionality between these
two measurements is called the "fundamental plane," and it
is a key feature of the distribution of stars and their velocities in elliptical
galaxies. We measured the average random velocities and half-light
radii for the merging galaxies and compared them with those of typical
elliptical galaxies. It turns out that the mergers are very similar
to the ellipticals. Only a few of the most luminous mergers deviate
a bit from the ellipticals. We believe that the current high rate
of star formation in these galaxies is largely responsible for this deviation,
and as this rate slows, these mergers will join their fellows on the fundamental
In summary, it appears that that when spirals merge, they do form elliptical
galaxies. This may answer the question, why do galaxies come in two shapes?
They all begin as spirals, and when galaxies collide and merge, the result
is an elliptical galaxy.