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Merging Spiral Galaxies Create Ellipticals

by Robert Joseph

elliptical galaxy
spiral galaxy

A spiral 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 galaxies.

merging galaxies

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 galaxies.

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 plane.

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.