What Happens When Galaxies Collide? Hubble Space Telescope Finds New Surprises
25 May 1993The NASA/ESA Hubble Space Telescope (HST) has discovered a new population of exceptionally bright and young star clusters at the heart of a head-on collision between two galaxies.The orbiting telescope also discovered a rotating, pinwheel-shaped disk at the center of the collision. In the Hubble photo, the disk resembles a full spiral galaxy, seen face on. Yet the disk is only ten thousand light-years across, about 1/20 the diameter of the whole galaxy.
The newly found clusters are concentrated near the core of the peculiar galaxy NGC 7252. They were apparently born in the aftermath of the collision of two disk-shaped galaxies, which occurred roughly one billion years ago. The two disk-shaped galaxies are in the process of merging into a single giant elliptical galaxy. Though several of the clusters were first spotted from ground-based telescopes, their true nature was uncertain until the Hubble observations.
This discovery provides some of the best evidence to date for explaining the origin of giant elliptical galaxies. For more than a half century, astronomers have theorized on how such galaxies formed. Some theories propose that ellipticals formed from collisions between disk-shaped galaxies — flattened stellar systems resembling our own Milky Way galaxy.
These results are being presented in a press conference today at NASA Headquarters by Dr. Bradley Whitmore of the Space Telescope Science Institute (STScI) in Baltimore, Maryland and Dr. Francois Schweizer of the Carnegie Institution of Washington. Co-investigators in the Hubble findings are Drs. Claus Leitherer, Kirk Borne, and Carmelle Robert of STScI. Dr. Schweizer's co-worker in the Hale Telescope research is Dr. Patrick Seitzer of the University of Michigan at Ann Arbor.
"I knew we had a major result within 10 seconds of looking at the Hubble picture," said Whitmore. "This may provide a clue to how all globular clusters formed." He added, "The Hubble observation also shows how tiny, disk-like structures may have formed near the centers of many other galaxies."
The striking image shows a spiral pattern at the galaxy's core, which is surrounded by bright star clusters.
The Hubble observations succeeded in resolving the globular clusters, that is, measuring their apparent sizes on the sky despite the vast distance of NGC 7252. The galaxy is 300 million light- years from the Earth, and located in the constellation Aquarius.
The clusters are typically about 0.04 arc seconds in diameter, which corresponds to the apparent size of a dime, seen from a distance of 60 miles. This means that the clusters are each about 60 light-years in diameter, the same size as the globular star clusters of our own Milky Way.
Astronomers using the 5-meter (200-inch) Hale Telescope at Palomar Observatory have confirmed that these are globular clusters by obtaining spectrograms of the two brightest ones. This work (described in a coordinated press release from the Carnegie Institution of Washington) shows that the two objects have spectra characteristic of young globular clusters, rather than other kinds of celestial objects.
The pinwheel structure found in the Hubble observations of the central disk of NGC 7252 consists of a system of interstellar gas and stars, orbiting the bright nucleus of NGC 7252. The gas and stars in the disk look like the swirling pattern formed by cream just mixed into a cup of coffee.
The globular clusters found in NGC 7252 probably resemble the progenitors of the globular clusters in our own Milky Way galaxy. Since globular clusters normally contain ancient, red giant stars, they provide a fossil record of the formation and evolution of galaxies. Globular clusters contain about one million stars each, arranged in a tight, spherical formation, and are generally found to be about 15 billion years old. But the 40 clusters found in NGC 7252 represent a rare exception to this rule.
According to Whitmore, the clusters near the center of NGC 7252 are "ultra-luminous," — up to a few hundred times brighter than the globular clusters in the Milky Way. He attributes this high luminosity, and their bright color, to the presence of numerous hot bluish stars in each of the clusters. Such blue stars are short-lived and not found in the globular clusters of the Milky Way.
From the presence of these short-lived blue stars, the research team estimates that the clusters in NGC 7252 are mostly between 50 and 500 million years old.
If the globular clusters in the Milky Way were as bright as the newly found clusters in NGC 7252, many of them would be brighter than the stars in the Big Dipper, and readily visible to the naked eye.
In the 1920's American astronomer Edwin Hubble classified galaxies according to their spiral or elliptical shape. A key difference is that stars are concentrated in a disk in spirals, but are distributed in a diffuse, roughly spherical distribution in ellipticals.
Since Edwin Hubble's time, astronomers have sought an explanation for why there are two different types of galaxies. During the past decade, the hypothesis that spiral galaxies can collide and merge to form elliptical galaxies has become increasingly popular.
NGC 7252 has been considered the prototypical example of a merger between two disk-shaped galaxies. The galaxy has a pair of long tails that are unambiguous evidence of the effects of gravitational tidal forces from a galaxy merger. The galaxy is nicknamed the "Atoms-for-Peace" * galaxy because its stars form a bizarre loop-like structure that resembles a schematic diagram of electrons orbiting an atomic nucleus.
If globular clusters can be born during galaxy collisions, it reinforces the theory that disk galaxies merge to make giant elliptical galaxies. One argument against this theory has been the fact that elliptical galaxies have more globular clusters than would be expected if disk galaxies were simply combined, since disk galaxies have relatively few clusters.
The new Hubble Space Telescope observation solves this dilemma by showing that when disk galaxies collide they can form new globular clusters. Rather than being a problem for the merger scenario, an increase in the number of globular clusters is a natural consequence of galaxy mergers.
Both the globular clusters and the "mini-spiral" were born as a result of the galaxy merger, which occurred about one billion years ago. This triggered an infall of the gas which fueled the birth of new stars around the center of the galaxy. The mini-spiral contains enough gas to make eight billion stars like our Sun. The disk was presumably fueled by the collision. The fact that the mini-spiral is rotating in a direction opposite to the rest of the galaxy — another clear indication that the material originated from a galaxy collision.
In a few billion years the gas in NGC 7252 will be exhausted and the galaxy will look like a normal elliptical galaxy with a small inner disk. This also fits with the merger scenario since similar disk-like features appear to exist in many elliptical galaxies.
This discovery is the latest in a series of HST discoveries which have uncovered disk and ring- like objects in the cores of galaxies. Previously, HST found a giant disk of cool gas and dust orbiting a suspected black hole in the active galaxy NGC 4261, and an edge-on "donut" of dust in the spiral galaxy M51.
In December 1953, U.S. President Dwight D. Eisenhower made his "Atoms for Peace" speech to foster peaceful applications of nuclear energy.