Galaxies dance before merger

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September 6, 1999
Web posted at: 2:30 p.m. EDT (1830 GMT)

By Robin Lloyd
CNN Interactive Senior Writer

(CNN) -- A trio of galaxies recently photographed by the Hubble Space Telescope is shown courting at a distance but they will merge to become one in a short cosmological time, an astronomer said.

The Hickson Compact Group 87, made up of one large spiral galaxy, an elliptical galaxy and at least one smaller spiral, are shown in a June image engaged in an intricate "dance" orchestrated by mutual gravitational forces acting between them.

But the dance will end in the next 500 million to billion years, said Jane Charlton, an astronomer and astrophysicist at Pennsylvania State University.

"What you see is not all you get," she said Monday. "You would think these things are moving around and not colliding, but in fact there is dark matter around them suffering dynamical friction.

"So they are being slowed and that is going to cause them to fall in to merger."

Over time, gravity will tug gas flows between the adjacent galaxies to the point where they crash into one another. Dark matter is invisible, but math models correlating speed and mass indicate that there is "missing mass" in the system that astronomers believe is dark matter or some tiny sub-nuclear particles.

The Wide Field and Planetary Camera 2 on NASA's Hubble Space Telescope provides a much better image than any ground-based telescope imager could.

Each month or so, the Hubble Heritage Team makes public a stunning image like the Hickson group, purely for aesthetic reasons.

But there usually is a scientific story that can be told just by looking over the image.

The Hickson group, in the constellation of Capricorn, is at a distance from Earth of 400 million light years. For comparison, our galaxy, the Milky Way, extends about 100,000 light years across.

The Hickson image shows a larger galaxy, 87a, disk-shaped and tilted so it is seen edge-on. To its right is an elliptical galaxy, 87b, that appears as a bright round blotch.

Elliptical galaxies are thought to be the result of a collision between two spiral galaxies. Gas is thrown out, the arms fall off and what's left is an elliptical galaxy.

The galaxies are leaving each other alone for the mean time, so it is thought that past interactions between them produced gas to light up the galaxies, Charlton said.

In the meantime, the pair of galaxies is being tormented by nearby gas-sucking black holes, Charlton said.

Black holes cannot be seen but are theorized to be nearby because the image shows the nuclei of the galaxies are very active and spewing gas.

A third group member, the nearby spiral galaxy 87c at the top of the image, may be undergoing a burst of active star formation, Charlton said. But it also is likely to merge in coming years with the larger galaxy beneath it in the image.

The elliptical galaxy to the right of 87a looks like a star but can be distinguished as a galaxy because it lacks "diffraction spikes," Charlton said. In contract, the spiked objects in the center of the image are stars -- probably in our own galaxy.

In the human time-scale, not much will change in the Hickson group, she said. But on the cosmological scale, the group is just a transient phenomenon.

"Sometimes new galaxies will join in and join this dance but in terms of these they will merge together," she said.

The smallest galaxy at the center of the image could be related to the other three or an unrelated background object, she said.

The colors blue and pink indicate regions of star formation in the galaxies. Dark bands on 87a came from interstellar dust creating shadows on the galaxy's background starlight. A faint tidal bridge of stars can be seen between the edge-on and elliptical galaxies.

The HST exposures of the Hickson group were acquired by Charlton, the Hubble Heritage Team at the Space Science Telescope Institute in Maryland and Sally Hunsberger of the Lowell Observatory in Arizona.