Big galaxy collisions can turn off the process of star formation

Washington, Oct 8 : Astronomers have found evidence that high-speed collisions between large galaxies can turn off the process of star formation.

The evidence was found in the form of a deep new image of the Virgo cluster, which revealed monumental tendrils of ionized hydrogen gas 400,000 light-years long connecting the elliptical galaxy M86 and the disturbed spiral galaxy NGC 4438.

Taken with the wide-field Mosaic imager on the National Science Foundation's Mayall 4-meter telescope at Kitt Peak National Observatory, the image and related spectroscopic measurements of the filament provide striking evidence of a previously unsuspected high-speed collision between the two galaxies.

"Our data show that this system represents the nearest recent collision between a large elliptical galaxy and a large spiral," said Jeffrey Kenney of Yale University.

"This discovery provides some of the clearest evidence yet for high-speed collisions between large galaxies, and it suggests that the consequences of such collisions are a plausible alternative to black holes in trying to explain the mystery of what process turns off star formation in the biggest galaxies," he added.

The Virgo cluster is located approximately 50 million light-years from Earth.

Previous studies had noticed disturbed H-alpha gas around each of the two galaxies, but no connection from the two had been inferred.

Some results have suggested that NGC 4438 collided with the small lenticular galaxy NGC 4435, but NGC 4435 has a much higher line-of-sight velocity as seen from Earth and appears undisturbed.

Spectroscopy of selected regions along the filament between M86 and NGC 4438, obtained with the Sparsepak Integral Field Unit instrument on the WIYN 3.5-meter telescope on Kitt Peak, shows a fairly smooth velocity gradient between the galaxies, supporting the collision scenario.

There are no obvious stars in the filaments.

As in most elliptical galaxies, most of the gas within M86 is extremely hot, and therefore radiates X-rays.

The X-ray distribution in M86 is irregular and sports a long plume, which had previously been interpreted as a tail of gas which is being stripped by ram pressure as M86 falls into the intracluster medium of the Virgo cluster.

The new H-alpha image from Kitt Peak suggests that most of the disturbances to the interstellar medium in M86 are instead due to the collision with NGC 4438.

In high velocity collisions, the kinetic energy of the collision can cause the gas to heat up so much that it cannot easily cool and form stars.

ANI