An unexpectedly high percentage of galaxies seen by the Infrared Astronomical Satellite (IRAS) are merging, colliding, or otherwise interacting with one another, IRAS scientists reported today at meeting of the American Astronomical Society in Baltimore, MD.
IRAS, an orbiting infrared telescope, last year observed about 20,000 galaxies, many of which emit more than half of their energy in far-infrared wavelengths. Recent studies indicate that as many as 25 percent of the most luminous of these infrared-bright galaxies may be part of interacting or merging groups of galaxies. Less than 5 percent of all visually identified galaxies are interacting.
The infrared energy from many of these interacting galaxies seems to be due to great bursts of massive star formation, Dr. Carol J. Lonsdale of NASA's JPL reported. Instead of producing one or two stars per year, these infrared galaxies may have been shocked into the release of immense amounts of gravitational energy and the production of up to hundred times more stars per year than isolated spiral galaxies. An alternative explanation for the tremendous infrared emission detected from some of the galaxy pairs could be that source such as black hole exists at the center of one of the merging galaxies.
One example of possibly colliding galaxy studied in detail by IRAS is Arp 220 (number 220 in astronomer Halton Arp's Catalog of Peculiar Galaxies). Dr. B. Thomas Soifer of Caltech reported that analysis of IRAS data shows that the intense infrared emission from this galaxy is probably the result of collisions between pair of galaxies that occurred in the last 100 million years.
Arp 220 is 100 times more luminous than normal spiral galaxies, with power output equal to that of distant quasars, equivalent to 2 trillion suns. Almost all of its power output (99 percent) is emitted at far-infrared wavelengths. While the source of this energy is not known, it is most likely coming from small core of few thousand light-years across within the colliding galaxy.
Characteristics displayed by Arp 220 are typical of some of the "unidentified" objects discovered by IRAS last year and reported by Dr. James R. Houck of Cornell University. This indicates that some of the other unidentified objects may also be infrared galaxies. Such galaxies would be much more distant and might be even more luminous than Arp 220.
Dr. Lonsdale of JPL also reported on IRAS studies of peculiar object named Arp 243 -- another example of the phenomenon of high infrared luminosity from merging galaxies. Arp 243, located about 300 million light-years from Earth, has been found to emit more than 95 percent of its total energy output at infrared wavelengths.
The presence of large numbers of very young, luminous stars has already been established directly from optical spectroscopy, so the infrared emission from these objects undoubtedly comes from great bursts of star formation triggered by the collision. #####
The following were also among the 14 new IRAS results presented at the AAS meeting:
- In continuing studies of comets observed by IRAS, an extensive infrared tail of ice and dust as long as 22 million miles was found on Comet Bowell. The comet was observed in part of its orbit beyond the orbit of Jupiter. (Dr. Russell G. Walker, Jamieson Science and Engineering.)
- Scientists have correlated IRAS observations of spiral galaxies in the Virgo, Coma, and Hercules galaxy clusters with radio and optical observations. The results of the study show that galaxies with more pronounced disks, younger stars and large amounts of interstellar gas show brighter far-infrared luminosities than elliptical galaxies deficient in gas and dust. (Dr. George Helou, JPL, and Dr. Erick Young, Steward Observatory, University of Arizona.)
- very young star roughly the mass and brightness of the sun has formed in the last 100,000 years in nearby (450 light-years away) cloud of molecular hydrogen gas called Lynds 255 (Dr. Charles Beichman, JPL.)
- Prelimary examination of the statistical properties of the roughly quarter-million infrared sources detected by IRAS in its 300-day mapping of 98 percent of the entire sky shows that most of the infrared objects emitting energy in the 12 and 25-micron region of the spectrum are old stars within our galaxy. Looking away from the plane of our galaxy, the sky as observed in the 60 to 100-micron region appears to be dominated not by nearby stars but by other galaxies. (Dr. Thomas J. Chester, JPL.)
Analysis of data taken late in the IRAS mission has yielded new information about and images of the zodiacal dust bands within our solar system. These bands of warm (200 degrees Kelvin or -100 degrees Fahrenheit) material orbit the sun near the inner edge of the main asteroid belt, between the orbits of Mars and Jupiter. Debris from comets and collisions between asteroids are possible sources of this rocky material. (Dr. T.N. Gautier, JPL.)
- IRAS observations of 47 Tucanae (a group of about million stars called globular cluster), have called into question our present understanding of these objects, thought to be the oldest in our galaxy. The suprisingly small amount of far-infrared emission from 47 Tucanae suggests that either old stars return less dusty material to the interstellar medium than previously thought, or this globular cluster has very efficient means, such as high-velocity cluster wind, for sweeping out the returned material. This globular cluster may have had its interstellar material swept away as it passed through the plane of the galaxy. (Dr. Fred Gillett, Kitt Peak National Observatory.)
IRAS was launched into polar orbit by NASA Jan. 25, l983, from Vandenberg Air Force Base in California. The orbiting telescope mapped 98 percent of the sky during the 10- month mission, measuring the infrared emission and location of more than 250,000 infrared sources.
IRAS is joint project of NASA, The Netherlands Space Agency (NIVR), and the United Kingdom's Science and Engineering Research Council. NASA's JPL is the management
center for the project.
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