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Discovering the Secrets of Dusty Galaxies
June 17, 2010   
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A Japanese-Polish team of scientists has determined that most infrared light sources from outside the Milky Way are in fact nearby galaxies, which appear normal under normal light. The surprising activity evidenced by these objects in the infrared spectrum could be explained by collisions with other galaxies.

The findings, based on data collected from the AKARI satellite launched in 2006 and jointly analyzed by the Japanese-Polish team, lead to more questions than they answer. What makes precisely these galaxies, and not their similar neighbors, emit such quantities of infrared radioactivity?

“We suspect that the key to this mystery lies in interaction between the galaxies,” says Katarzyna Małek, a doctoral candidate at the Theoretical Physics Center of the Polish Academy of Sciences (PAN).

Infrared radiation
Infrared, or thermographic, cameras often appear in war films and animal documentaries, in which they make it possible to observe animals and people in complete darkness. They register infrared radiation emitted by any heat source slightly warmer than absolute zero. Astronomers can discover even very cold objects thanks to infrared detectors—so cold in fact that they are undetectable in the visible light spectrum. Among them are planets orbiting other stars, cold stars, clouds of interstellar dust with stars forming in their midst, or dust-filled galaxies. Infrared imaging allows scientists to observe objects that are very distant on a cosmological scale.

Astronomical observations in the infrared spectrum are difficult to conduct from Earth, as the atmosphere blocks most of the cosmic infrared radiation and also emits light in the same spectrum. This explains why most of the data is collected by detectors placed on board orbiting satellites.

The pioneering attempt in this area was the Infrared Astronomical Satellite (IRAS), which was launched in January 1983 and collected data for 10 months, observing over 350,000 light sources. Most of them were located in the Milky Way, but tens of thousands of distant dust-filled galaxies were also observed. Some of the light sources have not been adequately explained to this day.

The AKARI mission
Nearly a quarter of a century after the findings of the Infrared Astronomical Satellite, the Japanese-built AKARI satellite was tasked with creating a higher-resolution map of the sky. Launched in 2006, the satellite was a joint effort by the Japanese Aerospace Exploration Agency (JAXA), the Japanese Institute of Space and Astronautical Science (ISAS), and the European Space Agency (ESA). The satellite was outfitted with detectors working in the far-infrared spectrum, a technology that did not even exist at the time of the IRAS launch.

AKARI has been observing the whole sky, with a particular focus on two regions. One of them is known as the AKARI Deep Field South (ADF-S), a 12-square-degree region near the southern ecliptic pole. In some ways this region is unique: the density of dust originating in our own galaxy is the lowest there, which means that radiation emanating from faraway sources suffers the least from absorption by this very same dust. This makes ADF-S a sort of window into the infrared universe outside of our own galaxy.

AKARI observed the ADF-S region in four bands of far-infrared light: 65, 90, 140 and 160 micrometers. It discovered more than 2,000 distinct radiation sources. However, scientists were left with the task of finding out exactly what these infrared objects were. Very distant galaxies? Or closer galaxies filled with vast quantities of cold, star-forming dust? Cold stars then? Perhaps super-massive black holes or other even more exotic objects?

Thanks to their analysis of data sent over by AKARI, the scientists were able to determine that most of the brightest objects were galaxies located in our proximity, in many respects similar to “normal” galaxies which can be observed in the visible light spectrum—with one important difference though.

“Among the objects observed by AKARI, galaxies which are in close contact with others, or have been in such a situation recently, are 10 times more frequent than those observed in the visible spectrum,” says Agnieszka Pollo, Ph.D., from the Andrzej Sołtan Institute for Nuclear Studies in ¦wierk near Warsaw.

This led the scientists to conclude that many of the galaxies visible in the infrared spectrum, which appear as existing alone under normal conditions, have recently been on a collision course with another similar object.

These interactions between galaxies disrupt their structures, which leads to the dust they contain becoming denser and leading to sudden formation of stars. However, the light from these young stars is absorbed by the dust around them, making its direct observation impossible. The dust “releases” the absorbed radiation in the form of infrared light, which indicates that the galaxy is subject to violent star-forming processes, hidden behind clouds of cold gas.

The Research Team
The Japanese-Polish research team which analyzed data collected from the AKARI satellite was composed of Katarzyna Małek (Theoretical Physics Center of the Polish Academy of Sciences), Agnieszka Pollo (Andrzej Sołtan Institute for Nuclear Studies in ¦wierk near Warsaw), Tsutomu T. Takeuchi (Nagoya University), Przemysław Bienias (College of Inter-Faculty Individual Studies in Mathematics and Natural Sciences, University of Warsaw), Mai Shirahata (JAXA), Shuji Matsuura (JAXA), and Mitsonobu Kawada (Nagoya University).
The researchers’ article, “Star Forming Galaxies in the AKARI Deep Field South: Identifications and SEDs,” appeared in a special issue of Astronomy & Astrophysics, which was entirely devoted to the AKARI mission.
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