Galaxies Discovered Buried In Dust
June 1, 2013

Researchers Discover 15 Previously Unidentified Extremely Dark Galaxies

redOrbit Staff & Wire Reports - Your Universe Online

Approximately 80 percent of all unidentifiable millimeter wave signals emitted in the universe actually originate from galaxies, according to new research published in Saturday´s edition of the Astrophysical Journal Letters.

Based on observations collected using Atacama Large Millimeter/sub-millimeter Array (ALMA) telescopes, professor Kouji Ohta and postdoctoral researcher Bunyo Hatsukade of Kyoto University in Japan and their colleagues said that they are able to pinpoint the exact locations of those galaxies in fine solid particles, or dust.

Using ALMA´s high resolution and sensitivity, they were able to observe the “Subaru/XMM-Newton Deep Survey Field” in the same direction as the constellation Cetus. Furthermore, the researchers said that they were able to identify 15 previously unknown, extremely dark galaxies, while also measuring the number density of galaxies with 10 times less luminosity than those previously detected using traditional instruments.

“Their densities well match the prediction by theories of galaxy formation,” they added. “Therefore, the researchers consider that they managed to capture more like ℠normal´ galaxies, which had been impossible to detect up to now, than extremely bright ℠submillimeter-luminous galaxies.´”

Thanks to ALMA and the Subaru Telescope, Ohta, Hatsukade and their colleagues are “now seeking to uncover the overall picture of galaxy formation and evolution while conducting observations of much darker galaxies.”

Typically, research on distant galaxies is conducted using visible and near infrared light. However, the study authors believe that this could cause many galaxies to be overlooked because much of that radiation is absorbed by cosmic dust — miniature, solid particles made up of silicon, carbon, iron and other elements.

However, stellar light absorbed by that dust is reradiated from the particles as millimeter and submillimeter waves, allowing astronomers to use these waves to find galaxies that had been undetectable using optical telescopes.

“This is because the more distant the galaxy is, the more luminous part of light we can see due to the shift of wavelength of light by the expansion of the universe. This effect is called ℠negative K correction´ and it compensates the source dimming in the distant universe,” the researchers said.

“In past observations, gigantic galaxies deeply covered in dust, where several hundreds to thousands of stars are actively forming per year, have been detected with millimeter/submillimeter waves,” they added. “To capture the overall picture of galaxies in the universe, it is important to observe ℠general galaxies´ which have moderate star-formation activities. However, it has not been possible to detect faint galaxies due to the low sensitivity of existing observation instruments.”

Such was the case in this latest study, in which Ohta, Hatsukade and their colleagues observed the 15 previously unidentified extremely dark galaxies in the “Subaru/XMM-Newtown Deep Survey Field” using the ALMA equipment. Thanks to their observations, the investigative team was able to successfully measure the number density of galaxies nearly 10 times darker than existing millimeter wave research.

“That means, the galaxies detected in this research are the faint but dust-rich galaxies and they are most likely to be similar in type to normal galaxies not detected before,” the researchers said. “Furthermore, the team concluded that approximately 80% of the sources of the cosmic background radiation within the millimeter/submillimeter wavebands are more "normal galaxies" like those detected by ALMA this time.”

“It is thanks to the high performance of ALMA, which is proudly said to be the best in the world, that observations like this have been made possible,” Hatsukade said. Ohta added that their work was “a big step towards getting the big picture of galaxy evolution as the objects connecting especially bright galaxies in millimeter/submillimeter waves and normal galaxies were detected with ALMA.”