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With the help of ALMA (Atacama Large Millimeter/submillimeter Array), scientists have observed the "Subaru/XMM-Newton Deep Survey Field" in the direction of the constellation Cetus, and succeeded in identifying 15 extremely dark galaxies which had been previously unknown.
In addition, they also successfully measured the number density of galaxies with 10 times less luminosity than ones previously observed with the conventional millimeter instruments.
Based on the observations with ALMA telescope, the research team led by Bunyo Hatsukade, a postdoc researcher, and Kouji Ohta, a professor, both from the Graduate School of Science, Kyoto University, have revealed that approximately 80 percent of the unidentifiable millimeter wave signals from the universe are actually emitted from galaxies.
ALMA telescope finds 15 extremely dark galaxies unidentified until now
Their densities well match the prediction by theories of galaxy formation. 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".
Using ALMA and the Subaru Telescope , the research team is now seeking to uncover the overall picture of galaxy formation and evolution while conducting observations of much darker galaxies.
Conventional research on distant galaxies has been carried out mainly with visible light and near infrared light. However, it is possible that many galaxies in the universe have been overlooked as much of that radiation is largely absorbed by cosmic dust. That is why millimeter and submillimeter wave observations are important.
ALMA telescope finds 15 extremely dark galaxies unidentified until now
Stellar light absorbed by dust is reradiated from the dust as millimeter/submillimeter waves. Therefore galaxies, even those that it has not been possible to observe with optical telescopes, can be detected using these wavebands. Furthermore, millimeter/submillimeter waves are suitable for observation of distant galaxies. 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.
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. 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.
The research team observed a field named "Subaru/XMM-Newtown Deep Survey Field," located in the direction of the constellation Cetus, with the ALMA telescope. As a result, they succeeded in finding 15 extremely dark galaxies that were unidentified until now.
"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," said Hatsukade.
With the ALMA observations the team successfully measured the number density of galaxies approximately 10 times darker than the millimeter wave research results up to now.
The new results agree well with the prediction by the theories of galaxy formation. 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.
In regards to this, Professor Ohta commented, "This is 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."
Furthermore, the team concluded that approximately 80 percent of the sources of the cosmic background radiation within the millimeter/submillimeter wavebands are more "normal galaxies" like those detected by ALMA this time.
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