31 May, 2013
Researchers from Kyoto University, using the Atacama Large Millimeter/ submillimeter Array (ALMA) revealed that approximately 80% of the unidentified millimeter wave light from the Universe is actually emitted from galaxies. Thanks to ALMA’s high resolving power and sensitivity astronomers were able to pinpoint the locations of those galaxies rich in fine solid particles (dust).
Bunyo Hatsukade, a postdoctoral researcher, and Kouji Ohta, a professor, both from the Graduate School of Science from Kyoto University, lead the team that observed with the ALMA telescope the “Subaru/XMM-Newton Deep Survey Field” region in the direction of the constellation Cetus, and worked on a “Population Census” of Galaxies buried in dust. They succeeded in identifying 15 extremely dark galaxies which had been previously unknown.
In addition, they also successfully measured the number density of galaxies which are 10 times less luminous than ones previously observed with conventional millimeter instruments. Their densities match well with the prediction by theories of galaxy formation. Therefore, the researchers consider that they managed to capture “normal” galaxies, which had been impossible to detect up to now, compared to extremely bright “submillimeter-luminous galaxies”. Using the ALMA and Subaru telescopes, the research team is now seeking to uncover the overall picture of galaxy formation and evolution while conducting observations of much darker galaxies.
Furthermore, the team concluded that approximately 80% of the sources of the cosmic background light  within the millimeter/submillimeter wavebands are “normal galaxies” like those detected by ALMA in this study. Past observations showed the total amount of light emitted from the Universe at the millimeter/submillimeter wavebands. However, spatial resolution was not sufficient to identify the sources of all the light; only 10 – 20% of them were identified.
To gain an overall picture of galaxies in the Universe requires a much higher sensitivity for observation. For this research, only a part of the ALMA telescope was used (23~25 antennas out of the 66 final ones). As the number of antennas in the ALMA telescope increases, its ability to observe will also improve. Hatsukade expressed his hopes, saying “I want to clarify the overall picture of galaxy evolution. So, using the ALMA telescope, I would like to make observations of much darker galaxies, and also study star formation activities and the amount of dust in those galaxies in detail.” Professor Ohta also mentioned, “We are also planning to make thorough observations with visible light and infrared radiation, using the Subaru telescope. This is in order to explore the nature of galaxies which become darker due to light-absorbing dust. But for observations of extremely dark galaxies, we might need the Thirty Meter Telescope with much larger light-gathering power.”
Conventional research on distant galaxies has been carried out mainly with visible light  and near infrared light . However, many galaxies in the Universe may have been overlooked because much of their light is largely absorbed by cosmic dust . That is why millimeter and submillimeter wave observations  are important. Stellar light absorbed by dust is reradiated from the dust as millimeter/submillimeter waves. Therefore galaxies, even those ones we have not been able to observe with optical telescopes, can be detected using these wavebands. Furthermore, millimeter/submillimeter waves are suitable for observation of distant galaxies. Galaxies which contain much dust are known to be bright in infrared light, but when these galaxies are distant, the expansion of the universe shifts the infrared light into millimeter/submillimeter wavelengths. Because of this, distant galaxies do not get dim when we observe in the millimeter/submillimeter wavelengths.
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 “normal 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.
“It is thanks to the high performance of ALMA that observations like this have been made possible,” said Hatsukade. Professor Ohta added, “This is a big step towards getting the big picture of galaxy evolution.”
 Some types of radiation seem to be dimly emitted not from a certain object, but from the overall Universe. This is called the “cosmic background radiation”. The most typical is “3 K Cosmic Microwave Background Radiation,” which is supposed to be a remnant of the Big Bang. Telescopes with high sensitivity and resolution are needed to capture it.
 Visible light is a part of electromagnetic wave which we can see with our eyes. The wavelength of the visible light is 0.4 to 0.7 micrometer.
 Near infrared light is a part of electromagnetic wave with the wavelength of 0.7 to 3 micrometer.
 Cosmic dust is small solid particles consisted of silicon, carbon, iron and other elements with the size smaller than 1 micrometer.
 Millimeter wave is a part of electromagnetic wave with the wavelength of 1 millimeter to 1 centimeter. Submillimeter wave has a bit shorter wavelength, 0.1 millimeter to 1 millimeter. Cold cosmic dust and gas emit millimeter/submillimeter wave.
The research team members are:
・ Bunyo Hatsukade: Postdoctoral fellow, Kyoto University and the Japan Society for the Promotion of Science
・ Kouji Ohta: Professor, Kyoto University
・ Akifumi Seko: Graduate student, Kyoto University
・ Kiyoto Yabe: Postdoctoral Fellow, National Astronomical Observatory of Japan
・ Masayuki Akiyama: Associate professor, Tohoku University
The research is described in the paper “FAINT END OF 1.3 mm NUMBER COUNTS REVEALED BY ALMA” in the Astrophysical Journal Letters published on June 1st, 2013.
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) in Taiwan and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).
ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
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