A research team led by Bunyo Hatsukade, a postdoc researcher, and
Kouji Ohta, a professor, both from the Graduate School of Science, Kyoto
University, revealed that approximately 80% of the unidentifiable
millimeter wave signals from the universe is actually emitted from
galaxies, based on the observations with ALMA (Atacama Large
Millimeter/submillimeter Array). ALMA's high resolving power and
sensitivity enables us to pinpoint the locations of those galaxies rich
in fine solid particles (dust).
With the ALMA telescope, the research team 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. 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.
Research background
Conventional research on distant galaxies have 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 (*1).
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 which 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.
Observations with ALMA
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."
Figure 1. This figure shows number density (red squares) for each
brightness of the galaxies derived from observation this time. Compared
to past observation results (blue squares), galaxies approximately ten
times as dark were detected. The curve shows the prediction by the
theories of galaxy formation. Credit: ALMA (ESO/NAOJ/NRAO) / Kyoto University
Figure 2. Artist's illustration of the observed field. In each close-up
view, left is the illustration of the optical (blue) and conventional
millimeter/submillimeter (red) image and right is the optical and ALMA
image.
Existing millimeter/submillimeter telescopes could not identify the
sources of the emission due to their low resolution, however, ALMA
pinpoints the galaxies which emit the emission. These objects were not
detected in the optical observations, which indicate that they are
heavily embedded in dust. Credit: ALMA (ESO/NAOJ/NRAO) / Kyoto University. Click to Enlarge(JPEG/ 1.5MB)
- Figure 2. Each part of artist's illustration
- SXDS1 31189: Image taken by existing millimeter/submillimeter telescopes overlaid on the optical image (artist's illustration)
- SXDS1 31189: Image taken by existing millimeter/submillimeter telescopes (artist's illustration)
- SXDS1 31189: Image taken by ALMA overlaid on the optical image (artist's illustration)
- SXDS1 31189: Image taken by ALMA (artist's illustration)
- SXDS3 110465: Image taken by existing millimeter/submillimeter telescopes overlaid on the optical image (artist's illustration)
- SXDS3 110465: Image taken by existing millimeter/submillimeter telescopes (artist's illustration)
- SXDS3 110465: Image taken by ALMA overlaid on the optical image (artist's illustration)
- SXDS3 110465: Image taken by ALMA (artist's illustration)
- SXDS1 1723: Image taken by existing millimeter/submillimeter telescopes overlaid on the optical image (artist's illustration)
- SXDS1 1723: Image taken by existing millimeter/submillimeter telescopes (artist's illustration)
- SXDS1 1723: Image taken by ALMA overlaid on the optical image (artist's illustration)
- SXDS1 1723: Image taken by ALMA (artist's illustration)
Furthermore, the team concluded that approximately 80% of the sources of the cosmic background radiation (*2)
within the millimeter/submillimeter wavebands are more "normal
galaxies" like those detected by ALMA this time.. Past observations
showed the total power of signals emitted from the universe with the
millimeter/submillimeter wavebands. However, spatial resolution was not
sufficient to identify the sources of all the signals; only 10 - 20% of
them were identified.
Future prospects
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, 23~25 antennas, were used. As the number of
antennas in the ALMA telescope increases, its observation ability will
also improve. Hatsukade expressed his hopes, saying "I want to clarify
the overall picture of galaxy evolution. So, using ALMA, I would like to
make observations of much fainter 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
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."
Notes
*1.
Cosmic dust is small solid particles consisted of silicon, carbon, iron and other elements with the size smaller than 1 micrometer.
Cosmic dust is small solid particles consisted of silicon, carbon, iron and other elements with the size smaller than 1 micrometer.
*2.
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. However, even when many dark celestial bodies emit radiation, the emission may be observed just as "cosmic background" radiation if the sensitivity and resolution of the telescope in use is insufficient. To clear up this misidentification, astronomers need telescopes with better sensitivity and resolution.
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. However, even when many dark celestial bodies emit radiation, the emission may be observed just as "cosmic background" radiation if the sensitivity and resolution of the telescope in use is insufficient. To clear up this misidentification, astronomers need telescopes with better sensitivity and resolution.
More Information
The research team members are:
・ Bunyo Hatsukade: Post-doctoral fellow, Kyoto University and the Japan Society for the Promotion of Science
・ Kouji Ota: Professor, Kyoto University
・ Akifumi Seko: Graduate student, Kyoto University
・ Kiyoto Yabe: Post-doctral fellow, National Astronomical Observatory of Japan, National Institutes of Natural Sciences
・ Masayuki Akiyama: Associate professor, Tohoku University
・ Bunyo Hatsukade: Post-doctoral fellow, Kyoto University and the Japan Society for the Promotion of Science
・ Kouji Ota: Professor, Kyoto University
・ Akifumi Seko: Graduate student, Kyoto University
・ Kiyoto Yabe: Post-doctral fellow, National Astronomical Observatory of Japan, National Institutes of Natural Sciences
・ 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.
Link
ALMA
- Click to Enlarge
ALMA array from the air
Credit: Clem & Adri Bacri-Normier (wingsforscience.com)/ESO
The Atacama Large Millimeter/submillimeter Array (ALMA), an
international astronomy facility, is a partnership of Europe, North
America and East Asia in cooperation with the Republic of Chile. ALMA is
funded in Europe by the European Southern Observatory (ESO), in North
America by the U.S. National Science Foundation (NSF) in cooperation
with the National Research Council of Canada (NRC) and the National
Science Council of Taiwan (NSC) and in East Asia by the National
Institutes of Natural Sciences (NINS) of Japan in cooperation with the
Academia Sinica (AS) in Taiwan. ALMA construction and operations are led
on behalf of Europe by ESO, on behalf of North America by the National
Radio Astronomy Observatory (NRAO), which is managed by Associated
Universities, Inc. (AUI) and on behalf of East Asia by the National
Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory
(JAO) provides the unified leadership and management of the
construction, commissioning and operation of ALMA.
