The following release was received from the Institute
for Cosmic Ray Research at the University of Tokyo and is reprinted
here in its entirety for the convenience of our readers: Original Article: (http://www.icrr.u-tokyo.ac.jp/2013/12/09140001.html)
Figure 1: The color composite Subaru images around the
largest OIIB1 (center), zoom-in image of OIIB1 (top right), and the
eleven other OIIBs (left and right). The size of each small panel
corresponds to 400 thousand light years. The picture of Andromeda galaxy
(by Robert Gendler) in the top right panel was scaled as if it were at
the same distance as OIIB. Credit: NAOJ, the University of Tokyo
(Suraphong YUMA)
Using wide-field imaging data of Subaru Telescope, a
team of international astronomers led by Dr. Suraphong Yuma (JSPS
Fellow) and Dr. Masami Ouchi (Associate Professor) at ICRR, the
University of Tokyo has discovered twelve galaxies expelling hot oxygen
gas extending up to 250 thousand light years, beyond the sizes of the
galaxies. These galaxies are located 9 billion light years from the
Earth. Some of these galaxies host a super massive black hole (SMBH),
while some show violent star formation with no SMBH. The vast amount of
energy produced by the SMBH or star formation activity is able to heat
up the gas in the galaxies, and drive the strong outflow of hot oxygen
gas. These distant galaxies are thought to be in their final phase of
galaxy growth, with star formation being quenched by the expulsion of
gas needed to produce new stars. This is the first discovery of the
large physical extents of these energetic oxygen-gas outflows driven by
SMBH and star formation.
Astronomers attempt to understand how galaxies like
our Milky Way have formed and evolved over time by observing galaxies at
the present and in the past. Some galaxies are converting gas to stars
from a large reservoir of gas in the galaxy, while others have ceased
their star formation and are just letting their stars dying out. In the
latter galaxies, known as elliptical galaxies, there are almost no stars
younger than several billion years old, and so the quenching of star
formation is believed to have taken place several billion years
previously. This quenching takes place in the final stage of galaxy
growth, but the physical mechanism is not clear. It is thought that the
gas can escape from distant galaxies through heating by SMBH and/or
star-formation. However, existing observations do not make it possible
to determine which quenching mechanism is the most important.
"Because oxygen in the Universe is only produced
within galaxies by nuclear fusion in the cores of stars, we conceived of
the idea to identify distant galaxies with gas outflows from their
extended profiles of oxygen gas. We therefore looked for distant
galaxies associated with large clouds of ionized oxygen emission, which
were efficiently located with Subaru's prime focus camera, Suprime-Cam,
at optical wavelengths," explains Dr. Yuma. "So far, outflows have been
studied separately in galaxies with either SMBH or star formation,
whereas our search technique works regardless of the energy source. Our
search was conducted in the Subaru/XMM-Newton Deep Survey (SXDS) field
that has the ideal Subaru data sets."
Dr. Alyssa Drake, who recently completed her Ph.D. at
Liverpool John Moores University, UK, picks up the story. "As the
Universe expands, the light emitted by galaxies is stretched out on its
way to us, changing the colors of galaxies and making them redder. We
can use this fact to calculate the distances to galaxies if we measure
their colors accurately.
From the search the team pinpointed 12 galaxies whose
oxygen gas emission extends over more than 100,000 light years,
corresponding to the size of the Milky Way. Due to their spatial extent,
they call these galaxies [OII] blobs or OIIB. Subaru images (Figure 1) show diffuse oxygen emission extending beyond the stellar components of the galaxies.
While OIIBs are producing tens to hundreds of stars
per year from gas, the star-formation rates of well-matured OIIBs are
lower than those of typical star-forming galaxies with the same stellar
mass. "This would be a signature of quenching star-formation activity in
the process of gas outflow," remarks Associate Prof. Ouchi. "According
to our estimates, about 3% of star-forming galaxies at this epoch are
right in the middle of the gas outflow phase, which is a new and
important insight for understanding galaxy formation."
Gas outflow has been confirmed in two OIIBs by
optical spectroscopic observations with the Subaru telescope and the
Very Large Telescope operated by the European Southern Observatory
(ESO). "The spectra present absorption features made of cold gas
outflowing from the galaxy. The intense activity of star formation
and/or SMBH would trigger the outflow," explains co-author Dr. Chris
Simpson, a senior lecturer at Liverpool John Moores University.
One of the OIIBs has a very fast outflow velocity of
600 km/s, suggesting a strong gas flow energized by a powerful SMBH.
"This object, dubbed OIIB1, is the largest OIIB that we found in this
study. OIIB1 is more than twice as large as Milky Way; its oxygen
emission extends over 250,000 light years," adds Dr. Yuma. This giant
blob has been detected in X-rays, a clear signature of SMBH activity.
Prof. Masayuki Umemura of Tsukuba University comments, "This kind of
object is very rare and can be a milestone to reveal gas outflow powered
by SMBH."
This research is published in the December 10, 2013
issue of The Astrophysical Journal. The work was supported by KAKENHI
(23244025) Grant-in-Aid for Scientific Research (A) through Japan
Society for the Promotion of Science (JSPS).
Publication Details:
Suraphong Yuma*, Masami Ouchi, Alyssa B. Drake, Chris
Simpson, Kazuhiro Shimazaku, Kimihiko Nakajima, Yoshiaki Ono, Rieko
Momose, Masayuki Akiyama, Masao Mori, and Masayuki Umemura, "First
Systematic Search for Oxygen-Line Blobs at High Redshift: Uncovering AGN
Feedback and Star Formation Quenching," Astrophysical Journal Online
Edition: 2013/12/10 (Japan time).
Member of the research team are
- Suraphong Yuma (The University of Tokyo)
- Masami Ouchi (The University of Tokyo)
- Alyssa B. Drake (Liverpool John Moores University)
- Chris Simpson (Liverpool John Moores University)
- Kazuhiro Shimasaku (The University of Tokyo)
- Kimihiko Nakajima (The University of Tokyo)
- Yoshiaki Ono (The University of Tokyo)
- Rieko Momose (The University of Tokyo)
- Masayuki Akiyama (Tohoku University)
- Masao Mori (University of Tsukuba)
- Masayuki Umemura (University of Tsukuba)
Souce: Subaru Telescope
