A team led by researchers from Osaka Sangyo University, with members
from Tohoku University, Japan Aerospace Exploration Agency (JAXA) and
others, has used the Suprime-Cam on the Subaru Telescope to create the
most-extensive map of neutral hydrogen gas in the early universe (Figure 1).
This cloud appears widely spread out across 160 million light-years in
and around a structure called the proto-supercluster. It is the largest
structure in the distant universe, and existed some 11.5 billion years
ago. Such a huge gas cloud is extremely valuable for studying
large-scale structure formation and the evolution of galaxies from gas
in the early universe, and merits further investigation.
Figure 1: The distribution of galaxies in the
proto-supercluster region 11.5 billion years ago (top left), and the
Subaru Telescope Suprime-Cam image used in this work (right, larger image).
Neutral hydrogen gas distribution is superposed on the Subaru image.
The red color indicates denser regions of the neutral hydrogen gas. Cyan
squares correspond to member galaxies in the proto-supercluster, while
objects without cyan squares are foreground galaxies and stars. The
distribution of neutral hydrogen gas does not align perfectly with the
galaxies. (Credit: Osaka Sangyo University/NAOJ)
"We are surprised because the dense gas structure is extended much
more than expected in the proto-supercluster," said Dr. Mawatari. "Wider
field observations with narrow-band filters are needed to grasp full
picture of this largest structure in the young Universe. This is exactly
the type of strong research that can be done with Hyper Suprime-Cam
(HSC) recently mounted at the Subaru Telescope. We intend to study the
gas – galaxy relation in various proto-superclusters using the HSC."
Understanding Matter Distribution in the Universe
Stars assembled to form galaxies, and galaxies are clustered to form
larger structures such as clusters or superclusters. Matter in the
current universe is structured in a hierarchical manner on scales of ~
100 million light-years. However, we cannot observe inhomogeneous
structure in any direction or distance over scales larger than that. One
important issue in modern astronomy is to clarify how perfectly the
large-scale uniformity and homogeneity in matter distribution is
maintained. In addition, astronomers seek to investigate the properties
of the seeds of large-scale structures (i.e., the initial matter
fluctuations) that existed at the beginning of the universe. Thus, it is
important to observe huge structures at various epochs (which
translates to distances). The study of gaseous matter as well as
galaxies is needed for an accurate and comprehensive understanding. This
is because local superclusters are known to be rich in gas.
Furthermore, it is clear that there are many newborn galaxies in ancient
(or distant) clusters. A detailed comparison between the spatial
distributions of galaxies and gas during the early epochs of the
universe is very important to understand process of galaxy formation
from the dim (low light-emitting) clumps of gas in the early universe.
In order to investigate early, dim gas clouds, astronomers take
advantage of the fact that light from bright distant objects gets dimmed
by foreground gas (giving an effect like a "shadow picture"). Since
neutral hydrogen in the gas cloud absorbs and dims light from background
objects at a certain wavelength, we can see characteristic absorption
feature in the spectrum of the background object. In many previous
observations, researchers used quasars (which are very bright and
distant) as background light sources. Because bright quasars are very
rare, opportunities for such observations are limited. This allows
astronomers to get information about the gas that lies only along the
line of sight between a single QSO and Earth in a wide survey area. It
has long been the goal to obtain "multi-dimensional" information of gas
(e.g., spatially resolve the gas clouds) rather than the
"one-dimensional" view currently available. This requires a new
approach.
Expanding the View
To widen their view of these objects in the early universe, Dr. Ken
Mawatari at Osaka Sangyo University and his colleagues recently
developed a scheme to analyze the spatial distribution of the neutral
hydrogen gas using imaging data of galaxies of the distant epoch (Figure 2).
There are two major advantages to this approach. First, instead of rare
quasars, the team uses numerous normal galaxies as background light
sources to investigate gas distribution at various places in the search
area. Second, they use imaging data taken with the narrow-band filter on
Suprime-cam. It is fine-tuned so that light with certain wavelengths
can be transmitted, to capture evidence of absorption by the neutral
hydrogen gas (the shadow picture effect). Compared with the traditional
scheme of observations based on spectroscopy of quasars, this new method
enables Mawatari and his collaborators to obtain wide-area gas
distribution information relatively quickly.
The researchers applied their scheme to the Subaru Telescope
Suprime-Cam imaging data taken in their previous large survey of
galaxies. The fields investigated in this work include the SSA22 field,
an ancestor of a supercluster of galaxies (proto-supercluster), where
young galaxies are formed actively, in the universe 11.5 billion years
ago in the early universe.
Figure 2: Schematic pictures of an analysis scheme of
previous work (left) and a new method (right). In the previous approach,
basically a single background light source (quasar) can be used in a
searched area. On the other hand, with the new scheme, it is easier to
spatially resolve the neutral hydrogen gas density by using many normal
galaxies in a searched area as background light sources. In the new
scheme, absorption strength by the neutral hydrogen gas is estimated by
measuring how much flux of the background galaxies becomes dimmed in the
narrow-band image, not by using spectrum. By combining this scheme with
the wide-area imaging ability of the Subaru Telescope, Mawatari, et al.
made the most-extensive map of neutral hydrogen gas ever created.
(Credit: Osaka Sangyo University/NAOJ)
New Maps of Neutral Hydrogen Distribution
The researchers' work resulted in very wide-area maps of the neutral hydrogen gas in the three fields studied (Figure 3).
It appears that the neutral hydrogen gas absorption is significantly
strong over the entire SSA22 proto-supercluster field compared with
those in the normal fields (SXDS and GOODS-N). It is clearly confirmed
that the proto-supercluster environment is rich in neutral hydrogen gas,
which is the major building block of galaxies.
Figure 3: Sky distribution of the neutral hydrogen gas
in the three fields studied in this work. While in the normal fields
(SXDS and GOODS-N) the neutral hydrogen gas density is consistent with
the average density in the entire universe at 11.5 billion years ago,
the neutral hydrogen gas density is higher than the average over the
entire SSA22 proto-supercluster field. Contours correspond to the
galaxies' number density. Bold, solid thin, and dashed contours mean the
average, high density, and low density regions, respectively. (Credit:
Osaka Sangyo University/NAOJ)
The team's work also revealed that gas distribution in the
proto-supercluster region does not align with the galaxies' distribution
perfectly (see Figure 1 and Figure 3).
While the proto-supercluster is rich in both galaxies and gas, there is
no local-scale dependency of gas amount correlated with the density of
galaxies inside the proto-supercluster. This result may mean that the
neutral hydrogen gas not only is associated with the individual galaxies
but also spreads out diffusely across intergalactic space only within
the proto-supercluster. Since the neutral hydrogen gas excess in the
SSA22 field is detected over the entire searched area, this overdense
gas structure is actually extended more than 160 million light-years. In
the traditional view of structure formation, matter density fluctuation
is thought to be smaller and large-scale high-density structure was
rarer in the early universe. The discovery that a gas structure that
extends across more than 160 million light-years (which is roughly same
as present-day superclusters in scale) already existed in the universe
11.5 billion years ago is a surprising result of this study.
By investigating spatial distribution of the neutral hydrogen gas in a
very large area, the researchers have provided a new window on the
relation between gas and galaxies in the young universe. The SSA22 huge
gas structure revealed by this work is considered a key object to test
the standard theory of structure formation, and so further investigation
is anticipated.
This research will be published in the journal of the British Royal Astronomical Society
(Monthly Notices of the Royal Astronomical Society, publisher Oxford
University Press) in its June, 2017 issue of the printed version
(Mawatari et al. 2017, MNRAS, 467, 3951, "Imaging of diffuse HI
absorption structure in the SSA22 protocluster region at z = 3.1"). This
work is supported by JSPS Grant-in-Aid JP26287034 and JP16H06713.
Source: Subaru Telescope
