A research group led by Hiroshima University has revealed a picture of
the increasing fraction of massive star-forming galaxies in the distant
universe. Massive star-forming galaxies in the distant universe, about 5
billion years ago, trace large-scale structure in the universe. In the
nearby universe, about 3 billion years ago, massive star-forming
galaxies are not apparent. This change in the way star-forming galaxies
trace the matter distribution is consistent with the picture of galaxy
evolution established by other independent studies.
Galaxies in the universe trace patterns on very large scales; there
are large empty regions (called "voids") and dense regions where the
galaxies exist. This distribution is called the cosmic web. The most
massive concentrations of galaxies are clusters. The formation of the
cosmic web is governed by the action of gravity on the invisible
mysterious "dark matter" that exists throughout the universe. The normal
baryonic material one can see falls into the dark matter halos and
forms galaxies. The action of gravity over about 14-billion-year history
of the universe makes the halos cluster together. The location of
galaxies or clusters in this enormous cosmic web tests our understanding
of the way structure forms in the universe.
Increasingly, deeper and more extensive observations with telescopes
like Subaru Telescope provide a clearer picture of the way galaxies
evolve within the cosmic web. Of course, one cannot see the dark matter
directly. However, one can use the galaxies that are seen to trace the
dark matter. It is also possible to use the way the gravity of clusters
of galaxies distort more distant background galaxies, weak gravitational
lensing, as another tracer.
The Hiroshima group combined these two tracers: galaxies and their
weak lensing signal to map the changing role of massive star-forming
galaxies as the universe evolves.
Weak lensing is a phenomenon that provides a powerful technique for
mapping the changing contribution of star-forming galaxies as tracers of
the cosmic web. The cluster of galaxies and surrounding dark matter
halo act as a gravitational lens. The lens bends the light passing
through from more distant galaxies and distorts the images of them. The
distortions of the appearance of the background galaxies provide a
two-dimensional image of the foreground dark matter distribution that
acts as a huge lens. The excellent imaging of the Subaru Telescope
covering large regions of the sky provides exactly the data needed to
construct maps of this weak lensing.
Dr. Yousuke Utsumi, a member of Hyper Suprime-Cam building team and a
project assistant professor at Hiroshima University, conducted a 1-hour
observation of a 4-deg2 patch of sky in the direction of the
constellation Cancer. Figure 1
shows a close-up view of a cluster of galaxies with the weak lensing
map tracing the matter distribution. The highest peaks in the maps
correspond the foreground massive clusters of galaxies that lie 5
billion light-years away.
To map the three-dimensional distribution of the foreground galaxies,
spectrographs on large telescopes like the 6.5-meter MMT disperse the
light with a grating. The expansion of the universe shifts the light to
the red and by measuring this shift one measures the distances to the
galaxies. Using spectroscopy places the galaxies in the cosmic web. The
observations locate star-forming galaxies and those that are no longer
forming stars.
Collaborators led by Dr. Margaret Geller (Harvard-Smithsonian Center
for Astrophysics) conducted spectroscopic measurements for galaxies. The
Hectospec instrument on the MMT enables measurements of redshifts for
250 galaxies at a time. The survey contains measurements for 12,000
galaxies.
The MMT redshift survey provides the map for the way all types of
galaxies might contribute to the weak lensing map. Because the MMT
survey provides distances to the galaxies, slices of the map at
different distances corresponding to different epochs in the history of
the universe can also be made and compared with the lensing map.
The MMT survey provides a predicted map of the cosmic web based on
the positions of galaxies in three-dimensional space. Research team
compared this map with the weak lensing map to discover the
similarities. Figure 2
shows that both the highest peak and the largest empty regions are
similar in the two maps. In other words, the matter distribution traced
by the foreground galaxies and the distribution traced by the Subaru
weak lensing map are similar. There are two complementary views of the
cosmic web in this patch of the universe.
Figure 2: Distribution of mass (left) and galaxies
(right) in the corresponding area. The conspicuous feature in the galaxy
distribution also is visible in the left side, mass distribution, while
the areas with no structure in the right also has no feature in the
left. (Credit: Hiroshima University/NAOJ). Hi-res image
If they slice up the three-dimensional map in different redshift or time
slices, they can examine the way the correspondence between these maps
and the weak lensing map changes for different slices (Figure 3).
Remarkably, the distribution of star-forming galaxies around a cluster
of galaxies in the more distant universe (5 billion years ago)
corresponds much more closely with the weak lensing map than a slice of
the more nearby universe (3 billion years ago). In other words, the
contribution of star-forming galaxies to the cosmic web is more
prominent in the distant universe. These maps are the first
demonstration of this effect in the weak lensing signal (Figure 4).
Figure 3: The distribution of galaxies with respect to
the distance. The panels show the three-dimensional distribution of the
galaxies, viewed from the observer on Earth. Red points represent
quiescent galaxies and blue points are star-forming galaxies. Boxes in
the cone are 3 and 5 billion light-years from the observer. The maps
next to the enclosed areas show the corresponding distribution of
galaxies. (Credit: Hiroshima University/NAOJ). Hi-res image
Figure 4: Close-ups of the cluster of galaxies at 3
billion light years (top) and 5 billion light years (bottom). These
panels show the distribution of mass (left), quiescent galaxies
(middle), and star forming galaxies (right), respectively. Three billion
years ago, it is hard to see any similarity between the star-forming
galaxies and the mass distribution, but there is much greater similarity
in the maps of 5 billion years ago. (Credit: Hiroshima University/NAOJ). Hi-res image
The research team provides a new window on galaxy evolution by
comparing the three-dimensional galaxy distribution mapped with a
redshift survey including star-forming galaxies to a weak lensing map
based on Subaru imaging.
"It turns out that the contribution of star-forming galaxies as
tracers of the mass distribution in the distant universe is not
negligible," said Dr. Utsumi. "The HSC weak lensing map should contain
signals from more distant galaxies in the 8 billion-year-old universe.
Deeper redshift surveys combined with similar weak lensing maps should
reveal an even greater contribution of star-forming galaxies as tracers
of the matter distribution in this higher redshift range. Using the next
generation spectrograph for the Subaru Telescope, Prime Focus
Spectrograph (PFS), we hope to extend our maps to the interesting era."
This research is published in the Astrophysical Journal in its
December 14, 2016 on-line version and December 20, 2016 in the printed
version, Volume 833, Number 2. The title of the paper is "A weak lensing
view of the downsizing of star-forming galaxies" by Y. Utsumi et al.,
which is also available in preprint from arXiv:1606.07439v2. This work
is supported by a JSPS Grant-in-Aid for Young Scientists (B)
(JP26800103) and a MEXT Grant-in-Aid for Scientific Research on
Innovative Areas (JP24103003).
Authors:
- Yousuke Utsumi: Hiroshima Astrophysical Science Center, Hiroshima University, Japan
- Margaret J. Geller: Smithsonian Astrophysical Observatory, USA
- Ian P. Dell'Antonio: Department of Physics, Brown University, USA
- Yukiko Kamata: National Astronomical Observatory of Japan (NAOJ), Japan
- Satoshi Kawanomoto: NAOJ, Japan
- Michitaro Koike: NAOJ, Japan
- Yutaka Komiyama: NAOJ, Japan; Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI), Japan
- Shintaro Koshida: Subaru Telescope, NAOJ, USA
- Sogo Mineo: NAOJ, Japan
- Satoshi Miyazaki: NAOJ, Japan; Department of Astronomical Science, SOKENDAI, Japan
- Junya Sakurai: NAOJ, Japan; Department of Astronomical Science, SOKENDAI, Japan
- Philip J. Tait: Subaru Telescope, NAOJ, USA
- Tsuyoshi Terai: Subaru Telescope, NAOJ, USA
- Daigo Tomono: Subaru Telescope, NAOJ, USA
- Tomonori Usuda: NAOJ, Japan; Department of Astronomical Science, SOKENDAI, Japan
- Yoshihiko Yamada: NAOJ, Japan
- Harus J. Zahid: Smithsonian Astrophysical Observatory, USA
Links:
- Press release from Hiroshima University
- Dark Matter Map Begins to Reveal the Universe's Early History
- Image of M31 Heralds the Dawn of HSC's Productivity
- Hyper Suprime-Cam Subaru Strategic Program
Source: Subaru Telescope
