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[Research] Tracing the Cosmic Web with star forming galaxies in the distant universe

 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 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 in two different kinds of maps 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 and dense regions where the galaxies are. 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". The normal baryonic material we can see falls into the dark matter halos and forms stars. The action of gravity over the fourteen billion year history of the universe makes the halos cluster together. The location of 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 provide a clearer and clearer picture of the way galaxies evolve within the cosmic web. Of course, we cannot see the dark matter directly. We can use the galaxies we see to trace the dark matter. We can also 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 powerful technique for mapping the changing contribution of star-forming galaxies as tracers of the cosmic web. A massive cluster of galaxies distorts the shapes of background galaxies behind of the cluster. The cluster acts as a gravitational lens and the phenomenon is called "weak lensing". The distortions of the background galaxies provide a 2D 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 are exactly the data needed to construct the weak lensing maps.

 Dr. Yousuke Utsumi, a member of Hyper Suprime-Cam builders and a project assistant professor at Hiroshima University, conducted a 1 hour observation with Hyper Suprime-Cam of a 4deg2 patch of sky at Cancer.  Figure 1 shows a close-up view of a cluster of galaxies with the weak lensing map. The highest peaks in the maps correspond the foreground massive clusters of galaxies.

 To map the 3D 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 we measure 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 3D positions of galaxies. We can compare 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; we have two complementary views of the cosmic web in this patch of the universe.

 If we slice up the 3D map in different redshift or time slices, we 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 becomes prominent in a distant universe. These maps are the first demonstration of this effect in the weak lensing signal (Figure 4)

 We provide a new window on galaxy evolution by comparing the 3D galaxy distribution mapped with a redshift survey including star-forming galaxies to a weak lensing map based on Subaru imaging. Dr. Utsumi says "It turns out that the contribution of star-forming galaxies as tracers of the mass distribution in the distant universe is not negligible. 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).

Figure 1. A close-up view of a cluster of galaxies observed with HSC (i-band as Red) and the Kitt-Peak Mayall 4-m telescope (R-band as Green, V band as Blue).

Figure 2. The weak lensing map (left) and the map revealed by galaxy distribution (right).

Figure 3. The distribution of galaxies in the 3D redshift survey. Red points represent quiescent galaxies and blue points are star-forming galaxies. The size of a point reflects the mass of the galaxy. Boxes in the cone are 3 and 5 billion light years from us (left bottom). The maps next to the boxes show the mass distribution constructed from the redshift survey.

Figure 4. Close-ups of the weak lensing map and the predicted maps from the redshift survey for quiescent galaxies and star-forming galaxies.  The upper panels are a picture of the region 3 billion years ago and lower panels show the picture 5 billion years ago. Three billion years ago, it is hard to see any similarity between the predicted map for star-forming galaxies and the weak lensing map, but there is much greater similarity in the maps 5 billion years ago.

Full Citation:

Authors: Yousuke Utsumi, Margaret J. Geller, Ian P. Dell'Antonio, Yukiko Kamata, Satoshi Kawanomoto, Michitaro Koike, Yutaka Komiyama, Shintaro Koshida, Sogo Mineo, Satoshi Miyazaki, Jyunya Sakurai, Philip J. Tait, Tsuyoshi Terai, Daigo Tomono, Tomonori Usuda, Yoshihiko Yamada, Harus J. Zahid
Title: A weak lensing view of the downsizing of star-forming galaxies
Journal: The Astrophysical Journal, Volume 833, Number 2 (December, 2016)
DOI: 10.3847/1538-4357/833/2/156

Media Contact:

Dr. Yousuke Utsumi
Assistant Professor (Special Appointment)
Hiroshima Astrophysical Science Center

youtsumi*hiroshima-u.ac.jp (Please change * into @)