The new maps of dark matter

A preliminary survey of the sky, made a new room off the field installed at the Japanese Subaru Telescope, allowed astronomers to reveal well-9 concentrations of dark matter. The findings, published in the Astrophysical Journal, are comforting to understand the role of dark energy in connection with the expansion of the Universe. The next step will be to expand the survey to cover at least a thousand square degrees of sky.


Credit: NAOJ/HSC Project

Researchers from the National Astronomical Observatory of Japan (NAOJ), the University of Tokyo and other institutions have begun a survey of the sky to study the distribution of dark matter using the Hyper Suprime-Cam, a new room off the field installed at the telescope Subaru located in Hawaii. The first results of the observations, which cover an area of 2.3 square degrees towards the constellation of Cancer, revealed well-9 concentrations of dark matter, each having a typical mass of a galaxy cluster. Analyze how it is spatially distributed dark matter and how it varies over time is essential for understanding the role of dark energy that controls the expansion of the Universe. Moreover, these data demonstrate that astronomers now have the technology and the tools most appropriate to study dark energy. The next step will be to expand the survey to cover at least a thousand square degrees of the sky in order to obtain additional data in order to unravel the mystery of dark energy and, therefore, of the cosmic expansion.
Map the spatial distribution of dark matter on large regions of the sky is of paramount importance to understand in more detail the properties of dark energy, the enigmatic component that is causing the accelerated expansion of the Universe. In fact, these preliminary results are showing that with current research techniques and with the Hyper Suprime-Cam, it is possible to explore the spatial distribution of dark matter and how it has evolved over time, reveal the mystery of dark energy and then track the history of the cosmic expansion in unprecedented detail.


Credit: NAOJ/HSC Project

Since 1929, when Edwin Hubble discovered the expansion of the universe, astronomers have begun to use a cosmological model that indicated a rate of expansion of space slowing over time. We know that gravity, until some time ago considered the only known force that acts between the galaxies, it is opposed to the expansion. But in the 90s, the observations of distant Type Ia supernovae showed that the Universe is expanding faster today than ever before. This discovery required the introduction of a new physical concept: or is there some kind of “dark energy” of repulsive nature, that tends to alienate the galaxies, or the physics of gravity requires some revision in the most fundamental level.
So to unravel the mystery of the accelerated expansion, it is important to analyze the relationship between the rate of expansion of the universe and that to which formed the astrophysical objects. For example, if the universe is expanding rapidly, it takes longer to matter to collapse and form galaxies. Conversely, if the Universe expands more slowly, the cosmic structures will be formed more easily. In fact, there is a direct relationship between the history of structure formation and history of the cosmic expansion. The problem is that most of the matter in the Universe is really “dark”, it does not emit light and therefore cannot be detected directly by telescopes.
A technique to overcome this problem is based on the “weak” gravitational lensing or “weak lensing”. A concentration of dark matter acts as a sort of “cosmic lens” which deflects the light rays coming from the most distant objects. Now, observing the deformation of distant objects caused by the effect of the gravitational lens, it is possible to determine the spatial distribution of dark matter interposed along the line of sight. This analysis of the effects of dark matter allows researchers to determine how has thickened over time. The process of aggregation of dark matter may be related with the history of the cosmic expansion and could reveal some physical properties of dark energy, its strength and evolution over time.
To obtain a sufficient amount of data, astronomers must observe galaxies that are located at least to more than a billion years light and which are distributed spatially in an area of the sky larger than a thousand square degrees (about 1/40 of ‘whole sky). The combination achieved by putting together the Subaru telescope, with its opening diameter of 8.2 meters, and the Suprime-Cam, the room earlier than the new one, which has a field of view equal to 1/10 of a square degree (about the size underlying the Full Moon), it was one of the most significant technological breakthroughs in research of faint and distant objects. However, even in the case of this powerful combination instrumental, explore a thousand square degrees of the sky, at a given depth, is not very realistic. “This is why we have spent ten years developing the Hyper Suprime-Cam, which has a higher image quality to Suprime-Cam and a field of view over seven times larger,” says Satoshi Miyazaki of the National Astronomical Observatory of Japan’s Advanced Technology Center, principal investigator of the research team and author of the study published in the Astrophysical Journal.
The Hyper Suprime-Cam was installed at the Subaru telescope in 2012. After the first test, starting in March 2014 was made publicly available to the astronomical community. Currently, there is an ongoing observation program “strategic”, consisting of more than 300 nights of observation planned in a period of 5 years. The room, equipped with 870 million pixels, provides images covering an area of sky comparable to that applied by nine full moons in a single exposure, with a very minimal distortion and with a resolution of 7/1000 of a degree (0, 5 seconds of arc).


Credit: NAOJ/HSC Project

The researchers analyzed the preliminary data provided by the Hyper Suprime-Cam to check its power in the exploratory map the distribution of dark matter with the technique of “weak lensing”. The data collected from an area of sky covering 2.3 square degrees, considering an exposure of about two hours, provided a series of sharp images of many galaxies. In this way, the scientists were able to construct a map of the spatial distribution of dark matter. The results of the observations led to the discovery of nine regions where the density of dark matter is high. In addition, for each of these nine regions it is estimated an equivalent mass of a typical cluster of galaxies. The reliability of the technique of “weak lensing”, and then the resulting maps of the distribution of dark matter, were also confirmed by observations made with other telescopes that show real galaxy clusters at the nine regions with high density identified by Hyper Suprime-Cam. Optical Identification of clusters, the astronomers used the archive Deep Lens Survey.
The number of galaxy clusters found by the Hyper Suprime-Cam exceeds that derived from predictions of current models that describe the primordial cosmic history of the Universe. So, as the map will be extended to cover at least a thousand square degrees, the data should reveal whether this excess is really real or whether it is an instrumental artifact. If the excess will be confirmed reliably, the conclusion might be that in the past there’s been so much dark energy as expected, which allowed the Universe to expand slowly and the stars and the galaxies formed quickly.
In short, using the technique of “weak lensing” to map the distribution of dark matter is also a way to find astronomical objects using their mass, know that there is “something” and its weight at the same time. It provides a direct measure of the mass that is not possible to obtain with other methods. Therefore, the maps that allow you to measure the mass of the dark matter distribution are an essential tool to understand in a more precise and detailed the history of the Universe.

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