Month: March 2018

Unprecedentedly Wide And Sharp Dark Matter Map

A research team of multiple institutes, including the National Astronomical Observatory of Japan and University of Tokyo, released an unprecedentedly wide and sharp dark matter map based on the newly obtained imaging data by Hyper Suprime-Cam on the Subaru Telescope. The dark matter distribution is estimated by the weak gravitational lensing technique. The team located the positions and lensing signals of the dark matter halos and found indications that the number of halos could be inconsistent with what the simplest cosmological model suggests. This could be a new clue to understanding why the expansion of the Universe is accelerating.

Mystery of the accelerated Universe

In the 1930’s, Edwin Hubble and his colleagues discovered the expansion of the Universe. This was a big surprise to most of the people who believed that the Universe stayed the same throughout eternity. A formula relating matter and the geometry of space-time was required in order to express the expansion of the Universe mathematically. Coincidentally, Einstein had already developed just such a formula. Modern cosmology is based on Einstein’s theory for gravity.

It had been thought that the expansion is decelerating over time because the contents of the Universe (matter) attract each other. But in the late 1990’s, it was found that the expansion has been accelerating since about 8 Giga years ago. This was another big surprise which earned the astronomers who found the expansion a Nobel Prize in 2011. To explain the acceleration, we have to consider something new in the Universe which repels the space.

The simplest resolution is to put the cosmological constant back into Einstein’s equation. The cosmological constant was originally introduced by Einstein to realize a static universe, but was abandoned after the discovery of the expansion of the Universe. The standard cosmological model (called LCDM) incorporates the cosmological constant. LCDM is supported by many observations, but the question of what causes the acceleration still remains. This is one of the biggest problems in modern cosmology.

Wide and deep imaging survey using Hyper Suprime-Cam

The team is leading a large scale imaging survey using Hyper Suprime-Cam (HSC) to probe the mystery of the accelerating Universe. The key here is to examine the expansion history of the Universe very carefully.

In the early Universe, matter was distributed almost but not quite uniformly. There were slight fluctuations in the density which can now be observed through the temperature fluctuations of the cosmic microwave background. These slight matter fluctuations evolved over cosmic time because of the mutual gravitational attraction of matter, and eventually the large scale structure of the present day Universe become visible. It is known that the growth rate of the structure strongly depends on how the Universe expands. For example, if the expansion rate is high, it is hard for matter to contract and the growth rate is suppressed. This means that the expansion history can be probed inversely through the observation of the growth rate.

It is important to note that growth rate cannot be probed well if we only observe visible matter (stars and galaxies). This is because we now know that nearly 80 % of the matter is an invisible substance called dark matter. The team adopted the ‘weak gravitation lensing technique.’ The images of distant galaxies are slightly distorted by the gravitational field generated by the foreground dark matter distribution. Analysis of the systematic distortion enables us to reconstruct the foreground dark matter distribution.

This technique is observationally very demanding because the distortion of each galaxy is generally very subtle. Precise shape measurements of faint and apparently small galaxies are required. This motivated the team to develop Hyper Suprime-Cam. They have been carrying out a wide field imaging survey using Hyper Suprime-Cam since March 2014. At this writing in February 2018, 60 % of the survey has been completed.

Unprecedentedly wide and sharp dark matter map

The team is now presenting the dark matter map based on the imaging data taken by April 2016. This is only 11 % of the planned final map, but it is already unprecedentedly wide. There has never been such a sharp dark matter map covering such a wide area.

Imaging observations are made through five different color filters. By combining these color data, it is possible to make a crude estimate of the distances to the faint background galaxies (called photometric redshift). At the same time, the lensing efficiency becomes most prominent when the lens is located directly between the distant galaxy and the observer. Using the photometric redshift information, galaxies are grouped into redshift bins. Using this grouped galaxy sample, dark matter distribution is reconstructed using tomographic methods and thus the 3D distribution can be obtained. Data for 30 square degrees are used to reconstruct the redshift range between 0.1 (~1.3 G light-years) and 1.0 (~8 G light-years). At the redshift of 1.0, the angular span corresponds to 1.0 G x 0.25 G light-years. This 3D dark matter mass map is also quite new. This is the first time the increase in the number of dark matter halos over time can be seen observationally.

What the dark matter halo count suggests and future prospects

The team counted the number of dark matter halos whose lensing signal is above a certain threshold. This is one of the simplest measurements of the growth rate. It is suggested that the number count of the dark matter halos is less than what is expected from LCDM. This could indicate there is a flaw in LCDM and that we might have to consider an alternative rather than the simple cosmological constant (Note 1).

The statistical significance is, however, still limited as the large error bars suggest. There has been no conclusive evidence to reject LCDM, but many astronomers are interested in testing LCDM because discrepancies can be a useful probe to unlock the mystery of the accelerating Universe. Further observation and analysis are needed to confirm the discrepancy with higher significance. There are some other probes of the growth rate and such analysis are also underway (e.g. angular correlation of galaxy shapes) in the team to check the validity of standard LCDM.

Hubble Observes Exoplanet Atmosphere In More Detail Than Ever Before

An international team of scientists has used the NASA/ESA Hubble Space Telescope to study the atmosphere of the hot exoplanet WASP-39b. By combining this new data with older data they created the most complete study yet of an exoplanet atmosphere. The atmospheric composition of WASP-39b hints that the formation processes of exoplanets can be very different from those of our own Solar System giants.

Investigating exoplanet atmospheres can provide new insight into how and where planets form around a star. “We need to look outward to help us understand our own Solar System,” explains lead investigator Hannah Wakeford from the University of Exeter in the UK and the Space Telescope Science Institute in the USA.

Therefore the British-American team combined the capabilities of the NASA/ESA Hubble Space Telescope with those of other ground- and space-based telescopes for a detailed study of the exoplanet WASP-39b. They have produced the most complete spectrum of an exoplanet’s atmosphere possible with present-day technology [1].

WASP-39b is orbiting a Sun-like star, about 700 light-years from Earth. The exoplanet is classified as a “Hot-Saturn,” reflecting both its mass being similar to the planet Saturn in our own Solar System and its proximity to its parent star. This study found that the two planets, despite having a similar mass, are profoundly different in many ways. Not only is WASP-39b not known to have a ring system, it also has a puffy atmosphere that is free of high-altitude clouds. This characteristic allowed Hubble to peer deep into its atmosphere.

By dissecting starlight filtering through the planet’s atmosphere [2] the team found clear evidence for atmospheric water vapour. In fact, WASP-39b has three times as much water as Saturn does. Although the researchers had predicted they would see water vapour, they were surprised by the amount that they found. This surprise, combined with the water abundance allowed to infer the presence of large amount of heavier elements in the atmosphere. This in turn suggests that the planet was bombarded by a lot of icy material which gathered in its atmosphere. This kind of bombardment would only be possible if WASP-39b formed much further away from its host star than it is right now.

“WASP-39b shows exoplanets are full of surprises and can have very different compositions than those of our Solar System,” says co-author David Sing from the University of Exeter, UK.

The analysis of the atmospheric composition and the current position of the planet indicate that WASP-39b most likely underwent an interesting inward migration, making an epic journey across its planetary system. “Exoplanets are showing us that planet formation is more complicated and more confusing than we thought it was. And that’s fantastic!,” adds Wakeford.

Having made its incredible inward journey WASP-39b is now eight times closer to its parent star, WASP-39, than Mercury is to the Sun and it takes only four days to complete an orbit. The planet is also tidally locked, meaning it always shows the same side to its star. Wakeford and her team measured the temperature of WASP-39b to be a scorching 750 degrees Celsius. Although only one side of the planet faces its parent star, powerful winds transport heat from the bright side around the planet, keeping the dark side almost as hot.

“Hopefully this diversity we see in exoplanets will help us figure out all the different ways a planet can form and evolve,” explains David Sing.

Looking ahead, the team wants to use the NASA/ESA/CSA James Webb Space Telescope — scheduled to launch in 2019 — to capture an even more complete spectrum of the atmosphere of WASP-39b. James Webb will be able to collect data about the planet’s atmospheric carbon, which absorbs light of longer wavelengths than Hubble can see [3]. Wakeford concludes: “By calculating the amount of carbon and oxygen in the atmosphere, we can learn even more about where and how this planet formed.”