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Mass of Dark Matter Revealed by Precise Measurements of the Galaxy
Author: National Astronomical Observatory of Japan
Published on Oct 8, 2012 - 7:26:43 AM

20121003-fig4-vera.jpg
Image of the Galaxy image seen from above, and the distribution of the precisely measured 52 stars (marked in red).
Oct. 5, 2012 - A research team, led by Associate Professor Mareki Honma(NAOJ) has reached the following two conclusions:

(1) The distance from the solar system to the galactic center is approximately 26,100 light years (*1);
(2) The galactic rotation velocity in the solar system (V0) (*2) is approximately 240km/s.

These conclusions are drawn based upon the precise distances to celestial objects in the Galaxy and their proper motions, a technical term to describe the stars' change in position. The International Astronomical Union has endorsed V0=220km/s; this value was announced in 1986. When the V0 value derived from this research is applied, the mass of dark matter in the galaxy is about 20% larger than what has been considered so far.

VERA (VLBI Exploration of Radio Astrometry) (*3) and other VLBI arrays in the world were utilized to get most of the precise distances to about 50 objects and the data on their proper motions. At VERA, triangulation (*4) was used to precisely measure the distances to the objects and their proper motions.

The research team plans to further increase the number for objects of which they measure the precise distances and proper motions. In addition, the team has hopes to deepen understanding of the Galaxy in which we all live by creating a 3D Galaxy map, and then determine motions and mass distribution in the Galaxy based upon the 3D map.

*1: A light year is a unit of distance, which is how far light can travel in one year at the speed of 300,000km/sec. It is equal to approximately 9.5 trillion km.

*2: The Galactic disk rotates around the Galactic center. It has been found that the rotation speed is almost constant, no matter whether it is near the center or near the outer edge.

*3: VERA stands for VLBI Exploration of Radio Astrometry while VLBI means Very Long Baseline Interferometry. This is a project to precisely measure distances to celestial objects with the principles of triangulation (see *4), and to identify the 3D structure of the Galaxy.

Construction work was completed in 2002, and astronomical observations to measure distances to objects have been done on a regular basis since 2007. The 20-meter radio telescopes used for VERA are located at the following four places: Oshu City in Iwate Prefecture, Satsumasendai City in Kagoshima Prefecture, Ogasawara Village in Tokyo, and Ishigaki City in Okinawa Prefecture.

*4: Triangulation is a method to observe an object from two fixed spots and determine a distance from the difference of the apparent direction. It is used as the most reliable method to determine distances to stars located outside the solar system.

The difference of direction as seen from two different places is called parallax, or trigonometric parallax. When the distance from two points is equal, the trigonometric parallax is in inverse proportion to the distance to the star. They took advantage of the fact that the earth revolves around the sun once over a year in order to measure the distance to the stars. That means, for example, that the direction from the earth in summer is different from the one in winter by approximately one thirty-millionth degree when observing a star in the Galactic center.

Even though the star is located in the Galaxy, the direction difference should be precisely measured because of the long distance to the star. The VERA project makes full use of the cutting-edge technologies of astronomical observation, so star location is measured with the super high accuracy of within one 360-millionth degree.

1. Research Overview

A research team, led by Associate Professor Mareki Honma from the National Astronomical Observatory of Japan (NAOJ), has succeeded in precisely determining the astronomical yardstick for the Galaxy based upon the precise distance measurements with VERA from NAOJ and other advanced radio telescopes. The new findings are that the distance from the sun to the Galactic center is 26,100 light-years, and that the Galactic rotation velocity in the solar system is 240km/s.

The Galactic rotation velocity from this research is higher than that of previously known (220km/s). This results in the conclusion that the mass of the Galaxy, especially that of dark matter, is about 20% larger than what has been previously considered.

2. What is Our Milky Way Galaxy Like as a Galaxy?

What is our Milky Way Galaxy like? --- How big? How heavy? What shape? We know now that the Galaxy is a spiral galaxy, but precise information including its size, shape, and rotation velocity, has not been made clear yet.

The biggest reason is that we cannot see the Galaxy from the outside since we stay inside the Galaxy. In order to see the whole shape of the Galaxy from the inside, it is necessary to precisely measure the distance of each one of the many Galactic objects, and make a "Galactic map" with an overhead view.

In that case, trigonometric parallax, or annual parallax, is utilized to measure the distance to an object without any "what if?" assumptions. The trigonometric parallax is the difference in position of an object, which is generated when the earth orbits around the sun (see Figure 1). However, the difference is extremely small; even that of Alpha Centauri, the nearest star from the Sun, is one arcsecond or less. Therefore, we could not measure any areas beyond 1,000 light-years away from the solar system by using the annual parallax because of the measurable limit of the parallax. The distance of 1,000 light-years is far smaller than the distance from the Sun to the Galactic center (approximately 26,100 light-years, as mentioned later). This means that measuring the area of the Galaxy has been a frontier left for modern astronomy.

3. Precise Triangulation by Radio Interferometers

VERA (VLBI Exploration of Radio Astrometry), with which we have continued our research, is a group of radio interferometers; 20-meter radio telescopes are installed in Oshu City in Iwate Prefecture, Satsumasendai City in Kagoshima Prefecture, Ogasawara Village in Tokyo, and Ishigaki City in Okinawa Prefecture (see Figure 2). This is a project to precisely measure distances to objects with the technology of Very Long Baseline Interferometry (VLBI), and to identify the 3D structure of the Galaxy. Mizusawa VLBI Observatory of NAOJ is operating VERA in cooperation with Kagoshima University and other organizations. The construction of VERA was finished in 2002, and astronomical observations to measure distances to stars have been regularly conducted since 2007.

VERA has completed observations of more than 100 radio objects (maser sources) in the Galaxy, and so far we have reported on the precise distances and motions of approximately 30 of those objects. This time, we determined the Galactic yardstick based upon the precise distance measurements with the observation results of 52 objects in total (Figures 3 and 4): 19 star-forming regions (newly-born stars) observed at VERA, and other objects observed by the Very Long Baseline Array (VLBA) of US equipment, and by the European VLBI Network (EVN). In this report, the latest measurement results of VERA were added, making it the world's first analysis of Galactic structure using more than 50 objects.

4. Galactic Yardsticks Precisely Determined

This research successfully managed to determine the Galactic yardsticks precisely: the R0 value of the distance to the Galactic center from the solar system, and theΘ0 value of the Galactic rotation velocity in the solar system. The distance to the Galactic center is R0=8.0 +/- 0.5 kpc (approximately 26,100 light-years +/-1,600 light-years), and the Galactic rotation velocity in the solar system isΘ0=240 +/- 14 km/s. (See Figure 5)

The value of Galactic rotation velocity from this research is larger than V0=220km/s, the one endorsed by the International Astronomical Union (IAU) since 1986. This finding now forces them to change the rotation speed and mass distribution of the Galaxy, as mentioned later.

On the other hand, the distance to the galactic center is almost equal, within an 8.5 kpc (approximately 27,700 light-years) margin of error, as that endorsed by the IAU since 1986. However, one of the most important points is that this measurement was directly and more precisely done with the triangulation method and is more precise. In addition to these yardsticks, it is also confirmed that the Galactic rotation velocity is almost constant between the distances of 10,000 and 50,000 light-years from the Galactic center.

5. More Dark Matter Exists

In general, galactic rotation velocity is determined by the balance with galactic gravity. Therefore measuring galactic rotation is equal to measuring Galaxy's mass.

When the Milky Way's mass within the solar system is measured with the latest Galactic rotation velocity from this research (Θ0=240 km/s), the amount should increase by no less than approximately 20%. It means that the total amount of dark matter in this area is larger than projected up until now.

The current main theory of dark matter is that it consists of elementary particles. At the moment, some experimental particle physicists have been carrying out dark-matter detection experiments to directly detect dark matter. Our research findings also impact any experiments with for dark matter search.

The findings of this research emphasize again that the precise measurements of the Milky Way Galaxy promoted by VERA are effective to determine the structure of the Galaxy. This is also a milestone achievement as the year 2012 is the 10th anniversary of VERA's completion, and also the turning point of the fifth year from its initial results. In the future, we will continue observations with VERA to increase the number of objects measured to several hundred in about 10 years. We expect to determine the basic structure of the Milky Way Galaxy more precisely. In addition to VERA, other observations are also available with VLBA and EVN, and with the GAIA satellite expected to launch in 2013. In light of this, our understanding of the Galaxy should drastically improve in the next 10 years.

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