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 Milky Way 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 240 km/s.
The Galactic rotation velocity from this research is higher than that of previously known (220 km/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.
Astronomical yardstick for the Galaxy based upon the precise distance measurements. Left: Image of our galaxy seen from an angel; Rght: Our galaxy seen from above. Image Credit:NAOJ
What is the Milky Way galaxy like?
What is our 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 Milky Way 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.
Photo: Looking up at the Milky Way Galaxy from VERA. The precise size, weight, and shape of the Mily Way galaxy are still unknown. Image Credit: NAOJ
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 arc-second 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 our galaxy has been a frontier left for modern astronomy.
Figure 1: Image of the annual parallax measurement. Image Credit: NAOJ
Because the Earth revolves around the Sun over a year, the star position (direction as viewed from the earth) changes slightly in summer and in winter. This change of star positions is called annual parallax. The parallax value is small for distant stars while large for nearby ones. Therefore, the distance to a star can be specified when its parallax value is measured.
Precise Triangulation by Radio Interferometers
VERA (VLBI Exploration of Radio Astrometry), with which the team has continued their research, is a group of four radio interferometers; installed on 20-meter radio telescopes (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 Milky Way. The construction of VERA was finished in 2002, and astronomical observations to measure distances to stars have been regularly conducted since 2007.
Figure 2: Layout of VERA telescopes. Simultaneously observing an object with telescopes at four places provides the same performance as a gigantic telescope of approximately 2,300 km in diameter, the same size as Japan. Image Credit: NAOJ
VERA has completed observations of more than 100 radio objects (maser sources) in the Milky Way galaxy, and so far the precise distances and motions of approximately 30 of those objects have been reported. This time, the Galactic yardstick was determined 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.
Figure 3: The distribution of the observed objects on the position-velocity diagram. The horizontal axis shows the galactic longitude while the vertical axis shows radial velocity. The black dots represent the 52 objects observed so far. The background image in red, green, and blue describes the distribution of molecular gas that leads to the formation of stars; this shows that the observed objects are located along the molecular gas. Image Credit: NAOJ
Figure 4: Image of the Galaxy image seen from above, and the distribution of the precisely measured 52 stars (marked in red). Image Credit: NAOJ
Galactic Yardsticks Precisely Determined
This research successfully managed to determine the Galactic yardsticks precisely: the the distance to the Galactic center from the Solar system, and the Galactic rotation velocity in the Solar System. The distance to the Galactic center is approximately 26,100 light-years +/-1,600 light-years, and the Galactic rotation velocity in the solar system is 240 +/- 14 km/s. (See Figure 5)
Figure 5: An Artist’s rendering of the face-on view of the Milky Way galaxy. The yardsticks of our galaxy determined from this analysis. Image Credit: NAOJ
The following two values were precisely measured: the distance to the Galactic center from the Solar system is 26,100 light-years; the Galactic rotation velocity in the solar system is 240 km/s. From the data on distance and velocity, it has been learned that the Solar System takes approximately 200 million years to revolve around the inner galaxy once.
The value of Galactic rotation velocity from this research is larger than 220 km/s, the one endorsed by the International Astronomical Union (IAU) since 1985. This finding now forces them to change the rotation speed and mass distribution of the Milky Way, as mentioned later. On the other hand, the distance to the Galactic center is almost equal, within an approximately 27,700 light-years margin of error, as that endorsed by the IAU since 1985. 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 (see Figure 6).
Figure 6: The rotation velocity of objects in the Galaxy obtained from this analysis. It has been found that a star at any location inside the Galaxy rotates at the almost constant speed of about 240 km/s. Image Credit: NAOJ
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 the galaxy’s mass. When the Milky Way’s mass within the Solar System is measured with the latest Galactic rotation velocity from this research (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. This research findings also impact any experiments 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, the team will continue observations with VERA to increase the number of objects measured to several hundred in about 10 years. They 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 Milky Way galaxy should drastically improve in the next 10 years.
Source: National Astronomical Observatory of Japan (NAOJ)