Mass of dark matter revealed by precise measurements of Milky Way galaxy

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…

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.
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.

EnlargeImage of the annual parallax measurement. 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.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.

EnlargeFigure 2: Layout of VERA telescopes. Simultaneously observing an object with telescopes at four places from Iwate Prefecture to Okinawa Prefecture provides the same performance as a gigantic telescope of approximately 2,300 km in diameter, the same size as Japan.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.

EnlargeFigure 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.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 the Θ0 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 is Θ0=240 +/- 14 km/s. (See Figure 5)

EnlargeFigure 4: Image of the Galaxy image seen from above, and the distribution of the precisely measured 52 stars (marked in red).
EnlargeFigure 5: An Artist’s rendering of the face-on view of the Galaxy. The yardsticks of the Milky Way Galaxy determined from this analysis. 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 the Galaxy once.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 1985. 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 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).

EnlargeFigure 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.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.

Provided by

National Astronomical Observatory of Japan

view popular
send feedback to editors

4.8 /5 (14 votes)

1
2
3
4
5

Move the slider to adjust rank threshold, so that you can hide some of the comments.

Display comments:
newest first

Rank

1
2
3
4
5

4.8 /5 (14 votes)

more news

Related Stories

Dec 05, 2011
0
Could dark matter not matter?

Oct 01, 2012
0
55 million light years from the Sun: Hubble portrays a dusty spiral galaxy

Nov 30, 2010
0
Milky Way stars move in mysterious ways

Dec 22, 2011
0
2012: Shadow of the Dark Rift

Relevant PhysicsForums posts

ways to decelerate satellites
Oct 12, 2012
How can decelerating force would be applied to satellite in order to slow it down?

Imaging Fomalhauts Debris Disk
Oct 12, 2012
This is my first attempt at imaging a debris disk around a star. I chose Fomalhaut because, as far as I can tell, there are no other stars visible at this time of year in the northern hemisphere with…

determine starting and ending days
Oct 11, 2012
for a place in the frigid zone, where the longitude is greater than 66.56 degrees, determine

(a) the starting day and ending day in a year that the sun never sets

(b) the starting day in a year…

Radio Telescope Question
Oct 10, 2012
Alright, I know about optical telescopes, but I haven’t a clue how a radio telescope forms an image. I’m used to pixels on CCD’s measuring photon counts, but how are the radio waves detected and an…

Glese581d and tectonics for life
Oct 08, 2012
http://en.wikipedia.org/wiki/Gliese_581_d

How far away are we in the study of exoplanets to find such detail as tectonic activity?

Edit.

ooops missed this url…

Moon orbiting backwards
Oct 07, 2012
If the Earth still rotated and stayed on its course around the sun, but the moon was orbiting in the opposite direction. What would be the consequences, and would you actually be able to see the Moon…

More from Physics Forums – General Astronomy

More news stories

Shuttle passes obstacle, heads toward LA museum

Slowly surmounting a key obstacle, the shuttle Endeavour maintained a heading Saturday through the streets of Los Angeles toward its retirement home at a museum.

Space & Earth / Space Exploration

1 hour ago |
5 / 5 (1) |
0

Soyuz rocket launches two Galileo satellites

A Soyuz rocket launched two Galileo satellites into orbit on Friday, marking a crucial step for Europe’s planned navigation system, operator Arianespace announced.

Space & Earth / Space Exploration

5 hours ago |
not rated yet |
0

Austrian daredevil to make new space jump bid

An Austrian daredevil hopes to make a new record-breaking attempt Sunday to jump from the edge of space, days after his initial bid was aborted at the last minute due to gusting winds.

Space & Earth / Space Exploration

5 hours ago |
4 / 5 (1) |
0

Scientists uncover diversion of Gulf Stream path in late 2011

(Phys.org)—At a meeting with New England commercial fishermen last December, physical oceanographers Glen Gawarkiewicz and Al Plueddemann from the Woods Hole Oceanographic Institution (WHOI) were alerted …

Space & Earth / Earth Sciences

22 hours ago |
4.4 / 5 (11) |
18
|

Shuttle Xing: Endeavour treks through LA streets

The shuttle Endeavour is rolling again early Saturday after an hourslong delay as crews prepared it to cross over a major Los Angeles interstate overpass.

Space & Earth / Space Exploration

5 hours ago |
not rated yet |
0

Bird flu debate: Should H5N1 experiments resume?

Virologists making mutated versions of the H5N1 bird flu halted their research in January after a U.S. government advisory panel suggested that their work, though well-intentioned, had the potential to endanger the public.

Medicine & Health / Medical research
1 hour ago |
5 / 5 (1) |
0

Retailers large and small find advantages to mobile payments

The way we pay for things is changing. Those credit-card keypads yielding paper receipts are giving way to a new group of mobile payment devices that merchants say charge cheaper swipe fees, and are faster and easier to use.

Technology / Telecom
1 hour ago |
not rated yet |
0

Dyson engineers labor toward hair-free turbine solution (w/ Video)

(Phys.org)—At such an innovation rich time of telepresence, 3-D printing, advanced cryptography, advanced medical imaging, and robotics, you need to wonder why in all this time we never got a break with …

Technology / Hi Tech & Innovation
16 hours ago |
4.8 / 5 (12) |
3
|

A complex logic circuit made from bacterial genes

(Phys.org)—By force of habit we tend to assume computers are made of silicon, but there is actually no necessary connection between the machine and the material. All that an engineer needs to do to make …

Biology / Other
17 hours ago |
5 / 5 (3) |
2
|

US thinks Iran behind cyberattack in Saudi: ex-official

The United States believes Iran was behind a major cyberattack on Saudi Arabia’s state oil company and a Qatari gas firm, a former US official who has worked on cybersecurity issues said.

Technology / Internet
5 hours ago |
not rated yet |
1

Long wait for answers in US tainted drug outbreak

Thousands of Americans who may have been injected with a tainted steroid will have to wait weeks to see if they have been infected with meningitis as investigators seek answers to a widening outbreak.

Medicine & Health / Diseases, Conditions, Syndromes
4 hours ago |
not rated yet |
0