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

Oct 05, 2012
Credit: NAOJ

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 , 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 , 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 (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.

Image 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 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.

Figure 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.

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.

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)

Figure 4: Image of the Galaxy image seen from above, and the distribution of the precisely measured 52 stars (marked in red).

Figure 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 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.

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 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 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.

Explore further: Grad student's aTmCam offers cosmic insight for dark energy survey

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User comments : 18

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Ophelia
3.7 / 5 (3) Oct 05, 2012
Other than cost, is there a reason we don't park some satellite out at an outer planet Lagrange point to do parallax measurements? Is it that the benefit of increased resolution of distances wouldn't be a big enough benefit?
El_Nose
4 / 5 (2) Oct 05, 2012
@Ophilia

in short no.

remember that we are taking 1 measurement from one side of the sun, 1 AU. And one from the other side of the sun, 1 AU. This is a total distance of 2 AU approximately, one side is actually a lil closer.

Of the five Lagrange points only L2 is farther from the sun. And it only adds 1.5 M km. so thats a total of 3 M km that could be used ... but 3M km is only a 1% increase in distance from the 298 M km that is 2 AU.

Even if we used satellite orbiting Mars to make this observation it would only add 1 AU to the parallax. However a satellite orbiting Jupiter would have a parallax of 10 AU.
Ophelia
3 / 5 (2) Oct 05, 2012
@ El Nose:
The question I asked was
is there a reason we don't park some satellite out at an outer planet Lagrange point to do parallax measurements?


I know you aren't getting much benefit going to Mars, but what about Saturn? Neptune, Uranus, Pluto?
Lurker2358
3.2 / 5 (5) Oct 05, 2012
Other than cost, is there a reason we don't park some satellite out at an outer planet Lagrange point to do parallax measurements? Is it that the benefit of increased resolution of distances wouldn't be a big enough benefit?


Well, I think you'd need more than one satellite on opposite sides of the solar system, because you want the opposite side measurements to be taken as close together in time as possible, after all, orbits that far out take decades or centuries,a nd the solar system is going to move a very long way during that time, which will throw off your observations and complicate the math even further. Therefore simultaneous or nearly simultaneous observations from opposite sides of the solar system are required.

Having a wider angle only increases the resolution if you have simultaneity or near simultaneity of the measurements, that is, looking at the same object(s) at the same time.
julianpenrod
1.5 / 5 (15) Oct 05, 2012
A lot can be concluded concerning the apparent intellectual ability of those making a number of the comments here with respect to the propostion of using a probe far out in the solar system to take parallax measurements. First, it doesn't have to be at a Lagrange point. Just set it anywhere in any orbit around the sun. It could be between Jupiter and Saturn. It could be in Jupiter's orbit but the othe side of the sun. There is nothing magic about Lagrange points! And you don't need two probes, one on either side of the sun! You need two points a distance apart, that's all! You can make the other side of every parallax measurement from earth! And, yes, why didn't "science" consider making a valuable stab in this direction? Becuase they want there to be enough uncertainty so they could peddle the lie of "dark matter"!
A2G
1 / 5 (11) Oct 05, 2012
Julian,

They are not lying about dark matter. They really believe in it. Therefore they are telling the truth about what they believe.

Dork matter does not exist, but they are not telling a "lie" as the dorks believe in it.
julianpenrod
1 / 5 (11) Oct 05, 2012
Unfortunately, it's still the same thing. They say they believe in dark matter. There is nothing that independently proves they believe in it. And if they believe in it despite that nothing definitively proves it and in the face of information that argues against it, then they are lying about being so reliable that they can be trusted.
julianpenrod
1.3 / 5 (9) Oct 05, 2012
Incidentally, the article seems to be making an enormous error. Thay say the sun is 26,100 light years from the galactic center almost equal to the old measurement, "within an 8.5 kpc (approximately 27,700 light years) margin of error". No measurement which is less than the margin of error is considered accurate or even necessarily accepted. Persumably the margin of error is something else.
Ophelia
5 / 5 (2) Oct 05, 2012
@lurker,

Let me rephrase what I'm asking a bit better. Distance measurements become important for establishing "cosmic ladders" and determining things like the Hubble Constant and the amount of dark energy. I believe the first steps on those ladders are established by parallax measurements, though I admit to being uncertain on that point. If so, however, will the increased resolution obtained in the distance measurements by increasing the base distance of the parallax measurment have much of an impact on these other matters?

Two satellites would of course be preferable, but would double the cost. One could be used in conjunction with earth-based scopes, but would it be worth it scientifically? That's what I'd like to know - how big of an impact would the increased resolution have?
Ophelia
5 / 5 (6) Oct 05, 2012
@julianpenrod
A lot can be concluded concerning the apparent intellectual ability of those making a number of the comments here with respect to the propostion of using a probe far out in the solar system to take parallax measurements. ...

Please, go on and explain in detail how stupid you think I am. Sorry if I sounded dumb as a post, but my Ph.D. in astrophysics got burned when the trailer went up in the meth explosion.

I can change the oil in my car, though. Can an egghead like you do it?
Widdekind
1 / 5 (1) Oct 06, 2012
Figure 6 may be interpreted differently than stated. Inside 10K lyr, no objects have galactic rotation velocities >200km/s. Visually, a general decrease in galactic orbital velocity towards the galactic center can be discerned, in the noisy data.
Fleetfoot
5 / 5 (4) Oct 06, 2012
Having a wider angle only increases the resolution if you have simultaneity or near simultaneity of the measurements, that is, looking at the same object(s) at the same time.


The stars move by such a small amount in 6 months that this isn't a limiting factor.
Fleetfoot
5 / 5 (1) Oct 06, 2012
Other than cost, is there a reason we don't park some satellite out at an outer planet Lagrange point to do parallax measurements? Is it that the benefit of increased resolution of distances wouldn't be a big enough benefit?


However a satellite orbiting Jupiter would have a parallax of 10 AU.


The overall accuracy depends both on the baseline length and the angular resolution of the measuring device. The latter depends on the size of the antenna array. The system above has a longest separation of 2270km and a baseline of 2AU. For comparable performance, a satellite would need to be 454km across. It would have to be a cluster of satellites with separations maintained to a tiny fraction of a wavelength to compete with this approach.
Fleetfoot
5 / 5 (3) Oct 06, 2012
A lot can be concluded concerning the apparent intellectual ability of those making a number of the comments here ..


Indeed.

you don't need two probes, one on either side of the sun! You need two points a distance apart, that's all! You can make the other side of every parallax measurement from earth!


Or just wait half an orbit and use the same probe to make the second measurement. Of course using an outer planet means that takes a bit longer ;-)

And, yes, why didn't "science" consider making a valuable stab in this direction? Becuase they want there to be enough uncertainty so they could peddle the lie of "dark matter"!


ROFL, completely the opposite of the truth, the system they used gives results orders of magnitude better than could be obtained with a single satellite orbiting another planet at a fraction of the price.

For the satellite version, see:

http://www.rssd.e...IPPARCOS
Fleetfoot
5 / 5 (6) Oct 06, 2012
Incidentally, the article seems to be making an enormous error. Thay say the sun is 26,100 light years from the galactic center almost equal to the old measurement, "within an 8.5 kpc (approximately 27,700 light years) margin of error". No measurement which is less than the margin of error is considered accurate or even necessarily accepted. Persumably the margin of error is something else.


It is badly written, a little earlier the article says the new value is 8.0 kpc (or 26,100 light years) with a margin of 0.5 kpc. The second part should say the old nominal value was 8.5 kpc (or 27,700 light years), within the error of the new determination.
RealScience
5 / 5 (4) Oct 06, 2012
@Ophelia: Your questions about one probe on earth and the other at some distant point in the solar system are excellent, and cost is indeed the problem.

Placing a probe at Saturn (~9 AU) would give a 8-10 AU baseline as the earth orbits the sun as opposed to the current 2 AU baseline. The improvement is pretty linear, so if the same quality measurement could be made this would result in 4x-5x better resolution (and 15 years later an 18 AU = 9x longer baseline could be obtained).

However this study used four 20-meter radio telescope linked to one another, and the current cost of sending that much hardware to Saturn would be astronomical.
Anda
4.3 / 5 (3) Oct 07, 2012
Dumb people like @a2g and @julianpenrod are pathetic. Being sure that something exists or not when no one can know it yet is pathetic.
We don't know if dark matter exists or not, but there sure is something that defies our understanding of the universe... and that's positive: it's the way that science goes on, improving our understanding and filling our human curiosity.
El_Nose
not rated yet Oct 08, 2012
The reason we don't put a satellite in orbit elsewhere in the solar system is because quite frankly you are not going to increase the resolution enough to add other stars to the study... you need a satellite near another star to make this better.

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