For a long time, we knew very little about the Milky Way, and we didn’t even know the exact location of the sun in the Milky Way. However, astronomers have recently used some observational data to create a new "map of the Milky Way" that will refresh our understanding of the Milky Way and the formation of galaxies. And with this map, we also know that the sun is almost exactly on the central plane of the Milky Way’s disk.
Hundreds of years ago, explorers made detailed maps across oceans and uncharted continents. Over the past half-century, human space probes have photographed much of the solar system. But while we know our own astronomical backyard, we don’t know enough about our cosmic community, the Milky Way. The reason is obvious. Just as "we don’t know the true face of Mount Lu, we can’t leave the Milky Way and look back at the panorama of the Milky Way."
Maybe you could dream that we would launch a spacecraft, take it out of our galaxy, and then go back and take a panoramic picture of the galaxy, but a spacecraft would only be able to do this after a journey of millions of years, which is obviously impractical. We still have a lot of unanswered questions about the galaxy, such as how many spiral arms the galaxy has, whether a large structure closest to the sun can be counted as a single spiral arm, and where our solar system is in the galaxy.
Recently, however, scientists are working to map the Milky Way from the inside out, so that for the first time, accurate maps of the Milky Way’s structure can be made. This promising future is the result of the collaborative efforts of several advanced radio and optical large telescope projects, including our Bar and Spiral Structure Legacy Survey of the Milky Way (BeSSeL). We have obtained an unprecedented 5,000 hours of observation time for the Very Long Baseline Array.
The preliminary results of our project provide a new picture of the Milky Way. In addition to better understanding the overall image of the Milky Way, we are also beginning to clarify why galaxies like the Milky Way exhibit spiral structures and how our galactic home is integrated with the entire universe.
Cosmic Neighborhood
In the early nineteenth century, William Parsons, the third Earl of Rousseau, built a 72-inch telescope – a "gigantic" telescope by the standards of the time. He observed and drew the M51 nebula (which we now call the Whirlpool Galaxy) with a distinct swirling arm pattern. However, without knowing how far M51 is or the scale of the Milky Way, it is not clear whether the vortex galaxy is a small structure in our Milky Way or a galaxy similar to the Milky Way.
Debate about these issues continued until the early twentieth century. It was not until the American scientist Edwin Hubble used techniques developed by Henrietta Leavitt to measure our distances to some bright stars that we learned that spiral galaxies and other spiral nebulae are outside the Milky Way and similar to the Milky Way. This discovery overturned the idea that the Milky Way is the entire universe.
By measuring the movement of gas throughout the Milky Way disk, the pancake-shaped region that makes up the main body of the Milky Way, astronomers have discovered that we actually live in a spiral galaxy. The main common types of galaxies include spiral galaxies and elliptical galaxies. The Milky Way, seen from a distance, may resemble the nearby spiral galaxy NGC 1300 and the Pinwheel Galaxy (M101). At the center of NGC 1300 is a bright, elongated structure that astronomers call the bar structure of the galaxy. Two blue spiral arms extend from both ends of the bar structure and gradually extend outward around the central bar.
Most spiral galaxies have bar structures, which are generally thought to have formed as a result of the attractive force instability of the galaxy’s dense disk. The central bar structure then rotates, creating a stirring effect that in turn may promote the formation of spiral arms (other processes, such as attractive force instability caused by massive clumps in the disk or attractive force disturbances in nearby galaxies, may also contribute to the formation of spiral arms). Spiral arms are easier to see in the blue light band, because spiral arms are huge star-forming star-producing houses that are more prone to emitting blue light. The Pinwheel Galaxy M101 is another galaxy that may resemble the Milky Way. Although the Pinwheel Galaxy does not have the central bar of NGC 1300, it has more spiral arms.
Astronomers have long believed that the Milky Way may have the characteristics of two galaxies, NGC 1300 and M101: a distinct long bar structure like NGC 1300, and multiple spiral arms like M101. But beyond these basic conclusions, there is still much controversy. For example, infrared observations by the Spitzer space telescope more than a decade ago suggested that the Milky Way may have only two main spiral arms. And radio-band observations of atomic hydrogen and carbon monoxide suggest that the Milky Way has four spiral arms. In extragalactic galaxies, the gas is concentrated in the spiral arms. In addition to the spiral arm feature, astronomers are also debating how far the sun is from the center of the galaxy, and what the sun’s vertical height is relative to the galactic midplane, the plane of the center of the disk.
About 70 years ago, scientists calculated the distance from some bright blue stars nearby. If these stars were marked on a map of the Milky Way, they would be part of three adjacent spiral arms. We call these three spiral arms the centaur arm, the local arm, and the Perseus arm. Around the same time, beginning in the 1950s, radio astronomers observed atomic hydrogen gas, which emits a characteristic radio signal with a wavelength of 21 centimeters. As this atomic hydrogen gas moves relative to the earth, its characteristic radio frequency is shifted by the Doppler effect, allowing astronomers to use the frequency shift to measure the speed of the gas’s movement and then map its location in the Milky Way.
Using this measurement method, astronomers adopted a sun-centered coordinate system: similar to the longitude and latitude of a globe, the Milky Way longitude (l) is zero in the direction toward the galactic center and increases clockwise within the galactic "equatorial" plane (the Milky Way is seen from the northern celestial sphere); the galactic latitude (b) represents the angle perpendicular to the plane of the galactic disk. The 21-centimeter characteristic radio signal of the hydrogen atom gas shows a continuous structure in the galactic longitude-velocity diagram, which most likely traced the galactic spiral arm structure. Later plots of the galactic longitude-velocity of carbon monoxide molecular gases also exhibited similar characteristics. However, this indirect mapping method may be ambiguous and not accurate enough to clearly show the spiral arm structure of the Milky Way.
A New Horizon
One reason we know so little about the structure of the Milky Way is that there is a lot of dust in the Milky Way. Dust absorbs visible light so efficiently that it obscures our view in most line-of-sight directions, preventing us from seeing very far. Another reason is that the size of the Milky Way is staggering: light from stars on the other side of the Milky Way takes more than 50,000 years to reach Earth. Such a distance makes it difficult to even tell which stars are close to us and which are far away.
Now, new optical telescopes operating in space, as well as new radio telescopes around the world, could allow us to better answer all kinds of questions about the Milky Way. The Gaia mission, launched in 2013 to measure the precise distances of nearly a billion stars in the Milky Way, will no doubt revolutionize our understanding of the different populations of stars that formed our galaxy. But because Gaia observes in the visible light band, which is easily absorbed and scattered by interstellar dust particles, Gaia may be affected by interstellar dust when observing very distant spiral arms. On the contrary, because radio waves easily pass through dust, radio telescopes can probe the entire disk, allowing us to use observations from such telescopes to map the overall structure of the disk.
At present, the two main observational projects to map the structure of the Milky Way are using the very long baseline interferometry (VLBI) technique in radio astronomy. The VERA (VLBI Exploration of Radio Astrometry) project in Japan uses four radio telescopes, ranging from northern Japan (Mizusawa, Iwate Prefecture) to the southernmost Okinawa Islands of Ishigaki and the easternmost Ogasawara Islands, across Japan. The Very Long Baseline Array used by our BeSSeL sky survey program includes 10 telescopes, ranging from Hawaii to New England to St. Croix in the U.S. Virgin Islands, across most of the Western Hemisphere.
Because the distance between the telescopes that make up the Very Long Baseline Array is nearly the same as the diameter of the Earth, the array can achieve angular resolution that far exceeds that of any other telescope at any wavelength. The researchers must observe simultaneously with all the telescopes in the array, and use the world’s best atomic clocks to synchronize the data recorded by computers at each site. They then transport the recorded data to a dedicated computer, which processes the signals collected by each telescope. If our eyes are sensitive to radio waves, the calibrated image is an ultra-high definition digital image that we can see at radio wavelengths that is parsed by almost the entire width of the Earth.
Such images have incredible angular resolution (better than 0.001 arc seconds: if the entire celestial sphere is divided into 360 degrees, then 1 arc second is 1/3600 of a degree). By contrast, the human eye can only resolve structures for about 40 arc seconds at most, and even the Hubble Space Telescope can only achieve a resolution of about 0.04 arc seconds.
Using the VLBI, we can measure the position of a bright star in radio waves relative to a background quasar (which is actually a bright, active black hole at the center of a distant galaxy) with an accuracy of close to 0.00001 arcseconds. In this way, we can measure very far distances by measuring the triangular parallax effect. The triangular parallax effect is that when viewed from different positions, nearby celestial objects appear correspondingly in different positions in the background starry sky. You can extend your arms forward, raise your thumb, and simulate this effect by alternately closing your left and right eyes to observe the thumb. Because our eyes are a few centimeters apart, when we alternately observe the thumb at an arm’s length with our left and right eyes, the thumb will be offset by about 6 degrees relative to the distant background object. If we know the distance between the two observation positions, and the angular displacement observed, we can easily calculate our distance from the observation target. This is the same principle that surveyors use to map cities.
Ideally, astronomers who want to map the structure of spiral arms should observe young, massive stars. These short-lived stars are usually associated with violent star formation in spiral arms, and such stars are so hot that they can ionize the surrounding gas, causing it to emit blue light. Therefore, in theory, these stars can serve as beacons for observing the spiral arms of galaxies in the visible light range.
But because these stars are surrounded by the Milky Way’s dust disk, we cannot easily observe them throughout the Milky Way. Fortunately, water and methanol molecules outside the ionized regions of these hot stars can serve as very bright radio sources, because they emit large amounts of natural "masers" that are barely attenuated by galactic dust. The term maser is an acronym for "microwave amplification by stimulated emission of radiation," which means that a maser is actually a laser in the radio band. In an astrophysical setting, maser radiation comes from a solar system-scale gas cloud of mass comparable to that of Jupiter. Maser sources appear as very bright point sources in radio images. Therefore, the maser source is an ideal target for triangulation parallax measurement.
A new picture of the Milky Way
Using the BeSSeL and VERA projects, astronomers have used triangulation parallax to measure the distances of about 200 young, hot stars. The data spans the Milky Way, covering about a third of the galaxy and revealing four very long spiral arms.
The resulting "map of the Milky Way" also shows that the Sun is very close to the Milky Way’s fifth spiral arm, which appears to be an isolated spiral arm known as the "local arm". Previously, this section of the spiral arm was called the "Orion Arm Spine" or "Local Arm Spine", that is, this spiral arm resembles a small accessory structure extending from the main spiral arms of other galaxies. However, the interpretation of this "arm spur" may be wrong. In our BeSSeL data, this spiral arm is isolated, orbiting the Milky Way less than a quarter of a turn. Although the local arm is shorter in length, in this arm the star formation rate can be comparable to that of the Perseus Arm of the same length. Interestingly, astronomers once thought of the Perseus Arm as one of the Milky Way’s two main spiral arms (the other being the shield-centaur arm). However, we found that the star formation rate decreased significantly as the Perseus Arm moved away from the Sun and towards the Milky Way. This suggests that the Perseus Arm does not appear to be a very obvious spiral arm to outside observers.
By mapping the three-dimensional positions of a large number of young stars and measuring and modeling their velocities, we can calculate the basic parameters of the Milky Way. We found that the distance from the sun to the center of the Milky Way is 8150 ± 150 parsecs (or 26,600 light-years). This is smaller than the 8500 parsecs recommended by the International Astronomical Union a few decades ago. In addition, we found that the Milky Way rotates at 236 km/s at the position of the sun, which is about 8 times the speed of the earth’s rotation around the sun. Based on these parameter values, we found that it takes about 212 million years for the sun to rotate around the Milky Way. This also means that the last time our solar system was where the Milky Way is now, dinosaurs were still roaming the earth.
The Milky Way’s inner disk, which lies within the sun’s position, is so thin that it is almost a flat plane. But the sun’s vertical height relative to this plane has been controversial. Recently, some astronomers have measured that the sun is 25 parsecs (82 light-years) above the inner disk, but our measurements differ significantly from this estimate. By fitting the plane in which massive stars with precise distances and positions are located, we can determine that the sun is only about 6 parsecs (20 light-years) above the plane. This height is only 0.07% of the distance from the sun to the center of the galaxy, which means that the sun is very close to the mid-plane of the galaxy. We also confirmed the previously observed galactic warping, in which the outer disk of the Milky Way gradually deviates from the inner disk and begins to curve upward on the north side and downward on the south side, somewhat like a curved potato chip.
In order to describe observations, astronomers usually divide the Milky Way into four solar-centered quadrants. Using this coordinate, we found spiral arms in the first three quadrants. To map the fourth quadrant, we need observation equipment located in the southern hemisphere. We are conducting a southern hemisphere observation project and plan to use telescopes in Australia and New Zealand to make observations.
While we await these observations, we can use observations from atomic hydrogen and carbon monoxide to extrapolate the known spiral arms to the fourth quadrant. The structures revealed by these observations are consistent with previous theoretical conjectures of the Norma-Outer, Shield-Centaur, Sagittarius-Carina, and Perseus arms. It should be noted, however, that we have only made one observation of the star-forming region far from the center of the Milky Way. The distance of this region we observed, combined with its position in the galactic meridian-velocity map of carbon monoxide, gives us some confidence in how to "map" the spiral arms at the other end of the galaxy.
However, we need more observations to validate our model. We now have a clearer picture of our galactic home. We may live in a four-arm spiral galaxy with a bright and symmetrical central bar. Our sun is almost entirely in the mid-plane of the Milky Way disk, but the sun is far from the center of the galaxy, about two-thirds of the galactic radius. In addition to the spiral arms that can circle the galaxy almost once, the galaxy has at least one additional spiral arm segment (the local arm), and each major spiral arm may have many forks. These spiral arm features make our galaxy appear fairly normal, but certainly not typical. About two-thirds of spiral galaxies have a central bar structure, so the Milky Way belongs to the bar spiral galaxy that makes up the vast majority of spiral galaxies. However, the Milky Way has four clear, well-defined, and fairly symmetrical arms, which makes the Milky Way unique because most spiral galaxies have far fewer arms and are relatively messy.
More mysteries
Although we have some new answers, there are still many important unanswered questions. Astronomers are still debating how the spiral arms arose. There are two competing theories on the issue. One theory holds that attractive force instability at the galactic scale creates a persistent spiral arm pattern of density waves. Another theory holds that some spiral arm fragments will be stretched over time due to instability at small scales, magnified, and then connected to form longer spiral arms. In the former theory, the spiral arms can last for billions of years, while in the latter theory, although the life of the spiral arms is short, the new spiral arms will appear multiple times throughout the evolutionary history of the galaxy.
Since the Milky Way does not have a clear date of birth, it is difficult to determine its exact age. The current popular view is that as many smaller protogalaxies that formed first in the history of the universe collided and merged, they gradually merged to form what is now the Milky Way.
The Milky Way was already a large galaxy about 5 billion years ago, but at that time it may have looked very different than it does now, because the merging process is likely to break up any existing spiral structure.
We need more observations to improve our existing picture of the structure of the Milky Way, and the next generation of VLBI-enabled radio telescope arrays will facilitate this. Such arrays in the pipeline include the Square Kilometer Array in Africa and the Next Generation Very Large Array in North America. Both are giant radio telescope arrays spanning an entire continent, and they are expected to be fully operational by the late 2020s. They will have a much larger signal-gathering area than existing arrays, and will therefore be able to detect faint radio emissions from stars, allowing us to see farther in the Milky Way. Ultimately, we hope to clearly map the building structure of the Milky Way Home to confirm or disprove the theory of the formation of the Milky Way’s magnificent spiral arm structure.
By Mark J. Reed, a senior radio astronomer at the Smithsonian Astrophysical Observatory at the Harvard-Smithsonian Center for Astrophysics, who was recently elected to the National Academy of Sciences. Zheng Xingwu, a professor of astronomy at Nanjing University, has spent the past few decades studying "pulses" and star formation in the universe.
Jun-Tai Shen, a professor in the Department of Astronomy at Shanghai Jiao Tong University, specializes in galactic dynamics, the structure of the Milky Way, and astronomical numerical simulations. His team has used gas dynamics methods to estimate the rotational speed of the pattern of the Milky Way’s rods and spiral arms based on the positions of the spiral arms obtained by the BeSSel sky survey. The relevant results are published in the international authoritative journal Astrophysical Journal.
Article Source: Global Science
