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Astrophysics

The Milky Way revisited

The galaxy's arms may be closer and not as spiral-shaped as had been thought

BRUNO GILLI / ESOThe Milky Way seen from Chile: a beautiful structure in discussionBRUNO GILLI / ESO

There are many myths and legends about the Milky Way, the group of dust, gas and approximately 400 billion stars, which remain relatively close to each other by the gravitational pull in which the solar system is inserted. The ancient Egyptians believed that the galaxy was a bifurcation of the Nile, a river in the sky. For many ancient peoples, the water was the galaxy’s core element and the stars were fixed in the sky. Some of Brazil’s indigenous people referred to the Milky Way as Tapirapé, the path of the tapir. The apparently milky aspect of the galaxy, which has been observed through the naked eye since time immemorial, was apparently the inspiration for the name given to this cluster of celestial objects. The name galaxy – “gala” in Greek means “milk” – derives from this analogy. Myths or fantasies about the Milky Way, such as the ones mentioned above, have been put to the test and destroyed ever since Galileo Galilei pointed his telescope at the celestial dome four centuries ago. Nowadays, there is a huge amount of scientific information on the galaxy, but, according to some astrophysicists, the idea that we know the galaxy in detail is misleading.

Two independent and recent research studies conducted by Brazilian researchers question the most widely held opinion on one of the most outstanding characteristics of our galaxy, namely, the spiral arm structures of the Milky Way. At the end of November, Augusto Damineli and Jacques Lépine, two astrophysicists of the University of São Paulo (USP), spent four days discussing the characteristics of the Milky Way’s spiral structure with 60 scientists from Europe, the United States, Japan, and Latin America. The event took place at a hotel located in the seaside resort of Bahia Inglesa, in Chile’s Atacama Desert. The objective of the event was to compare observations conducted by various research groups with the existing theories in this field. New data announced by Damineli suggests that the stellar regions associated with the galaxy’s spiral arm structures are up to 50% closer to the Earth than had been calculated by previous measurements. Perhaps the extension of the Milky Way itself is smaller than has been believed. Lepine’s study indicates that some parts of the spiral arms may be straight instead of spiraled and that the galaxy may have a small peripheral arm that has an unusual curvature turned outwards from the galaxy. “The shape of the spiral arms is determined by the orbiting of the stars around the galaxy’s center,” says Lépine, author of the book A Via Láctea, nossa ilha no universo [The Milky Way, our island in the universe] (published by Edusp). “The idea that the arms of our galaxy are almost perfect spirals is wrong.” So could it be that the shape of the Milky Way resembles a square?

Technically, the Milky Way is described as a barred spiral galaxy. It is surrounded by a halo comprised of low-density matter. The flat galactic disk includes the spiral arms and a sphere-shaped bulge in its core; this bulge resembles an American football. The galaxy has a concentration of stars that go through the bulge and create a structure of contours that resembles a bar. In this kind of galaxy, the arms generally “stem” from the tips of the bar. The older stars, whose color ranges between yellow and red, are concentrated in the core region. The newer stars, which have a higher mass and a bluish color, outline the arms. There is evidence of the existence of a black hole, a type of mysterious celestial object that sucks in all the matter that surrounds it and that does not emit any light, right in the core of the galaxy, in the center of the bulge. The parts of the galaxy did not form all at once. The Milky Way’s older stars are more than 13 billion years old, but the arms are a little more than half this age.

Although important points of consensus have been established in the last decades, there are many interpretation-related controversies and missing data on some of the Milky Way’s main characteristics. “Our schematic view of the galaxy has not changed much in the last 20 years; but we have increased our understanding of the galaxy’s details and mechanisms,” explains Portuguese astrophysicist André Moitinho, of the University of Lisbon, who also participated at the meeting in the Chilean desert. The total mass and the size of the Milky Way, parameters that seemed to have been reasonably well established for a long time, still raise questions periodically. Nobody knows the distance of the Sun and other stars in relation to the center of the galaxy, nor the speed at which matter rotates at each point of the galactic arms.

Of all these doubts, the issue that generates the most debates and revisions is the spiral structure of the Milky Way. After all, does the galaxy have two or four main arms? What do they look like and where exactly are they located? “I thought we would be moving towards a consensus on this issue after so many years of research,” says Damineli. “But the results of different methods of observation used to analyze the arms do not always converge.”

The best technique to establish a celestial object?s distance from the Earth is based on the calculation of the angle of the trigonometric parallax, a method that has been used for this purpose for nearly two centuries. The astronomer measures the variation of the apparent position of a star against a fixed background at two distinct moments of observation, generally at opposing points of the Earth’s orbit. The parallax is this alleged movement of the star, and is obtained by means of an angle, the key variable in a triangulation that allows astronomers to calculate the object’s distance from our planet. This method, however, has a limitation: it cannot be used to establish the location of objects that are too far away or whose brightness is very weak. In the case of the Milky Way, the stars located on the opposite side of the Sun, at the other corner of the galaxy, generally cannot be studied by using the parallax calculation.

Gamma Ray bubbles in the galaxy: action by black hole

GODDARD SPACE FLIGHT CENTER / NASA Gamma Ray bubbles in the galaxy: action by black holeGODDARD SPACE FLIGHT CENTER / NASA

Shorter distances
In his research work, Damineli and his collaborators, among whom the outstanding member was the then doctorate student Alessandro Moisés, used a modern variant of this method. They analyzed an enormous amount of spectrums and images obtained in the course of 14 years in the length of the closest infrared wave. The specters and images came from three telescopes installed in Chile (Blanco, Gemini and Soar); in addition, they resorted to the average infrared registers provided by the USA’s space agency Nasa’s Spitzer satellite. Based on all this data, the researchers calculated the distance of the galaxy’s 35 HII regions, most of which are enormous. Comprised of clouds of ionized gas (hydrogen), this kind of region is characterized by the intense formation of stars with huge mass. “The HII regions are considered good indicators of where the Milky Way’s arms should pass,” says Damineli. The USP group’s research study was published on line on November 25, in the electronic issue of Great Britain’s Monthly Notices of the Royal Astronomical Society and showed that many of these stellar nurseries are 50% closer to the Earth than had been suggested by research studies using the so-called kinematic method. By using the classic kinematic method, astrophysicists infer the distance of the gas that envelops stars by calculating the star?s speed of approach to or speed of separation from the solar system.

According to Damineli’s article, 14 of the 35 analyzed HII regions are closer than had been previously suggested by studies conducted under the kinematic method, while two regions are farther away. Results were non-conclusive (10 cases) for the other HII regions, or were identical to the findings of previous studies (nine cases). If the data of this study is correct, the Milky Way’s diameter – not to be confused with the size of the galaxy’s mass – could be smaller than the 100 thousand light years, as widely believed. “Knowing the distances of the objects is crucial to a better understanding of our galaxy and the entire Universe,” Damineli states. One light year corresponds to the distance travelled by light in one year, which is equivalent to approximately 9.5 trillion kilometers.

Lépine’s study used the kinematic method to build a map representing what could be the galaxy’s arms. Besides using a specific technique, the astrophysicist decided to analyze a different kind of indicator of the Milky Way’s spiral structure. A group of Chilean radio astronomers obtained the speed of 870 sources of carbon monosulfide gas emissions, which had been identified from measurements in the infrared spectrum conducted by the Itas space satellite. Based on these speeds, Lépine calculated the distance of the objects. Carbon monosulfide is a molecule associated with the existence of small HII regions, that is, zones in which there is a great density of young stars. “No other study on the classic HII regions employed more objects to draw the galaxy’s arms than ours,” says Lépine, whose article, written in partnership with Brazilian colleagues and one Russian colleague, has already been accepted for publication in the Monthly Notices of the Royal Astronomical Society.

The contours that emerge from Lépine’s mapping challenge the more traditional vision of the Milky Way. According to the study, the galaxy might have only two big arms in the center, but it undoubtedly has four arms in the vicinity of the sun. The most astonishing detail is that, according to the study, the arms do not form perfect logarithmic spirals. Some of the parts seem to have straight angles. Thus, the Milky Way’s arms could be somewhat diamond-shaped. “We frequently see this kind of structure in other galaxies,” says Lépine, one of the main researchers of a theme project of FAPESP on the formation and evolution of structures in the Universe. Another finding of the study is the apparent existence on the galaxy’s periphery of a small, unknown arm, named Sagitarian-Cepheu, which is found near these constellations. With the curvature turned outwards from the Milky Way, the arm is thought to lie at a distance of approximately 33 thousand light years from the center of the galaxy.

Image of an HII region: gas and star nursery

NASA / JPL-CALTECH / M. POVICH Image of an HII region: gas and star nurseryNASA / JPL-CALTECH / M. POVICH

View of the galaxy’s plane
Studying the Milky Way poses a unique difficulty that, by definition, no other galaxy will ever pose to astrophysicists. We are inside an object to be observed and, to make things even more complicated, at an unfavorable angle for visualization. The Sun is only five degrees above the plane of all the material that comprises the galaxy. “As we cannot travel to a nearby galaxy, turn halfway and take a photo of the Milky Way, we have to use other methods to build up an ‘image’ of the galaxy,” says Mark Reid, of the Harvard–Smithsonian Center for Astrophysics, of Cambridge, United States, and one of the foremost experts on the Milky Way. “We place a dot on the map of the Milky Way every time we measure the distance of a young star.” The astrophysicists believe that the shape of the arms is dictated essentially by the existence of enormous concentrations of gas and young stars in certain parts of the galaxy.

There are other obstacles that observation techniques seek to deal with to better understand the nature of the Milky Way. In our galaxy, as in any other, only part of its total matter can be seen in the visible beam of light of the electromagnetic spectrum. It is often necessary to resort to other wavelengths, such as X rays, ultraviolet or infrared rays, to study certain objects. The existence of dust in the midst of gases that comprise the interstellar space does not make this task any easier. The particles of dust absorb and spread the radiation emitted by the stars at different wavelengths, including that of the visible light. In practice, the phenomenon of extinction, as the effect caused by these fine particles is referred to, alters the brightness of many objects and makes it unfeasible to conduct observations in certain corners of and at certain distances from the galaxy. In infrared rays, the wavelength used in the studies conducted by Damineli and those by Lépine, the effect from the extinction is smaller.

Although the studies conducted by the two astrophysicists from USP do not point to the same configuration of the Milky Way’s arms, both astrophysicists agree on one point: their colleagues from the Spitzer telescope should correct the best-known illustration of the galaxy, namely, the beautiful map, divulged in early 2008, that shows the Milky Way with only two main spiral arms: Scutum-Centaurus and Perseus. Two other arms, Norma and Carina-Sagittarius, located between the bigger arms, were downgraded and are now considered secondary arms. They are duller and their contours are not as sharp. “They practically eliminated the Carina arm, the galaxy’s most visible region,” Damineloi complains. The illustration also shows a recently discovered mini arm, which is almost straight and runs parallel to the galaxy’s central bar, and a smaller arm (branch) of Orion, where the Sun is located. In the previous version of the map, prepared in 2005, also with the help of the Spitzer, the four main arms had the same status.

The criticism of the map, voiced by several astrophysicists, is that the symmetry of the structure is too perfect to be real – it is almost always the same. “The drawing reflects a more artistic view rather than a scientific one, and did not use the best indicators of the galaxy’s arms,” says France’s Delphine Russeil, from the Observatory of Marseille, who is also an expert on this matter. “When we analyze the existence of young objects in the Milky Way, everyone agrees that the galaxy has four arms, even though we don’t really know how the different parts of these structures interconnect if they are seen from the Northern and Southern Hemispheres.”

American astrophysicist Robert Benjamin, of the University of Wisconsin, and one of the people involved in the preparation of the controversial map, explains how the drawing was created. “It is extraordinarily difficult to encapsulate in a single image the results of more than 50 years of research work, conducted by us and other groups around the world,” says Benjamin. “Some star populations seem to indicate that there are two brighter arms and two duller arms. The map was our best attempt to try and reflect that data.” The Spitzer team’s objective is to periodically improve the illustration; a new version of the illustration is scheduled to be produced by the end of this year.

With Andromeda’s mass
The Milky Way’s arms are not the only detail that have stirred controversy. Recently, the Milky Way’s mass and its status as the second biggest galaxy in its cosmic region were questioned. Until a few years ago, all evidence indicated that the Andromeda had twice the amount of mass as that of the Milky Way, and was the biggest of the more than 45 galaxies that comprise the so-called local group. “It seems that the Milky Way and the Andromeda have approximately the same total mass,” states astrophysicist Mark Reid, of the Harvard-Smithsonian Center for Astrophysics. “This is the simplest and most direct interpretation of our data.” In early 2009, Reid announced measurements – considered as being quite accurate – which increased by approximately 15% the rotation speed attributed to the Milky Way. The study indicated that the galaxy rotated at approximately 966 thousand kilometers per hour instead of 805 thousand kilometers per hour, as had been previously believed.

If Reid’s calculations are correct – and very few people doubt that they are not – the indirect conclusion of the research study is that the galaxy must have double its total mass (the ordinary matter plus the mysterious dark mass) to rotate at that speed. The extra mass might mean bad news in the long term: our galaxy might crash into Andromeda in less time than originally predicted 5 billion years.

Another recent discovery more specifically, in November 2010, might generate heated discussion, similar to the kind generated by the Milky Way’s arms. Data from the Fermi satellite suggests the existence of two gigantic bubbles formed by gamma rays above and below the galaxy’s plane. The astonishing bubbles could be produced by the alleged activity of the black hole located in the galactic nucleus. This means that more discussions and debates will soon be coming up.

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The project
New physics in space – Formation and evolution of structures in the Universe (nº 2006/56213-9); Type Thematic Project; Coordinator Reuven Opher – IAG/USP; Investment R$ 1,178,363.33 and US$ 523,179.96.

Scientific articles
MOISÉS, A. P. et al. Spectrophotometric distances to Galactic HII regions. Monthly Notices of the Royal Astronomical Society. Published online 25 Nov. 2010.
LÉPINE, J. R. D. et al. The spiral structure of the galaxy revealed by CS sources and evidence for the 4:1 resonance. Monthly Notices of the Royal Astronomical Society. Forthcoming.

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