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An X in the Milky Way?

The center of the galaxy might have two crisscrossing bars

ESO PHOTO INTERVENTION / S. BRUNIERImage of the Milky Way with the X in the center, which resembles two stellar bars crossing each otherESO PHOTO INTERVENTION / S. BRUNIER

The concentration of bright stars in the center of spiral-shaped galaxies sometimes results in a galactic bulge, an oval-shaped structure that resembles the ball used in American football. The accumulation of material in the core of the Milky Way may have generated a bulge of unusual contours, highlighted by two stellar bars (and not only one) that cross over each other and outline an X. This controversial conclusion is described by Brazilian astrophysicist Roberto Saito, from the Pontifical Catholic University of Chile, and his Chilean, European, and American peers, in an article published in the September issue of The Astronomical Journal. “Depending on how we look at the galaxy, we see a three-dimensional bar that splits into two, forming an X or even a K,” says Saito. “The two bars lie diagonally – one is the main bar, the other one is the secondary bar.” According to a technique used by researchers to study the composition of the bulge, the X crosses the part closest to the center of the Milky Way and its ends are visible at between three and eight degrees above or below the plane of the galactic disk.

To map the inside of the Milky Way’s bulge, Saito analyzed the data, collected in three infra red wave lengths, from the 2Mass project, a survey of the entire sky visible in the Northern and Southern hemispheres. The survey was conducted in the late 1990’s and early 2000’s. In the midst of an avalanche of information, the astrophysicists were specifically looking for the location of a specific bright, metal-rich star. These stars are referred to as red clumps giants, and are used to infer the astronomic distances and tracers of specific galaxy structures. The distribution of these stars in a map that divides the bulge of the Milky Way into 170 square sectors resulted in the double cross bar seen in the center of the galaxy. Since early 2010, a new survey in the nearby infra red has monitored the central region of our galaxy with the Vista telescope, installed in Chile and operated by the European Organization for Astronomical Research in the Southern Hemisphere (ESO). According to Saito, the preliminary results of this initiative apparently corroborate the hypothesis of the existence of an X-shaped structure in the heart of the Milky Way.

Since the mid 1990’s, astrophysicists have suspected that the Milky Way, like two-thirds of the spiral-shaped galaxies, have a bar in their bulge, whose total extension is probably equivalent to somewhere between 15% and 20% of the galaxy’s diameter. In the last decade, the suspicion has become a certainty and nowadays the issue being discussed is related to the characteristics of this bar – or bars, as defended by Saito. If the conclusions presented in the study are correct, the bulge of the Milky Way is not the first bulge to hide two bars of bright stars in the shape of an X. Galaxies NGC 128, 3625, 4469 and 4710 are examples of galaxies whose centers might also be shaped in the same way.

Info_Via LacteaSERGIO VÁSQUEZ/PUC FROM CHILEHowever, as we are inside the object being observed, some of the Milky Way’s characteristics are more difficult to be seen than the characteristics of the neighboring galaxies. To make things even more difficult, the angle of our view of the Milky Way is not the best. Another obstacle is the existence of grains of dust found in the midst of the gases that take up the space between the stars. These fine particles absorb and spread the radiations emitted by the stars in various wave lengths, especially in visible and ultraviolet light waves, creating a phenomenon referred to as extinction. Certain regions of the galaxy, such as the bulge, are virtually inaccessible to telescopes. Observations made in infra red undergo less interference from interstellar dust. This is why infra red waves are extensively used in studies on the Milky Way.

Chaotic Dynamics
Scientists who study the Milky Way have not reached a consensus on the nature of the Milky Way’s bulge. Jacques Lépine, of the Institute of Astronomy, Geophysics and Atmosphere Sciences of the University of São Paulo (IAG-USP), is skeptical about the possible existence of an X in the center of the Milky Way. He prefers to believe that the difficulty to correct the interference of the extinction phenomenon in Saito’s work could explain the X issue. “The chaotic dynamics of the old stars (the yellowish ones) that are part of the bulge would not allow a structure of that kind to survive,” says Lépine. “In other galaxies, where it is easier to see the bulge, we see nothing other than a box-shaped structure.” In Lépine’s opinion, the Milky Way has only one bar, which is nearly aligned with the center of the galaxy and with the Sun.

Augusto Damineli, his colleague at IAG-USP, has a different opinion. “The X in the Milky Way’s bulge seems to be a robust result, even though the mapping of the galaxy needs to be refined,” he says. “Other galaxies have a structure of this kind, which is described (in scientific papers) as being the result of numerical results.” There is no simple explanation for the existence of X-shaped bulges, according to Damineli. If the center of the Milky Way actually has a structure formed of two bars that cross each other and create a concentration of stars whose contours resemble the letter X, the use of the word bulge, which automatically reminds people of rounded shapes, may be inappropriate to describe the central region of some galaxies.

Scientific article
Saito, R. K. et al. Mapping the X-shaped milky way bulge. The Astronomical Journal. v. 142, n. 3, p. 76. Sep. 2011.