If we could observe our galaxy from outside, hundreds of thousands of light years, we would see several spiral arms that form an approximately circular disk. But when placing it on the edge, we would check that the disc is not perfect: in addition to having a center bulged by the concentration of stars and gas, the galactic plane is warped, like a wooden board that has warped in the rain.
This deformity of the Milky Way has been known since the late twentieth century, but since astronomers can only observe the edges of the galaxy from within, they have not been able to describe it accurately. A new map published in Nature Astronomy located in three dimensions the position of 1,339 stars of the Milky Way to offer the first faithful view of the warping.
The shape it has is "as if one takes a flexible plastic disc and, raising one end, bends the other down", illustrates Francisco Garzón, an astronomer at the Institute of Astrophysics of the Canary Islands oblivious to the investigation. Several decades ago, it was discovered that the galaxy was not flat, because of the distribution of hydrogen gas in its periphery. But the new study is not based on gas observations; instead, map the position of individual stars that serve as reference points distributed by the disk.
The research team, made up of scientists from China and Australia, focused on stars called Cepheid variables. They are stars that pulsate radially (like a lighthouse), with a very stable period that is directly proportional to their luminosity. This allows knowing the intrinsic brightness of the star by just counting the time interval between its pulsations. Then, you can calculate your distance from Earth, comparing the observable brightness with the real one. These properties of Cepheids, discovered by the calculator Henrietta Leavitt between 1908 and 1912, they have become ideal objects to measure astronomical distances.
"It is notoriously difficult to determine distances from the Sun to areas of the outer disk of the Milky Way without knowing what the shape of that disk is," explains Xiaodian Chen, the lead author of the study, from the Chinese Academy of Sciences in Beijing. To achieve this, he and his companions chose Cepheids that are between four and 20 times more massive than our Sun, and up to 100,000 times brighter. Some are observable in the visible spectrum and others only in the infrared.
The information they analyzed comes from the WISE astronomical space telescope, launched by NASA in December 2009. In addition, they relied on data from the Gaia space probe, the astrometry mission of the European Space Agency, active since 2013. They used this data to eliminate noise from the original sample and thus produce a clean map that "is not blurred by clustered objects," according to Garzón.
The model confirms that the irregularities of the galactic edge are due to the interaction between the gravitational and centrifugal forces of the dense core of stars. They are deformities that were not present when the Milky Way was younger. The new map, for its accuracy, allows to discard some models on the formation and evolution of galaxies, as well as to refine the predictions of others.
For the first time, the investigation reveals a precession in the own axis of the warping of the galaxy, previously unknown. This means that, in addition to bending the edges of the galaxy in opposite directions, they also twist perpendicular to the disk at the ends. To visualize this phenomenon, the Milky Way can be considered as a disk composed of concentric rings, and each ring bends differently, so that they reach more closed angles as they move away from the center.
This map covers a radius of 20 kilopársecs from the galactic center, more or less, or about 65,230 light years. It is the most complete picture of the shape of the Milky Way, but it is known that the actual extent of the galaxy is greater, with a radius of at least 25 kilopársecs. Our Sun is about eight kilopársecs from the center. Although warping does not occur in all galaxies, it is not unique to the Milky Way. In some galaxies that have a favorable orientation -the ones that are singing to us- it is observed that the stars draw a kind of S in the sky However, none has been studied as accurately as ours.