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Astrophysics

The coupling of the old dwarfs

Brazil has contributed to a study witnessing the onset of the merging process of a group of small galaxies

NASA, ESA, S. Gallagher and J. English HCG 31: four ancient dwarf galaxies are merging to form a single large elyptical galaxy NASA, ESA, S. Gallagher and J. English

After aiming its most modern observation instruments at a not too distant region of the Universe, where there is a group of four ancient dwarf galaxies, and taking measurements with different wave lengths to try to discover the age of the stars in this fine system, which is so small that it would fit within the MilkyWay, a team of international astrophysicists realized that it had detected something unexpected. The scientists had run into an event that must have been common in the Universe’s early days, several billions of years ago, when large galaxies started gaining mass as a result of the merging of smaller galaxies, but that was now literally out of its habitual context. An analysis of a series of images taken by the Hubble and other telescopes in space confirms an idea already advocated some years ago by astrophysicists Claudia Mendes de Oliveira, from the University of São Paulo (USP), to the effect that the quartet of dwarf galaxies, known as the Hickson Compact Group 31 (HCG 31), is merging 166 million light-years away. This is a relatively modest distance from Earth (one light-year equals the distance covered by light in one year within a vacuum).

The merging of old galaxies, resulting in a single large elyptical galaxy, began only some 10 million years ago, which is akin to yesterday on the cosmic time scale. This is perhaps the most eloquent information on how extemporaneous this phenomenon seems. It was described in detail in an article published by Canadian, American and one Brazilian astrophysicist in the February issue of The Astronomical Journal. “We believe that these dwarf galaxies are merging for the first time,” states Claudia. “This type of merging must be very rare nowadays.” Drawing a parallel with palentology, the international team of astrophysicists compared the discovery of merging galaxies in a corner of the Universe nearby to the recovery of a dinosaur fossil in one’s back yard.

In the picture on the right, a composition produced from observations by the Hubble and by other satellites, such as the Spitzer, the shiny and distorted object in the mid-lefthand side actually depicts two dwarf galaxies merging. From this clash of matter new stars are born, hot and massive. These emit ultraviolet radiation and warm up the gas clouds around them, making them shine. The spectacle is reminiscent of a fireworks display in space. Still on the lefthand side, but above these two galaxies, there is a third one, connected to the two former ones by a bridge of star clusters. Its shape is reminiscent of a cigar. Finally, on the lower righthand corner, one can see the fourth galaxy of this compact group, linked to the others by a belt of stars. The extremely shiny object right in the center of the picture is a star that is closer than HCG31 and that has no connection whatsoever with the group of galaxies. “There is still a lot of gas in the system and the merging of the galaxies should last for a billion years,” comments Claudia. The quartet of small galaxies, though smaller than the Milky Way, has an amount of hydrogen, the raw material of the stars, that is five times greater than in our galaxy.

The age of the stars in the HCG 31 system is the main evidence that this process of merging matter is recent. The older ones were formed some 10 billion years ago, indicating that the agglomeration is really very old. However, the younger ones, comprising some sets of 100 thousand intensely brilliant stars, are 10 million years old at most. “We know that this system has been around for quite a while,” says the astronomer Sarah Gallagher from the University of Western Ontario (Canada), one of the study’s authors. “Most of the other dwarf galaxies interacted billions of years ago, but these are only starting to join.” Prior studies indicate that as they are being pulled together by the force of gravity, the quartet’s galaxies are on a collision route at the speed of 60 kilometers a second, which is extremely slow. In the system, the distance from one galaxy to the next is of the order of 75 thousand light-years, or three quarters of the Milky Way. The scientists speculate that the merging of the galaxies took almost 10 billion years to start because HCG 31 is in an area of the Universe with low density matter. It is as if it were in a sort of rural area of the Cosmos, where the force of gravity is not so strong, being far removed from the busier centers.

The inclusion of Claudia de Mendes de Oliveira in the list of authors of the Astronomical Journal article’s is an acknowledgement of her work on compact groups of galaxies, one of the subjects in which she has specialized. She was invited take part in this study by scientists from abroad. The study in question mentions eight other scientific articles by the USP astrophysicist, generally written jointly with other Brazilian colleagues. “In 2004, we published an article that showed that the HCG 31 galaxies were merging,” Claudia tells us. In 1992, she completed her doctorate in Canada under the guidance of Paul Hickson, from the University of British Columbia. He was the astrophysicist who 10 years earlier had identified for the first time this type of system. In the corners of the Cosmos in which there are agglomerations of a few galaxies, all of them close to each other and interacting in gravitational terms, the scientists say that there is a compact group of galaxies. These systems are regarded as laboratories for the study of galaxy evolution. Outbreaks of star formation may take place, also nuclear activity, radio and X-ray emission and, of course, the merging of galaxies. More than one hundred compact groups have been discovered in recent decades. Some systems, such as the Stephan Quintet, have yielded beautiful images. At present, the astrophysicist from USP is focusing on mapping and classifying 25 compact groups of galaxies.

Scientific article
Gallagher, S. C. et al. Hierarchical structure formation and modes of star formation in Hickson Compact Group 31. The Astronomical Journal. v. 139, p. 545-56. 2010.

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