October 12, 2003

Devouring of smaller galaxies is a natural phenomenon called galactic cannibalism, a process Sagittarius is going through

NOBODY knows why the Greeks called the constellation of Sagittarius an arrow three millenniums ago. Perhaps, due to its location and direction that was visible with the naked eye, and then attributed it to one of the zodiac signs that many people are bracketed with. From the astrological point of view, the people bearing this sign are believed to be honest, hardworking and straightforward. This could be one of the reasons for calling it an arrow which hits the dot.

They might not have called it Sagittarius had they known what was happening to the constellation of stars lying between Scorpiconous and Capricornus, containing bright second and third magnitude stars. On September 25, 2003, astronomers and scientists from the University of Virginia and Massachusetts finally said that Sagittarius is being devoured by our galaxy, the Milky Way. The final report of the investigation into the galactic change is due to appear on December 20, 2003, in the issue of the Astrophysical Journal authored by the team leader, Steven Majewski.

Sagittarius, the dwarf galaxy, is the nearest galaxy and 48,980 light years away from the outermost diffuse limits of the Milky Way. Until 1994, it was not known whether it was a dwarf galaxy or a satellite to our Milky Way, 10,000 times heavier than the Sagittarius. Since then, the study continued and for weeks, astronomers and scientists continued observing the movements of the debris that had fallen into the outer fringes of the Milky Way, specially after 1998. It is for the first time in human history that we have witnessed a galaxy devouring another galaxy. Devouring of smaller galaxies is a natural phenomenon called galactic cannibalism, a process Sagittarius is going through.

The study is part of a very fundamental question tantalizing scientists from the very beginning of the human race: How the Universe came into being; how galaxies were formed and what will be the fate of the Universe. Scientists believe that the Universe came into being through the ‘Big Bang’ about 15 billion years ago. The moment of creation is said to be an astronomical figure 10-57th of a second, or in other words, a decimal point followed by 57 zeros and one part of a second. This followed matter and energy being released into space, creating cosmic gases, solidifying matter and forming stars, gas clouds, cosmic dust and finally, the galaxies. Our galaxy, the Milky Way, too, came into being from a cloud of gas at the same moment and took a spiral spin, moving in a direction set at the time of its creation. The ‘Bang’ was so huge that galaxies continue to form, running away from each other, expanding the Universe at a tremendous speed. Nobody knows when and what the fate of the Universe would be.

These questions have been so intriguing that scientists have been making efforts to go as deep into it as they can. Every observation adds a new dimension to the research, but the phenomenon remains the same, more or less.

We know that since the Big Bang, new galaxies are getting born, others dying and larger galaxies devouring their smaller neighbours. The theory is that dwarf galaxies were the first to come into being. Many of them then clubbed together to form larger galaxies, or were devoured by larger galaxies. The annihilation of the dwarf galaxies is a rampant practice. The cannibalism by the Milky Way is part of the phenomenon. The Milky Way gaining a huge mass is part of this mechanism and it got larger by devouring smaller galaxies. Before 1994, Sagittarius, as a dwarf galaxy, was not known. It was found diametrically opposed to the galactic centre of the Sun, only 15 kiloparsecs away from the centre (a kiloparsec equals to 3,262 light years, and a light year is 6,570,000kms per year).

In 1998, a team of astronomers saw a stream of matter, an outshoot from the centre to the southwest of the Sagittarius. A scientist in the Astrophysical Journal of Letters predicted: “Theoretical models predict the presence of another symmetric stream, extending to the northwest, that could be so long as to encircle our galaxy completely. However, this stream would be even more difficult to identify since it would cross the disc of the Milky Way and be hidden by the Galactic Centre.”

The four-year study of the galaxy was very extensive, establishing the causes of the debris that were found in the Milky Way. It was first thought that the Sagittarius galaxy has disintegrated and had reached the point of destruction. The presumption was that the debris in the Milky Way were nothing but of the Sagittarius that has been swallowed

Scientists became curious about the event as it offered, for the first time in the history of the human race, a chance to observe the destruction of a dwarf galaxy and learn the phenomenon of the formation of large galaxies first-hand. This also made it possible for astronomers to look for the stars of the Sagittarius galaxy that might have entered the Milky way and would be straying on its rims. But it had problems, the first being how to distinguish them.

The Institute de Astrfisica de Canarias (IAC) and the University of Geneva, along with other scientists, took up the hard study by using the 2.5-metre Issac Newton telescope at La Palma, and aided by their dynamical models, were able to identify the existence of young stars at a distance of 56 kiloparsecs from the centre of the Milky Way which belong to the Sagittarius galaxy. The debris detected was confirmed by observations to belong to the Sagittarius, as predicted at the early stage of detection. The debris, scientists believe, has completely encircled our galaxy.

The discovery’s first findings were checked and re-verified by astronomers, who sometimes theorized that large galaxies are formed by devouring smaller satellite galaxies. But never before has the human race been able to watch the mechanism in practice. Presently, besides others, there are two main objectives before the astrophysicists: The Sagittarius galaxy is in the advanced process of destruction, and much of it has already been cannibalized by the Milky Way. The other being that how can the debris of a cannibalized galaxy affect our system, specially the stars that now roam about the Milky Way?

The destruction of Sagittarius may be a part of the cosmological phenomenon and for some it may be a question of little importance, but the fact remains that the findings can guide us about the future of our own Earth, Sun and the Milky Way, hoped to be survive for another five billion years. This sounds quite an astounding time duration, but it has relevance with the present. After all, our Sun also came into being 4.5 billion years ago and became the source of life on Earth. The evolutionary process thereafter, reaching to the present stage, is in itself a history of the Universe and understanding it would be a vital key to human life. The introduction of new stars in the galaxy, the increase in its mass and most importantly, the establishment of an hypothesis of cannibalism is bound to create new pulls affecting many phenomena of the life cycle elsewhere in the Milky Way, our extended home.

For Greeks, who thought it an ‘arrow on the dot’ and those who consider the zodiac sign of Sagittarius may find it an interesting option to apprehend, but the fact remains that for the coming 48,930 years or so, their projected effects will remain unchanged. This is a twist in Nature.

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