It is not easy to imagine the dimensions of a galaxy. Ours, without going any further, with its nearly 100,000 extreme light years, is so large that even the solar system, with all its planets, is lost in its immensity … To become a simple point between hundreds of billions of similar points, most stars that also carry their own planetary systems in tow. Thus, like an immense island of stars that spiral through space, the Milky Way moves on a cosmic route issued by the omnipresent force of gravity. Which, by the way, makes about 600 km per second, or what is the same, about two million km per hour.
But the Milky Way does not travel alone. Two and a half million light years away (here next to astronomical terms) another giant spiral galaxy, Andromeda, follows the same route. And both, Andromeda and the Milky Way, are in turn the largest and most massive members of a ‘local group’ of at least thirty galaxies.
Around the two giants, in addition, dozens of dwarf galaxies, ‘small’ satellites that have just a fraction of the size and mass of the two huge spirals. Our Milky Way has, that we know so far, about 70 of these galactic satellites, but astronomers continue to discover new every year. And the neighbor Andromeda seems to have up to three times.
Satellite swarm
All that swarm of dwarf galaxies around the largest provides valuable clues on how the Milky Way and Andromeda have evolved over billions of years. And it turns out that these little satellites tell a very different story for each of them. Stories that we can now know thanks to an ambitious observation program carried out with the Hubble Space Telescope and in which, for the first time, all the dwarf galaxies known around Andromeda have been invented.
Perfectly distinguishable in the sky, the majestic Andromeda is available to any amateur astronomer. But a much more powerful telescope is needed to see a swarm of almost three dozen satellite galaxies surrounding the giant, like bees around a hive.
Together, all these small galaxies constitute a bustling ‘ecosystem’ that the Hubble space telescope is studying in an unprecedented detail. The ambitious program used observations made over more than 1,000 orbits of the telescope, and thanks to them it was possible to build a precise 3D map of all dwarf galaxies that buzz around Andromeda, and also verify some of the utmost importance: the efficient thing that had been in the formation of new stars during the almost 14 billion years of existence of the universe.
Different from us
In his study, recently published in ‘The astrophysical journal’, An international team of almost forty researchers reveals in Andromeda a much larger galactic ecosystem and very different from our own galaxy. Which indicates that both giants did not evolve in the same way over time.
The history of our Milky Way, in effect, despite having ‘absorbed’ at least six other minor galaxies and having starred in a major collision, has been relatively placid compared to Andromeda’s, with a much more violent past and that was most likely affected by an important fusion with another great galaxy about one billion years ago. That encounter, and the fact that Andromeda is about twice more massive than our Milky Way, could explain its abundant and diverse population of dwarf galaxies.
Knowing with the same detail the satellite system of the Milky Way is extraordinarily difficult, since we are within our galaxy and we do not have the perspective sufficient. And it cannot be achieved with other large galaxies because they are too far so that our best instruments can study their small satellite galaxies in detail. In fact, beyond Andromeda, the closest galaxy of comparable dough is almost 12 million light years.
Therefore, Andromeda is the only great galaxy whose surroundings we can study in detail. And that has allowed researchers to decipher what drives the evolution of these small galaxies. “We see,” says Principal Author Alessandro Savino, from the University of California in Berkeley – that time during which satellites can continue to form new stars really depends on how massive they are and how close they are from the Andromeda Galaxy. It is a clear indication of how the growth of a small galaxy is disturbed by the influence of a massive galaxy like Andromeda ».
A ‘significant’ event
«Everything in the Andromeda system appears dispersed, and is very asymmetric and disturbed. Which indicates that something significant happened not long ago -the principal researcher Daniel Weisz, from the University of California in Berkeley, says. You always tend to use what we understand in our own galaxy to extrapolate more generally to the other galaxies of the universe. And he has always worried about whether we are learning on the Milky Way can also apply to other galaxies. Or is there more diversity among external galaxies? Do they have similar properties? Our work has shown that low -dough galaxies in other ecosystems have followed different evolutionary paths than what we know about the satellite galaxies of the Milky Way ».
For example, half of Andromeda’s satellite galaxies seem to be confined in a plane, all in orbit in the same direction. “Which is quite rare,” says Weisz. Actually, it was a surprise to find the satellites in that configuration, and we still do not completely understand why they appear in that way ».
Star formation
Andromeda’s brightest satellite galaxy is Messier 32 (M32). It is a compact ellipsoidal galaxy that could be the remaining nucleus of a larger galaxy that collided with Andromeda about one billion years ago. After the meeting, and after being stripped of a part of its gas reserves and many of its stars, which remained of that galaxy continued throughout its orbit. M32 contains older stars than one billion years, but there is a strong evidence that there was a ‘wave’ of star formation just at that time. In addition to M32, there seems to be a unique population of dwarf galaxies in Andromeda that do not have the same on the Milky Way. All of them formed most of their stars very early, but then they did not stop, but continued to form stars, at a very low rate, for a much longer time.
“The formation of stars,” explains Savino, “really continued until a very later time, which is not at all what is expected for these dwarf galaxies. This does not appear in computer simulations. No one knows what to do with that for now ».
«We discovered,” And the way in which things come together a lot to understand the history of this galaxy ».
In another five years, Hubble himself or the most modern James Webb space telescope will be able to obtain a second set of observations, which will allow astronomers to make a dynamic reconstruction of the 36 dwarf galaxies of Andromeda, and thereby understand with much more detail the story of our huge neighbor and its complex ecosystem over billions of years.
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