Galactic ancestors

Australian astronomers believe they have solved one of the mysteries of galactic evolution that concerns the fate of massive and compact galaxies that were present in the early Universe. A careful analysis of the galactic components has identified the missing population associated to spheroidal compact systems eliminating some issues that arose from the models of galactic merging. The results were published in the Astrophysical Journal.

NGC 1332 Galaxy


Credit: G. Savorgnan

A group of astronomers from the Swinburne University of Technology in Australia believe they have found the answer to one of the greatest mysteries of galactic evolution that concerns the fate of galaxies that were present during primordial cosmic history. After ‘dissected’ a set of galaxies (including lenticular and spirals galaxies), the researchers were able to locate the missing relative population to those spheroidal compact systems thus eliminating some of the problems that arose from the models of galactic merging.
“We wanted to address one of the biggest puzzles of galaxy evolution, a mystery remained unexplained for a decade,” says Alister Graham Professor of Astronomy at Swinburne and author of the study accepted for publication in Astrophysical Journal. “Scientists have tried to explain how galaxies compact and far more evolved than the complex anatomy we see in galaxies today. Also, since the light takes a finite time to propagate in space, we see these distant galaxies as they appeared in the Universe of origins. Instead, we look at the present time only a few spheroidal stellar systems. ”
According to the most commonly accepted theory, fusion (merging) of galaxies would lead to their destruction by turning them into elliptical galaxies larger. However, there have been galactic collisions sufficient to take account of the scarcity of these systems spheroidal compact. Astronomers have therefore eliminated the problems that emerge with the theory because they believe they have identified the missing galaxies.
“They hide from view,” says Bililign Dullo co-author of the research. “The spheroids are masked from the disks of stars that were formed from much of the hydrogen gas and from smaller galaxies over a very long time.” In addition, the number of these stellar systems hidden is roughly equal to the number of compact galaxies in the early Universe. “Unlike the large dinosaurs that existed when the Earth was still young, the ‘galactic dinosaurs’ of our Universe are not extinct,” adds Graham. “They are simply embedded in huge disks of stars relatively thin.”

NGC 1277 Lenticular Galaxy


Credit: G. Savorgnan

Because of the enormity of modern galactic survey, it has become common practice to treat the galaxies as individual entities. Now, thanks to a more careful analysis of the individual components, i.e. the spheroid inner and outer disk, the researchers were able to uncover the missing population. But what all this means in terms of galactic evolution? Media INAF has asked Giulia Savorgnan doctoral student and co-author of the work: “The galaxies, much like humans, through very different stages in their lives. When we look at very distant galaxies, we see them as they appeared during their ‘youth’. At those times they presented themselves as objects spheroidal, quite simple. However, with the passage of time they have evolved into more complex objects. Many of them have formed a disk around the spheroid and in some galaxies were born complex structures that we astronomers call bars, internal disks, spiral arms, and so on. My analysis allows decomposing the ‘older’ galaxies (i.e. those closest to us) in their different constituents, as if they were ‘Lego’. This way, I can look at individual components and understand the different mechanisms of formation of each ‘brick’. In other words, we have just the ‘cut’ instead of considering them as individual objects, which explains why they were lost. This type of analysis is called ‘galaxy decomposition’ seeing I have renamed ‘galaxy vivisection’ ‘.
Coming closer to home, according to the authors also spheroid center of our Milky Way seems to have existed, at least in part, when the Universe was still young. We know that some stars are 12 billion years old, not much younger than the present Universe. The question that should be answered is which part of the galactic bulge has subsequently formed by other processes.

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