Stellar Megalopolises Reveal Their Origin and Nature of Their Outskirts

Hence, Buitrago and his team, composed by the IAC researcher Ignacio Trujillo among other members, set out to investigate the nature of the outskirts of a sample of massive elliptical galaxies from when the Universe was half its current age, roughly 6.2 billion years ago. Focusing their research on faint features at great distances from the galactic centre, they could only work with the deepest ever image of the Universe, the Hubble Ultra Deep Field (HUDF).

The Hubble Ultra-Deep Field (HUDF) is the most profound image of a small region of space. It collected light from objets looking back until approximately 13 billion years.

Credit; HUDF

Using the six galaxies that matched their criteria and are recorded in this image, the researchers were able to demonstrate for the first time the existence of extended stellar envelopes in individual massive elliptical galaxies at that period in time.

The quality of the information collected in the HUDF, enabled the team to characterise the individual galactic haloes and to place them in the context of the evolutionary history of this type of galaxies. Moreover, Buitrago and his team concluded that, for their sample of massive elliptical galaxies at half the age of the Universe, the outer parts were, like in disc galaxies, formed mainly due to the merging of other galaxies. “In our work”, says Ignacio Trujillo, “we could observe galactic cannibalism in action. The larger galaxies were devouring the smaller ones at a high rate. Since its formation, almost half of the stars we see today in these massive galaxies are due to the fusion with other smaller galaxies”, concludes.

The results emerged from the comparison of the sample with mathematical simulations based on the current model of galaxy formation and evolution. The team saw that, in this very case, the simulation and the real data matched very well and that it was possible to derive parallelisms. “In elliptical galaxies, we cannot say ‘this is the galactic bulge and this is the halo’,” says Buitrago. “All the stars form a huge spheroid, like an immense rugby ball. But when we use a computer simulation, we can track the origin of every part of the simulated galaxy and compare with our real galaxies. Through this method, we identified the process behind the dramatic increase of these galaxies outer parts, and were able to explain how their size evolves.”

The results of the study, the most detailed to date and recently published in Monthly Notices of the Royal Astronomical Society, contribute to a better understanding of how the larger galaxies of the Universe evolved.

 

 

Contacts and sources:

 Instituto de Astrofísica e Ciências do Espaço (IA)