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The images reveal the stellar envelopes, as well as potential satellite galaxies.
Credit: Ignacio Trujillo (IAC) with images from the HUDF 2012 programme.
For elliptical galaxies, however, this is much harder because these galaxies look much like a smooth, featureless cloud of stars. Fernando Buitrago, of Instituto de Astrofísica e Ciências do Espaço (IA) and Faculdade de Ciências da Universidade de Lisboa (FCUL) says: “With elliptical galaxies, there is direct evidence of merging of satellite galaxies going on, but it is hard to ascertain that the processes that have been happening for these galaxies to grow their outer envelopes are the same that we see occurring in disc galaxies like our own.”
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.
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 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.