by Staff Writers
Santa Cruz de Tenerife, Spain (SPX) Mar 07, 2017
Galaxies have dramatically grown in size since the early Universe, and elliptical galaxies, in particular, are the largest galaxies in both size and mass. What is the main driver behind the late growth of their outer parts was the question that motivated this study.
With disc galaxies, like our Milky Way, it is fairly easy to identify their distinct parts: the central bulge, the disc with its spiral arms, and a halo of stars enveloping the whole. Astronomers can tell, for instance, that the stellar halo of a disc galaxy is mostly formed by stars from satellite galaxies that it has swallowed.
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 Astrofisica e Ciencias do Espaco (IA) and Faculdade de Ciencias 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."
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).
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.
Princeton NJ (SPX) Mar 03, 2017
The universe has come into sharper focus with the release this week of new images from the one of the largest telescopes in the world. A multinational collaboration led by the National Astronomical Observatory of Japan that includes Princeton University scientists has published a "cosmic census" of a large swath of the night sky containing roughly 100 million stars and galaxies, including some o ... read more
Instituto de Astrofisica de Canarias
Stellar Chemistry, The Universe And All Within It
|The content herein, unless otherwise known to be public domain, are Copyright 1995-2017 - Space Media Network. All websites are published in Australia and are solely subject to Australian law and governed by Fair Use principals for news reporting and research purposes. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA news reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. All articles labeled "by Staff Writers" include reports supplied to Space Media Network by industry news wires, PR agencies, corporate press officers and the like. Such articles are individually curated and edited by Space Media Network staff on the basis of the report's information value to our industry and professional readership. Advertising does not imply endorsement, agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement|