Galaxie elliptique NGC 4150. Crédit : NASA, ESA, R.M. Crockett (Université d’Oxford, Royaume-Uni), S. Kaviraj (Imperial College London et Université d’Oxford, Royaume-Uni), J. Silk (Université d’Oxford), M. Mutchler (Space Telescope Science Institute, Baltimore, États-Unis), R. O’Connell (Université de Virginie, Charlottesville, États-Unis) et le Comité de surveillance scientifique du WFC3.
Les galaxies elliptiques ont une forme régulière, ellipsoïdale. Elles contiennent généralement une proportion beaucoup plus importante d’étoiles plus âgées que les galaxies spirales.
Quatre classes sont utilisées pour classer galaxies: spirale ; spirale barrée; elliptique et irrégulière. Les galaxies elliptiques ont une apparence ellipsoïdale ou sphérique lisse, et elles sont beaucoup moins structurées que les galaxies spirales. Les galaxies elliptiques se trouvent généralement dans des amas de galaxies.
Il existe une idée fausse très répandue selon laquelle les astronomes pensaient que les galaxies elliptiques étaient les précurseurs évolutionnaires des galaxies spirales, car Hubble lui-même qualifiait les galaxies elliptiques de ” type précoce ” et les galaxies spirales de ” type tardif “. En fait, Hubble a simplement utilisé ces noms pour indiquer les différences de structure entre les galaxies elliptiques et spirales, et il n’était pas sûr de la façon dont les différents types de galaxies avaient évolué.
Les galaxies elliptiques ont une forme régulière, ellipsoïdale. Elles contiennent généralement une proportion beaucoup plus importante d’étoiles plus anciennes que les galaxies spirales. Crédit : ESA/Hubble, ;” data-gt-translate-attributes=”[{” attribute=””>NASA, ESA, R.M. Crockett (University of Oxford, U.K.), S. Kaviraj (Imperial College London and University of Oxford, U.K.), J. Silk (University of Oxford), M. Mutchler (Space Telescope Science Institute, Baltimore, USA), R. O’Connell (University of Virginia, Charlottesville, USA), and the WFC3 Scientific Oversight Committee
Current thinking amongst astronomers is that most elliptical galaxies formed from the collisions and subsequent mergers of spiral galaxies. The typical ages of the stellar populations of elliptical and spiral galaxies provide evidence for this theory, because the stars in elliptical galaxies are typically much older and redder than those in spiral galaxies. Whilst spiral galaxies have rich reservoirs of the dust and gas that fuel star formation, elliptical galaxies appear to have virtually exhausted that fuel, and so there is very little raw material for the formation of new stars. Therefore, it seems likely that elliptical galaxies are largely populated by stars that formed within active spiral galaxies. Due to their very low rate of star formation and their populations of old, red stars, elliptical galaxies are sometimes colloquially referred to as ‘red and dead’ by astronomers.
Elliptical Galaxy. Credit: NASA, ESA, R.M. Crockett (University of Oxford, U.K.), S. Kaviraj (Imperial College London and University of Oxford, U.K.), J. Silk (University of Oxford), M. Mutchler (Space Telescope Science Institute, Baltimore, USA), R. O’Connell (University of Virginia, Charlottesville, USA), and the WFC3 Scientific Oversight Committee
Recently, the Hubble Space Telescope and ESO’s Very Large Telescope (VLT) were used to help solve an astrophysical mystery that had centered on the most massive elliptical galaxies. These gigantic ‘red and dead’ galaxies were not always so inactive. In fact, earlier in the history of the Universe, the evolutionary precursors to these massive galaxies were full of gas and forming stars at a prodigious rate. Thanks to Hubble and the VLT, astronomers were able to show for the first time how star formation in these ‘dead’ galaxies actually sputtered out billions of years ago. Hubble has also captured beautiful imagery of elliptical galaxies throughout its more than 30-year history. This includes detailed views of the elliptical galaxies NGC 4150 (see image at the top of this page) and NGC 2768.
