The multiple faces of today’s galaxies: their morphologies tell their evolution

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The large morphological classes of galaxies have fascinated astronomers since their discovery. A new analysis led by Louis Quilley, PhD student (Sorbonne University), and Valérie de Lapparent, researcher at the Institut d’Astrophysique de Paris (IAP), establishes a physical link between the order of the different types of the famous Hubble sequence, and the systematic evolution of galaxies.

This analysis shows for the first time that the trajectory of galaxies across the green plain, the transition zone through which galaxies evolve from forming many new stars to a quiescent state, is closely related to morphological types, making the Hubble sequence an inverse evolutionary sequence. These results were published on October 27, 2022 in Astronomy and Astrophysics.

Less than a century earlier, in 1926, the American astronomer Edwin Hubble published a morphological classification of galaxies that grouped together the different objects he had observed according to their shape and the details they contained. An order is chosen and constitutes a sequence ranging from elliptical galaxies, to lenticular, spiral and irregular. Later enriched by Gérard de Vaucouleurs and Allan Sandage, the Hubble sequence remains a reference tool for classifying galaxies, still used today by astronomers. At the time, astronomers wondered if this sequence of types reflected the evolutionary stage of galaxies. At the end of the century, numerical simulations showed that the first galaxies along the sequence, the ellipticals, could be the result of mergers of spiral galaxies, more advanced on the sequence.

To conduct the study, astrophysicists used the EFIGI (Extraction of Idealized Galaxy Shapes in Imaging) catalog of 4458 nearby galaxies, whose visual classification, coordinated by IAP astronomers was published in 2011. This classification uses digital images from the Sloan Digital Sky Survey (SDSS) and benefits from the richness of detail they provide for nearby galaxies. The new analysis also takes advantage of the new SourceXtractor++ software, developed for the Euclid space telescope (scheduled for launch in 2023), which allows to reliably and accurately measure the distribution of light within each galaxy. These optical measurements were complemented by ultraviolet data obtained by the Galaxy Evolution Explorer (GALEX) space telescope, in order to calculate the colors of galaxies, and by the same token, to evaluate at what rate they form stars. Finally, the Z-PEG software based on the theoretical scenarios of galaxy evolution, PEGASE.2, all developed at IAP, has been used to estimate the mass of each galaxy.

The work of Louis Quilley and Valérie de Lapparent (CNRS, Sorbonne University) brings today a decisive answer: the Hubble sequence is a reversed sequence of galaxy evolution.