In the primitive universe, about half of the observed galaxies have ceased to form stars: they are “extinguished” and no longer grow. Astronomers describe them as “quiescent”, “quadnéd” or even “red and dead” galaxies. They appear red, because they no longer contain young and bright stars of blue. Only older and smaller stars remain with red shades.
An important part of quiescent galaxies is among the massive galaxies, often observed with elliptical morphologies. The formation of these red and dead galaxies normally takes a lot of time, because they must first accumulate a large number of stars before the stellar formation process stops completely.
The exact cause of the Quenching remains an enigma. “Finding the first examples of massive quiescent galaxies (MQG) in the primordial universe is essential, because this sheds light on their possible training mechanisms,” explains Pascal Oesch, associate professor in the Astronomy Department of the Faculty of Sciences at the University of Geneva (Unige) and co-author of a study that has just appeared in the astrophysical journal, according to a press release from Unige.
Identified with increasingly remote eras
The quest for these systems is thus a major objective of astronomers for years. Thanks to technological advances, in particular spectroscopy in the close infrared, astronomers have gradually been able to identify the massive quiescent galaxies (MQG) with increasingly remote cosmic eras. Their abundance defies the theoretical models, which provided for a longer training time.
With the James Webb telescope (JWST), this contradiction between theory and observations intensified, several MQGs having been confirmed to 1.2 billion years after the Big Bang. A relatively short period on an astronomical time scale. Today, the new study led by Unige reveals that they have formed even earlier and faster.
In Cycle 2 of the JWST, the Rubies large field program (The Red Unknowns: Bright Infrared Extragalactic Survey or Les Unknown Rouge: extragalactic survey in light infrared), one of the largest European programs dedicated to extragalactic research with the Nirspect instrument, obtained spectroscopic observations of several thousand galaxies, including hundreds of newly discovered sources thanks to the first images of the JWST.
100 times more abundant than expected
Among these new spectra, scientists have identified the most distant massive quiescent galaxy (MQG) ever observed, about 700 million years after the Big Bang. The specter obtained with Nirspec/Prism reveals a surprisingly old stellar population in a still young universe. A detailed modeling of the spectrum and imaging data shows that this galaxy has formed a stellar mass of more than 10 billion (1010) Solar masses during the first 600 million years after the Big Bang, before suddenly ceasing its stars formation, thus confirming its quiescent state.
“The discovery of this galaxy, named Rubies-Uds-Qg-Z7, implies that the massive galaxies quiescent during the first billion years of the universe are more than 100 times more abundant than what the current models provide,” explains Andrea Weibel, doctoral student in the Astronomy Department of the Faculty of Sciences of Unige and first author of the study.
This suggests that certain key factors of theoretical models – such as the effects of stellar winds or the intensity of material flows induced by stellar formation and massive black holes – must be reassessed. Finally, the size of Rubies-Uds-Qg-Z7 is only about 650 years of approximately, which implies a high stars density, comparable to the highest central densities measured in the quiescent galaxies at slightly lower distances. These galaxies are likely to become the nuclei of the oldest and most massive elliptical galaxies observed in the local universe.
“The discovery of Rubies-Uds-Qg-Z7 provides the first solid proof that the centers of certain close massive elliptical galaxies have perhaps already in place for the first hundreds of millions of years of the universe,” concludes Anna de Graaff, a post-doctoral researcher at the Max Planck Institute of Astronomy of Heidelberg, initiator of the Rubies and Second Author of the article.
