Discovering a young galaxy, less than 6 billion years old, but whose stars are as mature as those of our Milky Way: this is the puzzle that astronomers have just uncovered.
This paradox was revealed thanks to a rare phenomenon, an Einstein cross. This exceptional gravitational lensing configuration allowed astronomers to probe the depths of a distant galaxy like never before, revealing a surprising stellar composition that challenges our models of galaxy formation.
An Einstein cross with the strange elliptical galaxy J1453g at its center
Credit: Quirino D'Amato.
An Einstein cross occurs when a massive object, such as a galaxy, aligns almost perfectly with a more distant light source, warping the space-time around it. The light from the background object is then bent and amplified, appearing as four images arranged in a cross. This phenomenon, predicted by Einstein's general relativity, provides astronomers with a cosmic magnifying glass to study objects that are normally too faint or too distant.
In this specific case, the lensing galaxy is J1453g, a massive elliptical galaxy seen at a time when the Universe was less than 6 billion years old. Behind it, a bright quasar, powered by a supermassive black hole, is multiplied into four points of light. By analyzing the arrangement of these images, the researchers were able to map the mass distribution of J1453g with unprecedented precision, particularly the composition of its stars.
The results stunned scientists: the core of J1453g contains a proportion of massive stars similar to that of the Milky Way, a mature spiral galaxy that is much younger. Yet current models predict that the cores of young elliptical galaxies should be dominated by low-mass stars, formed rapidly during intense bursts of star formation.
This observation challenges our understanding of galaxy evolution. If an early elliptical galaxy can host such evolved stars, it implies that star formation processes are far more intricate than previously thought. Galactic mergers, for instance, could stir up matter and produce massive stars at very early stages.
The research team, led by Quirino D'Amato of INAF, published these results in the journal
Nature Astronomy. This work opens a new window into the Universe's adolescence. By combining gravitational lensing with telescopes like the JWST, astronomers hope to detect other Einstein crosses and thus probe the mysteries of galaxy formation in remote epochs.