Are quiescent disks in early-universe galaxies hiding a much more violent reality? Recent research suggests that they might and the evidence is in one of the universe’s strongest natural magnifying lenses.
The galaxy in question, which is being called “Cosmic Grapes,” formed only 930 million years after the Big Bang. Its striking photograph would never have been possible thanks to one of the strongest gravitational lensing events ever observed. Here in this event predicted by Einstein’s theory of general relativity, a large foreground object a cluster of galaxies bends spacetime, twisting and magnifying the light from a source farther back. “This object is known as one of the most strongly gravitationally lensed distant galaxies ever discovered,” said Seiji Fujimoto, lead author of the study. The phenomenon magnified the galaxy’s apparent size and brightness, allowing instruments to observe details otherwise beyond their grasp.
Astronomical application of gravitational lensing is that it can overcome the resolving power of even the finest telescopes. In the thin-lens approximation generally used in observation studies, light beams are modeled as curved straight lines at the lens plane by an angle that can be measured. But magnification depends on the geometry, mass distribution, and the important point source size. Amplification in high-redshift compact galaxies can be enormous, exhibiting detail on tens of parsecs scales. Here, the lensing, combined with over 100 hours of observing time, offered a resolution of just 10 parsecs about 30 light-years in a galaxy 13 billion light-years from us.
Earlier Hubble Space Telescope observations had shown the Cosmic Grapes as a smooth disk-like smear. But when ALMA and the James Webb Space Telescope (JWST) let loose their combined power on the object behind the lens, the scene was revolutionized. Instead of a uniform star distribution, astronomers saw a rotating disk inhabited with at least 15 giant dense clumps between 10 and 60 parsecs in diameter that regulated approximately 70 percent of the galaxy’s ultraviolet light. Co-author Mike Boylan-Kolchin said, “Our observations reveal that some early galaxies’ young starlight is dominated by several massive, dense, compact clumps rather than one smooth distribution of stars.”
These are not random distortion; they are dense clumps of intense star formation activity with conditions to supply rapid stellar birth. Their presence is in contradiction with conventional galaxy development theories, which predict that the initial disks would be uniform because gas and stars would intermix in a chaotic manner. It is impossible to reproduce such a huge quantity of clumps with modern simulations in a rotating galaxy at this point in the evolution of the universe, which means the physics of feedback how young stars and supernovae inject energy into the environment would need to be very fundamentally rewritten.
Technically, observations also exhibit the synergy between gravitational lensing and next-generation observatories. Interferometric arrays within ALMA can observe cold dust and gas with millimeter-wave emission lines like [CII], while infrared imaging in JWST pierces through dust to reveal hidden populations of stars. Combined with the magnification provided by lensing, these tools can investigate the inner machinery of galaxies at cosmic dawn, relating small-scale to large-scale rotation a previously unseen link in an average galaxy at this early stage.
The Cosmic Grapes is no oddity in its global properties. It is well within the “main sequence” of mass, size, chemical composition, and star-formation rate of star-forming galaxies. This means that most early galaxies that appear smooth in lower-resolution surveys might be concealing such substructure themselves. If this is the case, then simplicity of early galactic disks is an observationally generated property, being smeared out by angular resolution boundaries.
The discovery also highlights the statistical importance of lensing for high-redshift applications. As shown in tests of magnification bias, the finite size of sources and of ellipticities of lenses can highly truncate the highest accessible magnification, affecting how often such close looks are possible. But with the upcoming wide-area surveys of Euclid and the Roman Space Telescope, the number of strong galaxy–galaxy lenses is expected to rise by orders of magnitude, greatly expanding the target sample for this kind of work.
Temporarily at least, the Cosmic Grapes offers us a singular and enlightening example: a galaxy in the early morning of cosmic history, its internal structure revealed by the collective power of nature’s own eyes and mankind’s most advanced instruments.
