Cosmology
Dark matter
About 85% of the matter in the Universe appears to exist in a form that neither emits nor absorbs light, and it doesn't resemble the matter we encounter in our daily lives. This dark matter remains unidentified, but it plays a crucial role in our understanding of cosmology and the formation of large-scale structures in the Universe. Every large galaxy, including our own Milky Way, is believed to be enveloped by a vast dark matter halo—far more massive and extensive than the galaxy itself.
One of the unresolved challenges in modern cosmology is understanding the distribution of dark matter on scales smaller than individual galaxies. The prevailing dark matter theory suggests that galaxies like the Milky Way should be surrounded by numerous dark matter clumps, or subhalos, in the dwarf-galaxy mass range. However, only a few tentative detections of such dark subhalos have been made. Our team is using observations to study the properties of these subhalos, through the gravitational lensing distortions they produce in the images of distant galaxies and quasars.
Ongoing work has suggested that dark matter halos could potentially host the formation of small, extremely faint Population III galaxies at high redshifts. In rare cases, gravitational lensing (see below) could amplify the fluxes of these objects to detectable levels. However, detecting such galaxies in randomly chosen regions of the sky requires large-area surveys. Instruments like the James Webb Space Telescope (JWST), the Roman Space Telescope (RST), and Euclid have the capability to detect these faint galaxies, though their effectiveness varies. The RST, in particular, shows promise for photometric detections of Population III galaxies, while wide-field surveys offer better chances for these detections compared to focusing on individual cluster lenses. However, spectroscopic detections remain difficult due to the low number densities of these galaxies, unless very high star formation efficiencies are assumed.
The luminous parts of the galaxy 4144, a disk galaxy fairly similar to our own home galaxy, the Milky Way. Like all large galaxies, NGC 4144 is believed to be sitting inside a halo of dark matter, with large numbers of dark matter clumps (subhalos) in the dwarf-galaxy mass range. Image: NASA.
Gravitational lensing
Galaxies and galaxy clusters can bend the path of light from background objects, causing them to appear distorted and magnified. This phenomenon, known as gravitational lensing, allows astronomers to study faint, distant light sources that would otherwise be out of reach and to explore the distribution of matter within the lensing objects themselves. By combining observations with simulations, our team is leveraging gravitational lensing effects to search for galaxies in the early Universe, and to study dark matter on subgalactic scales.
One of the most remarkable outcomes of gravitational lensing is the ability to observe individual stars at incredible distances, magnified by factors of thousands. A significant recent observation is on a star named WHL 0137-LS, also known as Earendel, at a redshift of 6.2 ± 0.1, corresponding to roughly 900 million years after the Big Bang. This star, magnified by a galaxy cluster lens (WHL0137-08) located at redshift 0.566, is among the most distant and highly magnified stars ever discovered. The magnification and observed brightness of Earendel have remained relatively stable over 3.5 years of imaging and follow-up, with a delensed absolute UV magnitude consistent with a star more than 50 times the mass of the Sun. This stability suggests a persistent magnification effect, which is quite rare for such distant stars.
Gravitational lensing continues to broaden our understanding of the early Universe by allowing us to observe and analyze objects that would otherwise be invisible and providing new perspectives on the nature of dark matter and the formation of cosmic structures.
By curving spacetime, the yellow galaxy in the centre distorts the appearance of a more distant, blue galaxy and makes it appear as a large, horseshoe-shaped structure. Gravitational lensing effects of this type can be used to study distant astronomical objects at large magnification, and to study the distribution of matter within the lens galaxy itself. Image: ESA/Hubble & NASA.