🔭 Why so few dwarf galaxies?

With the James Webb Space Telescope, our view of the early Universe is becoming clearer, but a recent observation contradicts some expectations: small-sized galaxies appear strangely few in number in the early cosmic ages.

Under the direction of Xuheng Ma from the University of Wisconsin, a team analyzed data from the James Webb Telescope's UNCOVER program. To study these very distant objects, the researchers used the galaxy cluster Abell 2744 as a natural lens. They thus exploited the gravitational lensing effect, which amplifies light from distant background regions.


During the analysis, astronomers focused on the luminosity function, a statistical tool that counts galaxies based on their brightness. Generally, faint galaxies are supposed to be the most abundant. However, the data reveals a phenomenon: below a certain low luminosity threshold, the number of galaxies begins to decline instead of increasing.


This scarcity could originate from the hostile conditions of the young Universe. Indeed, the intense radiation emitted by the first massive stars would have heated the surrounding gas, preventing small, low-mass galaxies from retaining it. Deprived of this raw material, they could not generate enough stars, thus remaining too faint to be detected by our instruments.

These observations question our modeling of the reionization epoch, that period when the Universe transitioned from an opaque state to a transparent one. If ultrafaint galaxies are less present than expected, they cannot then be the main actors of this metamorphosis. We should rather look towards larger and already well-formed galaxies to explain this cosmic transition.

Current interpretations rely on gravitational lensing models that could be refined. To validate this trend, complementary observations using other clusters and future large sky surveys will be essential. The James Webb Telescope continues its exploration, hinting at new discoveries about galaxy evolution.