2019MNRAS.484.4379P

Recent observations of lensed sources have shown that the faintest (M_UV ~-15 mag) galaxies observed at z = 6-8 appear to be extremely compact. Some of them have inferred sizes of less than 40 pc for stellar masses between 10⁶ and 10⁷M^_☉, comparable to individual super star clusters or star cluster complexes at low redshift. High-redshift, low-mass galaxies are expected to show a clumpy, irregular morphology and if star clusters form in each of these well-separated clumps, the observed galaxy size would be much larger than the size of an individual star-forming region. As supernova explosions impact the galaxy with a minimum delay time that exceeds the time required to form a massive star cluster, other processes are required to explain the absence of additional massive star-forming regions. In this work, we investigate whether the radiation of a young massive star cluster can suppress the formation of other detectable clusters within the same galaxy already before supernova feedback can affect the galaxy. We find that in low-mass (M₂₀₀ ≲10¹⁰ M_☉) haloes, the radiation from a compact star-forming region with an initial mass of 10⁷ M_☉ can keep gas clumps with Jeans masses larger than ~10⁷ M_☉ warm and ionized throughout the galaxy. In this picture, the small intrinsic sizes measured in the faintest z = 6-8 galaxies are a natural consequence of the strong radiation field that stabilizes massive gas clumps. A prediction of this mechanism is that the escape fraction for ionizing radiation is high for the extremely compact, high-z sources.