Does radiative feedback make faint z > 6 galaxies look small?

Abstract

Recent observations of lensed sources have shown that the faintest (\u2060MUV≈−15mag\u2060) 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⊙\u2060, 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 (\u2060M200≲1010M⊙\u2060) haloes, the radiation from a compact star-forming region with an initial mass of 107M⊙ can keep gas clumps with Jeans masses larger than ≈107M⊙ 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.