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

Abstract

Recent observations of lensed sources have shown that the faintest (MUV ∼ -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 107MΘ, 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 ≤ 1010MΘ) haloes, the radiation from a compact star-forming region with an initial mass of 107MΘ can keep gas clumps with Jeans masses larger than ∼107MΘ 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.