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

Abstract

Recent observations of lensed sources have shown that the faintest ($M_{\mathrm{UV}}\approx -15\mathrm{mag}$) 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 $10^6$ and $10^7{\mathrm{M}}_{\odot }$, 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 ($M_{200}\lesssim 10^{10}{\mathrm{M}}_{\odot }$) haloes, the radiation from a compact star forming region with an initial mass of $10^7{\mathrm{M}}_{\odot }$ can keep gas clumps with Jeans masses larger than $\approx 10^7{\mathrm{M}}_{\odot }$ 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 stabilises 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.

arXiv comment16 pages, 13 figures, accepted for publication in MNRAS, formatting error in previous version did not display certain characters in equations, Fig. 8 replaced to be consistent with published version

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