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