  Intracluster light (ICL) is diffuse light from stars that are gravitationally
bound not to individual member galaxies, but to the halo of galaxy clusters.
Leading theories predict that the ICL fraction, defined by the ratio of the ICL
to the total light, rapidly decreases with increasing redshift, to the level of
a few per cent at z > 1. However, observational studies have remained
inconclusive about the fraction beyond redshift unity because, to date, only
two clusters in this redshift regime have been investigated. One shows a much
lower fraction than the mean value at low redshift, whereas the other possesses
a fraction similar to the low-redshift value. Here we report an ICL study of
ten galaxy clusters at 1 \lesssim z \lesssim 2 based on deep infrared imaging
data. Contrary to the leading theories, our study finds that ICL is already
abundant at z \lesssim 1, with a mean ICL fraction of approximately 17\%.
Moreover, no significant correlation between cluster mass and ICL fraction or
between ICL color and cluster-centric radius is observed. Our findings suggest
that gradual stripping can no longer be the dominant mechanism of ICL
formation. Instead, our study supports the scenario wherein the dominant ICL
production occurs in tandem with the formation and growth of the brightest
cluster galaxies and/or through the accretion of preprocessed stray stars.
