Abstract
Over the last three decades, photometric galaxy selection using the
Lyman-break technique has transformed our understanding of the high-z Universe,
providing large samples of galaxies at 3 < z < 8 with relatively small
contamination. With the advent of the James Webb Space Telescope, the
Lyman-break technique has now been extended to z ~ 17. However, the purity of
the resulting samples has not been tested. Here we use a simple model, built on
the robust foundation of the dark matter halo mass function, to show that the
expected level of contamination rises dramatically at z > 10, especially for
luminous galaxies, placing stringent requirements on the selection process. The
most luminous sources at z > 12 are likely at least ten thousand times rarer
than potential contaminants, so extensive spectroscopic followup campaigns may
be required to identify a small number of target sources.
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