We study the high-energy properties of GRB 181123B, a short gamma-ray burst
(sGRB) at redshift z≈1.75. We show that, despite its nominal short
duration with T90 < 2 s, this burst display evidence of a temporally extended
emission (EE) at high energies and that the same trend is observed in the
majority of sGRBs at z > 1. We discuss the impact of instrumental selection
effects on the GRB classification, stressing that the measured T90 is not an
unambiguous indicator of the burst physical origin. By examining their
environment (e.g. stellar mass, star formation, offset distribution), we find
that these high-z sGRBs share many properties of long GRBs at a similar
distance and are consistent with a short-lived progenitor system. If produced
by compact binary mergers, these sGRBs with EE may herald a larger population
of sGRBs in the early universe.Comment: 10 pages, 6 figures, 2 tables. Submitted to ApJ