We present the discovery of the optical afterglow and host galaxy of the Swift short-duration gamma-ray burst , GRB 181123B .
Observations with Gemini-North starting at \approx 9.1 hr after the burst reveal a faint optical afterglow with i \approx 25.1 mag , at an angular offset of 0.59 \pm 0.16 ^ { \prime \prime } from its host galaxy .
Using grizYJHK observations , we measure a photometric redshift of the host galaxy of z = 1.77 ^ { +0.30 } _ { -0.17 } .
From a combination of Gemini and Keck spectroscopy of the host galaxy spanning 4500-18000 Å , we detect a single emission line at 13390 Å , inferred as H \beta at z = 1.754 \pm 0.001 and corroborating the photometric redshift .
The host galaxy properties of GRB 181123B are typical to those of other SGRB hosts , with an inferred stellar mass of \approx 1.7 \times 10 ^ { 10 } M _ { \odot } , mass-weighted age of \approx 0.9 Gyr and optical luminosity of \approx 0.9 L ^ { * } .
At z = 1.754 , GRB 181123B is the most distant secure SGRB with an optical afterglow detection , and one of only three at z > 1.5 .
Motivated by a growing number of high- z SGRBs , we explore the effects of a missing z > 1.5 SGRB population among the current Swift sample on delay time distribution models .
We find that log-normal models with mean delay times of \approx 4 - 6 Gyr are consistent with the observed distribution , but can be ruled out to 95 \% confidence with an additional \approx 1 - 5 Swift SGRBs recovered at z > 1.5 .
In contrast , power-law models with \propto t ^ { -1 } are consistent with the redshift distribution and can accommodate up to \approx 30 SGRBs at these redshifts .
Under this model , we predict that \approx 1 / 3 of the current Swift population of SGRBs is at z > 1 .
The future discovery or recovery of existing high- z SGRBs will provide significant discriminating power on their delay time distributions , and thus their formation channels .