With afterglows came host galaxy identifications and thus redshifts, allowing us to establish the distance scale to these events for the first time. Unlikely long GRBs, which have massive star progenitors and thus live in dense regions of their hosts, we have only a single example of an absorption redshift for a short-duration GRB (GRB130603B). The remaining host associations are done probabilistically, an issue that we will return to in the next section.
The median redshift of short GRBs is ~ 0.5, significantly smaller than the median redshift of long GRBs (<z> ~ 2; Figure 8). The cause of this smaller average redshift is likely a combination of two factors: first, short GRBs are more challenging to detect than long ones (fewer photons for a fixed fluence), and so detector sensitivity almost certainly plays a role. But because of their old progenitor system, we expect an offset of ~ 1 Gyr between the peak of cosmic star formation (z ~ 3) and the time of merger.