  We present results on the environmental dependence of the star-forming galaxy
main sequence in 11 galaxy cluster fields at $1.0 < z < 1.5$ from the Gemini
Observations of Galaxies in Rich Early Environments Survey (GOGREEN) survey. We
use a homogeneously selected sample of field and cluster galaxies whose
membership is derived from dynamical analysis. Using [OII]-derived star
formation rates (SFRs), we find that cluster galaxies have suppressed SFRs at
fixed stellar mass in comparison to their field counterparts by a factor of 1.4
$\pm$ 0.1 ($\sim3.3\sigma$) across the stellar mass range: $9.0 < \log(M_{*}
/M_{\odot}) < 11.2$. We also find that this modest suppression in the cluster
galaxy star-forming main sequence is mass and redshift dependent: the
difference between cluster and field increases towards lower stellar masses and
lower redshift. When comparing the distribution of cluster and field galaxy
SFRs to the star-forming main sequence, we find an overall shift towards lower
SFRs in the cluster population, and note the absence of a tail of high SFR
galaxies as seen in the field. Given this observed suppression in the cluster
galaxy star-forming main sequence, we explore the implications for several
scenarios such as formation time differences between cluster and field
galaxies, and environmentally-induced star formation quenching and associated
timescales.
