Calibrating [OII] star-formation rates at z>1 from dual H\alpha-[OII]
  imaging from HiZELS
  We investigate the relationship between H\alpha\ and [OII](\lambda 3727)
emission in faint star-forming galaxies at z=1.47 with dust uncorrected star
formation rates (SFRs) down to 1.4 Msun/yr, using data in two narrow-bands from
WFCAM/UKIRT and Suprime-Cam/Subaru. A stacking analysis allows us to
investigate H\alpha\ emission flux from bright [OII] emitters as well as faint
ones for which H\alpha\ is not individually detected, and to compare them with
a large sample of local galaxies. We find that there is a clear, positive
correlation between the average H\alpha\ and [OII] luminosities for [OII]
emitters at z=1.47, with its slope being consistent with the local relation.
[OII] emitters at z=1.47 have lower mean observed ratios of H\alpha/[OII]
suggesting a small but systematic offset (at 2.8\sigma\ significance) towards
lower values of dust attenuation, A(H\alpha)~0.35, than local galaxies. This
confirms that [OII] selection tends to pick up galaxies which are significantly
less dusty on average than H\alpha\ selected ones, with the difference being
higher at z=1.47 than at z=0. The discrepancy of the observed line ratios
between [OII] emitters at z=1.47 and the local galaxies may in part be due to
the samples having different metallicities. However, we demonstrate that
metallicity is unlikely to be the main cause. Therefore, it is important to
take into account that the relations for the dust correction which are derived
using H\alpha\ emitter samples, and frequently used in many studies of high-z
galaxies, may overestimate the intrinsic SFRs of [OII]-selected galaxies, and
that surveys of [OII] emission galaxies are likely to miss dusty populations.
