Hubble Space Telescope Hα imaging of star-forming galaxies at z ≃ 1-1.5: evolution in the size and luminosity of giant H II regions

We present Hubble Space Telescope/Wide Field Camera 3 narrow-band imaging of the H alpha emission in a sample of eight gravitationally lensed galaxies at z = 1-1.5. The magnification caused by the foreground clusters enables us to obtain a median source plane spatial resolution of 360 pc, as well as providing magnifications in flux ranging from similar to 10x to similar to 50x. This enables us to identify resolved star-forming H II regions at this epoch and therefore study their H alpha luminosity distributions for comparisons with equivalent samples at z similar to 2 and in the local Universe. We find evolution in the both luminosity and surface brightness of H II regions with redshift. The distribution of clump properties can be quantified with an H II region luminosity function, which can be fit by a power law with an exponential break at some cut-off, and we find that the cut-off evolves with redshift. We therefore conclude that clumpy galaxies are seen at high redshift because of the evolution of the cut-off mass; the galaxies themselves follow similar scaling relations to those at z = 0, but their H II regions are larger and brighter and thus appear as clumps which dominate the morphology of the galaxy. A simple theoretical argument based on gas collapsing on scales of the Jeans mass in a marginally unstable disc shows that the clumpy morphologies of high-z galaxies are driven by the competing effects of higher gas fractions causing perturbations on larger scales, partially compensated by higher epicyclic frequencies which stabilize the disc.