Hubble Space Telescope H alpha imaging of star-forming galaxies at z similar or equal to 1-1.5: evolution in the size and luminosity of giant H II regions

摘要

We present HST/WFC3 narrowband imaging of the Hα 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 360pc, as well as providing magnifications in flux ranging from ∼ 10× to ∼ 50×. This enables us to identify resolved star-forming Hii regions at this epoch and therefore study their Hα luminosity distributions for comparisons with equivalent samples at z ∼ 2 and in the local Universe. We find evolution in the both luminosity and surface brightness of Hii regions with redshift. The distribution of clump properties can be quantified with\udan Hii 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\udconclude 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\udz = 0, but their Hii regions are larger and brighter and thus appear as clumps which dominate the morphology of the galaxy. A simple theoretical argument based on gas\udcollapsing on scales of the Jeans mass in a marginally unstable disk shows that the clumpy morphologies of high-z galaxies are driven by the competing effects of higher\udgas fractions causing perturbations on larger scales, partially compensated by higher epicyclic frequencies which stabilise the disk.