HST/ACS Observations of Star Formation Driven Outflows in Nearby Edge-on Spiral Galaxies: Dependence of Halo Morphology on Star Formation Activity

(Abridged). We present new high spatial resolution narrowband imaging observations of extraplanar diffuse ionized gas (eDIG) in four late-type, actively star forming edge-on spirals, obtained with ACS on-board HST. Our F658N (H-alpha) observations reveal a multitude of structures on both small and large scales. Whereas all four galaxies have been studied with ground-based telescopes before, here the small scale structure of the extended emission line gas is presented for the first time at a spatial resolution of 0.05", corresponding to 5.0 pc at the mean distance to the target galaxies. The eDIG morphology is very different for all four targets, probably as a result of their different levels of star formation activity. We find that the morphology of the eDIG, in particular the break-up of diffuse emission into filaments in galaxy halos, shows a strong dependence on the level of star formation activity per unit area, and eDIG can be arranged into a morphological sequence. NGC4634 and NGC5775 have the highest SF rate per unit area in our sample and the observed morphology suggests that the break-up of the smooth eDIG layer into individual resolved filaments occurs only above a certain threshold of SF activity per unit area. Combined with ground-based data for samples that span a larger range of galaxy mass our results indicate that the gravitational potential also plays an important role in the eDIG morphology. In low-mass galaxies the gas can be expelled due to shallow gravitational potentials more easily and couple with strong star formation driven outflows on a local scale. This is in contrast to the more massive galaxies, which show smooth eDIG layers, unless they are powered by a superwind, as in the case of nucleated starburst galaxies.