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

TL;DR

This study uses high-resolution HST/ACS imaging to analyze how star formation activity influences the morphology of extraplanar ionized gas in edge-on spiral galaxies, revealing a dependence on star formation rate and galaxy mass.

Contribution

First high-resolution imaging of small-scale eDIG structures in multiple galaxies, showing morphology depends on star formation activity and gravitational potential.

Findings

eDIG morphology varies with star formation rate per unit area

Filamentary structures form only above a certain star formation threshold

Galaxy mass influences the ability of gas to escape and form outflows

Abstract

(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.