The Morphological Diversity of DIG in Halos of Edge-on Spirals as Revealed by HST/ACS

¹ Department of Astronomy, University of Florida, 211 Bryant Space Science Center, PO Box 112055, Gainesville, FL 32611, USA ;
e-mail: jrossa@astro.ufl.edu
² CSIRO/ATNF - Paul Wild Observatory , Locked Bag 194, Narrabri, NSW 2390, Australia
³ Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstrasse 150/NA7, 44780 Bochum, Germany
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Abstract

We present new results on extraplanar DIG (eDIG), based on high spatial resolution narrowband imaging observations of four late-type, actively star-forming edge-on spirals, obtained with ACS on-board HST. Our Hα 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 very first time at a spatial resolution of 0.05′′, corresponding to 5 pc at the mean distance to our galaxies. The eDIG morphology is very different for all four targets, as a result of their different star formation activity and galaxy mass. There is a very smooth DIG morphology observed in two of the galaxies (NGC 4634 and NGC 5775), whereas the other two (NGC 4700 and NGC 7090) show a much more complex morphology with intricate filaments, and bubbles and supershells. 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.