The Relationship Between the Dense Neutral and Diffuse Ionized Gas in the Thick Disks of Two Edge-On Spiral Galaxies

TL;DR

This study uses high-resolution optical imaging to explore the multiphase interstellar medium in the thick disks of two edge-on spiral galaxies, revealing distinct dust and ionized gas structures and their implications for galaxy feedback and star formation.

Contribution

It provides detailed observations of dust and ionized gas in galaxy thick disks, highlighting their distinct phases and the role of stellar feedback in shaping these structures.

Findings

Dust filaments extend up to 2-2.5 kpc from the galactic plane.

Diffuse ionized gas appears smooth and lacks filamentary structure.

Dense, dusty matter correlates with star formation activity in the thick disk.

Abstract

We present high-resolution, optical images (BVI + Halpha) of the multiphase interstellar medium (ISM) in the thick disks of the edge-on spiral galaxies NGC 4013 and NGC 4302. Our images from the Hubble Space Telescope, Large Binocular Telescope, and WIYN 3.5-m reveal an extensive population of filamentary dust absorption seen to z ~ 2-2.5 kpc. Many of these dusty thick disk structures have characteristics reminiscent of molecular clouds found in the Milky Way disk. Our Halpha images show the extraplanar diffuse ionized gas (DIG) in these galaxies is dominated by a smooth, diffuse component. The strongly-filamentary morphologies of the dust absorption have no counterpart in the smoothly distributed Halpha emission. We argue the thick disk DIG and dust-bearing filaments trace physically distinct phases of the thick disk ISM, the latter tracing a dense, warm or cold neutral medium. The dense, dusty matter in the thick disks of spiral galaxies is largely tracing matter ejected from the thin disk via energetic feedback from massive stars. The high densities of the gas may be a result of converging gas flows. This dense material fuels some thick disk star formation, as evidenced by the presence of thick disk H II regions.