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

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.

20 pages, 10 figures, 6 tables. Accepted for publication in AJ

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Cited by in corpus (9)

• Investigating Disk-Halo Flows and Accretion: A Kinematic and Morphological Analysis of Extraplanar HI in NGC 3044 and NGC 4302
• The HI Kinematics of NGC 4013: a Steep and Radially Shallowing Extra-planar Rotational Lag
• Comparison of the extraplanar H$\alpha$ and UV emissions in the halos of nearby edge-on spiral galaxies
• GASP XXXII. Measuring the diffuse ionized gas fraction in ram-pressure stripped galaxies
• Near-Infrared Structure of Fast and Slow Rotating Disk Galaxies
• Halpha and [S II] emission from warm ionized gas in the Scutum-Centaurus Arm
• Extraplanar H II Regions in Spiral Galaxies. I. Low-Metallicity Gas Accreting through the Disk-Halo Interface of NGC 4013
• The Panchromatic Hubble Andromeda Treasury XX: The Disk of M31 is Thick
• Extraplanar H II Regions in Spiral Galaxies. II. In Situ Star Formation in the Interstellar Thick Disk of NGC 4013