The Radial and Vertical Structure of Molecular Gas in the Edge-on Galaxy NGC 4565

Authors

• Grace Krahm, Ohio State University
• Adam K. Leroy, Ohio State University
• Jiayi Sun, University of Kentucky
• Kijeong Yim, Chungnam National University
• Eric W. Koch, National Radio Astronomy Observatory Socorro
• Tony Wong, University of Illinois at Urbana-Champaign
• Deanne Fisher, Swinburne University of Technology
• Erik Rosolowsky, University of Alberta
• Karin Sandstrom, University of California San Diego Health
• Dyas Utomo, Ohio State University
• Jesse van de Sande, University of New South Wales
• Marie Martig, Liverpool John Moores University
• Amelia Fraser-McKelvie, Dimensions (ASTRO 3D)
• Michael R. Hayden, University of Oklahoma

Abstract

We present high-resolution (0 (Formula presented) .″ 94 ≈ 55 pc) Atacama Large Millimeter/submillimeter Array (ALMA) CO(2–1) and¹³CO(2–1) observations of the highly inclined ( (Formula presented) i∼87.°5 ) galaxy NGC 4565 covering out to galactocentric radius R_gal ≳ ± 17 kpc. The combination of sensitivity and resolution enables the detection of CO emission well into the H i-dominated outer disk while isolating individual molecular clouds across the full extent of the galaxy. Although often described as an edge-on Milky Way analog, the molecular gas profile of NGC 4565 has a central gap that is more similar to M31. The ¹³CO/¹²CO ratio remains consistent at 0.086 ± 0.009 from R_gal = 5–13 kpc. Based on fits to position–velocity slices, the molecular disk remains thin, with an FWHM scale height of 79.1 ± 1.6 pc measured from the vertical intensity profile with little evidence for vertical flaring. Molecular clouds in NGC 4565 show sizes, linewidths, and surface densities consistent with those found in similar environments in PHANGS-ALMA galaxies and in M31. We identify a prominent star-forming complex on the ring—an overdensity of molecular gas we term the “East Ring Pileup.” This feature hosts a compact, multiwavelength-bright region, which we call “the Jewel.” Effects of galaxy inclination on molecular cloud radius, velocity dispersion, surface density, and virial parameter appear as second-order effects that are strongest in velocity dispersion. At this resolution, GMCs are preferentially aligned with the disk of the galaxy and horizontally elongated by a factor of ∼2.

Document Type

Article

Publication Date

7-1-2026

Notes/Citation Information

Publisher Copyright: © 2026. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the https://creativecommons.org/licenses/by/4.0/. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Digital Object Identifier (DOI)

10.3847/1538-3881/ae710b

Archival?

Archival

Repository Citation

Krahm, Grace; Leroy, Adam K.; Sun, Jiayi; Yim, Kijeong; Koch, Eric W.; Wong, Tony; Fisher, Deanne; Rosolowsky, Erik; Sandstrom, Karin; Utomo, Dyas; van de Sande, Jesse; Martig, Marie; Fraser-McKelvie, Amelia; and Hayden, Michael R., "The Radial and Vertical Structure of Molecular Gas in the Edge-on Galaxy NGC 4565" (2026). Physics and Astronomy Faculty Publications. 768.
https://uknowledge.uky.edu/physastron_facpub/768