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

TL;DR

This study uses high-resolution ALMA observations to analyze the molecular gas structure in the edge-on galaxy NGC 4565, revealing a thin molecular disk, cloud properties consistent with similar galaxies, and a prominent star-forming region.

Contribution

First detailed high-resolution molecular gas analysis of NGC 4565, revealing its thin disk, molecular cloud characteristics, and star-forming structures in an edge-on galaxy.

Findings

Molecular disk remains thin with a scale height of ~79 pc.

Molecular clouds have properties similar to those in other PHANGS-ALMA galaxies.

Identified a prominent star-forming complex called the East Ring Pileup.

Abstract

We present high-resolution (0.94" $\approx$ 55 pc) ALMA CO(2-1) and 13CO(2-1) observations of the highly inclined (i~87.5 deg) galaxy NGC 4565 covering out to galactocentric radius Rgal > $\pm$ 17 kpc. The combination of sensitivity and resolution enables the detection of CO emission well into the HI-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 which is more similar to M31. The 13CO/12CO ratio remains consistent at 0.086 $\pm$ 0.009 from Rgal = 5-13 kpc. Based on fits to position-velocity slices, the molecular disk remains thin, with a FWHM scale height of 79.1 $\pm$ 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.