Molecular cloud matching in CO and dust in M33

I. High-resolution hydrogen column density maps from Herschel

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
² School of Astronomy, Institute for Research in Fundamental Sciences (IPM), PO Box 19395-5531 Tehran, Iran
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 19 December 2023
Accepted: 22 April 2024

Abstract

This study is aimed to contribute to a more comprehensive understanding of the molecular hydrogen distribution in the galaxy M33 by introducing novel methods for generating high angular resolution (18.2″, equivalent to 75 pc for a distance of 847 kpc) column density maps of molecular hydrogen (N_H2). M33 is a local group galaxy that has been observed with Herschel in the far-infrared (FIR) wavelength range from 70 to 500 μm. Previous studies have presented total hydrogen column density maps (N_H), using these FIR data (partly combined with mid-IR maps), employing various methods. We first performed a spectral energy distribution (SED) fit to the 160, 250, 350, and 500 μm continuum data obtain N_H, using a technique similar to one previously reported in the literature. We also use a second method which involves translating only the 250 μm map into a N_H map at the same angular resolution of 18.2″. An N_H2 map via each method is then obtained by subtracting the H I component. Distinguishing our study from previous ones, we adopt a more versatile approach by considering a variable emissivity index, β, and dust absorption coefficient, κ₀. This choice enables us to construct a κ₀ map, thereby enhancing the depth and accuracy of our investigation of the hydrogen column density. We address the inherent biases and challenges within both methods (which give similar results) and compare them with existing maps available in the literature. Moreover, we calculate a map of the carbon monoxide CO(1 − 0)-to-molecular hydrogen (H₂) conversion factor (X_CO factor), which shows a strong dispersion around an average value of 1.8 × 10²⁰ cm⁻²/(K km s⁻¹) throughout the disk. We obtain column density probability distribution functions (N-PDFs) from the N_H, N_H2, and N_H I maps and discuss their shape, consisting of several log-normal and power-law tail components.