Molecular Cloud Matching in CO and Dust in M33 I. High-Resolution Hydrogen Column Density Maps from Herschel

TL;DR

This paper introduces novel high-resolution methods to map molecular hydrogen in galaxy M33 using Herschel FIR data, considering variable dust properties, and compares results with existing maps, also deriving the CO-to-H2 conversion factor.

Contribution

It presents a new versatile approach incorporating variable dust emissivity and absorption coefficients to improve hydrogen column density mapping in M33.

Findings

High-resolution $N_{H_2}$ maps achieved at 75 pc scale.

Derived a $X_{CO}$ factor with strong dispersion around $1.8×10^{20}$ cm$^{-2}$/K km s$^{-1}$.

Compared methods and validated results against existing literature.

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) column density maps of molecular hydrogen ($N_{\rm H_2}$). M33 is a local group galaxy that has been observed with Herschel in the far-infrared wavelength range from 70 to 500 $\mu$m. Previous studies have presented total hydrogen column density maps ($N_{\rm H}$), using these FIR data (partly combined with mid-IR maps), employing various methods. We first performed a spectral energy distribution fit to the 160, 250, 350, and 500 $\mu$m continuum data obtain $N_{\rm H}$, using a technique similar to one previously reported in the literature. We also use a second method which involves translating only the 250 $\mu$m map into a $N_{\rm H}$ map at the same angular resolution. An $N_{\rm H_2}$ map via each method is then obtained by subtracting the HI component. Distinguishing our study from previous ones, we adopt a more versatile approach by considering a variable emissivity index, $\beta$ and dust absorption coefficient, $\kappa_0$. This choice enables us to construct a $\kappa_0$ 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-to-H$_2$ conversion factor ($X_\mathrm{CO}$ factor), which shows a strong dispersion around an average value of $1.8\times10^{20}\,\mathrm{cm^{-2}/(K\,km\,s^{-1})}$ throughout the disk. We obtain column density probability distribution functions (N-PDFs) from the $N_{\rm H}$, $N_{\rm H_2}$, and $N_{HI}$ maps and discuss their shape.