Cool gas and dust in M 33: Results from the HERschel M 33 extended survey (HERM33ES)

摘要

We present an analysis of the first space-based far-IR-submm observations of M 33, which measure the emission from the cool dust and resolve the giant molecular cloud complexes. With roughly half-solar abundances, M 33 is a first step towards young low-metallicity galaxies where the submm may be able to provide an alternative to CO mapping to measure their H_2 content. In this Letter, we measure the dust emission cross-section σ using SPIRE and recent CO and H i observations; a variation in σ is present from a near-solar neighborhood cross-section to about half-solar with the maximum being south of the nucleus. Calculating the total H column density from the measured dust temperature and cross-section, and then subtracting the H i column, yields a morphology similar to that observed in CO. The H_2/H i mass ratio decreases from about unity to well below 10 and is about 15 averaged over the optical disk. The single most important observation to reduce the potentially large systematic errors is to complete the CO mapping of M 33.