GMC Dust Masses and CO-to-dust Conversion Factors in the Whirlpool Galaxy

In this work, we present derived dust masses and new estimated CO-to-dust conversion factors for 33 giant molecular clouds (GMCs) in the nearby spiral galaxy M51. Our methodology is based on a combined analysis of CO integrated line emission from the SMA and thermal broadband infrared photometry using the Herschel Space Observatory and the Spitzer Space Telescope. We measured dust masses using a custom version of a spectral energy distribution (SED) fitting routine. Specifically, we performed a two-component modified blackbody fit to four photometric measurements which adopts a dust emissivity of beta = 2, following Galametz et al. (2011). The first component used a fixed dust temperature of 20 K to model the cold dust, and the second component used a fixed dust temperature of 50 K to model the warm dust. We measured a mean GMC total dust masses of 5.8×10⁵ solar masses for the two components. We also measured CO integrated line intensities, luminosities, gas masses and CO-to-dust conversion factors (alpha'/CO-dust) for the GMCs following Colombo et al. (2014) and Forbrich et al. (2020). Our derived CO-to-dust conversion factors had a range of 0.10 to 1.97 and are approximately a factor of three larger than those reported in Forbrich et al. (2020) for M31. Finally, we find evidence that the mean lifetime of the GMCs is 1.78 Gyr, and their SFRs (estimated from Halpha photometry) have no significant correlation to the measured dust masses. We speculate that this may be due to a lack of precise estimates for the physical conditions within our selected GMCs and suggest that further investigation is required.

The SAO REU program is funded in part by the National Science Foundation REU and Department of Defense ASSURE programs under NSF Grant no. AST-1852268, and by the Smithsonian Institution.