paper

The Spatially Resolved Dust-to-Metals Ratio in M101

arXiv:1808.07164 · doi:10.3847/1538-4357/aadc5f

Abstract

The dust-to-metals ratio describes the fraction of the heavy elements contained in dust grains, and its variation provides key insights into the life cycle of dust. We measure the dust-to-metals ratio in M101, a nearby galaxy with a radial metallicity (Z) gradient spanning 1 dex. We fit the dust spectral energy distribution from 100 to 500 with five variants of the modified blackbody dust emission model in which we vary the temperature distribution and how emissivity depends on wavelength. Among them, the model with a single temperature blackbody modified by a broken power-law emissivity gives the statistically best fit and physically most plausible results. Using these results, we show that the dust-to-gas ratio is proportional to . This implies that the dust-to-metals ratio is not constant in M101, but decreases as a function of radius, equivalent to a lower fraction of metals trapped in dust at low metallicity (large radius). The dust-to-metals ratio in M101 remains at or above what would be predicted by the minimum depletion level of metals observed in the Milky Way. Our current knowledge of metallicity-dependent CO-to-H conversion factor suggests that variations in the conversion factor cannot be responsible for the dust-to-metals ratio trends we observe. This change of dust-to-metals ratio is significantly correlated with molecular hydrogen fraction, which suggests that the accretion of gas phase metals onto existing dust grains could be a mechanism contributing to a variable dust-to-metals ratio.

arXiv comment29 pages, 18 figures; accepted by ApJ

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