METAL: The Metal Evolution, Transport, and Abundance in the Large Magellanic Cloud Hubble program. III. Interstellar Depletions, Dust-to-Metal, and Dust-to-Gas Ratios Versus Metallicity

TL;DR

This study investigates how interstellar depletions, dust-to-gas, and dust-to-metal ratios depend on metallicity across different galaxies, revealing consistent element depletion patterns and discrepancies in dust measurements from various methods.

Contribution

It provides a comparative analysis of depletion patterns and dust ratios in the Milky Way, LMC, and SMC, highlighting their dependence on metallicity and density, and discusses implications for cosmic chemical evolution.

Findings

Depletion relations are consistent across galaxies down to 20% solar metallicity.

Dust-to-gas ratio increases with gas density in the ISM.

Discrepancies exist between depletion-based and emission-based dust measurements.

Abstract

The metallicity and gas density dependence of interstellar depletions, the dust-to-gas (D/G), and dust-to-metal (D/M) ratios have important implications for how accurately we can trace the chemical enrichment of the universe; either by using FIR dust emission as a tracer of the ISM; or by using spectroscopy of damped Lyman-alpha systems (DLAs) to measure chemical abundances over a wide range of redshifts. We collect and compare large samples of depletion measurements in the Milky Way (MW), LMC (Z=0.5 Zsun), and SMC (Z=0.2 Zsun). The relation between the depletions of different elements do not strongly vary between the three galaxies, implying that abundance ratios should trace depletions accurately down to 20% solar metallicity. From the depletions, we derive D/G and D/M. The D/G increases with density, consistent with the more efficient accretion of gas-phase metals onto dust grains in the denser ISM. For log N(H) > 21 cm^-2, the depletion of metallicity tracers (S, Zn) exceeds -0.5 dex, even at 20% solar metallicity. The gas fraction of metals increases from the MW to the LMC (factor 3) and SMC (factor 6), compensating the reduction in total heavy element abundances and resulting in those three galaxies having the same neutral gas-phase metallicities. The D/G derived from depletions are a factor of 2 (LMC) and 5 (SMC) higher than the D/G derived from FIR, 21 cm, and CO emission, likely due to the combined uncertainties on the dust FIR opacity and on the depletion of carbon and oxygen.