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Astro Seminar

Constraining the lifecycle of dust and metals in neutral gas from the UV to radio

Speaker: Julia Roman-Duval (STSci)
Date: Wednesday 8 July 2020
Time: 15:00
Venue: Zoom

A key component of the baryon cycle in galaxies is the depletion of metals from the gas to the dust phase in the neutral ISM. Our understanding of the dust and gas contents of galaxies via dust emission-based gas mass estimates and of the chemical enrichment of the Universe via spectroscopy of damped Lyman-a systems (DLAs) critically relies on the characterization of the variations of the dust-to-metal mass ratio (D/M, i.e., fraction of metals in the dust) with environment (metallicity, density, galaxy morphology). In this talk, I will present results from several independent efforts to characterize the dust-to-gas and dust-to-metal ratios in the Magellanic Clouds and other nearby low-metallicity galaxies using multi-wavelength techniques. An analysis of the gas-to-dust ratio variations in the LMC and SMC (with metallicities 0.5 and 0.2 solar, respectively) based on the modeling of the resolved SED from all-sky FIR and 21 cm surveys shows that the dust abundance increases by factors 3-7 between the diffuse ISM and dense molecular clouds, albeit with significant degeneracies between dust opacity, CO-dark molecular gas, and dust-to-gas ratio. Two large HST programs, METAL (LMC at 50% solar metallicity) and METAL-Z (IC 1613 at 15% solar and Sextans A at 8% solar metallicity) are aimed at resolving those degeneracies by constraining the dust-to-metal ratio using absorption spectroscopy in the UV. The gas-phase fractions (i.e., depletions) of key components of dust grains (Si, Mg, Fe, Ni) but also other volatile elements (Zn, S) decrease with increasing hydrogen column density and volume density, and strongly support dust growth in the ISM via accretion of gas-phase metals onto dust grains. Depletion patterns differ between the Milky Way, the LMC, and SMC, with differences in dust-to-metal ratio offsetting almost exactly the total (gas + dust) metallicity differences between these galaxies, leading to the surprising finding that, for a given hydrogen column density, gas in the Milky Way, LMC, and SMC have the same gas-phase metallicities, despite the large differences in stellar mass and total metallicities for these galaxies. With the very recent METAL-Z program, we are investigating these trends at even lower metallicities, below the critical metallicity (0.2 Zsun) at which chemical evolution models predict D/M decreases steeply.