METAL: The Metal Evolution, Transport, and Abundance in the Large Magellanic Cloud Hubble Program. II. Variations of Interstellar Depletions and Dust-to-gas Ratio within the LMC

Abstract

A key component of the baryon cycle in galaxies is the depletion of metals from the gas to the dust phase in the neutral interstellar medium (ISM). The METAL (Metal Evolution, Transport, and Abundance in the Large Magellanic Cloud) program on the Hubble Space Telescope acquired UV spectra toward 32 sight lines in the half-solar metallicity LMC, from which we derive interstellar depletions (gas-phase fractions) of Mg, Si, Fe, Ni, S, Zn, Cr, and Cu. The depletions of different elements are tightly correlated, indicating a common origin. Hydrogen column density is the main driver for depletion variations. Correlations are weaker with volume density, probed by C i fine-structure lines, and distance to the LMC center. The latter correlation results from an east–west variation of the gas-phase metallicity. Gas in the east, compressed side of the LMC encompassing 30 Doradus and the southeast H i over-density is enriched by up to +0.3 dex, while gas in the west side is metal deficient by up to −0.5 dex. Within the parameter space probed by METAL, no correlation with molecular fraction or radiation-field intensity are found. We confirm the factor of three to four increase in dust-to-metal and dust-to-gas ratios between the diffuse (log N(H) ∼ 20 cm−2) and molecular (log N(H) ∼ 22 cm−2) ISM observed from far-infrared, 21 cm, and CO observations. The variations of dust-to-metal and dust-to-gas ratios with column density have important implications for the sub-grid physics of chemical evolution, gas and dust mass estimates throughout cosmic times, and for the chemical enrichment of the Universe measured via spectroscopy of damped Lyα systems.

Dependence of pulsation mode of Cepheids on metallicity

“…The metallicity of the galaxies is measured by various works that yield different values. Specifically, the value of 12 + log(O/H) is measured to be 8.13 by Russell & Dopita (1990), 7.96 by Carlos Reyes et al (2015 and 7.80 by Maggi et al (2019) for SMC, 8.37 by Russell & Dopita (1990), 8.33 by Carlos Reyes et al (2015, 8.50 by Roman-Duval et al (2021) for LMC, 8.71 by Shaver et al (1983), 8.70 by Esteban & Peimbert (1995), 8.66 by Snow & Witt (1996), 8.73 by Asplund et al (2009) for the Milky Way, 8.75 by Garnett et al (1997) and8.76 by Toribio San Cipriano et al (2016) for M33, and 9.00 by Zaritsky et al (1994) and 8.90 by Sanders et al (2012) for M31. The result is plotted in Figure 4, where the horizontal line indicates the range of metallicity measured by different works.…”

Large Metallicity Variations in the Galactic Interstellar Medium

“…To measure the ISM metallicity it is essential to quantify the amount of metals that are missing from the observable gas-phase but instead are incorporated into dust grains, which is the phenomenon of dust depletion [6,43,44,7,8,9,45]. The dust-corrected (total of gas and dust) abundances can defined as…”

“…Here no other effects such as nucleosynthesis or ionization are taken into account. The depletion δ X is linearly proportional to the overall strength of depletion [8,9,45]. The overall strength of depletion can be represented in different ways, for example from the observed relative abundances, like in the "relative method", or with a specific parameter F * , like in the "F * method".…”

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

“…Indeed, Zn could behave like an α-process element (Ernandes et al 2018), with the stellar [Zn/Fe] ratio being enhanced in some stellar populations in an age and metallicity-dependent way (Duffau et al 2017;da Silveira et al 2018;Delgado Mena et al 2019). Based on a small sample in the Large Magellanic Cloud (LMC, 50% solar metallicity, Russell & Dopita 1992) and the Small Magellanic Cloud (SMC, 20% solar metallicity, Russell & Dopita 1992), De Cia (2018 show that the calibration of iron depletions, which correlate tightly with the depletions of other elements (Jenkins 2009;Roman-Duval et al 2021), as a function of [Zn/Fe] does not appear to change significantly between the Milky Way, LMC, and SMC, where interstellar depletions can be estimated from the gas and stellar abundances (Jenkins 2009;Tchernyshyov et al 2015;De Cia 2018;Roman-Duval et al 2021), as opposed to inferred from abundance ratios. Nevertheless, only a few depletion measurements were available in the LMC until the METAL (GO-14675) large HST program obtained UV spectra of 32 sight-lines in the LMC (Roman-Duval et al 2019.…”

“…In this paper, we compile recent depletion measurements in the Milky Way, LMC, and SMC and compare the relations between depletions of different elements and their abundance ratios between these three galaxies. From the depletions, we compute D/G and D/M and examine the relation between depletions, D/M, D/G, and hydrogen column density, which has been shown to be a driver of the D/M and D/G (Roman-Duval et al 2017;Chiang et al 2018Chiang et al , 2021Roman-Duval et al 2021). Additionally, we examine the metallicity dependence of depletions, D/M, and D/G by also including D/G estimates in nearby galaxies obtained from FIR measurements.The results presented in this paper lay the groundwork for deriving calibrations of depletions as a function of abundance ratios that can be applied to DLAs in order to estimate the metal and dust content of the universe over cosmic times, which will be presented in the upcoming METAL IV paper (Roman-Duval et al, in prep).…”