Dust in High‐Redshift Galaxies

Abstract

Measurements of Zn and Cr abundances in 18 damped Lyman α systems (DLAs) at absorption redshifts z abs = 0.692 − 3.390 (but mostly between z ≃ 2 and 3) show that metals and dust are much less abundant in high redshift galaxies than in the Milky Way today. Typically, [Zn/H] ≃ −1.2; as Zn tracks Fe closely in Galactic stars of all metallicities and is only lightly depleted onto interstellar grains, we conclude that the overall degree of metal enrichment of damped Lyman α galaxies ≈ 13.5 Gyr ago (H 0 = 50 km s −1 Mpc −1 , q 0 = 0.05) was ∼ 1/15 solar.Values of [Cr/Zn] span the range from ≃ 0 to < ∼ − 0.65 which we interpret as evidence for selective depletion of Cr onto dust in some DLAs. On average Cr and other refractory elements are depleted by only a factor of ≈ 2, significantly less than in local interstellar clouds. We propose that this reflects an overall lower abundance of dust-which may be related to the lower metallicities, likely higher temperature of the ISM, and higher supernova rates in these young galaxies-rather than an "exotic" composition of dust grains.Combining a metallicity Z DLA ≃ 1/15Z ⊙ with a dust-to-metals ratio ≈ 1/2 of that in local interstellar clouds, we deduce that the "typical" dust-to-gas ratio in damped Lyman α galaxies is ≈ 1/30 of the Milky Way value. This amount of dust will introduce an extinction at 1500 Å of only A 1500 ≈ 0.1 in the spectra of background QSOs. Similarly, we expect little reddening of the broad spectral energy distribution of the high-z field galaxies now being found routinely by deep imaging surveys. Even such trace amounts of dust, however, can explain the weakness of Lyman α emission from star-forming regions. We stress the approximate nature of such general statements; in reality, the range of metallicities and dust depletions encountered indicates that some sight-lines -3through high-redshift galaxies may be essentially dust-free, while others could suffer detectable extinction.Finally, we show that, despite claims to the contrary, these conclusions are not inconsistent with recent high resolution observations of DLAs with the Keck telescope. We point out that the star-formation histories of high-z galaxies are not necessarily the same as that of the Milky Way and that, if depletions of some elements onto dust are not taken into account correctly, it is possible to misinterpret the clues to early nucleosynthesis provided by non-solar element ratios.

THE PRIMORDIAL DEUTERIUM ABUNDANCE OF THE MOST METAL-POOR DAMPED Lyα SYSTEM∗

“…Another possibility is that deuterium may be preferentially depleted onto dust grains (Jura 1982;Draine 2004Draine , 2006 (Pettini et al 1997;Vladilo 2004;Akerman et al 2005), and very low-metallicity DLAs are not expected to harbor a significant amount of dust (see Murphy & Bernet 2016, and references therein). Ultimately, this issue will be clarified by extending the number of precision measures of D/H over a wider range of metallicity than covered by the present sample.…”

Evidence for a magnitude-dependent bias in the Hamburg/ESO survey for damped Lyman–$\alpha$ systems

“…1); (ii) a comparison between optically-and radioselected DLA samples (Ellison et al 2001) does not provide strong constraints because of the small size of the radio-selected sample. Instead, the evidence for dust in DLA systems is mostly indirect (Pettini et al 1997;Boissé et al 1998;Ledoux et al 2003).…”

Zn and Cr Abundances in Damped Lyman Alpha Systems from the CORALS Survey

“…In Figure 3 we plot this ratio against the metallicity [Zn/H] for each of the CORALS DLAs (coloured) together with analogous measurements (black) from the compilations by Khare et al (2004) and Kulkarni et al (2005), after rescaling their values to the same solar abundances used here. Figure 3 shows the trend of increasing Cr depletion with increasing metallicity which was previously noted by Pettini et al (1997a) and shown by Prochaska & Wolfe (2002) to be a general feature of refractory elements in DLAs. In systems with [Zn/H] < ∼ − 1.5, [Cr/Zn] is approximately solar-indicating that there is little dust depletion at such low metallicities-while when [Zn/H] > 1, up to ∼90% of the Cr can be 'missing' from the gas phase and presumably be in solid form.…”