Measurements of Zn and Cr abundances in 18 damped Lyman \alpha systems ( DLAs ) at absorption redshifts z _ { abs } = 0.692 - 3.390 ( but mostly between z \simeq 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 ] \simeq - 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 \alpha galaxies \approx 13.5 Gyr ago ( H _ { 0 } = 50 km s ^ { -1 } Mpc ^ { -1 } , q _ { 0 } = 0.05 ) was \sim 1 / 15 solar .

Values of [ Cr/Zn ] span the range from \simeq 0 to \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } -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 \approx 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 } \simeq 1 / 15 Z _ { \sun } with a dust-to-metals ratio \approx 1 / 2 of that in local interstellar clouds , we deduce that the “ typical ” dust-to-gas ratio in damped Lyman \alpha galaxies is \approx 1 / 30 of the Milky Way value .
This amount of dust will introduce an extinction at 1500 Å of only A _ { 1500 } \approx 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 \alpha 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 through 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 .