Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Metal Abundances at z < 1.5: Fresh Clues to the Chemical Enrichment
  History of Damped Lyman alpha Systems   We explore the redshift evolution of the metal content of damped Lyman alpha
systems (DLAs) with new observations of four absorbers at z < 1.5 . The main
conclusion is that the column density--weighted mean metallicity, [<Zn/H>] =
-1.03 +/- 0.23 (on a logarithmic scale), is not significantly higher at z < 1.5
than at earlier epochs, despite the fact that the comoving star formation rate
density of the universe was near its maximum value at this redshift. For three
of the four DLAs our observations include absorption lines of Si, Mn, Cr, Fe,
and Ni, as well as Zn. We argue that the relative abundances of these elements
are consistent with a moderate degree of dust depletion which, once accounted
for, leaves no room for the enhancement of the alpha-elements over iron seen in
metal poor stars in the Milky Way. This is contrary to previous assertions that
DLAs have been enriched solely by Type II supernovae, but can be understood if
the rate of star formation in the systems studied proceeded more slowly than in
the early history of our Galaxy. These results add to a growing body of data
all pointing to the conclusion that known DLAs do not trace the galaxy
population responsible for the bulk of star formation. Possible reasons are
that sight-lines through metal rich gas are systematically underrepresented
because the background QSOs are reddened, and that the most actively star
forming galaxies are also the most compact, presenting too small a
cross-section to have been probed yet with the limited statistics of current
samples.

9
The categories of this paper are: nlin
15
The categories of this paper are: nucl-ex
12
The categories of this paper are: nucl-th
11
The categories of this paper are: hep-lat
28
The categories of this paper are: alg-geom
4
The categories of this paper are: cond-mat
29
The categories of this paper are: funct-an
26
The categories of this paper are: bayes-an
33
The categories of this paper are: plasm-ph
24
The categories of this paper are: dg-ga
18
The categories of this paper are: econ
36
The categories of this paper are: cmp-lg
2
The categories of this paper are: math
16
The categories of this paper are: stat
17
The categories of this paper are: q-fin
20
The categories of this paper are: acc-phys