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We present 4 new measurements of nitrogen abundances and one upper limit in damped
Ly α absorbers (DLAs) obtained by means of high
resolution (FWHM≃7 km s-1) UVES/VLT spectra. In addition to these measurements we have compiled data from all DLAs
with measurements of nitrogen and
α-capture elements (O, S or Si) available
in the literature, including
all HIRES/Keck and UVES/VLT
data for a total of 32 systems, i.e.
the largest sample investigated so far.
We find that [ N/α] ratios
are distributed in two groups: 75% of the DLAs show a mean value of
[N/α]=−0.87
with a scatter
of 0.17 dex, while the remaining 25% shows ratios clustered at
[N/α]=−1.45 with an even
lower dispersion of 0.05 dex.
The high
[N/α]≃−0.9
plateau is consistent with the one observed
in metal-poor \ion{H}{ii} regions of blue compact dwarf (BCD) galaxies
([N/α]=−0.73±0.13),
while the [ N/α]≃−1.5 values are
the lowest ever observed in any astrophysical site.
These low [ N/α] ratios are real
and are not due to ionization effects. They provide crucial evidence
against the primary production of N by massive stars
as being responsible for the plateau
at −0.9/
-0.7 dex observed in DLAs and BCD galaxies.
The transition between
the low-N ([N/α]≃−1.5) and
high-N ([ N/α]≃−0.9) DLAs
occurs at a nitrogen abundance of [N/H]≃−2.8,
suggesting that the separation may result from some peculiarity of the
nitrogen enrichment history.
The [N/α]≃−1.5
values and their low dispersion are consistent with a
modest production of primary N in massive stars;
however, due to the limited sample, specially for the low-N DLAs, we cannot exclude
a primary origin
in intermediate mass stars as responsible for the low N abundances observed.
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