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Issue A&A
Volume 403, Number 1, May III 2003
Page(s) 55 - 72
Section Galactic structure and dynamics
DOI 10.1051/0004-6361:20030273



A&A 403, 55-72 (2003)
DOI: 10.1051/0004-6361:20030273

Early stages of nitrogen enrichment in galaxies: Clues from measurements in damped Lyman  $\alpha$ systems

M. Centurión1, P. Molaro1, G. Vladilo1, C. Péroux1, S. A. Levshakov2 and V. D'Odorico1

1  Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy
2  Department of Theoretical Astrophysics, Ioffe Physico-Technical Institute, Politechnicheskaya Str. 26, 194021 St. Petersburg, Russia

(Received 13 November 2002 / Accepted 30 January 2003)

Abstract
We present 4

new measurements of nitrogen abundances and one upper limit in damped Ly $\alpha$ absorbers (DLAs) obtained by means of high resolution ( $FWHM \simeq 7$ km s -1) UVES/VLT spectra.

In addition to these measurements we have compiled data from all DLAs with measurements of nitrogen and $\alpha$-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/ $\alpha$] ratios are distributed in two groups: 75% of the DLAs show a mean value of $\rm [N/\alpha] = -0.87$ with a scatter of 0.17 dex, while the remaining 25% shows ratios clustered at $\rm [N/\alpha] = -1.45$ with an even lower dispersion of 0.05 dex. The high $\rm [N/\alpha] \simeq -0.9$ plateau is consistent with the one observed in metal-poor $\ion{H}{ii}$ regions of blue compact dwarf (BCD) galaxies ( $\rm [N/\alpha] =-0.73 \pm 0.13$), while the [ $\rm N/\alpha] \simeq -1.5$ values are the lowest ever observed in any astrophysical site. These low [ N/ $\alpha$] 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 ( $\rm [N/\alpha]\simeq-1.5$) and high-N ([ $\rm N/\alpha] \simeq -0.9$) DLAs occurs at a nitrogen abundance of $\rm [N/H] \simeq -2.8$, suggesting that the separation may result from some peculiarity of the nitrogen enrichment history. The $\rm [N/\alpha]\simeq-1.5$ 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.


Key words: cosmology: observations -- galaxies: abundances -- galaxies: evolution -- quasars: absorption lines

Offprint request: M. Centurión, centurio@ts.astro.it

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© ESO 2003


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