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Title:
The UCSD HIRES/Keck I Damped Lyα Abundance Database. IV. Probing Galactic Enrichment Histories with Nitrogen
Authors:
Prochaska, Jason X.; Henry, Richard B. C.; O'Meara, John M.; Tytler, David; Wolfe, Arthur M.; Kirkman, David; Lubin, Dan; Suzuki, Nao
Affiliation:
AA(Observatories of the Carnegie Institute of Washington, 813 Santa Barbara Street, Pasadena, CA 91101; ), AB(Department of Physics and Astronomy, University of Oklahoma, Norman, OK 73019; ), AC(Visiting Astronomer, W. M. Keck Telescope. The Keck Observatory is a joint facility of the University of California and the California Institute of Technology.), AD(Visiting Astronomer, W. M. Keck Telescope. The Keck Observatory is a joint facility of the University of California and the California Institute of Technology.), AE(Department of Physics, and Center for Astrophysics and Space Sciences, University of California, San Diego, C-0424, La Jolla, CA 92093; , , , , , ; Visiting Astronomer, W. M. Keck Telescope. The Keck Observatory is a joint facility of the University of California and the California Institute of Technology.), AF(Department of Physics, and Center for Astrophysics and Space Sciences, University of California, San Diego, C-0424, La Jolla, CA 92093; , , , , , ; Visiting Astronomer, W. M. Keck Telescope. The Keck Observatory is a joint facility of the University of California and the California Institute of Technology.), AG(Department of Physics, and Center for Astrophysics and Space Sciences, University of California, San Diego, C-0424, La Jolla, CA 92093; , , , , , ), AH(Department of Physics, and Center for Astrophysics and Space Sciences, University of California, San Diego, C-0424, La Jolla, CA 92093; , , , , , )
Publication:
The Publications of the Astronomical Society of the Pacific, Volume 114, Issue 799, pp. 933-954. (PASP Homepage)
Publication Date:
09/2002
Origin:
UCP
PASP Keywords:
Galaxies: Abundances, Galaxies: Evolution, nuclear reactions, nucleosynthesis, abundances, Galaxies: Quasars: Absorption Lines
DOI:
10.1086/342354
Bibliographic Code:
2002PASP..114..933P

Abstract

We present 14 N0 measurements from our High Resolution Echelle Spectrometer/Keck database of damped Lyα abundances. These data are combined with measurements from the recent and past literature to build a homogeneous, uniform set of observations. We examine photoionization diagnostics such as Fe++ and Ar0 in the majority of the complete sample and assess the impact of ionization corrections on N/α and α/H values derived from observed ionic column densities of N0, Si+, H0, and S+. Our final sample of 19 N/α, α/H pairs appears bimodal; the majority of systems show N/α values consistent with metal-poor emission regions in the local universe, but a small subsample exhibits significantly lower N/α ratios. Contrary to previous studies of N/α in the damped systems, our sample shows little scatter within each subsample. We consider various scenarios to explain the presence of the low N/α sight lines and account for the apparent bimodality. We favor a model where at least some galaxies undergo an initial burst of star formation with suppressed formation of intermediate-mass stars. We found that a power-law initial mass function with slope 0.10 or a mass cut of ~5-8 Msolar would successfully reproduce the observed low-nitrogen damped Lyα values. If the bimodal distribution is confirmed by a larger sample of measurements, this may present the first observational evidence for a top-heavy initial mass function in some early stellar populations. See http://kingpin.ucsd.edu/~hiresdla.
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