The dependence of some metallicity calibrations of strong-line ratios on nitrogen enrichment history

0.21 MB·English

by  Y. C. Liang

The Central Engine of Active Galactic Nuclei
ASP Conference Series, Vol. ***, 2007
eds. L. C. Ho and J.-M. Wang
The dependence of some metallicity calibrations of
strong-line ratios on nitrogen enrichment history
Y. C. Liang1, S. Y. Yin1,2
1National Astronomical Observatories, Chinese Academy of Sciences,
7 20A Datun Road, Chaoyang District, Beijing 100012, China
0 2Department of Physics, Hebei Normal University, Shijiazhuang 050016,
0
China; email: [email protected], [email protected]
2

n
Abstract. WeshowthedependenceoftheBayesianandtheN2-derivedoxy-
a
J gen abundances on the N-enrichment history of the star-forming galaxies. We
select 531 metal-poor and ∼20,000 metal-rich galaxies from the SDSS database
9
for this study. Their “accurate” O/H abundances are obtained from Te and P-

1 method, respectively. The discrepancies of the Bayesian and N2-derived abun-
v dances from these two “accurate” abundances show obvious correlations with
1 log(N/O) abundance ratios.
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2
1
0
7 1. Introduction
0
/
h Metallicity isoneof thefundamentalparameters tostudythepropertyofgalaxy
p and its evolution history. The [N ii]/Hα strong emission-line ratio in the ion-
-
o ized gas in galaxies is a useful tracer to indicate the metallicities of the galaxies
r (as N2=log([N ii]/Hα); Pettini & Pagel 2004, PP04; Yin et al. 2006a, Y06a;
t
s Liang et al. 2006, L06). Nitrogen is mainly synthesized in intermediate- and
a
low-mass stars. The N2 index will therefore become stronger with ongoing
:
v star formation and galaxy evolution until reaching very high metallicities, i.e.
Xi 12+log(O/H)∼9.0, where [N ii] starts to become weaker due to the very low
electron temperature caused from strong cooling by metal ions. TheN2 index is
r
a not affected very much by dust extinction due to the close wavelength positions
of the two related lines. The near infrared spectroscopic instruments can gather
these two lines for the galaxies with intermediate and high redshifts. TheN2 in-
dex has helped to provide important information on the metallicities of galaxies
at high-z (Erb et al. 2006). However, the resulting log(O/H) abundances from
N2 (and other methods including [N ii] line) may involve the specific history of
N-enrichment in the galaxies. We check this effect in more details in this paper.
2. The dependence on log(N/O) abundance ratios of the oxygen
abundances derived from strong emission-lines
The MPA/JHU group used the photoionization models to simultaneously fit
most prominent emission lines (including [N ii] line), and then use the Bayesian
techniquetocalculatethelikelihooddistributionofthemetallicity ofeachgalaxy
in the SDSS database (Tremonti et al. 2004, T04), given as (O/H) in this
Bay
1
2
Figure 1. (a) Comparison between the log(O/H) and log(O/H) for
Te Bay
the 531 metal-poor SDSS sample galaxies. The long-dashed line represents a
+0.34dex discrepancy from the equal-value line (the solid line) to show the
overestimatesoftheBayesianabundancesforalmosthalfofthesamplegalax-
ies (∼227). (b) The offset between the log(O/H) and log(O/H) abun-
Bay Te
dancesasafunctionofthelog(N/O)abundanceratiosforthesamplegalaxies.
(c) The offset between the log(O/H) and log(O/H) abundances as a
N2,P Te
function of log(N/O) for the metal-poor galaxies. See Y06a for more details.
(d) The discrepancy between the log(O/H) and log(O/H) abundances
Bay P05
as a function of log(N/O) for the ∼20,000 SDSS metal-rich galaxies, which
showsthat,forthegalaxieswithaboutlog(N/O)<−0.95,thediscrepancyde-
pendsonlog(N/O)obviously,butforthosewithlog(N/O)≥−0.95,thedepen-
dence is very weak. (e) The discrepancy between the log(O/H) (by using
N2
PP04’s calibration) and log(O/H) abundances as a function of log(N/O)
P05
for the SDSS metal-rich galaxies, which shows two-phase correlations for the
about log(N/O)<−0.95 and ≥−0.95, respectively.
study. We select 531 (dr4) metal-poor and ∼20,000 (dr2) metal-rich galaxies
from the SDSS database for this study. We obtain their metallicities of (O/H)
Te
and (O/H) for the metal-poor sample galaxies (Y06a,b), and (O/H) and
N2,P N2
(O/H) for the metal-rich galaxies by using the calibrations given by PP04
P05
andPilyugin&Thuan(2005, P05), respectively. Themetal-rich samplegalaxies
have 12+log(O/H) >8.5 and 12+log(O/H) >8.25 (Y06b). Figs.1a-e show
Bay P05
clearly the dependence of these abundances on log(N/O).
In summary, the strong-line ratio calibrations of metallicities involved [N ii]
emission-line may be related to the N-enrichment history of the galaxies, which
should be kept in mind when compare the resulted abundances with others.
Acknowledgments. ThisworkwassupportedbytheNaturalScienceFoun-
dation of China (NSFC) Foundation under No.10403006,10373005,10433010.
References
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