410 research outputs found
Metallicities of 0.3<z<1.0 Galaxies in the GOODS-North Field
We measure nebular oxygen abundances for 204 emission-line galaxies with
redshifts 0.3<z<1.0 in the Great Observatories Origins Deep Survey North
(GOODS-N) field using spectra from the Team Keck Redshift Survey (TKRS). We
also provide an updated analytic prescription for estimating oxygen abundances
using the traditional strong emission line ratio, R_{23}, based on the
photoionization models of Kewley & Dopita (2003). We include an analytic
formula for very crude metallicity estimates using the [NII]6584/Halpha ratio.
Oxygen abundances for GOODS-N galaxies range from 8.2< 12+log(O/H)< 9.1
corresponding to metallicities between 0.3 and 2.5 times the solar value. This
sample of galaxies exhibits a correlation between rest-frame blue luminosity
and gas-phase metallicity (i.e., an L-Z relation), consistent with L-Z
correlations of previously-studied intermediate-redshift samples. The zero
point of the L-Z relation evolves with redshift in the sense that galaxies of a
given luminosity become more metal poor at higher redshift. Galaxies in
luminosity bins -18.5<M_B<-21.5 exhibit a decrease in average oxygen abundance
by 0.14\pm0.05 dex from z=0 to z=1. This rate of metal enrichment means that
28\pm0.07% of metals in local galaxies have been synthesized since z=1, in
reasonable agreement with the predictions based on published star formation
rate densities which show that ~38% of stars in the universe have formed during
the same interval. (Abridged)Comment: AASTeX, 49 pages, 16 figures, accepted for publication in The
Astrophysical Journa
The N/O Plateau of Blue Compact Galaxies: Monte Carlo Simulations of the Observed Scatter
Chemical evolution models and Monte Carlo simulation techniques have been
combined for the first time to study the distribution of blue compact galaxies
on the N/O plateau. Each simulation comprises 70 individual chemical evolution
models. For each model, input parameters relating to a galaxy's star formation
history (bursting or continuous star formation, star formation efficiency),
galaxy age, and outflow rate are chosen randomly from ranges predetermined to
be relevant. Predicted abundance ratios from each simulation are collectively
overplotted onto the data to test its viability. We present our results both
with and without observational scatter applied to the model points. Our study
shows that most trial combinations of input parameters, including a simulation
comprising only simple models with instantaneous recycling, are successful in
reproducing the observed morphology of the N/O plateau once observational
scatter is added. Therefore simulations which include delay of nitrogen
injection are no longer favored over those which propose that most nitrogen is
produced by massive stars, if only the plateau morphology is used as the
principal constraint. The one scenario which clearly cannot explain plateau
morphology is one in which galaxy ages are allowed to range below 250 Myr. We
conclude that the present data for the N/O plateau are insufficient by
themselves for identifying the portion of the stellar mass spectrum most
responsible for cosmic nitrogen production.Comment: 41 pages, 15 figures; accepted by ApJ, to appear Aug. 20, 200
Oxygen and nitrogen abundances in Virgo and field spirals
The oxygen and nitrogen abundances in the HII regions of the nine Virgo
spirals of the sample from Skillman et al (1996) and in nine field spiral
galaxies are re-determined with the recently suggested P - method. We confirm
that there is an abundance segregation in the sample of Virgo spirals in the
sense that the HI deficient Virgo spirals near the core of the cluster have
higher oxygen abundances in comparison to the spirals at the periphery of the
Virgo cluster. At the same time both the Virgo periphery and core spirals have
counterparts among field spirals. We conclude that if there is a difference in
the abundance properties of the Virgo and field spirals, this difference
appears to be small and masked by the observational errors.Comment: 16 pages, 10 figures, accepted for publication in Astronomy and
Astrophysic
Chemical Properties of Star-Forming Emission Line Galaxies at z=0.1 - 0.5
We measure oxygen and nitrogen abundances for 14 star-forming emission line
galaxies (ELGs) at 0.11<z<0.5 using Keck/LRIS optical spectroscopy. The targets
exhibit a range of metallicities from slightly metal-poor like the LMC to
super-solar. Oxygen abundances of the sample correlate strongly with rest-frame
blue luminosities. The metallicity-luminosity relation based on these 14
objects is indistinguishable from the one obeyed by local galaxies, although
there is marginal evidence (1.1sigma) that the sample is slightly more
metal-deficient than local galaxies of the same luminosity. The observed
galaxies exhibit smaller emission linewidths than local galaxies of similar
metallicity, but proper corrections for inclination angle and other systematic
effects are unknown. For 8 of the 14 objects we measure nitrogen-to-oxygen
ratios. Seven of 8 systems show evidence for secondary nitrogen production,
with log(N/O)> -1.4 like local spirals. These chemical properties are
inconsistent with unevolved objects undergoing a first burst of star formation.
The majority of the ELGs are presently ~4 magnitudes brighter and ~0.5 dex more
metal-rich than the bulk of the stars in well-known metal-poor dwarf
spheroidals such as NGC 205 and NGC 185, making an evolution between some ELGs
and metal-poor dwarf spheroidals improbable. However, the data are consistent
with the hypothesis that more luminous and metal-rich spheroidal galaxies like
NGC 3605 may become the evolutionary endpoints of some ELGs. [abridged]Comment: 41 pages, w/12 figures, uses AASTeX aaspp4.sty, psfig.sty; To appear
in The Astrophysical Journa
The Distribution Of Heavy Elements In Spiral And Elliptical Galaxies
This review recaps significant results as they apply to non-dwarf galaxies,
including the Milky Way, spiral disks and bulges, and elliptical and lenticular
galaxies. Conclusions that span the galaxy types treated here are as follows.
All galaxies, on average, have heavy element abundances (metallicities) that
systematically decrease outward from their galactic centers while their global
metallicities increase with galaxy mass. Abundance gradients are steepest in
normal spirals and are seen to be progressively flatter going in order from
barred spirals, lenticulars, and ellipticals. For spiral galaxies, local
metallicity appears to be correlated with total (disk plus bulge) surface
density. Observed abundance patterns indicate that N production is dominated by
primary processes at low metallicity and secondary processes at high
metallicity; C production increases with increasing metallicity; and O, Ne, S,
and Ar are produced in lockstep independent of metallicity. In elliptical
galaxies, nuclear abundances are in the range [Z/H] = 0.0 to 0.4, but the
element mixture is not scaled-solar. In large elliptical galaxies [Mg/Fe] is in
the range 0.3 to 0.5, decreasing to ~0 in smaller elliptical galaxies. Other
light elements track the Mg enhancement, but the heavier Ca tracks Fe. Velocity
dispersion appears to be a key parameter in the modulation of [Mg/Fe], but the
cause of the connection is unclear.Comment: 55-page manuscript plus 16 figures. Invited review to appear in the
Publications Of The Astronomical Society Of The Pacifi
The Abundances of Nitrogen and Oxygen in Damped Lyman Alpha Systems
We take a fresh look at the abundance of nitrogen in DLAs with oxygen
abundances between 1/10 and 1/100 of solar. This is a metallicity regime poorly
sampled in the local universe and where QSO absorbers may hold clues to both
the nucleosynthetic origin of N and the chemical evolution of high redshift
galaxies. We combine new VLT UVES observations with others from the literature
to form a sample of 10 DLAs in which the abundances of N and of one of the two
alpha-capture elements O or S have been measured. We confirm earlier reports
that the (N/O) ratio exhibits a larger range of values than other ratios of
heavy elements in DLAs; however, all 10 DLAs fall in the region of the (N/O)
vs. (O/H) plot delimited by the primary and secondary levels of nitrogen
production. Our results provide empirical evidence in support of the proposal
that intermediate mass stars (with masses less than 7 solar masses) are the
main source of primary nitrogen, so that its release into the ISM lags behind
that of oxygen, which is produced by Type II SNae. A high proportion (40%) of
the DLAs in our sample have apparently not yet attained the full primary level
of N enrichment; this finding may be an indication that the metallicity regime
we are studying preferentially picks out galaxies which have only recently
condensed out of the IGM and begun forming stars. Alternatively, the delay in
the release of N following an episode of star formation may increase with
decreasing metallicity, if stars of progressively lower masses than 4 solar
masses can synthesize N in their hotter interiors.Comment: 17 pages, LaTeX, 7 Postscript Figures. Accepted for publication in
Astronomy & Astrophysic
The oxygen abundance distribution in M101
The well-observed spiral galaxy M101 was considered. The radial distributions
of oxygen abundances determined in three different ways (with the classic Te -
method, with the R23 - method, and with the P -- method) were compared. It was
found that the parameters (the central oxygen abundance and the gradient) of
the radial O/H(P) abundance distribution are close to those of the O/H(Te)
abundance distribution. The parameters of the O/H(R23) abundance distribution
differ significantly from those of the O/H(Te) abundance distribution: the
central O/H(R23) oxygen abundance is higher by around 0.4dex and the gradient
is steeper by a factor of around 1.5 as compared to those values in the O/H(Te)
abundance distribution.
The dispersion in O/H(P) abundance at fixed radius is rather small, around
0.08 dex, and is equal to that in O/H(Te) abundance. The dispersion in O/H(R23)
abundance at fixed radius is appreciably larger, around 0.16 dex, compared to
that in O/H(Te) abundance. It has been shown that the extra dispersion in
O/H(R23) abundances is an artifact and reflects scatter in excitation parameter
P at fixed radius.Comment: 7 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
The Metallicity-Luminosity Relation, Effective Yields, and Metal Loss in Spiral and Irregular Galaxies
I present results on the correlation between galaxy mass, luminosity, and
metallicity for a sample of spiral and irregular galaxies having well-measured
abundance profiles, distances, and rotation speeds. Additional data for low
surface brightness galaxies from the literature are also included for
comparison. These data are combined to study the metallicity-luminosity and
metallicity-rotation speed correlations for spiral and irregular galaxies. The
metallicity luminosity correlation shows its familiar form for these galaxies,
a roughly uniform change in the average present-day O/H abundance of about a
factor 100 over 11 magnitudes in B luminosity. However, the O/H - V(rot)
relation shows a change in slope at a rotation speed of about 125 km/sec. At
faster V(rot), there appears to be no relation between average metallicity and
rotation speed. At lower V(rot), the metallicity correlates with rotation
speed. This change in behavior could be the result of increasing loss of metals
from the smaller galaxies in supernova-driven winds. This idea is tested by
looking at the variation in effective yield, derived from observed abundances
and gas fractions assuming closed box chemical evolution. The effective yields
derived for spiral and irregular galaxies increase by a factor of 10-20 from
V(rot) approximately 5 km/sec to V(rot) approximately 300 km/sec, asympotically
increasing to approximately constant y(eff) for V(rot) > 150 km/sec. The trend
suggests that galaxies with V(rot) < 100-150 km/sec may lose a large fraction
of their SN ejecta, while galaxies above this value tend to retain metals.Comment: 40 pages total, including 7 encapsulated postscript figures. Accepted
for publication in 20 Dec 2002 Ap
Dust Formation In Early Galaxies
We investigate the sources and amount of dust in early galaxies. We discuss
dust nucleation in stellar atmospheres using published extended atmosphere
models, stellar evolution tracks and nucleation conditions and conclude that
the (TPAGB) phase of intermediate mass stars is likely to be the most promising
site for dust formation in stellar winds. The implications of chemical
evolution models for high redshift galaxies are investigated and we show there
is no difficulty in producing dusty galaxies at redshifts above 5 if supernovae
are a dominant source of interstellar dust. If dust does not condense
efficiently in SNe then significant dust masses can only be generated at by
galaxies with a high star formation efficiency. We find the visual optical
depth for individual star forming clouds can reach values greater than 1 at
very low metallicity (1/100 solar) provided that the mass-radius exponent of
molecular clouds is less than two. Most of the radiation from star formation
will emerge at IR wavelengths in the early universe provided that dust is
present. The (patchy) visual optical depth through a typical early galaxy will
however, remain less than 1 on average until a metallicity of 1/10 solar is
reached.Comment: in press MNRAS, 17 pages with 19 figs, corrected typo
On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass
We report 4.5 micron luminosities for 27 nearby (D < 5 Mpc) dwarf irregular
galaxies measured with the Spitzer Infrared Array Camera. We have constructed
the 4.5 micron luminosity-metallicity (L-Z) relation for 25 dwarf galaxies with
secure distance and interstellar medium oxygen abundance measurements. The 4.5
micron L-Z relation is 12+log(O/H) = (5.78 +/- 0.21) + (-0.122 +/- 0.012)
M_[4.5], where M_[4.5] is the absolute magnitude at 4.5 micron. The dispersion
in the near-infrared L-Z relation is smaller than the corresponding dispersion
in the optical L-Z relation. The subsequently derived stellar mass-metallicity
M-Z relation is 12+log(O/H) = (5.65 +/- 0.23) + (0.298 +/- 0.030) log Mstar.
and extends the SDSS M-Z relation to lower mass by about 2.5 dex. We find that
the dispersion in the M-Z relation is similar over five orders of magnitude in
stellar mass, and that the relationship between stellar mass and interstellar
medium metallicity is similarly tight from high-mass to low-mass systems. We
find a larger scatter at low mass in the relation between effective yield and
total baryonic mass. In fact, there are a few dwarf galaxies with large yields,
which is difficult to explain if galactic winds are ubiquitous in dwarf
galaxies. The low scatter in the L-Z and M-Z relationships are difficult to
understand if galactic superwinds or blowout are responsible for the low
metallicities at low mass or luminosity. Naively, one would expect an ever
increasing scatter at lower masses, which is not observed.Comment: Accepted, Ap.J.; 18 pages (AASTeX 5.2; emulateapj.cls) with 12
figures. Full paper with figures at http://www.astro.umn.edu/~hlee/papers
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