2,548 research outputs found
The Oxygen Abundance of Nearby Galaxies from Sloan Digital Sky Survey Spectra
We have derived the oxygen abundance for a sample of nearby galaxies in the
Data Release 5 of the Sloan Digital Sky Survey (SDSS) which possess at least
two independent spectra of one or several HII regions with a detected
[OIII]4363 auroral line. Since, for nearby galaxies, the [OII]3727 nebular line
is out of the observed wavelength range, we propose a method to derive (O/H)_ff
abundances using the classic Te method coupled with the ff relation. (O/H)_7325
abundances have also been determined, based on the [OII]7320,7330 line
intensities, and using a small modification of the standard Te method. The
(O/H)_ff and (O/H)_7325 abundances have been derived with both the one- and
two-dimensional t_2 - t_3 relations. It was found that the (O/H)_ff abundances
derived with the parametric two-dimensional t_2 - t_3 relation are most
reliable. Oxygen abundances have been determined in 29 nearby galaxies, based
on 84 individual abundance determinations in HII regions. Because of our
selection methods, the metallicity of our galaxies lies in the narrow range 8.2
< 12 + log (O/H) < 8.4. The radial distribution of oxygen abundances in the
disk of the spiral galaxy NGC 4490 is determined for the first time.Comment: 39 pages, 10 figures, 4 tables, accepted for publication in the
Astrophysical Journa
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
Near-Infrared Spectroscopy of 0.4<z<1.0 CFRS Galaxies: Oxygen Abundances, SFRs and Dust
Using new J-band VLT-ISAAC and Keck-NIRSPEC spectroscopy, we have measured
Halpha and [NII] line fluxes for 0.47<z<0.92 CFRS galaxies which have [OII],
Hbeta and [OIII]a line fluxes available from optical spectroscopy, to
investigate how the properties of the star forming gas in galaxies evolve with
redshift. We derive the extinction and oxygen abundances for the sample using a
method based on a set of ionisation parameter and oxygen abundance diagnostics,
simultaneously fitting the [OII], Hbeta,[OIII], Halpha, and [NII] line fluxes.
The individual reddening measurements allow us to accurately correct the
Halpha-based star formation rate (SFR) estimates for extinction. Our most
salient conclusions are: a) in all 30 CFRS galaxies the source of gas
ionisation is not due to AGN activity; b) we find a range of 0<AV<3, suggesting
that it is important to determine the extinction for every single galaxy in
order to reliably measure SFRs and oxygen abundances in high redshift galaxies;
c) high values of [NII]/Halpha >0.1 for most (but not all) of the CFRS galaxies
indicate that they lie on the high-metallicity branch of the R23 calibration;
d) about one third of the 0.47<z<0.92 CFRS galaxies in our sample have lower
metallicities than local galaxies with similar luminosities and star formation
rates; e) comparison with a chemical evolution model indicates that these low
metallicity galaxies are unlikely to be the progenitors of metal-poor dwarf
galaxies at z~0.Comment: Accepted for publication in the Astrophysical Journa
Calibration of Nebular Emission-line Diagnostics: II. Abundances
(Abridged) We examine standard methods of measuring nebular chemical
abundances, including estimates based on direct T_e measurements, and also
bright-line diagnostics. We use observations of 4 LMC HII regions whose
ionizing stars have classifications ranging from O7 to WN3. We assume a 2-zone
T_e structure to compute ionic abundances. We compare with photoionization
models tailored to the properties of the individual objects, and emphasize the
importance of correctly relating T_e in the two zones, which can otherwise
cause errors of ~0.2 dex in abundance estimates. There are no spatial
variations to within 0.1 - 0.15 dex in any of the objects, even one hosting 3
WR stars.
Our data agree with the modeled R23 and S23 diagnostics of O and S. We
present the first theoretical tracks for S23, which are in excellent agreement
with a larger dataset. However, contrary to earlier suggestions, S23 is much
more sensitive to the ionization parameter than is R23, because S23 does not
sample S IV. We therefore introduce S234 = ([SII]+[SIII]+[SIV])/H-beta.
Predicted and observed spatial variations in S234 are dramatically reduced in
contrast to S23. The intensity of [SIV]10.5 microns is easily estimated from a
simple relation between [SIV]/[SIII] and [OIII]/[OII]. This method of
estimating S234 yields excellent agreement with our models, hence we give a
theoretical calibration for S234. The double-valued structure of S23 and S234
remains an important problem as for R23, and presently we consider the S
diagnostics reliable only at Z < 0.5 Z_sol. However, the slightly larger
dynamic range and excellent compatibility with theoretical predictions suggest
the S diagnostics to be more effective abundance indicators than R23.Comment: Accepted to ApJ. 24 pages, 11 figures, uses emulateapj.st
The oxygen abundance calibrations and N/O abundance ratios of ~40,000 SDSS star-forming galaxies
Using a large sample of 38,478 star-forming galaxies selected from the Second
Data Release of the Sloan Digital Sky Survey database (SDSS-DR2), we derive
analytical calibrations for oxygen abundances from several
metallicity-sensitive emission-line ratios: [N II]/H_alpha, [O III]/[N II], [N
II]/[O II], [N II]/[S II], [S II]/H_alpha, and [O III]/H_beta. This consistent
set of strong-line oxygen abundance calibrations will be useful for future
abundance studies. Among these calibrations, [N II]/[O II] is the best for
metal-rich galaxies due to its independence on ionization parameter and low
scatter. Dust extinction must be considered properly at first. These
calibrations are more suitable for metal-rich galaxies (8.4<12+log(O/H)<9.3),
and for the nuclear regions of galaxies. The observed relations are consistent
with those expected from the photoionization models of Kewley & Dopita (2002).
However, most of the observational data spread in a range of ionization
parameter q from 1*10^7 to 8*10^7 cm s^{-1}, corresponding to logU= -3.5 to
-2.5, narrower than that suggested by the models. We also estimate the (N/O)
abundance ratios of this large sample of galaxies, and these are consistent
with the combination of a "primary" and a dominant "secondary" components of
nitrogen.Comment: 17 pages, 10 figures, 1 table. ApJ in pres
Strongly entangled light from planar microcavities
The emission of entangled light from planar semiconductor microcavities is
studied and the entanglement properties are analyzed and quantified.
Phase-matching of the intra-cavity scattering dynamics for multiple pump beams
or pulses, together with the coupling to external radiation, leads to the
emission of a manifold of entangled photon pairs. A decomposition of the
emitted photons into two parties leads to a strong entanglement of the
resulting bipartite system. For the quantification of the entanglement, the
Schmidt number of the system is determined by the construction of Schmidt
number witnesses. It is analyzed to which extend the resources of the
originally strongly entangled light field are diminished by dephasing in
propagation channels.Comment: 9 pages, 5 figures, extended versio
Chemical Evolution of Galaxies
Chemical evolution of galaxies brings together ideas on stellar evolution and
nucleosynthesis with theories of galaxy formation, star formation and galaxy
evolution, with all their associated uncertainties. In a new perspective
brought about by the Hubble Deep Field and follow-up investigations of global
star formation rates, diffuse background etc., it has become necessary to
consider the chemical composition of dark baryonic matter as well as that of
visible matter in galaxies.Comment: 6 pages, AAS LaTeX macros v5.0, Millennium Essay to appear in PASP,
Feb 200
Modelling the Pan-Spectral Energy Distribution of Starburst Galaxies: III. Emission Line Diagnostics of Ensembles of Evolving HII Regions
We build, as far as theory will permit, self consistent model HII regions
around central clusters of aging stars. These produce strong emission line
diagnostics applicable to either individual HII regions in galaxies, or to the
integrated emission line spectra of disk or starburst galaxies. The models
assume that the expansion and internal pressure of individual HII regions is
driven by the net input of mechanical energy from the central cluster, be it
through winds or supernova events. This eliminates the ionization parameter as
a free variable, replacing it with a parameter which depends on the ratio of
the cluster mass to the pressure in the surrounding interstellar medium. These
models explain why HII regions with low abundances have high excitation, and
demonstrate that at least part of the warm ionized medium is the result of
overlapping faint, old, large, and low pressure HII regions. We present line
ratios (at both optical and IR wavelengths) which provide reliable abundance
diagnostics for both single HII regions or for integrated galaxy spectra, and
we find a number that can be used to estimate the mean age of the cluster stars
exciting individual HII regions.Comment: 22 pages. 18 figures. Accepted for publication in Astrophysical
journal Supplements. Electronic tabular material is available on request to
[email protected]
Theoretical models for classical Cepheids. VIII. Effects of helium and heavy elements abundance on the Cepheid distance scale
Previous nonlinear fundamental pulsation models for classical Cepheids with
metal content Z <= 0.02 are implemented with new computations at super-solar
metallicity (Z=0.03, 0.04) and selected choices of the helium-to-metal
enrichment ratio DeltaY/Delta Z. On this basis, we show that the location into
the HR diagram of the Cepheid instability strip is dependent on both metal and
helium abundance, moving towards higher effective temperatures with decreasing
the metal content (at fixed Y) or with increasing the helium content (at fixed
Z). The contributions of helium and metals to the predicted Period-Luminosity
and Period-Luminosity-Color relations are discussed, as well as the
implications on the Cepheid distance scale. Based on these new results, we
finally show that the empirical metallicity correction suggested by Cepheid
observations in two fields of the galaxy M101 may be accounted for, provided
that the adopted helium-to-metal enrichment ratio is reasonably high (Delta
Y/Delta Z ~ 3.5).Comment: 23 pages, including 6 postscript figures, accepted for publication on
Ap
Spectrophotometry of HII Regions, Diffuse Ionized Gas and Supernova Remnants in M31: The Transition from Photo- to Shock-Ionization
We present results of KPNO 4-m optical spectroscopy of discrete emission-line
nebulae and regions of diffuse ionized gas (DIG) in M31. Long-slit spectra of
16 positions in the NE half of M31 were obtained over a 5-15 kpc range in
radial distance from the center of the galaxy. The spectra have been used to
confirm 16 supernova remnant candidates from the Braun & Walterbos (1993)
catalog. The slits also covered 46 HII regions which show significant
differences among the various morphological types (center-brightened, diffuse,
rings). Radial gradients in emission-line ratios such as [OIII]/H and
[OII]/[OIII] are observed most prominently in the center-brightened HII
regions. These line ratio trends are either much weaker or completely absent in
the diffuse and ring nebulae. The line ratio gradients previously seen in M31
SNRs (Blair, Kirshner, & Chevalier 1981; 1982) are well reproduced by our new
data. The spectra of center-brightened HII regions and SNRs confirm previous
determinations of the radial abundance gradient in M31. We use diagnostic
diagrams which separate photoionized gas from shock-ionized gas to compare the
spectral properties of HII regions, SNRs and DIG. This analysis strengthens
earlier claims (Greenawalt, Walterbos, & Braun 1997) that the DIG in the disk
of M31 is photoionized by a dilute radiation field.Comment: 45 pages, 9 figures, 7 tables, to appear in the Astronomical Journal
(December 1999
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