1,594 research outputs found
Two extremely metal-poor emission-line galaxies in the Sloan Digital Sky Survey
We present spectroscopic observations with the 3.6m ESO telescope of two
emission-line galaxies, J2104-0035 and J0113+0052, selected from the Data
Release 4 (DR4) of the Sloan Digital Sky Survey (SDSS). From our data we
determine the oxygen abundance of these systems to be respectively 12+logO/H =
7.26+/-0.03 and 7.17+/-0.09, making them the two most metal-deficient galaxies
found thus far in the SDSS and placing them among the five most metal-deficient
emission-line galaxies ever discovered. Their oxygen abundances are close to
those of the two most metal-deficient emission-line galaxies known,
SBS0335-052W with 12+logO/H = 7.12+/-0.03 and I Zw 18 with 12+logO/H =
7.17+/-0.01.Comment: 5 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysic
Luminous Blue Variable Stars In The Two Extremely Metal-Deficient Blue Compact Dwarf Galaxies DDO 68 and PHL 293B
We present photometric and spectroscopic observations of two luminous blue
variable (LBV) stars in two extremely metal-deficient blue compact dwarf (BCD)
galaxies, DDO 68 with 12+logO/H = 7.15 and PHL 293B with 12+logO/H = 7.72.
These two BCDs are the lowest-metallicity galaxies where LBV stars have been
detected, allowing to study the LBV phenomenon in the extremely low metallicity
regime, and shedding light of the evolution of the first generation of massive
stars born from primordial gas. We find that the strong outburst of the LBV
star in DDO 68 occurred sometime between February 2007 and January 2008. We
have compared the properties of the broad line emission in low-metallicity LBVs
with those in higher metallicity LBVs. We find that, for the LBV star in DDO
68, broad emission with a P Cygni profile is seen in both H and He I emission
lines. On the other hand, for the LBV star in PHL 293B, P Cygni profiles are
detected only in H lines. For both LBVs, no heavy element emission line such as
Fe II was detected. The Halpha luminosities of LBV stars in both galaxies are
comparable to the one obtained for the LBV star in NGC 2363 (Mrk 71) which has
a higher metallicity 12+logO/H = 7.89. On the other hand, the terminal
velocities of the stellar winds in both low-metallicity LBVs are high, ~800
km/s, a factor of ~4 higher than the terminal velocities of the winds in
high-metallicity LBVs. This suggests that stellar winds at low metallicity are
driven by a different mechanism than the one operating in high-metallicity
winds.Comment: 26 pages, 5 figures, accepted for publication in the Astrophysical
Journa
The chemical composition of metal-poor emission-line galaxies in the Data Release 3 of the Sloan Digital Sky Survey
We have re-evaluated empirical expressions for the abundance determination of
N, O, Ne, S, Cl, Ar and Fe taking into account the latest atomic data and
constructing an appropriate grid of photoionization models with state-of-the
art model atmospheres. Using these expressions we have derived heavy element
abundances in the 310 emission-line galaxies from the Data Release 3 of
the Sloan Digital Sky Survey (SDSS)with an observed Hbeta flux F(Hbeta)> 1E-14
erg s^{-1} cm^{-2} and for which the [O III] 4363 emission line was detected at
least at a 2sigma level, allowing abundance determination by direct methods.
The oxygen abundance 12 + log O/H of the SDSS galaxies lies in the range from ~
7.1 (Zsun/30) to 8.5 (0.7 Zsun). The SDSS sample is merged with a sample of 109
blue compact dwarf (BCD) galaxies with high quality spectra, which contains
extremely low-metallicity objects. We use the merged sample to study the
abundance patterns of low-metallicity emission-line galaxies. We find that
extremely metal-poor galaxies (12 + log O/H < 7.6, i.e. Z < Zsun/12) are rare
in the SDSS sample. The alpha element-to-oxygen abundance ratios do not show
any significant trends with oxygen abundance, in agreement with previous
studies, except for a slight increase of Ne/O with increasing metallicity,
which we interpret as due to a moderate depletion of O onto grains in the most
metal-rich galaxies. The Fe/O abundance ratio is smaller than the solar value,
by up to 1 dex at the high metallicity end. (abridged)Comment: 17 pages, 12 figures. Accepted for publication in the Astronomy and
Astrophysic
Tol 2240-384 - a new low-metallicity AGN candidate
Active galactic nuclei (AGNs) have typically been discovered in massive
galaxies of high metallicity. We attempt to increase the number of AGN
candidates in low metallicity galaxies. We present VLT/UVES and archival
VLT/FORS1 spectroscopic and NTT/SUSI2 photometric observations of the
low-metallicity emission-line galaxy Tol 2240-384 and perform a detailed study
of its morphology, chemical composition, and emission-line profiles. We
determine abundances of nitrogen, O, Ne, S, Cl, Ar, and Fe by analyzing the
fluxes of narrow components of the emission lines using empirical methods. We
verify with a photoionisation model that the physics of the narrow-line
component gas is similar to that in common metal-poor galaxies. Image
deconvolution reveals two high-surface brightness regions in Tol 2240-384
separated by 2.4 kpc.The brightest southwestern region is surrounded by intense
ionised gas emission on a spatial scale of ~5 kpc. The profiles of the strong
emission lines in the UVES spectrum are asymmetric and all these lines apart
from Halpha and Hbeta can be fitted by two Gaussians of FWHM ~75-92 km/s
separated by ~80 km/s implying that there are two regions of ionised gas
emitting narrow lines. The shapes of the Halpha and Hbeta lines are more
complex. In particular, the Halpha emission line consists of two broad
components of FWHM ~700 km/s and 2300 km/s, in addition to narrow components of
two regions revealed from profiles of other lines. The extraordinarily high
luminosity of the broad Halpha line of 3x10e41 erg/s cannot be accounted for by
massive stars at different stages of their evolution. The broad Halpha emission
persists over a period of 7 years, which excludes supernovae as a powering
mechanism of this emission. This emission most likely arises from an accretion
disc around a black hole of mass ~10e7 Msun.Comment: 15 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
Neon and Argon optical emission lines in ionized gaseous nebulae: Implications and applications
In this work we present a study of the strong optical collisional emission
lines of Ne and Ar in an heterogeneous sample of ionized gaseous nebulae for
which it is possible to derive directly the electron temperature and hence the
chemical abundances of neon and argon. We calculate using a grid of
photoionization models new ionization correction factors for these two elements
and we study the behaviour of Ne/O and Ar/O abundance ratios with metallicity.
We find a constant value for Ne/O, while there seems to be some evidence for
the existence of negative radial gradients of Ar/O over the disks of some
nearby spirals. We study the relation between the intensities of the emission
lines of [NeIII] at 3869 \AA and [OIII] at 4959 \AA and 5007 \AA. This relation
can be used in empirical calibrations and diagnostic ratios extending their
applicability to bluer wavelengths and therefore to samples of objects at
higher redshifts. Finally, we propose a new diagnostic using [OII], [NeIII] and
Hdelta emission lines to derive metallicities for galaxies at high z.Comment: 12 pages, 12 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Near-infrared spectroscopy of a large sample of low-metallicity blue compact dwarf galaxies
We present near-infrared (NIR) spectroscopic observations in the wavelength
range 0.90-2.40mum of eighteen low-metallicity blue compact dwarf (BCD)
galaxies and six HII regions in spiral and interacting galaxies. Hydrogen and
helium emission lines are detected in all spectra, while H2 and iron emission
lines are detected in most spectra. The NIR data for all objects have been
supplemented by optical spectra. In all objects, except perhaps for the highest
metallicity ones, we find that the extinctions A(V) in the optical and NIR
ranges are similar, implying that the NIR hydrogen emission lines in
low-metallicity BCDs do not reveal more star formation than seen in the
optical. We conclude that emission-line spectra of low-metallicity BCDs in the
0.36-2.40mum wavelength range are emitted by a relatively transparent ionized
gas. The H2 emission line fluxes can be accounted for by fluorescence in most
of the observed galaxies. We find a decrease of the H2 2.122mum emission line
relative to the Brgamma line with increasing ionization parameter. This
indicates an efficient destruction of H2 by the stellar UV radiation. The
intensities of the [FeII] 1.257mum and 1.644mum emission lines in the spectra
of all galaxies, but one, are consistent with the predictions of Cloudy stellar
photoinization models. There is thus no need to invoke shock excitation for
these lines, and they are not necessarily shock indicators in low-metallicity
high-excitation BCDs. The intensity of the HeI 2.058mum emission line is lower
in high-excitation BCDs with lower neutral gas column densities and higher
turbulent motions.Comment: 11 pages, 6 figures, accepted for publication in MNRAS. arXiv admin
note: text overlap with arXiv:1104.081
Deep VLT spectroscopy of the blue compact dwarf galaxies Tol 1214-277 and Tol 65
We present VLT spectroscopic observations with different spectral resolutions
and different slit orientations of the two metal-deficient blue compact dwarf
(BCD) galaxies Tol 1214-277 and Tol 65. The oxygen abundances in the brightest
HII regions of Tol 1214-277 and Tol 65 are found to be 12+log O/H = 7.55+/-0.01
and 7.54+/-0.01, or Zsun/24. The nitrogen-to-oxygen abundance ratios in the two
galaxies are log N/O =-1.64+/-0.03 and -1.60+/-0.02 and lie in the narrow range
found for other most metal-deficient BCDs. The helium mass fraction derived in
several HII regions in both galaxies is consistent with a high primordial
helium mass fraction, Yp ~ 0.244. We confirm the detection of the
high-ionization forbidden emission line [FeV]4227 in the spectrum of Tol
1214-277. Additionally, weak [NeIV]4725, [FeVI]5146, 5177, and [FeVII]5721,
6087 emission lines are detected in the high-resolution spectrum of Tol
1214-277. The detection of these lines implies the presence of hard radiation
with photon energy in the range ~ 4-8 Ryd. Emission lines are detected in the
spectra of eight galaxies in the fields of Tol 1214-277 and Tol 65. Seven of
these galaxies are background objects, while one galaxy has a redshift close to
that of Tol 1214-277. Situated at a projected distance of ~ 14.5 kpc from Tol
1214-277, this galaxy is probably a companion of the BCD.Comment: 16 pages, 8 Postscript figure
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