1,862 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
New southern blue compact dwarf galaxies in the 2dF Galaxy Redshift Survey
Aiming to find new extremely metal-deficient star-forming galaxies we
extracted from the Two-Degree Field Galaxy Redshift Survey (2dFGRS) 100K Data
Release 14 emission-line galaxies with relatively strong [OIII] 4363 emission.
Spectroscopic and photometric studies of this sample and, in addition, of 7
Tololo and 2 UM galaxies were performed on the basis of observations with the
ESO 3.6m telescope. All sample galaxies qualify with respect to their
photometric and spectroscopic properties as blue compact dwarf (BCD) galaxies.
Additionally, they show a good overlap with a comparison sample of 100
well-studied emission-line galaxies on the 12+log(O/H) vs. log(Ne/O), log(Ar/O)
and log(Fe/O) planes. From the analysis of the 2dFGRS subsample we report the
discovery of two new extremely metal-deficient BCDs with an oxygen abundance
12+log(O/H) < 7.6 and of another seven galaxies with 12+log(O/H) < 7.8.
Furthermore, we confirm previous oxygen abundance determinations for the BCDs
Tol 1304-353, Tol 2146-391, UM 559 and UM 570 to be 12+log(O/H) < 7.8.Comment: 26 pages, 65 figures, 5 tables, uses psfig.sty, Accepted for
publication in Astronomy and Astrophysics. The paper with high-resolution
figures can be downloaded at
http://www.uni-sw.gwdg.de/~papade/Publications/Papaderos2006_2dF.pd
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
An investigation of the luminosity-metallicity relation for a large sample of low-metallicity emission-line galaxies
(abridged) We present 8.2m VLT spectroscopic observations of 28 HII regions
in 16 emission-line galaxies and 3.6m ESO telescope spectroscopic observations
of 38 HII regions in 28 emission-line galaxies. These emission-line galaxies
were selected mainly from the Data Release 6 (DR6) of the Sloan Digital Sky
Survey (SDSS) as metal-deficient galaxy candidates. We collect photometric and
high-quality spectroscopic data for a large uniform sample of star forming
galaxies including new observations. Our aim is to study the
luminosity-metallicity (L-Z) relation for nearby galaxies, especially at its
low-metallicity end and compare it with that for higher-redshift galaxies. From
our new observations we find that the oxygen abundance in 61 out of the 66 HII
regions of our sample ranges from 12+logO/H=7.05 to 8.22. Our sample includes
27 new galaxies with 12+logO/H<7.6 which qualify as extremely metal-poor
star-forming galaxies (XBCDs). Among them are 10 HII regions with
12+logO/H<7.3. The new sample is combined with a further 93 low-metallicity
galaxies with accurate oxygen abundance determinations from our previous
studies, yielding in total a high-quality spectroscopic data set of 154 HII
regions. 9000 more galaxies with oxygen abundances, based mainly on the
Te-method, are compiled from the SDSS. Our data set spans a range of 8 mag with
respect to its absolute magnitude in SDSS g (-12>Mg>-20) and nearly 2 dex in
its oxygen abundance (7.0<12+logO/H<8.8), allowing us to probe the L-Z relation
in the nearby universe down to the lowest currently studied metallicity level.
The L-Z relation established on the basis of the present sample is consistent
with previous ones obtained for emission-line galaxies.Comment: 27 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
Star-formation rate in compact star-forming galaxies
We use the data for the Hbeta emission-line, far-ultraviolet (FUV) and
mid-infrared 22 micron continuum luminosities to estimate star formation rates
averaged over the galaxy lifetime for a sample of about 14000 bursting
compact star-forming galaxies (CSFGs) selected from the Data Release 12 (DR12)
of the Sloan Digital Sky Survey (SDSS). The average coefficient linking
and the star formation rate SFR_0 derived from the Hbeta luminosity at zero
starburst age is found to be 0.04. We compare s with some commonly used
SFRs which are derived adopting a continuous star formation during a period of
~100 Myr, and find that the latter ones are 2-3 times higher. It is shown that
the relations between SFRs derived using a geometric mean of two star-formation
indicators in the UV and IR ranges and reduced to zero starburst age have
considerably lower dispersion compared to those with single star-formation
indicators. We suggest that our relations for determination are more
appropriate for CSFGs because they take into account a proper temporal
evolution of their luminosities. On the other hand, we show that commonly used
SFR relations can be applied for approximate estimation within a factor of ~2
of the averaged over the lifetime of the bursting compact galaxy.Comment: 11 pages, 7 figures, accepted for publication in Astrophysics and
Space Scienc
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