413 research outputs found
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
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
Primordial 4He abundance: a determination based on the largest sample of HII regions with a methodology tested on model HII regions
We verified the validity of the empirical method to derive the 4He abundance
used in our previous papers by applying it to CLOUDY (v13.01) models. Using
newly published HeI emissivities, for which we present convenient fits as well
as the output CLOUDY case B hydrogen and HeI line intensities, we found that
the empirical method is able to reproduce the input CLOUDY 4He abundance with
an accuracy of better than 1%. The CLOUDY output data also allowed us to derive
the non-recombination contribution to the intensities of the strongest Balmer
hydrogen Halpha, Hbeta, Hgamma, and Hdelta emission lines and the ionisation
correction factors for He. With these improvements we used our updated
empirical method to derive the 4He abundances and to test corrections for
several systematic effects in a sample of 1610 spectra of low-metallicity
extragalactic HII regions, the largest sample used so far. From this sample we
extracted a subsample of 111 HII regions with Hbeta equivalent width EW(Hbeta)
> 150A, with excitation parameter x = O^{2+}/O > 0.8, and with helium mass
fraction Y derived with an accuracy better than 3%. With this subsample we
derived the primordial 4He mass fraction Yp = 0.254+/-0.003 from linear
regression Y-O/H. The derived value of Yp is higher at the 68% confidence level
(CL) than that predicted by the standard big bang nucleosynthesis (SBBN) model,
possibly implying the existence of different types of neutrino species in
addition to the three known types of active neutrinos. Using the most recently
derived primordial abundances D/H = (2.60+/-0.12)x10^{-5} and Yp =
0.254+/-0.003 and the chi^2 technique, we found that the best agreement between
abundances of these light elements is achieved in a cosmological model with
baryon mass density Omegab h^2 = 0.0234+/-0.0019 (68% CL) and an effective
number of the neutrino species Neff = 3.51+/-0.35 (68% CL).Comment: 23 pages, 14 figures, accepted for publication in Astronomy and
Astrophysic
A new determination of the primordial He abundance using the HeI 10830A emission line: cosmological implications
We present near-infrared spectroscopic observations of the high-intensity HeI
10830 emission line in 45 low-metallicity HII regions. We combined these NIR
data with spectroscopic data in the optical range to derive the primordial He
abundance. The use of the HeI 10830A line, the intensity of which is very
sensitive to the density of the HII region, greatly improves the determination
of the physical conditions in the He^+ zone. This results in a considerably
tighter Y - O/H linear regression compared to all previous studies. We
extracted a final sample of 28 HII regions with Hbeta equivalent width
EW(Hbeta)>150A, excitation parameter O^2+/O>0.8, and with helium mass fraction
Y derived with an accuracy better than 3%. With this final sample we derived a
primordial He mass fraction Yp = 0.2551+/-0.0022. The derived value of Yp is
higher than the one predicted by the standard big bang nucleosynthesis (SBBN)
model. Using our derived Yp together with D/H = (2.53+/-0.04)x10^-5, and the
chi^2 technique, we found that the best agreement between these light element
abundances is achieved in a cosmological model with a baryon mass density
Omega_b h^2 = 0.0240+/-0.0017 (68% CL), +/-0.0028 (95.4% CL), +/-0.0034 (99%
CL) and an effective number of neutrino species Neff = 3.58+/-0.25 (68% CL),
+/-0.40 (95.4% CL), +/-0.50 (99% CL). A non-standard value of Neff is preferred
at the 99% CL, implying the possible existence of additional types of neutrino
species.Comment: 18 pages, 11 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society. arXiv admin note: text overlap with
arXiv:1308.210
SBS 0335-052E+W: deep VLT/FORS+UVES spectroscopy of the pair of the lowest-metallicity blue compact dwarf galaxies
(abridged) We present deep archival VLT/FORS1+UVES spectroscopic observations
of the system of two blue compact dwarf (BCD) galaxies SBS 0335-052E and SBS
0335-052W. Our aim is to derive element abundances in different HII regions of
this unique system of galaxies and to study spatial abundance variations. We
determine abundances of helium, nitrogen, oxygen, neon, sulfur, chlorine, argon
and iron. The oxygen abundance in the brighter eastern galaxy varies in the
range 7.11 to 7.32 in different HII regions supporting previous findings and
suggesting the presence of oxygen abundance variations on spatial scales of
~1-2 kpc. The oxygen abundance in the brightest region No.1 of SBS 0335-052W is
7.22+/-0.07, consistent with previous determinations.Three other HII regions
are much more metal-poor with an unprecedently low oxygen abundance of
12+logO/H=7.01+/-0.07 (region No.2), 6.98+/-0.06 (region No.3), and 6.86+/-0.14
(region No.4). These are the lowest oxygen abundances ever derived in
emission-line galaxies. Helium abundances derived for the brightest HII regions
of both galaxies are mutually consistent. We derive weighted mean He mass
fractions of 0.2485+/-0.0012 and 0.2514+/-0.0012 for two different sets of HeI
emissivities. The N/O abundance ratio in both galaxies is slightly higher than
that derived for other BCDs with 12+logO/H<7.6. This implies that the N/O in
extremely metal-deficient galaxies could increase with decreasing metallicity.Comment: 20 pages, 11 figures, accepted for pulication in Astronomy and
Astrophysic
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