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Register a new userOxygen abundance variations in the system of the two blue compact dwarf galaxies SBS 0335-052E and SBS 0335-052W
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Added by
Polichronis Papaderos
Publication date
2006
fields
Physics
and research's language is
English
Authors
P. Papaderos
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We present 3.6m ESO telescope spectroscopic observations of the system of the two blue compact dwarf galaxies SBS 0335-052W and SBS 0335-052E. The oxygen abundance in SBS 0335-052W is 12 + log O/H = 7.13 +/- 0.08, confirming that this galaxy is the most metal-deficient emission-line galaxy known. We find that the oxygen abundance in SBS 0335-052E varies from region to region in the range from 7.20 to 7.31, suggesting the presence of an abundance gradient over a spatial scale of 1 kpc. Signatures of early carbon-type Wolf-Rayet stars are detected in cluster 3 of SBS 0335-052E, corresponding to the emission of three to eighteen WC4 stars, depending on the adopted luminosity of a single WC4 star in the CIV 4658 emission line.
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We present an X-ray study of the three most metal-deficient blue compact dwarf (BCD) galaxies known in the local Universe, based on deep Chandra observations of SBS 0335-052 (0.025 solar abundance), SBS 0335-052W (0.02 solar abundance) and I Zw 18 (0.02 solar abundance). All three are detected, with more than 90% of their X-ray emission arising from point-like sources. The 0.5-10.0 keV luminosities of these point sources are in the range (1.3-8.5)x1e39 erg/s. We interpret them to be single or a collection of high-mass X-ray binaries, the luminosities of which may have been enhanced by the low metallicity of the gas. There are hints of faint extended diffuse X-ray emission in both SBS 0335-052 and I Zw 18, probably associated with the superbubbles visible in both BCDs. The spectrum of I Zw 18 shows a OVIII hydrogen-like emission line. The best spectral fit gives an O overabundance of the gas in the X-ray point source by a factor of ~7 with respect to the Sun, or a factor of ~350 with respect to the O abundance determined for the HII region.
We present two-dimensional spectroscopy of the extremely metal-deficient blue compact dwarf (BCD) galaxy SBS 0335-052E aiming to studyphysical conditions, element abundances and kinematical properties of the ionised gas in this galaxy. Observations were obtained in the spectral range 3620-9400A with the imaging spectrograph GIRAFFE installed on the UT2 of the Very Large Telescope (VLT). These observations are the first ones carried out so far with GIRAFFE in the ARGUS mode which allows to obtain simultaneously 308 spectra covering a 11.4x7.3 region. We produced images of SBS 0335-052E in the continuum and in emission lines of different stages of excitation. We find that while the maximum of emission in the majority of lines, including the strong lines Hbeta 4861A, Halpha 6563A, [OIII] 4363,5007A, [OII] 3726,3729A, coincides with the youngest south-eastern star clusters 1 and 2, the emission of HeII 4686A line is offset to the more evolved north-west clusters 4,5. This suggests that hard ionising radiation responsible for the HeII 4686A emission is not related to the most massive youngest stars, but rather is connected with fast radiative shocks. This conclusion is supported by the kinematical properties of the ionised gas from the different emission lines as the velocity dispersion in the HeII 4686A line is systematically higher, by ~50%-100%, than that in other lines. The variations of the emission line profiles suggest the presence of an ionised gas outflow in the direction perpendicular to the galaxy disk. (abridged)
(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.
The results of deep long-slit spectroscopy of the extremely low-metallicity blue compact dwarf (BCD) galaxy SBS 0335-052 are presented. Down to intensity levels of 10^{-3 ... -4} of Hbeta, unprecedented for spectroscopy of extra-galactic giant H II regions, we detect numerous weak permitted and forbidden nebular lines in the brightest part of the galaxy. With varying degrees of confidence, the detections include lines of high-ionization ions like Fe^{4+} -Fe^{6+}, implying very hard ionizing radiation. Two broad emission features, possibly from Wolf-Rayet stars, and stellar He II 4200 absorption are seen in the same region. The large spatial extent of He II 4686 emission (implying the presence of sufficient ionizing photons with energies above 54 eV) and the spatial distribution of the electron temperature suggest that at least some part of the hard radiation is associated with shocks. Extended Halpha emission is detected over ~ 6 - 8 kpc, a much larger area than in previous studies, suggesting that hot ionized gas is spread out far away from the central ionizing clusters. This shows that nebular line and continuous emission can significantly modify the colours of these extended regions and must be taken into account in studies of the underlying stellar population.
We present high-quality spectroscopic observations of the two most-metal deficient blue compact galaxies known, I Zw 18 and SBS 0335-052 to determine the helium abundance. The underlying stellar absorption strongly influences the observed intensities of He I emission lines in the brightest NW component of I Zw 18, and hence this component should not be used for primordial He abundance determination. The effect of underlying stellar absorption, though present, is much smaller in the SE component. Assuming all systematic uncertainties are negligible, the He mass fraction derived in this component is Y = 0.243+/-0.007. The high signal-to-noise ratio spectrum (> 100 in the continuum) of SBS 0335-052 allows us to measure the helium mass fraction with a precision better than 2% -- 5% in nine different regions along the slit. Assuming all systematic uncertainties are negligible, the weighted mean He mass fraction in SBS 0335-052 is Y = 0.2437+/-0.0014 when the three He I 4471, 5876 and 6678 emission lines are used, and is 0.2463+/-0.0015 when the He I 4471 emission line is excluded. The weighted mean helium mass fraction in the two most metal-deficient BCGs I Zw 18 and SBS 0335-052, Y=0.2462+/-0.0015, after correction for the stellar He production results in a primordial He mass fraction Yp = 0.2452+/-0.0015. The derived Yp leads to a baryon-to-photon ratio of (4.7+/-1.0) 10^{-10}, consistent with the values derived from the primordial D and 7Li abundances, and supporting the standard big bang nucleosynthesis theory. For the most consistent set of primordial D, 4He, and 7Li abundances we derive an equivalent number of light neutrino species 3.0+/-0.3 (95% C.L.).
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