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Register a new userHelium abundance in the most metal-deficient blue compact galaxies: I Zw 18 and SBS 0335-052
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Y. I. Izotov
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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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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 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.
We present the results of HI mapping with the NRAO VLA of one of the most metal-deficient blue compact dwarf (BCD) galaxies known, SBS 0335-052, with an oxygen abundance of only 1/40 that of the Sun. We study the structure and dynamics of the neutral gas in this chemically young object with a spatial resolution of 20.5x15 (~ 5.4 x 3.9 kpc at an assumed distance of 54.3 Mpc), a sensitivity at the 2sigma detection level of ~2.0K or 7.5x10^19 cm^-2 and a velocity resolution of 21.2 km s^-1. We detected a large HI complex associated with this object with an overall size of about 66 by 22 kpc and elongated in the East-West direction. There are two prominent, slightly resolved peaks visible in the integrated HI map, separated in the East-West direction by 22 kpc (84). The eastern peak is nearly coincident with the position of the optical galaxy SBS 0335-052. The western peak is about a factor of 1.3 brighter in the HI line and is identified with a faint blue compact dwarf galaxy, SBS 0335-052W, with m_B = 19.4, and a metallicity close to the lowest values known for BCDs, about 1/50 that of the Sun. The radial velocities of both systems are similar, suggesting that the two BCDs SBS 0335-052 and SBS 0335-052W constitute a pair of dwarf galaxies embedded in a common HI envelope. Alternatively, the BCDs can be the nuclei of two distinct interacting primordial HI clouds. The estimated total dynamical mass, assuming the BCDs form a bound system, is larger than 6x10^9 Msun. This is to be compared to a total gaseous mass M_gas = 2.1x10^9 Msun, and a total stellar mass Mstar < 10^8 Msun. Hence, the mass of the SBS 0335-052 system is dominated by dark matter (abridged).
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 4-micron ISAAC imaging and spectroscopy of the extremely metal-poor dwarf galaxy SBS 0335-052, aimed at a better understanding of the dust in this low-metallicity galaxy. The 4-micron emission turns out to be very compact, confined to the brightest pair of Super Star Clusters (SSCs). The Ks-L$^prime$ color is extremely red, and the $L^prime$ emission is consistent with the extrapolation of the ISO mid-infrared spectral energy distribution (SED). From hydrogen recombination lines and a fit to the near-/mid-infrared SED, we confirm a visual extinction of >= 15 mag. Our data suggest that the sites of the optical and infrared emission are distinct: the optical spectral lines come from an almost dustless region with a high star formation rate and a few thousand OB stars. This region lies along the line-of-sight to a very dusty central star cluster in which there are more than three times as many massive stars, completely hidden in the optical. From the extinction, we derive an upper limit for the dust mass of 10^5 M$_odot$ which could be produced by recent supernovae.
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