Valentin A. Lipovetsky

The Primordial Helium Abundance: Systematic Effects and a New Determination

We present new high-quality spectrophotometric observations of 27 supergiant H II regions in 23 low-metallicity blue compact galaxies (BCGs) with oxygen abundance 12 + log (O/H) between 7.22 and 8.17 (Z☉/50 ≤ Z ≤ Z☉/6), most of which were selected from the first and second Byurakan objective-prism surveys. Combining this new data with our previous sample of 10 low-metallicity BCGs, we extract a subsample of 27 H II regions in 24 BCGs most appropriate for the determination of the primordial helium abundance Yp. We find that the most metal-deficient BCG known, I Zw 18 (Z ~ Z☉/50), cannot be used for this purpose because of its abnormally low He I line intensities. We have examined critically the systematic effects that may influence the determination of Yp. We find that the effects of the corrections for neutral helium and for underlying stellar absorption in the He I lines, possible deviations from case B recombination theory, fluorescent enhancement of the He I line intensities, temperature fluctuations in H II regions, Wolf-Rayet stellar winds, and supernova shock waves to be small. The main effect comes from the particular set of atomic data used. The best set of atomic data for use in the determination of Yp is composed of Smitss He I emissivities and Kingdon & Ferlands collisional enhancement correction factors. This set gives the smallest dispersion of the data in the Y-O/H and Y-N/H planes and corrects best the data for collisional enhancement effects. By extrapolating the Y versus O/H and Y versus N/H linear regressions to O/H = N/H = 0, we obtain Yp = 0.243 ± 0.003 for both regressions, considerably larger than the values derived before. In the framework of the standard hot big bang nucleosynthesis model with a number of neutrino families Nν = 3 and a neutron half-lifetime τn = 887 s, our new Yp gives a baryon-to-photon number ratio η=3.51.0−0.7 × 10-10, or a baryonic mass fraction Ωh250=0.05±0.01. Our Yp determination is fully consistent with measurements of the primordial 7Li in Galactic halo stars and of (D +3He) in the local interstellar medium and the solar system, and consistent at the 2 σ level with the primordial D abundance derived in a quasar absorption system by Tytler, Fan, & Burles. We derive a slope dY/dZ = 1.7 ± 0.9, considerably smaller than the slopes obtained before and consistent with chemical evolution models for star-forming dwarf galaxies with an outflow of well-mixed material. Extrapolation to solar metallicity with such a slope gives the correct solar helium mass fraction within the errors.

SBS 0335–052, A Probable Nearby Young Dwarf Galaxy: Evidence Pro and Con*

The results of Multiple Mirror Telescope spectrophotometry of the extremely low-metallicity blue compact galaxy (BCG) SBS 0335-052 are presented. The oxygen abundance in central brightest part of the galaxy is found to be 12 + log(O/H) = 7.33+-0.01. The N/O, Ne/O, S/O and Ar/O abundance ratios are close to those derived in other BCGs, suggesting that heavy element enrichment in the HII region is due to massive star evolution. We detect auroral [OIII] 4363 emission in the inner part of HII region with a diameter of 3.6 kpc and find that the HII region inside this diameter is hot, T_e~20000K. The oxygen abundance in this region is nearly constant (12 + log(O/H) = 7.1 - 7.3), implying effective mixing of ionized gas on short time-scales. We also discuss the origin of blue underlying extended low-intensity emission detected in SBS 0335-052. The (V-I) and (R-I) color distributions suggest that a significant contribution to the extended envelope is due to ionized gas emission. However, the observed equivalent width of Hbeta emission in the extended envelope is 2-3 times lower than the value expected in the case of pure gaseous emission. These findings suggest that, along with the blue young (~10^7yr) stellar clusters, an older stellar population with age ~10^8yr may be present in the extended envelope of SBS 0335-052, having a total mass conclude that SBS 0335-052 is a young nearby dwarf galaxy with age ~10^8 yr.

Hubble Space Telescope Observations of the Blue Compact Dwarf SBS 0335–052: A Probable Young Galaxy

We present HST WFPC2 V and I images and GHRS UV spectrophotometry of the spectral regions around Lyα and OI λ1302 of the extremely metal-deficient ( Z ∼ Z⊙/41 ) blue compact dwarf (BCD) galaxy SBS 0335–052. All the star formation in the BCD occurs in six superstar clusters (SSC) with ages ≤ 3-4 Myr. As there is no evident sign of tidal interaction, star formation in the BCD is probably triggered by stochastic cloud collisions in the HI envelope. Dust is clearly present and mixed spatially with the SSCs. There is a supershell of radius ∼ 380 pc, delineating a large supernova cavity. The instantaneous star formation rate is ∼ 0.4 M⊙ yr −1 . Strong narrow Lyα emission is not observed. Rather there is low intensity broad (FWZI = 20 Å) Lyα emission superposed on even broader Lyα absorption by the HI envelope. This broad low-intensity emission is caused by resonant scattering of Lyα photons. The absence of strong Lyα emission may be due partly to dust absorption, but mainly to multiple scattering which removes Lyα photons from the small HST aperture. As the HI cloud is seen nearly edge-on, geometrical effects may play also a role as photons escape more easily in a direction perpendicular to the plane than along it. The BCD appears to be a young galaxy, undergoing its very first burst of star formation. This conclusion is based on the following evidence: 1) the underlying extended low-surfacebrightness component is very irregular and filamentary, suggesting that a significant part of the emission comes from ionized gas; 2) it has very blue colors (–0.34 ≤ (V–I)0 ≤ 0.16), consistent with gaseous emission colors; 3) the OI λ1302 line is not detected in absorption in the GHRS spectrum, setting an upper limit for N(O)/N(H) in the HI envelope of the BCD of more than 3000 times smaller than the value in Orion. Subject headings: galaxies – young: interstellar matter – nebulae: HII regionsWe present HST WFPC2 V and I images and GHRS UV spectrophotometry of the spectral regions around Ly

The KPNO International Spectroscopic Survey. I. Description of the Survey

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SBSG 0335–052W: An Extremely Low Metallicity Dwarf Galaxy*

and OI 1302 of the extremely metal-deficient (Z~Zsun/41) blue compact dwarf (BCD) galaxy SBS 0335-052. All the star formation in the BCD occurs in six super-star clusters (SSC) with ages =< 3-4 Myr. Dust is clearly present and mixed spatially with the SSCs. There is a supershell of radius ~380 pc, delineating a large supernova cavity. The instantaneous star formation rate is ~0.4 Msun yr^-1. Strong narrow Ly

A new family of non-linear filters for background subtraction of wide-field surveys

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The Hamburg/SAO survey for emission-line galaxies

emission is not observed. Rather there is low intensity broad (FWZI = 20 A) Ly

Spectroscopy and Photometry of Stellar Objects from the Second Byurakan Survey

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The Primordial helium abundance from a new sample of metal-deficient blue compact galaxies

emission superposed on even broader Ly

Heavy element abundances in a new sample of low-metallicity blue compact galaxies

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