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We present low- and high-resolution Spitzer/IRS spectra, supplemented by IRAC and MIPS measurements, of 22 blue compact dwarf (BCD) galaxies. The BCD sample spans a wide range in oxygen abundance [12+Log(O/H) between 7.4 and 8.3], and hardness of the interstellar radiation field (ISRF). The IRS spectra provide us with a rich set of diagnostics to probe the physics of star and dust formation in very low-metallicity environments. We find that metal-poor BCDs have harder ionizing radiation than metal-rich galaxies: [OIV] emission is roughly 4 times as common as [FeII] emission. They also have a more intense ISRF, as indicated by the 71 to 160micron luminosity ratio. Two-thirds of the sample (15 BCDs) show PAH features, although the fraction of PAH emission normalized to the total infrared (IR) luminosity is considerably smaller in metal-poor BCDs (~0.5%) than in metal-rich star-forming galaxies (~10%). We find several lines of evidence for a deficit of small PAH carriers at low metallicity, and attribute this to destruction by a hard, intense ISRF, only indirectly linked to metal abundance. Our IRS spectra reveal a variety of H2 rotational lines, and more than a third of the objects in our sample (8 BCDs) have >=3sigma detections in one or more of the four lowest-order transitions. The warm gas masses in the BCDs range from 10^3 to 10^8 Msun, and can be comparable to the neutral hydrogen gas mass; relative to their total IR luminosities, some BCDs contain more H2 than SINGS galaxies., Accepted by ApJ: 70 pages in draft form, 6 tables, 22 figures

We present 6.5-meter MMT spectrophotometry of 20 H II regions in 13 extremely metal-poor emission-line galaxies selected from the Data Release 5 of the Sloan Digital Sky Survey to have [O III] 4959/Hbeta < 1 and [N II] 6583/Hbeta < 0.05. The electron temperature-sensitive emission line [O III] 4363 is detected in 13 H II regions allowing a direct abundance determination. The oxygen abundance in the remaining H II regions is derived using a semi-empirical method. The oxygen abundance of the galaxies in our sample ranges from 12+logO/H ~ 7.1 to ~ 7.8, with 10 H II regions having an oxygen abundance lower than 7.5. The lowest oxygen abundances, 12+logO/H = 7.14+/-0.03 and 7.13+/-0.07, are found in two H II regions of the blue compact dwarf galaxy SDSSJ0956+2849=DDO 68, making it the second most-metal deficient emission-line galaxy known, after SBS 0335-052W., 28 pages, 3 figures. Accepted for publication in the Astrophysical Journal

We have used the Balmer and Paschen jumps to determine the temperatures of the H+ zones of a total sample of 47 H II regions. The Balmer jump was used on MMT spectrophotometric data of 22 low-metallicity H II regions in 18 blue compact dwarf (BCD) galaxies and of one H II region in the spiral galaxy M101. The Paschen jump was used on spectra of 24 H II emission-line galaxies selected from the Data Release 3 of the Sloan Digital Sky Survey (SDSS). To derive the temperatures, we have used a Monte Carlo technique varying the electron temperature in the H+ zone, the extinction of the ionized gas and that of the stellar population, the relative contribution of the ionized gas to the total emission and the star formation history to fit the spectral energy distribution of the galaxies. For the MMT spectra, the fit was done in the wavelength range of 3200 - 5200A which includes the Balmer discontinuity, and for the SDSS spectra, in the wavelength range of 3900 - 9200A which includes the Paschen discontinuity. We find for our sample of H II regions that the temperatures of the O+2 zones determined from the nebular to auroral line intensity ratio of doubly ionized oxygen [O III] (4959+5007)/4363 do not differ, in a statistical sense, from the temperatures of the H+ zones determined from fitting the Balmer and Paschen jumps and the SEDs. We cannot rule out small temperature differences of the order of 3-5%., 33 pages, 12 figures, accepted for publication in the Astrophysical Journal

We present V and I photometry of the resolved stars in the most metal-deficient blue compact dwarf galaxy known, I Zw 18 (Zsun/50), using Hubble Space Telescope/Advanced Camera for Surveys (ACS) images, the deepest ones ever obtained for this galaxy. The resulting I vs. V-I color-magnitude diagram (CMD) reaches limiting magnitudes V=I=29 mag. It reveals a young stellar population of blue main-sequence (MS) stars (age, Comment: 29 pages, 13 Postscript figures, accepted for publication in the Astrophysical Journal

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., Comment: emulateapj.cls used, 7 pages, 7 figures + 1 table, accepted for publication in ApJ

We use spectroscopic observations of a sample of 82 HII regions in 76 blue compact galaxies to determine the primordial helium abundance Yp and the slope dY/dZ from the Y-O/H linear regression. To improve the accuracy of the dY/dZ measurement, we have included new spectrophotometric observations of 33 HII regions which span a large metallicity range, with oxygen abundance 12+log(O/H) varying between 7.43 and 8.30 (Zsun/30, Comment: 53 pages, 3 Postscript figures, accepted for publication in the Astrophysical Journal

We present deep JHK images of IZw18, the most metal-deficient Blue Compact Dwarf galaxy known, and analyze them in conjunction with archival HST/WFPC2 optical images. To investigate the stellar populations, we have divided the main body of IZw18 into eight regions, and fit the optical, near-infrared (NIR), and hybrid optical-NIR colors of these and the C component with evolutionary synthesis models. The composite best fit is obtained for an age for evolved stellar populations of = 0.4) while the latter is blue (V-I ~ -0.4)., 29 pages, 9 figures, accepted for publication in ApJ (May, 2003)

We present V and I photometry of the resolved stars in the cometary blue compact dwarf galaxy UGC 4483 using Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) images. The resulting I vs. (V-I) color-magnitude diagram (CMD) reaches limiting magnitudes V = 27.5 mag and I = 26.5 mag for photometric errors less than 0.2 mag. It reveals not only a young stellar population of blue main-sequence stars and blue and red supergiants, but also an older evolved population of red giant and asymptotic giant branch stars. The measured magnitude I = 23.65 +/- 0.10 mag of the red giant branch tip results in a distance modulus (m-M) = 27.63 +/- 0.12, corresponding to a distance of 3.4 +/- 0.2 Mpc. The youngest stars are associated with the bright H II region at the northern tip of the galaxy. The population of older stars is found throughout the low-surface-brightness body of the galaxy and is considerably more spread out than the young stellar population, suggesting stellar diffusion. The most striking characteristics of the CMD of UGC 4483 are the very blue colors of the red giant stars and the high luminosity of the asymptotic giant branch stars. Both of these characteristics are consistent with either: 1) a very low metallicity ([Fe/H] = -2.4 like the most metal-deficient globular clusters) and an old age of 10 Gyr, or 2) a higher metallicity ([Fe/H] = -1.4 as derived from the ionized gas emission lines) and a relatively young age of the oldest stellar population in UGC 4483, not exceeding ~ 2 Gyr. Thus our data do not exclude the possibility that UGC 4483 is a relatively young galaxy having formed its first stars only ~ 2 Gyr ago.

New Far Ultraviolet Spectroscopic Explorer (FUSE) far-UV spectroscopy of the nearby metal-deficient (Z☉/8) cometary blue compact dwarf (BCD) galaxy Markarian (Mrk) 59 is discussed. The data are used to investigate element abundances in its interstellar medium. The H I absorption lines are characterized by narrow cores that are interstellar in origin and by broad wings that are stellar in origin. The mean interstellar H I column density is ~7 × 1020 cm-2 in Mrk 59. No H2 lines are seen and N(H2) is 1015 cm-2 at the 10 σ level. The lack of diffuse H2 is due to the combined effect of a strong UV radiation field that destroys the H2 molecules and a low metallicity that leads to a scarcity of dust grains necessary for H2 formation. P Cygni profiles of the S VI λλ933.4, 944.5 and O VI λλ1031.9, 1037.6 lines are seen, indicating the presence of very hot O stars and a stellar wind terminal velocity of ~1000 km s-1. By fitting the line profiles with multiple components having each a velocity dispersion b = 7 km s-1 and spanning a radial velocity range of 100 km s-1, some of which can be saturated, we derive heavy element abundances in the neutral gas. We find log N(O I)/N(H I) = -5.0 ± 0.3 or [O I/H I] = -1.5 for the neutral gas, about a factor of 10 below the oxygen abundance of the supergiant H II region, implying self-enrichment of the latter.

We present high-quality Keck telescope spectroscopic observations of the two metal-deficient blue compact dwarf (BCD) galaxies Tol 1214-277 and Tol 65. These data are used to derive the heavy-element and helium abundances. We find that the oxygen abundances in Tol 1214-277 and Tol 65 are the same, 12+logO/H=7.54+/-0.01, or Zsun/24, despite the different ionization conditions in these galaxies. The nitrogen-to-oxygen abundance ratio in both galaxies is logN/O=-1.64+/-0.02 and lies in the narrow range found for the other most metal-deficient BCDs. We use the five strongest HeI emission lines 3889, 4471, 5876, 6678 and 7065, to correct self-consistently their intensities for collisional and fluorescent enhancement mechanisms and to derive the He abundance. Underlying stellar absorption is found to be important for the HeI 4471 emission line in both galaxies, being larger in Tol 65. The weighted He mass fractions in Tol 1214-277 and Tol 65 are respectively Y=0.2458+/-0.0039 and 0.2410+/-0.0050 when the three HeI emission lines, 4471, 5876 and 6678, are used, and are, respectively, 0.2466+/-0.0043 and 0.2463+/-0.0057 when the HeI 4471 emission line is excluded. These values are in very good agreement with recent measurements of the He mass fraction in others of the most metal-deficient BCDs by Izotov and coworkers. We find that the combined effect of the systematic uncertainties due to the underlying HeI stellar absorption lines, ionization and temperature structure of the HII region and collisional excitation of the hydrogen emission lines is likely small, not exceeding ~2% (the error is 2sigma). Our results support the validity of the standard big bang model of nucleosynthesis.

The similarity between observed velocity structures of Al III and singly ionised species in damped Lyα systems (DLAs) suggests the presence of ionised gas in the regions where most metal absorption lines are formed. To explore the possible implications of ionisation effects we construct a simplified two-region model for DLAs consisting of an ionisation bounded region with an internal radiation field and a neutral region with a lower metal content. Within this framework we find that ionisation effects are important. If taken into account, the element abundance ratios in DLAs are quite consistent with those observed in Milky Way stars and in metal-poor H II regions in blue compact dwarf galaxies. In particular we cannot exclude the same primary N origin in both DLAs and metal-poor galaxies. From our models no dust depletion of heavy elements needs to be invoked; little depletion is however not excluded.

We analyze long-slit spectral observations of 39 Wolf-Rayet (WR) galaxies with heavy element mass fraction ranging over 2 orders of magnitude, from Zsun/50 to 2Zsun. Nearly all galaxies in our sample show broad WR emission in the blue region of the spectrum (the blue bump) consisting of an unresolved blend of N III 4640, C III 4650, C IV 4658 and He II 4686 emission lines. Broad C IV 5808 emission (the red bump) is detected in 30 galaxies. Additionally, weaker WR emission lines are identified, most often the N III 4512 and Si III 4565 lines, which have very rarely or never been seen and discussed before in WR galaxies. These emission features are characteristic of WN7-WN8 and WN9-WN11 stars respectively. We derive the numbers of early WC (WCE) and late WN (WNL) stars from the luminosities of the red and blue bumps, and the number of O stars from the luminosity of the Hbeta emission line. Additionally, we propose a new technique for deriving the numbers of WNL stars from the N III 4512 and Si III 4565 emission lines. This technique is potentially more precise than the blue bump method because it does not suffer from contamination of WCE and early WN (WNE) stars and nebular gaseous emission. The N(WR)/N(O+WR) ratio decreases with decreasing metallicity, in agreement with predictions of evolutionary synthesis models. The N(WC)/N(WN) ratios and the equivalent widths of the blue bump EW(4650) and of the red bump EW(5808) derived from observations are also in satisfactory agreement with theoretical predictions., 49 pages, 9 figures, to appear in Astrophys. J

We present high-quality ground-based spectroscopic observations of 54 supergiant H II regions in 50 low-metallicity blue compact galaxies with oxygen abundances 12 + log O/H between 7.1 and 8.3. We use the data to determine abundances for the elements N, O, Ne, S, Ar and Fe. We also analyze Hubble Space Telescope (HST) Faint Object Spectrograph archival spectra of 10 supergiant H II regions to derive C and Si abundances in a subsample of 7 BCGs. The main result of the present study is that none of the heavy element-to-oxygen abundance ratios studied here (C/O, N/O, Ne/O, Si/O, S/O, Ar/O, Fe/O) depend on oxygen abundance for BCGs with 12 + log O/H < 7.6 (Z < Zsun/20). This constancy implies that all these heavy elements have a primary origin and are produced by the same massive (M > 10Msun) stars responsible for O production. The dispersion of the C/O and N/O ratios in these galaxies is found to be remarkably small, being only +/-0.03 dex and +/-0.02 dex respectively. This very small dispersion is strong evidence against any time-delayed production of C and primary N in the lowest-metallicity BCGs (secondary N production is negligible at these low metallicities). The absence of a time-delayed production of C and N is consistent with the scenario that galaxies with 12 + log O/H < 7.6 are undergoing now their first burst of star formation, and that they are therefore young, with ages not exceeding 40 Myr. If very low metallicities BCGs are indeed young, this would argue against the commonly held belief that C and N are produced by intermediate-mass (3Msun < M < 9Msun) stars at very low metallicities, as these stars would not have yet completed their evolution in these lowest metallicity galaxies., 37 pages, 5 EPS figures, to appear in ApJ, February 1999

We use a sample of 45 low-metallicity H II regions to determine the primordial helium abundance Y p with a precision of better than 1%. The data includes new spectrophotometric observations of 15 blue compact galaxies (BCGs) with oxygen abundance 12 ) log (O/H) between 7.83 and 8.35 (Z _ /13 " Z " most of which were selected from the First Byurakan and the University of Michigan objective Z _ /4), prism surveys. We have included many low-metallicity BCGs in our sample, including the two most metal-de-cient galaxies known, I Zw 18 and SBS 0335(052 We have carefully investi- (Z _ /55) (Z _ /43). gated the physical e†ects that may make the He I line intensities deviate from their recombination values, such as collisional and Nuorescent enhancements, underlying He I stellar absorption, and absorp- tion by Galactic interstellar Na I. By extrapolating the Y versus O/H and Y versus N/H linear regres- sions to O/H \ N/H \ 0, we obtain and 0.245 ^ 0.001, respectively, in agreement Y p \ 0.244 ^ 0.002 with the study of Izotov, Thuan, & Lipovetsky, but higher than previous determinations (Y p \ 0.230E0.234). Part of the di†erence comes from the fact that previous investigators have used the north- west component of I Zw 18 in the determination of This component is subject to strong underlying Y p . He I stellar absorption that reduces the He I line intensities by 5%E25%. The derived Y is 0.233 ^ 0.008 from the He I j6678 line. Instead, by using the southeast component of I Zw 18, which is much less subject to underlying He I stellar absorption, we obtain Y \ 0.242 ^ 0.009. The mean Y of the two most metal-de-cient BCGs, I Zw 18 and SBS 0335(052, is in excellent agreement with the Y1 \ 0.245 ^ 0.004, derived from the linear regressions. We derive a slope dY /dZ \ 2.3 ^ 1.0, considerably smaller than Y p those derived before. With this smaller slope and taking into account the errors, chemical evolution models with an outNow of well-mixed material can be built for star-forming dwarf galaxies that satisfy all the observational constraints. Our gives consistent with the lower limit set Y p ) b h 50 0.058 ^ 0.007, by dynamical measurements and X-ray observations of clusters of galaxies. It is also consistent, within the framework of standard big bang nucleosynthesis theory, with measurements of primordial 7Li in galactic halo stars, and at the 1 p level with the D/H abundance measured in absorption systems toward quasars by Tytler & Burles. Subject headings: galaxies: abundances E galaxies: irregular E galaxies: ISM E H II regions E ISM: abundances

We present high signal-to-noise 4 m KPNO Mayall telescope optical spectrophotometry of the two brightest northwest (NW) and southeast (SE) star-forming components in I Zw 18, the most metal-deficient blue compact dwarf (BCD) galaxy known. In addition, the flux-calibrated spectrum of the faint C component, a blue irregular star-forming region ~22'' northwest of the NW component, is presented. The Hβ equivalent width increases from the C component, through the NW component, to the SE component, indicating an age sequence and propagating star formation from the oldest C component (age ~2 × 108 yr) to the youngest SE component (age ~5 × 106 yr). The spatial distribution of the nebular He II λ4686 emission is investigated. It is confined to the older NW component, suggesting that hot post-main-sequence stars may be responsible for this emission. We derive from the He I λ4471 and λ6678 emission lines a helium mass fraction Y in the SE component equal to 0.242 ± 0.009. This value is higher than the value Y = 0.230 ± 0.009 derived by Skillman & Kennicutt and Y = 0.231 ± 0.010 by Pagel and coworkers for the SE component, although all values are consistent within the errors. By contrast, the helium mass fraction in the NW component, which has been used until now in the primordial helium abundance determination, is lower, equal only to 0.233 ± 0.008 from the He I λ6678 emission line and to 0.199 ± 0.008 from the He I λ4471 emission line. We argue that the lower Y-values and the large discrepancy between the values derived from the two lines in the NW component are caused by strong underlying stellar absorption in the He I lines. This absorption is much weaker in the SE component, and its Y-value should be the one to use for primordial helium abundance determination. The new Y-value in I Zw 18 is in agreement with the primordial helium mass fraction Yp = 0.243 ± 0.003 derived earlier by our group by extrapolation of the linear regressions of the Y-O/H and Y-N/H relations for a large sample of BCDs to O/H = N/H = 0.

We present Hubble Space Telescope UV spectrophotometry of three extremely metal-deficient blue compact dwarf (BCD) galaxies, SBS 0335-052 (Z ~ Z☉/40), Tol 65 (Z ~ Z☉/24), and T1214-277 (Z ~ Z☉/23). Broad damped Lyα absorption is seen in the first two BCDs. For SBS 0335-052, the H I column density derived by fitting the Lyα absorption profile is N(H I) = (7.0 ± 0.5) × 1021 cm-2, which is the highest H I column density derived thus far for a BCD and which is ~2 times larger than that in I Zw 18. As for Tol 65, N(H I) = (2.5 ± 1.0) × 1021 cm-2, which is also in the high range of H I column densities derived for BCDs and which is only ~1.4 times smaller than that in I Zw 18. The interstellar absorption line O I λ1302 has also been detected in both galaxies. Comparison with high-resolution quasar spectra implies that in SBS 0335-052, the O I λ1302 line, along with other heavy element interstellar absorption lines such as Si II λ1304 and S II λ1251, λ1254, and λ1259, are not saturated, which allows us to derive abundances. Assuming that these lines originate in the H I gas, we derive abundances of oxygen, silicon, and sulfur, respectively, as 37,000, 4000, and 116 times lower than the solar values. The oxygen abundance is a whole 37 times lower than in the neutral gas of I Zw 18. However, these highly discrepant deficiency factors of different elements suggest that the absorption lines are produced, not in the H I, but in the H II gas. Adopting that hypothesis, the derived abundances from the UV absorption lines are then consistent with that derived from the optical emission lines (Z ~ Z☉/40). The conclusion that the heavy element absorption lines originate in the H II region is supported by the detection of several systems of blueshifted S II λ1259, Si II λ1260, O I λ1302, Si II λ1304, and C II λ1335 absorption lines originating in fast-moving clouds with velocities up to ~1500 km s-1 and also by the presence of heavy element absorption lines with excited lower levels. If this conclusion holds, then the H I cloud in SBS 0335-052 is truly primordial, unpolluted by heavy elements. This would alleviate the need for postulating previous enrichment by widespread Population III stars and poses the problem of how such a cloud could last for a Hubble time without making stars before the present epoch. Because the O I λ1302 is likely to be saturated in Tol 65, we could derive only a lower limit for the oxygen abundance, ~6000 times lower than the solar value. Contrary to the situation in the two previous BCDs, a strong Lyα line is seen in the spectrum of T1214-277, which makes it the lowest metallicity BCD with detected Lyα emission. Its equivalent width of 70 A is the largest found in star-forming galaxies. The Lyα emission is not redshifted with respect to the H II gas velocity, so that the escape of Lyα photons in T1214-277 is not dictated by the velocity structure of the H I gas but probably by its porosity. The absence of Lyα emission in SBS 0335-052 and Tol 65 is probably caused by a combination of dust extinction, redistribution of the Lyα photons by multiple scattering over the whole area of the H I cloud, and the geometry of the cloud. The spectrum of SBS 0335-052 shows stellar Si IV λ1394 and λ1403 lines with P Cygni profiles, which suggests the presence of numerous hot supergiant stars. A strong stellar N V λ1240 line with a P Cygni profile is also seen in the spectrum of T1214-277, which suggests that a large population of massive (M 60 M☉) stars is present. The stellar wind terminal velocity is very low in SBS 0335-052, being only ~500 km s-1. It is higher, ~2000 km s-1 in T1214-277, in the low range of terminal velocities found for other BCDs. These BCDs are the two most metal-deficient galaxies known with P Cygni profiles. The presence of such profiles raises the question of how to set up a wind in stellar atmospheres devoid of heavy elements as in SBS 0335-052.

We report the discovery of broad Wolf-Rayet emission lines in the Multiple Mirror Telescope (MMT) spectrum of the NW component of I Zw 18, the lowest-metallicity blue compact dwarf (BCD) galaxy known. Two broad Wolf-Rayet (W-R) bumps at the wavelengths $\lambda$4650 and $\lambda$5800 are detected indicating the presence of WN and WC stars. The total numbers of WN and WC stars inferred from the luminosities of the broad He II $\lambda$4686 and C IV $\lambda$5808 lines are equal to 17(+/-)4 and 5(+/-)2, respectively. The W-R to O stars number ratio is equal to about 0.02, in satisfactory agreement with the value predicted by massive stellar evolution models with enhanced mass loss rates. The WC stars in the northwest component of I Zw 18 can be responsible for the presence of the nebular He II $\lambda$4686 emission line, however the observed intensity of this line is several times larger than model predictions, and other sources of ionizing radiation at wavelengths shorter than 228\AA are necessary.

We present HST WFPC2 V and I images and GHRS UV spectrophotometry of the spectral regions around Ly$_alpha$ 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$\alpha$ emission is not observed. Rather there is low intensity broad (FWZI = 20 A) Ly$\alpha$ emission superposed on even broader Ly$\alpha$ absorption by the HI envelope. This broad low-intensity emission is probably caused by resonant scattering of Ly$\alpha$ photons. 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-surface-brightness 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 1302 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.

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., Comment: 27 pages and 8 Postscript figures, uses aaspp4.sty. Submitted to Astrophysical Journal

We use high-quality spectrophotometric observations of 10 low-metallicity blue compact galaxies (BCGs) with oxygen abundance ranging from 12 + log(O/H) = 7.37 to 8.04 to determine the primordial helium abundance. We take special care into investigating the physical effects which may effect such a determination. We find that underlying stellar absorption, enhancement of helium lines due to self-absorption, and corrections for neutral and doubly ionized helium to be negligible. The main physical mechanism changing the He I line intensities from their recombination values is collisional excitation. To correct for it, we calculate the electron number density in the He(+) zone by a self-consistent procedure which constrains the He I lambda 5876/lambda 4471, lambda 6678/lambda 4471 and lambda 7065/lambda 4471 line ratios to have their recombination values, after correction for collisional enhancement. The density sensitive He I lambda 7065 line plays an important role in this respect. The usual practice of using the electron number density derived from the (S II) lambda 6717/lambda 6731 ratio underestimates the collisional enhancement correction and hence overestimates the helium abundance by approximately 5%. We find that Brocklehurst's (1972) He I recombination emissivities give a smaller dispersion of the data points in the Y versus O/H and Y versus N/H planes and within the derived He abundances from different lines in a given galaxy than Smits's old (1991a, b) emissivities. The dispersion is comparable when Smits's new (1994) emissivities are used. By taking the mean of the intercepts of both Y versus O/H and Y versus N/H linear regressions at O/H = N/H = 0, we determine a primordial helium mass fraction Yp = 0.229 +/- 0.004, with Brocklehurst's emissivities, and Yp = 0.240 +/- 0.005 with Smits's new emissivities. Taking the mean Y of the four most metal-deficient BCGs (Z less than Z(solar)/15) in our sample gives Yp = 0.233 +/- 0.003 with Brocklehurst's emissivities. Adding the four known BCGs with Z less than Z(solar)/15 from the literature yields: Yp = 0.232 +/- 0.002. These determinations are consistent with the lower limit of Yp = 0.236 required by standard big bang nucleosynthesis theory. We obtain a slope dY/dZ = 5.8 +/- 1.7 with Brocklehurst's emissivities and dY/dZ = 5.8 +/- 4.4 with Smits' new emissivities, larger than the values predicted by closed-box models, but consistent with those given by chemical evolution models with differential galactic winds.