Trinh X. Thuan

Heavy-Element Abundances in Blue Compact Galaxies

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 seven 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≤Z☉/20). This constancy implies that all of these heavy elements have a primary origin and are produced by the same massive (M≥10 M☉) stars responsible for O production. The dispersion of the ratios C/O and N/O in these galaxies is found to be remarkably small, being only ±0.03 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+logO/H≤7.6 are now undergoing their first burst of star formation, and that they are therefore young, with ages not exceeding 40 Myr. If very low metallicity BCGs are indeed young, this would argue against the commonly held belief that C and N are produced by intermediate-mass (3 M☉≤M≤9 M☉) stars at very low metallicities, as these stars would not have yet completed their evolution in these lowest metallicity galaxies. In higher metallicity BCGs (7.6 7.6; (3) by the time intermediate-mass stars have evolved and released their nucleosynthetic products (100-500 Myr), all galaxies have become enriched to 7.6 8.2, secondary N production becomes important. BCGs show the same O/Fe overabundance with respect to the Sun (~0.4 dex) as Galactic halo stars, suggesting the same chemical enrichment history. We compare heavy elements yields derived from the observed abundance ratios with theoretical yields for massive stars and find general good agreement. However, the theoretical models are unable to reproduce the observed N/O and Fe/O. Further theoretical developments are necessary, in particular to solve the problem of primary nitrogen production in low-metallicity massive stars. We discuss the apparent discrepancy between abundance ratios N/O measured in BCGs and those in high-redshift damped Lyα galaxies, which are up to 1 order of magnitude smaller. We argue that this large discrepancy may arise from the unknown physical conditions of the gas responsible for the metallic absorption lines in high-redshift damped Lyα systems. While it is widely assumed that the absorbing gas is neutral, we propose that it could be ionized. In this case, ionization correction factors can boost N/O in damped Lyα galaxies into the range of those measured in BCGs.

The chemical composition of metal-poor emission-line galaxies in the data release 3 of the sloan digital sky survey

We have re-evaluated empirical expressions for the abundance determination of N, O, Ne, S, Cl, Ar and Fe taking into account the latest atomic data and constructing an appropriate grid of photoionization models with state-of-the art model atmospheres. Using these expressions we have derived heavy element abundances in the ∼310 emission-line galaxies from the Data Release 3 of the Sloan Digital Sky Survey (SDSS) with an observed Hβ flux F(Hβ) > 10 −14 erg s −1 cm −2 and for which the [O iii] λ4363 emission line was detected at least at a 2σ level, allowing abundance determination by direct methods. The oxygen abundance 12 + log O/H of the SDSS galaxies lies in the range from ∼7.1 (Z� /30) to ∼8.5 (0.7 Z� ). The SDSS sample is merged with a sample of 109 blue compact dwarf (BCD) galaxies with high quality spectra, which contains extremely low-metallicity objects. We use the merged sample to study the abundance patterns of low-metallicity emission-line galaxies. We find that extremely metal-poor galaxies (12 + log O/H –1.6, implying that they have a different nature than the subsample of high-redshift damped Lyα systems with log N/ Oo f∼–2.3 and that their ages are larger than 100–300 Myr. We confirm the apparent increase in N/O with decreasing EW(Hβ), already shown in previous studies, and explain it as the signature of gradual nitrogen ejection by massive stars from the most recent starburst.

Oxygen abundance determination in H II regions: The strong line intensities-abundance calibration revisited

The problem of oxygen abundance determination in H II regions based on the strong oxygen line intensities-oxygen abundance empirical calibration is revisited. A compilation of spectroscopic data of H II regions in spiral and irregular galaxies with a measured [O III] λ4363 line intensity has been carried out, resulting in a sample containing more than 700 individual measurements. Methods are devised to select out only high-precision measurements from that original sample. Te-based oxygen abundances have been recomputed in the same way for all H II regions with high-precision measurements. That sample of Te abundances is used to recalibrate the empirical relations between the oxygen abundance and the strong oxygen line intensities for both high (the upper branch of the O/H-R23 diagram) and low (the lower branch) metallicities, within the framework of the P method, where P is the excitation parameter. Concerning high-metallicity H II regions, an alternative way for deriving oxygen abundances using only measurements of the strong nebular oxygen lines is proposed. The method is based on a tight correlation between the flux in the auroral [O III] λ4363 line and the fluxes in the nebular [O II] λλ3727, 3729 and [O III] λλ4959, 5007 lines, called the ff relation. This relation is also used to select out high-metallicity H II regions with high-precision measurements. In contrast to previous work, the new upper branch P calibration is based only on abundances. It is found that P abundances usually agree well with the ff abundances, although faint (log R23 -0.5) low-excitation (P 0.25) H II regions may show systematic differences that can be as large as ~0.1 dex. As for the newly derived low-metallicity P calibration, it is shown to be robust. The calibrations derived from the sample containing all abundance determinations and from that containing only recent (since 1995) measurements are found to be in very good agreement. For both low- and high-metallicity H II regions, the new calibration gives P abundances that agree with abundances to within 0.1 dex.

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.

15 Micron Infrared Space Observatory Observations of the 1415+52 Canada-France Redshift Survey Field: The Cosmic Star Formation Rate as Derived from Deep Ultraviolet, Optical, Mid-Infrared, and Radio Photometry

The Canada-France Redshift Survey 1452+52 field has been deeply imaged with the Infrared Space Observatory using ISOCAM through the LW3 filter (12-18 μm). Careful data analysis and comparison with deep optical and radio data have allowed us to generate a catalog of 78 15 μm sources with both radio and optical identifications. They are redder and lie at higher redshift than I-band-selected galaxies, with most of them being star-forming galaxies. We have considered the galaxies detected at radio and 15 μm wavelengths, which potentially include all strong and heavily extincted starbursts, up to z=1. Spectral energy distributions (SEDs) for each of the sources have been derived using deep radio, mid-IR, near-IR, optical, and UV photometry. The sources were then spectrally classified by comparing with SEDs of well-known nearby galaxies. By deriving their far-IR luminosities by interpolation, we can estimate their star formation rate (SFR) in a way that does not depend sensitively on the extinction. Between 35% and 85% of the star formation at z≤1 is related to IR emission, and the global extinction is in the range AV=0.5-0.85. While heavily extincted starbursts with SFRs in excess of 100 M☉ yr-1 constitute less than 1% of all galaxies, they contribute about 18% of the SFR density out to z=1. Their morphologies range from S0 to Sab, and more than a third are interacting systems. The SFR derived by far-IR fluxes is likely to be ~2.9 times higher than those previously estimated from UV fluxes. The derived stellar mass formed since the redshift of 1 could be too high when compared with the present-day stellar mass density. This might be due to an initial mass function in distant star-forming galaxies different from the solar neighborhood one or an underestimate of the local stellar mass density.

Systematic Effects and a New Determination of the Primordial Abundance of 4He and dY/dZ from Observations of Blue Compact Galaxies

We use spectroscopic observations of a sample of 82 H II 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 H II regions that span a large metallicity range, with oxygen abundance 12 + log(O/H) varying between 7.43 and 8.30 (Z☉/30 ≤ Z ≤ Z☉/4). Most of the new galaxies were selected from the First Byurakan, the Hamburg/SAO, and the University of Michigan objective prism surveys. For a subsample of seven H II regions, we derive the He mass fraction taking into account known systematic effects, including collisional and fluorescent enhancements of He I emission lines, collisional excitation of hydrogen emission, underlying stellar He I absorption, and the difference between the temperatures Te(He II) in the He+ zone and Te(O III) derived from the collisionally excited [O III] lines. We find that the net result of all the systematic effects combined is small, changing the He mass fraction by less than 0.6%. By extrapolating the Y versus O/H linear regression to O/H = 0 for seven H II regions of this subsample, we obtain Yp = 0.2421 ± 0.0021 and dY/dO = 5.7 ± 1.8, which corresponds to dY/dZ = 3.7 ± 1.2, assuming the oxygen mass fraction to be O = 0.66Z. In the framework of the standard big bang nucleosynthesis theory, this Yp corresponds to Ωbh2 = 0.012, where h is the Hubble constant in units of 100 km s-1 Mpc-1. This is smaller at the 2 σ level than the value obtained from recent deuterium abundance and microwave background radiation measurements. The linear regression slope dY/dO = 4.3 ± 0.7 (corresponding to dY/dZ = 2.8 ± 0.5) for the whole sample of 82 H II regions is similar to that derived for the subsample of seven H II regions, although it has a considerably smaller uncertainty.

THE PRIMORDIAL ABUNDANCE OF 4He: EVIDENCE FOR NON-STANDARD BIG BANG NUCLEOSYNTHESIS

We present a new determination of the primordial helium mass fraction Yp , based on 93 spectra of 86 low-metallicity extragalactic H II regions, and taking into account the latest developments concerning systematic effects. These include collisional and fluorescent enhancements of He I recombination lines, underlying He I stellar absorption lines, collisional and fluorescent excitation of hydrogen lines, and temperature and ionization structure of the H II region. Using Monte Carlo methods to solve simultaneously for the above systematic effects, we find the best value to be Yp = 0.2565 ± 0.0010 (stat.) ± 0.0050 (syst.). This value is higher at the 2σ level than the value given by standard big bang nucleosynthesis, implying deviations from it. The effective number of light neutrino species N ν is equal to 3.68+0.80 –0.70 (2σ) and 3.80+0.80 –0.70 (2σ) for a neutron lifetime τ n equal to 885.4 ± 0.9 s and 878.5 ± 0.8 s, respectively, i.e., it is larger than the experimental value of 2.993 ± 0.011.

A Spectroscopic Study of a Large Sample Of Wolf-Rayet Galaxies

We analyze long-slit spectral observations of 39 Wolf-Rayet (WR) galaxies with heavy element mass fraction ranging over 2 orders of magnitude, from Z☉/50 to 2Z☉. 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 λ4640, C III λ4650, C IV λ4658, and He II λ4686 emission lines. Broad C IV λ5808 emission (the red bump) is detected in 30 galaxies. Additionally, weaker WR emission lines are identified, most often the N III λ4512 and Si III λ4565 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 Hβ emission line. Additionally, we propose a new technique for deriving the numbers of WNL stars from the N III λ4512 and Si III λ4565 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. It is found that the relative number of WR stars N(WR)/N(O+WR) decreases with decreasing metallicity, in agreement with predictions of evolutionary synthesis models. The relative number ratios N(WC)/N(WN) and the equivalent widths of the blue bump EW(λ4650) and of the red bump EW(λ5808) derived from observations are also in satisfactory agreement with theoretical predictions, except for the most metal-deficient WR galaxies. A possible source of disagreement is too low a line emission luminosity adopted for a single WCE star in low-metallicity models. We assemble a sample of 30 H II regions with detected He II λ4686 nebular emission to analyze the possible connection of this emission with the hard UV radiation of the WR stars. The theoretical predictions satisfactorily reproduce the observed intensities and equivalent widths of the He II λ4686 nebular emission line. However, galaxies with nebular He II λ4686 emission do not always show WR emission. Therefore, in addition to the ionization of He+ in the H II region by WR stars, other mechanisms for the origin of He II λ4686 such as radiative shocks probably need to be invoked.

The Primordial Abundance of 4He: A Self-consistent Empirical Analysis of Systematic Effects in a Large Sample of Low-Metallicity H II Regions

We determine the primordial helium mass fraction Yp using 93 spectra of 86 low-metallicity extragalactic H II regions. This sample constitutes the largest and most homogeneous high-quality data set in existence for the determination of Yp. For comparison, and to improve the statistics in our investigation of systematic effects affecting the Yp determination, we have also considered a sample of 271 low-metallicity H II regions selected from Data Release 5 of the Sloan Digital Sky Survey. Although this larger sample shows more scatter, it gives results that are consistent at the 2 ? level with our original sample. We have considered known systematic effects that may affect the 4He abundance determination. They include different sets of He I line emissivities and reddening laws, collisional and fluorescent enhancements of He I recombination lines, underlying He I stellar absorption lines, collisional excitation of hydrogen lines, temperature and ionization structure of the H II region, and deviation of He I and H emission-line intensities from case B. However, the most likely value of Yp depends on the adopted set of He I line emissivities. Using Monte Carlo methods to solve simultaneously the above systematic effects, we find a primordial helium mass fraction of Yp = 0.2472 ? 0.0012 when using the He I emissivities from Benjamin and coworkers and 0.2516 ? 0.0011 when using those from Porter and coworkers. The first value agrees well with the value given by standard big bang nucleosynthesis (SBBN) theory, while the value obtained with the likely more accurate emissivities of Porter and coworkers is higher at the 2 ? level. This latter value, if confirmed, would imply slight deviations from SBBN.

Helium Abundance in the Most Metal-deficient Blue Compact Galaxies: I Zw 18 and SBS 0335–052*

We present high-quality spectroscopic observations of the two most metal-deficient blue compact galaxies known, I Zw 18 and SBS 0335-052. We use the data to determine the heavy-element and helium abundances. The oxygen abundances in the northwest and the southeast components of I Zw 18 are found to be the same within the errors, 7.17 ± 0.03 and 7.18 ± 0.03, respectively, although marginally statistically significant spatial variations of oxygen abundance might be present. In contrast, we find a statistically significant gradient of oxygen abundance in SBS 0335-052. The largest oxygen abundance, 12 + log O/H = 7.338 ± 0.012, is found in the region 06 to the northeast of the brightest part of the galaxy, and it decreases toward the southwest to values of ~7.2, comparable to that in I Zw 18. The underlying stellar absorption strongly influences the observed intensities of He I emission lines in the brightest northwest 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 southeast component. Assuming all systematic uncertainties are negligible, the He mass fraction Y = 0.243 ± 0.007 derived in this component is in excellent agreement with recent measurements by Izotov & Thuan, suggesting the robustness of the technique applied in measurements of the helium abundance in low-metallicity blue compact galaxies. 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. We show that, while underlying stellar absorption in SBS 0335-052 is important only for the He I 4471 A emission line, other mechanisms such as collisional and fluorescent enhancements are influencing the intensities of all He I emission lines and should be properly taken into account. When the electron number density derived from [S II] emission lines is used in SBS 0335-052, the correction of He I emission lines for collisional enhancement leads to systematically different He mass fractions for different He I emission lines. This unphysical result implies that the use of the electron number density derived from [S II] emission lines, being characteristic of the S+ zone but not of the He+ zone, will lead to an incorrect inferred value of Y. In the case of SBS 0335-052 it leads to a significant underestimate of the He mass fraction. In contrast, the self-consistent method using the five strongest He I emission lines in the optical spectrum for correction for collisional and fluorescent enhancements shows excellent agreement of the He mass fraction derived from the He I 5876 A and He I 6678 A emission lines in all nine regions of SBS 0335-052 used for the He abundance determination. 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 A emission lines are used, and it is 0.2463 ± 0.0015 when the He I 4471 A emission line is excluded. These values are in very good agreement with recent measurements of the He mass fraction in SBS 0335-052 by Izotov and coworkers. The weighted mean helium mass fraction in the two most metal-deficient blue compact galaxies, 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 × 10-10 and to a baryon mass fraction in the universe Ωbh = 0.068, 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 Nν = 3.0 ± 0.3 (2 σ).