Low-metallicity Blue Compact Galaxies Research Articles

The Deuterium Balmer Series

AbstractThe first detection and identification of deuterium Balmer lines were recently reported in H ii regions, using high spectral resolution data secured at CFHT and VLT. The Di lines appear as faint, narrow emission features in the blue wings of the H i Balmer lines and can be distinguished from high-velocity Hi emission. The identification as deuterium and the excitation mechanism as fluorescence are both established beyond doubt. The deuterium Balmer series might lead to a new, optical method of deuterium abundance measurement in the interstellar medium. This may be the only way to observe atomic deuterium in objects like the Magellanic Clouds or low metallicity blue compact galaxies.

Embedded Clusters in the Low Metallicity Galaxy SBS 0335-052

We present new infrared observations of the low-metallicity blue compact galaxy SBS 0335-052. We find that the NIR emission is consistent with star formation occurring in clusters younger than 5 Myr. We detect optically hidden star formation at 4 μm obscured by more than 15 mag. of visual extinction. We quantify the possible contribution from an evolved stellar population and from warm dust. We finally discuss the possible origin of the dust observed.

Collisional excitation of hydrogen and the determination of the primordial helium abundance from H II regions

This paper investigates the effect of collisional enhancement of the hydrogen lines on the derivation of the helium abundances in low metallicity H ii regions. For this, we have constructed a grid of photoionization models relevant for the analysis of giant H ii regions in blue compact galaxies. We show that the effect of collisional excitation on the Hα/Hβ ratio can be quite important (up to 8% or more). The impact of this effect on the determination of the helium mass fraction has been tracked on four low-metallicity blue compact galaxies for which Keck spectra are available and which are among the best objects for the quest for the pregalactic helium abundance. We find that taking into account the effects of collisional excitation of hydrogen results in an upward correction of the helium mass fraction Y by up to 5% . However, combined with other systematic effects usually not considered in the determination of the helium abundance in low-metallicity galaxies, the resulting uncertainty should be much less.

H [CSC]ii[/CSC] Regions and Abundances in the “Dark Galaxy” DDO 154 and the Chemical Evolution of Dwarf Irregular Galaxies

We present Hα imaging and optical spectrophotometry of H II regions in low surface brightness dwarf irregular DDO 154. The possesses a very small population of faint discrete H II regions and larger diffuse H II regions and ionized shells. We confirm very low star formation rate and extremely long gas consumption times reported previously by van Zee, Haynes, & Salzer. The current star formation rate is ~2–4 times lower than its average past rate, confirming previous characterization of DDO 154 as a quiescent dwarf irregular galaxy. Spectrophotometry of two of brightest H II regions yields a relatively low oxygen abundance of 0.055 ± 0.008 (O/H)⊙, in agreement with previous determination by van Zee et al., and in accordance with previously determined metallicity-luminosity relationship for dwarf irregular galaxies. We also find an N/O ratio of 0.037 ± 0.003, which is marginally higher than typical value of 0.025 found in low-metallicity blue compact galaxies. Although DDO 154 has been labeled the dark galaxy and is a prototype for low surface brightness galaxies with large H I content, its chemical abundances are consistent with an average, low-mass, dwarf irregular galaxy. Assuming that neutral gas is chemically homogeneous, we derive an effective oxygen yield of roughly 50% of solar value, a value that is close to theoretically favored values for true oxygen yield. Thus, it is possible that DDO 154 is evolving nearly as a closed system. On other hand, if abundances in extended H I disk are lower than in H II regions, derived value of effective yield has been artificially inflated, and DDO 154 may have experienced significant loss of metal-enriched gas.

SBSG 0335−052W: An Extremely Low Metallicity Dwarf Galaxy

We present Multiple Mirror Telescope (MMT) and Keck II telescope spectrophotometry and 3.5m Calar Alto telescope R, I photometry of the western component of the extremely low-metallicity blue compact galaxy SBS 0335-052. The components, separated by 22 kpc, appear to be members of a unique, physically connected system. It is shown that SBS 0335-052W consists of at least three stellar clusters and has the same redshift as SBS 0335-052. The oxygen abundance in its two brightest knots is extremely low, 12+log(O/H)= 7.22+/-0.03 and 7.13+/-0.08, respectively. These values are lower than in SBS 0335-052 and are nearly the same as those in I Zw 18. The (R-I) color profiles are very blue in both galaxies due to the combined effects of ionized gas and a young stellar population emission. We argue that SBS 0335-052W is likely to be a nearby, young dwarf galaxy.

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<12+log O/H<8.2), the abundance ratios Ne/O, Si/O, S/O, Ar/O, and Fe/O retain the same constant value they had at lower metallicities. By contrast, there is an increase of C/O and N/O along with their dispersions at a given O. We interpret this increase as due to the additional contribution of C and primary N production in intermediate-mass stars, on top of that by high-mass stars. The above results lead to the following timeline for galaxy evolution: (1) all objects with 12+logO/H≤7.6 began to form stars less than 40 Myr ago; (2) after 40 Myr, all galaxies have evolved so that 12+logO/H>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<12+logO/H<8.2. The delayed release of primary N at these metallicities greatly increases the scatter in N/O; (4) later, when galaxies get enriched to 12+logO/H>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 Primordial Abundance of4He Revisited

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

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 Smits's He I emissivities and Kingdon & Ferland's 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 (SBS—the Second Byurakan Survey) are presented. The oxygen abundance in the central, brightest part of the galaxy is found to be 12 + log (O/H) = 7.33 ± 0.01, only slightly greater than the oxygen abundance in the most metal-deficient BCG, I Zw 18. We show that the N/O, Ne/O, S/O, and Ar/O abundance ratios in SBS 0335-052 are close to those derived for other BCGs, suggesting that heavy-element enrichment in the H II region is due to massive-star evolution. However, we find an O/Fe abundance ratio close to that in the Sun, at variance with values derived for other BCGs. The helium abundance derived from the He I 4471, 5876, and 6678 Å emission lines, taking into account collisional and fluorescent enhancement, is Y = 0.245 ± 0.006, close to the value of the primordial helium abundance, Yp = 0.243 ± 0.003, derived by Izotov, Thuan, & Lipovetsky. We detect auroral [O III] λ4363 emission in the inner part of the H II region with a diameter of 14'', or 3.6 kpc, and find that the H II region inside this diameter is hot, Te ~ 20,000 K. The oxygen abundance in this region is nearly constant [12 + log (O/H) = 7.1-7.3], with a gradual decrease to the outer part of H II region, implying effective mixing of ionized gas on short timescales. We study the distribution of the nebular He II λ4686 emission line and find that it is not produced by main-sequence O stars or Wolf-Rayet stars. Possible excitation mechanisms for this line, such as massive X-ray binaries and shocks, are discussed. We also discuss the origin of blue, underlying, extended low-intensity emission detected in SBS 0335-052 in V, R, and I images. The blue V - I and R - I color distributions suggest that a significant contribution to the extended low-intensity envelope is due to ionized gas emission. This is evidence that SBS 0335-052 is a young galaxy experiencing its very first burst of star formation. However, we find that the observed equivalent width of Hβ emission in the extended envelope is 2-3 times lower than the value expected in the case of pure gaseous emission. Furthermore, we find that the widths of Hγ and Hβ are narrower than the instrumental profiles; this could be explained by the presence of underlying stellar absorption from A stars. These findings suggest that, along with the young ( ~107 yr) blue stellar clusters in the center of the galaxy, an older stellar population of age ~108 yr may be present in the extended envelope of SBS 0335-052 having a total mass of ~107 M☉, 2 orders of magnitude smaller than the neutral gas mass but comparable with the total mass of stars in blue young stellar clusters observed in the center of the galaxy. We conclude that SBS 0335-052 is a young nearby dwarf galaxy with age ~108 yr.

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

view Abstract Citations (190) References (77) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Heavy Element Abundances in a New Sample of Low-Metallicity Blue Compact Galaxies Thuan, Trinh X. ; Izotov, Yuri I. ; Lipovetsky, Valentin A. Abstract We present high-quality spectrophotometric observations of 15 supergiant H II regions in 14 new low-metallicity blue compact galaxies (BCGs) selected mainly from the First and Second Byurakan Surveys and with oxygen abundance 12 + log O/H between 7.37 and 8.04 (Z_sun_5/35 <= Z <= Z_sun_/7). We use the data to determine abundances for the elements N, O, Ne, S, Ar, and, for the first time in BCGs, Fe; discuss their origin; and constrain current nucleosynthesis stellar models. The main result of the present study is that none of the heavy element-to-oxygen abundance ratios studied here (N/O, Ne/O, S/0, Ar/O, Fe/O) depend on oxygen abundance. We conclude that all these heavy elements have a primary origin and are produced by the same massive (M >=10 M_sun_) stars. The dispersion of the N/O ratio is found to be remarkably small, being only +/-0.08 in the log. This can only be understood if primary N is produced in massive stars, not in intermediate-mass (4 M_sun_ <= M <= 9 M_sun_) stars as commonly thought. BCGs show the same O/Fe overabundance with respect to the Sun (~0.34 in the log) as galactic halo stars, suggesting the same chemical enrichment history, and supporting the scenario of an early enrichment of the galactic halo by massive Population III stars. We have compared the observed heavy element abundance ratios with theoretical yields from current massive star nucleosynthesis models from Weaver & Woosley (1993). The small dispersion in the heavy element abundance ratios suggests that there is not a large IMF variation between BCGs with different metallicities. Publication: The Astrophysical Journal Pub Date: May 1995 DOI: 10.1086/175676 Bibcode: 1995ApJ...445..108T Keywords: Abundance; Compact Galaxies; H Ii Regions; Heavy Elements; Line Spectra; Massive Stars; Nuclear Fusion; Sky Surveys (Astronomy); Argon; Flux Density; Neon; Nitrogen; Oxygen; Spectrum Analysis; Stellar Spectrophotometry; Sulfur; Astronomy; GALAXIES: ABUNDANCES; GALAXIES: COMPACT; ISM: H II REGIONS; NUCLEAR REACTIONS; NUCLEOSYNTHESIS; ABUNDANCES full text sources ADS | data products SIMBAD (22) NED (14)

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

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.