Primordial 4He abundance: a determination based on the largest sample of HII regions with a methodology tested on model HII regions

We verified the validity of the empirical method to derive the 4He abundance used in our previous papers by applying it to CLOUDY (v13.01) models. Using newly published HeI emissivities, for which we present convenient fits as well as the output CLOUDY case B hydrogen and HeI line intensities, we found that the empirical method is able to reproduce the input CLOUDY 4He abundance with an accuracy of better than 1%. The CLOUDY output data also allowed us to derive the non-recombination contribution to the intensities of the strongest Balmer hydrogen Halpha, Hbeta, Hgamma, and Hdelta emission lines and the ionisation correction factors for He. With these improvements we used our updated empirical method to derive the 4He abundances and to test corrections for several systematic effects in a sample of 1610 spectra of low-metallicity extragalactic HII regions, the largest sample used so far. From this sample we extracted a subsample of 111 HII regions with Hbeta equivalent width EW(Hbeta) > 150A, with excitation parameter x = O^{2+}/O > 0.8, and with helium mass fraction Y derived with an accuracy better than 3%. With this subsample we derived the primordial 4He mass fraction Yp = 0.254+/-0.003 from linear regression Y-O/H. The derived value of Yp is higher at the 68% confidence level (CL) than that predicted by the standard big bang nucleosynthesis (SBBN) model, possibly implying the existence of different types of neutrino species in addition to the three known types of active neutrinos. Using the most recently derived primordial abundances D/H = (2.60+/-0.12)x10^{-5} and Yp = 0.254+/-0.003 and the chi^2 technique, we found that the best agreement between abundances of these light elements is achieved in a cosmological model with baryon mass density Omegab h^2 = 0.0234+/-0.0019 (68% CL) and an effective number of the neutrino species Neff = 3.51+/-0.35 (68% CL).Comment: 23 pages, 14 figures, accepted for publication in Astronomy and Astrophysic

Excitation properties of galaxies with the highest [OIII]/[OII] ratios: No evidence for massive escape of ionizing photons

The possibility that star-forming galaxies may leak ionizing photons is at the heart of many present-day studies that investigate the reionization of the Universe. We test this hypothesis on local blue compact dwarf galaxies of very high excitation. We assembled a sample of such galaxies by examining the spectra from Data Releases 7 and 10 of the Sloan Digital Sky Survey. We argue that reliable conclusions cannot be based on strong lines alone, and adopt a strategy that includes important weak lines such as [OI] and the high-excitation HeII and [ArIV] lines. Our analysis is based on purely observational diagrams and on a comparison of photoionization models with well-chosen emission-line ratio diagrams. We show that spectral energy distributions from current stellar population synthesis models cannot account for all the observational constraints, which led us to mimick several scenarios that could explain the data. These include the additional presence of hard X-rays or of shocks. We find that only ionization-bounded models (or models with an escape fraction of ionizing photons lower than 10%) are able to simultaneously explain all the observational constraints.Comment: accepted in Astronomy & Astrophysic

A new determination of the primordial He abundance using the HeI 10830A emission line: cosmological implications

We present near-infrared spectroscopic observations of the high-intensity HeI 10830 emission line in 45 low-metallicity HII regions. We combined these NIR data with spectroscopic data in the optical range to derive the primordial He abundance. The use of the HeI 10830A line, the intensity of which is very sensitive to the density of the HII region, greatly improves the determination of the physical conditions in the He^+ zone. This results in a considerably tighter Y - O/H linear regression compared to all previous studies. We extracted a final sample of 28 HII regions with Hbeta equivalent width EW(Hbeta)>150A, excitation parameter O^2+/O>0.8, and with helium mass fraction Y derived with an accuracy better than 3%. With this final sample we derived a primordial He mass fraction Yp = 0.2551+/-0.0022. The derived value of Yp is higher than the one predicted by the standard big bang nucleosynthesis (SBBN) model. Using our derived Yp together with D/H = (2.53+/-0.04)x10^-5, and the chi^2 technique, we found that the best agreement between these light element abundances is achieved in a cosmological model with a baryon mass density Omega_b h^2 = 0.0240+/-0.0017 (68% CL), +/-0.0028 (95.4% CL), +/-0.0034 (99% CL) and an effective number of neutrino species Neff = 3.58+/-0.25 (68% CL), +/-0.40 (95.4% CL), +/-0.50 (99% CL). A non-standard value of Neff is preferred at the 99% CL, implying the possible existence of additional types of neutrino species.Comment: 18 pages, 11 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society. arXiv admin note: text overlap with arXiv:1308.210

LBT observations of compact star-forming galaxies with extremely high [OIII]/[OII] flux ratios: HeI emission-line ratios as diagnostics of Lyman continuum leakage

We present Large Binocular Telescope spectrophotometric observations of five low-redshift (z<0.070) compact star-forming galaxies (CSFGs) with extremely high emission-line ratios O32 = [OIII]5007/[OII]3727, ranging from 23 to 43. Galaxies with such high O32 are thought to be promising candidates for leaking large amounts of Lyman continuum (LyC) radiation and, at high redshifts, for contributing to the reionization of the Universe. The equivalent widths EW(Hbeta) of the Hbeta emission line in the studied galaxies are very high, ~350-520A, indicating very young ages for the star formation bursts, <3 Myr. All galaxies are characterized by low oxygen abundances 12+logO/H = 7.46 - 7.79 and low masses Mstar~10^6-10^7 Msun, much lower than the Mstar for known low-redshift LyC leaking galaxies, but probably more typical of the hypothetical population of low-luminosity dwarf LyC leakers at high redshifts. A broad Halpha emission line is detected in the spectra of all CSFGs, possibly related to expansion motions of supernova remnants. Such rapid ionized gas motions would facilitate the escape of the resonant Ly$\alpha$ emission from the galaxy. We show that high O32 may not be a sufficient condition for LyC leakage and propose new diagnostics based on the HeI 3889/6678 and 7065/6678 emission-line flux ratios. Using these diagnostics we find that three CSFGs in our sample are likely to have density-bounded HII regions and are thus leaking large amounts of LyC radiation. The amount of leaking LyC radiation is probably much lower in the other two CSFGs.Comment: 16 pages, 8 figures, accepted for publication in Monthly Notices of the Royal Astronomical Societ

Nonaxisymmetric MHD instabilities of Chandrasekhar states in Taylor-Couette geometry

We consider axially periodic Taylor-Couette geometry with insulating boundary conditions. The imposed basic states are so-called Chandrasekhar states, where the azimuthal flow $U_\phi$ and magnetic field $B_\phi$ have the same radial profiles. Mainly three particular profiles are considered: the Rayleigh limit, quasi-Keplerian, and solid-body rotation. In each case we begin by computing linear instability curves and their dependence on the magnetic Prandtl number Pm. For the azimuthal wavenumber m=1 modes, the instability curves always scale with the Reynolds number and the Hartmann number. For sufficiently small Pm these modes therefore only become unstable for magnetic Mach numbers less than unity, and are thus not relevant for most astrophysical applications. However, modes with m>10 can behave very differently. For sufficiently flat profiles, they scale with the magnetic Reynolds number and the Lundquist number, thereby allowing instability also for the large magnetic Mach numbers of astrophysical objects. We further compute fully nonlinear, three-dimensional equilibration of these instabilities, and investigate how the energy is distributed among the azimuthal (m) and axial (k) wavenumbers. In comparison spectra become steeper for large m, reflecting the smoothing action of shear. On the other hand kinetic and magnetic energy spectra exhibit similar behavior: if several azimuthal modes are already linearly unstable they are relatively flat, but for the rigidly rotating case where m=1 is the only unstable mode they are so steep that neither Kolmogorov nor Iroshnikov-Kraichnan spectra fit the results. The total magnetic energy exceeds the kinetic energy only for large magnetic Reynolds numbers Rm>100.Comment: 12 pages, 14 figures, submitted to Ap

SBS 0335-052E+W: deep VLT/FORS+UVES spectroscopy of the pair of the lowest-metallicity blue compact dwarf galaxies

(abridged) We present deep archival VLT/FORS1+UVES spectroscopic observations of the system of two blue compact dwarf (BCD) galaxies SBS 0335-052E and SBS 0335-052W. Our aim is to derive element abundances in different HII regions of this unique system of galaxies and to study spatial abundance variations. We determine abundances of helium, nitrogen, oxygen, neon, sulfur, chlorine, argon and iron. The oxygen abundance in the brighter eastern galaxy varies in the range 7.11 to 7.32 in different HII regions supporting previous findings and suggesting the presence of oxygen abundance variations on spatial scales of ~1-2 kpc. The oxygen abundance in the brightest region No.1 of SBS 0335-052W is 7.22+/-0.07, consistent with previous determinations.Three other HII regions are much more metal-poor with an unprecedently low oxygen abundance of 12+logO/H=7.01+/-0.07 (region No.2), 6.98+/-0.06 (region No.3), and 6.86+/-0.14 (region No.4). These are the lowest oxygen abundances ever derived in emission-line galaxies. Helium abundances derived for the brightest HII regions of both galaxies are mutually consistent. We derive weighted mean He mass fractions of 0.2485+/-0.0012 and 0.2514+/-0.0012 for two different sets of HeI emissivities. The N/O abundance ratio in both galaxies is slightly higher than that derived for other BCDs with 12+logO/H<7.6. This implies that the N/O in extremely metal-deficient galaxies could increase with decreasing metallicity.Comment: 20 pages, 11 figures, accepted for pulication in Astronomy and Astrophysic

On the universality of luminosity-metallicity and mass-metallicity relations for compact star-forming galaxies at redshifts 0 < z < 3

We study relations between global characteristics of low-redshift (0 < z < 1) compact star-forming galaxies, including absolute optical magnitudes, Hbeta emission-line luminosities (or equivalently star-formation rates), stellar masses, and oxygen abundances. The sample consists of 5182 galaxies with high-excitation HII regions selected from the SDSS DR7 and SDSS/BOSS DR10 surveys adopting a criterion [OIII]4959/Hbeta > 1. These data were combined with the corresponding data for high-redshift (2 < z < 3) star-forming galaxies. We find that in all diagrams low-z and high-z star-forming galaxies are closely related indicating a very weak dependence of metallicity on stellar mass, redshift, and star-formation rate. This finding argues in favour of the universal character of the global relations for compact star-forming galaxies with high-excitation HII regions over redshifts 0 < z < 3.Comment: 13 pages, 10 figures, accepted for publication in MNRA

New candidates for extremely metal-poor emission-line galaxies in the SDSS/BOSS DR10

We present a spectroscopic study of eight extremely low-metallicity candidate emission-line galaxies with oxygen abundances possibly below 12 +log O/H = 7.35. These galaxies were selected from Data Release 10 of the Sloan Digital Sky Survey/Baryon Oscillation Spectroscopic Survey (SDSS/BOSS DR10). We will call these extremely metal-deficient galaxies XMD galaxies. The electron temperature-sensitive emission line [O~{\sc iii}] $\lambda$4363 is detected in three galaxies and marginally detected in two galaxies, allowing for abundance determination by a "direct" method. Because of large uncertainties in the [O {\sc iii}]$\lambda$4363\AA\ line fluxes, we also calculated oxygen abundance in these galaxies together with the remaining three galaxies using a strong-line semi-empirical method. This method gives oxygen abundances higher than 7.35 for three galaxies with detected [O {\sc iii}]$\lambda$4363\AA\ line and lower than 7.35 for the remaining five objects of the sample. The newly-discovered galaxies represent excellent targets for follow-up spectroscopic observations with the largest telescopes to improve the oxygen abundance determination and to increase the number of these very rare low-metallicity objects. The extreme location of the most massive and luminous XMD galaxies and XMD candidates in the stellar mass-metallicity diagram implies that these galaxies may be genuine young objects. With stellar masses of up to $\sim$ 10$^7$ - 10$^8$$M_{\odot}$, the galaxies are not chemically enriched and strongly deviate to lower metallicity as compared to the relation obtained for a large sample of low-redshift, star-forming galaxies.Comment: 10 pages, 5 figures, 4 tables, accepted in Astronomy and Astrophysic