One of the key questions in observational cosmology is the identification of the sources responsible for ionization of the Universe after the cosmic 'Dark Ages', when the baryonic matter was neutral. ...The currently identified distant galaxies are insufficient to fully reionize the Universe by redshift z ≈ 6 (refs 1-3), but low-mass, star-forming galaxies are thought to be responsible for the bulk of the ionizing radiation. As direct observations at high redshift are difficult for a variety of reasons, one solution is to identify local proxies of this galaxy population. Starburst galaxies at low redshifts, however, generally are opaque to Lyman continuum photons. Small escape fractions of about 1 to 3 per cent, insufficient to ionize much surrounding gas, have been detected only in three low-redshift galaxies. Here we report far-ultraviolet observations of the nearby low-mass star-forming galaxy J0925+1403. The galaxy is leaking ionizing radiation with an escape fraction of about 8 per cent. The total number of photons emitted during the starburst phase is sufficient to ionize intergalactic medium material that is about 40 times as massive as the stellar mass of the galaxy.
Following our first detection reported in Izotov et al., we present the detection of Lyman continuum (LyC) radiation of four other compact star-forming galaxies observed with the Cosmic Origins ...Spectrograph (COS) onboard the Hubble Space Telescope. These galaxies, at redshifts of z ∼ 0.3, are characterized by high emission-line flux ratios O iii λ5007/O ii λ3727 ≳ 5. The escape fractions of the LyC radiation f
esc(LyC) in these galaxies are in the range of ∼6–13 per cent, the highest values found so far in low-redshift star-forming galaxies. Narrow double-peaked Ly α emission lines are detected in the spectra of all four galaxies, compatible with predictions for LyC leakers. We find escape fractions of Ly α, f
esc(Ly α) ∼ 20–40 per cent, among the highest known for Ly α emitting galaxies. Surface brightness profiles produced from the COS acquisition images reveal bright star-forming regions in the centre and exponential discs in the outskirts with disc scalelengths α in the range ∼0.6–1.4 kpc. Our galaxies are characterized by low metallicity, ∼1/8–1/5 solar, low stellar mass ∼(0.2–4) × 109 M⊙, high star formation rates, SFR ∼ 14–36 M⊙ yr−1, and high SFR densities, Σ ∼ 2–35 M⊙ yr−1 kpc−2. These properties are comparable to those of high-redshift star-forming galaxies. Finally, our observations, combined with our first detection reported in Izotov et al., reveal that a selection for compact star-forming galaxies showing high O iii λ5007/O ii λ3727 ratios appears to pick up very efficiently sources with escaping LyC radiation: all five of our selected galaxies are LyC leakers.
Abstract
We report the discovery of the most metal-poor dwarf star-forming galaxy (SFG) known to date, J0811+4730. This galaxy, at a redshift z = 0.04444, has a Sloan Digital Sky Survey (SDSS) g-band ...absolute magnitude Mg = −15.41 mag. It was selected by inspecting the spectroscopic data base in the Data Release 13 (DR13) of the SDSS. Large Binocular Telescope/Multi-Object Double spectrograph (LBT/MODS) spectroscopic observations reveal its oxygen abundance to be 12 + log O/H = 6.98 ± 0.02, the lowest ever observed for an SFG. J0811+4730 strongly deviates from the main sequence defined by SFGs in the emission line diagnostic diagrams and the metallicity–luminosity diagram. These differences are caused mainly by the extremely low oxygen abundance in J0811+4730, which is ∼10 times lower than that in main-sequence SFGs with similar luminosities. By fitting the spectral energy distributions of the SDSS and LBT spectra, we derive a stellar mass of M⋆ = 106.24–106.29 M⊙, and we find that a considerable fraction of the galaxy stellar mass was formed during the most recent burst of star formation.
We present near-infrared (NIR) spectroscopic observations of the high-intensity He i λ10830 Å emission line in 45 low-metallicity H ii
regions. We combined these NIR data with spectroscopic data in ...the optical range to derive the primordial He abundance. The use of the He i λ10830 Å line, the intensity of which is very sensitive to the density of the H ii 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 H ii regions with Hβ equivalent width EW(Hβ) ≥ 150 Å, excitation parameter O2 +/O ≥ 0.8, and with helium mass fraction Y derived with an accuracy better than 3 per cent. With this final sample we derived a primordial 4He mass fraction Y
p = 0.2551 ± 0.0022. The derived value of Y
p is higher than the one predicted by the standard big bang nucleosynthesis model. Using our derived Y
p together with D/H = (2.53 ± 0.04) × 10−5, and the χ2 technique, we found that the best agreement between these light element abundances is achieved in a cosmological model with a baryon mass density Ωb
h
2 = 0.0240 ± 0.0017 (68 per cent confidence level, CL), ± 0.0028 (95.4 per cent CL), ± 0.0034 (99 per cent CL) and an effective number of neutrino species N
eff = 3.58 ± 0.25 (68 per cent CL), ± 0.40 (95.4 per cent CL), ± 0.50 (99 per cent CL). A non-standard value of N
eff is preferred at the 99 per cent CL, implying the possible existence of additional types of neutrino species.
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 ($Z_\odot$/30) to ~8.5 (0.7 $Z_\odot$). 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 < 7.6, i.e. Z < $Z_\odot$/12) are rare in the SDSS sample. The α element-to-oxygen abundance ratios do not show any significant trends with oxygen abundance, in agreement with previous studies, except for a slight increase of Ne/O with increasing metallicity, which we interpret as due to a moderate depletion of O onto grains in the most metal-rich galaxies. The Fe/O abundance ratio is smaller than the solar value, by up to 1 dex at the high metallicity end. We also find that Fe/O increases with decreasing Hβ equivalent width EW(Hβ). We interpret this as a sign of strong depletion onto dust grains, and gradual destruction of those grains on a time scale of a few Myr. All the galaxies are found to have log N/O > –1.6, implying that they have a different nature than the subsample of high-redshift damped Lyα systems with log N/O of ~–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.
We report the discovery of the high-ionization Ne v λ3426 Å emission line in the spectra of five blue compact dwarf (BCD) galaxies. Adding the three previously known BCDs with Ne v emission, the ...entire sample of such galaxies now contains eight objects. The detection of this line implies the presence of intense hard ionizing radiation. Such radiation cannot be reproduced by models of high-mass X-ray binaries or massive stellar populations. Other mechanisms, such as active galactic nucleus (AGN) and/or fast radiative shocks, are needed. We consider that fast radiative shocks are the most likely mechanism. The observed Ne v λ3426/He ii λ4686 flux ratios in all eight galaxies can be reproduced by radiative shock models with shock velocities in the ∼300-500 km s−1 range, and with the shock ionizing contribution being ∼10 per cent of the stellar ionizing contribution. However, we cannot rule out that this 10 per cent part is produced by an AGN rather than by radiative shocks.
Aims. We have recently reported the discovery of five low redshift Lyman continuum (LyC) emitters (LCEs) with absolute escape fractions fescLyC that range from 6 to 13%, higher than previously found ...and that more than double the number of low redshift LCEs. We use these observations to test theoretical predictions about a link between the characteristics of the Lyman-alpha (Lyα) line from galaxies and the escape of ionizing photons. Methods. We analyse the Lyα spectra of eight LCEs of the local Universe observed with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope (our five leakers and three galaxies from the litterature), and compare their strengths and shapes to the theoretical criteria and comparison samples of local galaxies: the Lyman Alpha Reference Survey, Lyman-break analogs, Green Peas, and the high-redshift strong LyC leaker Ion2. Results. Our LCEs are found to be strong Lyα emitters, with high equivalent widths, EW(Lyα) > 70 Å, and large Lyα escape fractions, fescLyα> 20%. The Lyα profiles are all double-peaked with a small peak separation, in agreement with our theoretical expectations. They also have no underlying absorption at the Lyα position. All these characteristics are very different from the Lyα properties of typical star-forming galaxies of the local Universe. A subset of the comparison samples (2−3 Green Pea galaxies) share these extreme values, indicating that they could also be leaking. We also find a strong correlation between the star formation rate surface density and the escape fraction of ionizing photons, indicating that the compactness of star-forming regions plays a role in shaping low column density paths in the interstellar medium of LCEs. Conclusions. The Lyα properties of LCEs are peculiar: Lyα can be used as a reliable tracer of LyC escape from galaxies, complementing other indirect diagnostics proposed in the literature.
ABSTRACT
Using the Large Binocular Telescope (LBT)/Multi-Object Dual Spectrograph (MODS), we have obtained optical spectroscopy of one of the most metal-poor dwarf star-forming galaxies (SFGs) in the ...local Universe, J2229+2725. This galaxy with a redshift z = 0.0762 was selected from the Data Release 16 of the Sloan Digital Sky Survey (SDSS). Its properties derived from the LBT observations are most extreme among SFGs in several ways. Its oxygen abundance 12 + logO/H = 7.085 ± 0.031 is among the lowest ever observed for an SFG. With its very low metallicity, an absolute magnitude Mg = −16.39 mag, a low stellar mass M⋆ = 9.1 × 106 M⊙, and a very low mass-to-light ratio M⋆/Lg ∼ 0.0166 (in solar units), J2229+2725 deviates strongly from the luminosity–metallicity relation defined by the bulk of the SFGs in the SDSS. J2229+2725 has a very high specific star formation rate sSFR ∼ 75 Gyr−1, indicating very active ongoing star formation. Three other features of J2229+2725 are most striking, being the most extreme among lowest metallicity SFGs: (1) a ratio O32 = I(O iiiλ5007)/I(O iiλ3727) ∼ 53, (2) an equivalent width of the H β emission line EW(H β) of 577 Å, and (3) an electron number density of ∼1000 cm−3. These properties imply that the starburst in J2229+2725 is very young. Using the extremely high O32 in J2229+2725, we have improved the strong-line calibration for the determination of oxygen abundances in the most metal-deficient galaxies, in the range 12 + logO/H $\lesssim$ 7.3.
ABSTRACT
We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of nine low-mass star-forming galaxies at redshifts, z, in the range 0.3179–0.4524, with ...stellar masses $M_\star \, \lt $ 108 M⊙ and very high specific star-formation rates sSFR ∼150–630 Gyr−1, aiming to study the dependence of leaking Lyman continuum (LyC) emission on stellar mass and some other characteristics of the galaxy. We detect LyC emission in four out of nine galaxies with escape fractions, fesc(LyC), in the range of 11–35 per cent, and establish upper limits for fesc(LyC) in the remaining five galaxies. We observe a narrow Ly α emission line with two peaks in seven galaxies and likely more complex Ly α profiles in the two remaining galaxies. The velocity separation between the peaks Vsep varies in the range from ∼229 to ∼512 km s−1. Our additional data on low-mass galaxies confirm and strengthen the tight anticorrelation between fesc(LyC) and Vsep found for previous low-redshift galaxy samples with higher stellar masses. Vsep remains the best indirect indicator of LyC leakage. It is better than O32 on which fesc(LyC) depends weakly, with a large scatter. Finally, contrary to expectations, we find no increase of fesc(LyC) with decreasing galaxy stellar mass M⋆.
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