Natalia G. Guseva

We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of five star-forming galaxies at redshifts z in the range 0.2993-0.4317 and with high emission-line flux ratios O32=[OIII]5007/[OII]3727 ~ 8-27 aiming to detect the Lyman continuum (LyC) emission. We detect LyC emission in all galaxies with the escape fractions fesc(LyC) in a range of 2-72 per cent. A narrow Ly-alpha emission line with two peaks in four galaxies and with three peaks in one object is seen in medium-resolution COS spectra with a velocity separation between the peaks Vsep varying from ~153 km/s to ~345 km/s. We find a general increase of the LyC escape fraction with increasing O32 and decreasing stellar mass M*, but with a large scatter of fesc(LyC). A tight anti-correlation is found between fesc(LyC) and Vsep making Vsep a good parameter for the indirect determination of the LyC escape fraction. We argue that one possible source driving the escape of ionizing radiation is stellar winds and radiation from hot massive stars.

We report the detection of the Lyman continuum (LyC) radiation of the compact star-forming galaxy (SFG) J1154+2443 observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope. This galaxy, at a redshift of z = 0.3690, is characterized by a high emission-line flux ratio O₃₂ = [O iii]λ5007/[O ii]λ3727 = 11.5. The escape fraction of the LyC radiation f_esc(LyC) in this galaxy is 46 per cent, the highest value found so far in low-redshift SFGs and one of the highest values found in galaxies at any redshift. The narrow double-peaked Lyα emission line is detected in the spectrum of J1154+2443 with a separation between the peaks V_sep of 199 km s⁻¹, one of the lowest known for Lyα-emitting galaxies, implying a high f_esc(Lyα). Comparing the extinction-corrected Lyα/Hβ flux ratio with the case B value we find f_esc(Lyα) = 98 per cent. Our observations, combined with previous detections in the literature, reveal an increase of O₃₂ with increasing f_esc(LyC). We also find a tight anticorrelation between f_esc(LyC) and V_sep. The surface brightness profile derived from the COS acquisition image reveals a bright star-forming region in the centre and an exponential disc in the outskirts with a disc scale length α = 1.09 kpc. J1154+2443, compared to other known low-redshift LyC leakers, is characterized by the lowest metallicity, 12+logO/H = 7.65±0.01, the lowest stellar mass M_⋆ = 10^8.20 M_⊙, a similar star formation rate SFR = 18.9 M_⊙ yr⁻¹ and a high specific SFR of 1.2× 10⁻⁷ yr⁻¹.

We report the detection of the Lyman continuum (LyC) radiation of the compact star-forming galaxy (SFG) J1154+2443 observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope. This galaxy, at a redshift of z=0.3690, is characterized by a high emission-line flux ratio O32=[OIII]5007/[OII]3727=11.5. The escape fraction of the LyC radiation fesc(LyC) in this galaxy is 46 per cent, the highest value found so far in low-redshift SFGs and one of the highest values found in galaxies at any redshift. The narrow double-peaked Lya emission line is detected in the spectrum of J1154+2443 with a separation between the peaks Vsep of 199 km/s, one of the lowest known for Lya-emitting galaxies, implying a high fesc(Lya). Comparing the extinction-corrected Lya/Hb flux ratio with the case B value we find fesc(Lya) = 98 per cent. Our observations, combined with previous detections in the literature, reveal an increase of O32 with increasing fesc(LyC). We also find a tight anticorrelation between fesc(LyC) and Vsep. The surface brightness profile derived from the COS acquisition image reveals a bright star-forming region in the centre and an exponential disc in the outskirts with a disc scale length alpha=1.09 kpc. J1154+2443, compared to other known low-redshift LyC leakers, is characterized by the lowest metallicity, 12+logO/H=7.65+/-0.01, the lowest stellar mass M*=10^8.20 Msun, a similar star formation rate SFR=18.9 Msun/yr and a high specific SFR of 1.2x10^-7 yr^-1.

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 M_g = -15.41 mag. It was selected by inspecting the spectroscopic data base in the Data Release 13 (DR13) of the SDSS. LBT/MODS spectroscopic observations reveal its oxygen abundance to be 12 + log O/H = 6.98 +/- 0.02, the lowest ever observed for a 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* = 10^6.24 - 10^6.29 Msun (statistical uncertainties only), and we find that a considerable fraction of the galaxy stellar mass was formed during the most recent burst of star formation.

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.

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 = [O iii]λ5007/[O ii]λ3727, 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(Hβ) of the Hβ emission line in the studied galaxies are very high, ∼ 350 – 520Å, 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 M_⋆ ∼ 10⁶ – 10⁷ M_⊙, much lower than the M_⋆ 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 Hα 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 Lyα 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 He i λ3889/λ6678 and λ7065/λ6678 emission-line flux ratios. Using these diagnostics we find that three CSFGs in our sample are likely to have density-bounded H ii 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.

We present a spectroscopic study of metal-deficient dwarf galaxy candidates, selected from the SDSS DR12. The oxygen abundances were derived using the direct method in galaxies with the electron temperature-sensitive emission line [OIII]4363A measured with an accuracy better than 30%. The oxygen abundances for the remaining galaxies with larger uncertainties of the [OIII]4363A line fluxes were calculated using a strong-line semi-empirical method by Izotov and Thuan. The resulting sample consists of 287 low-metallicity candidates with oxygen abundances below 12+logO/H=7.65 including 23 extremely metal-deficient (XMD) candidates with 12+log O/H<7.35. Ten out of sixteen XMDs known so far (or ~60%) have been discovered by our team using the direct method. Three XMDs were found in the present study. We study relations between global parameters of low-metallicity galaxies, including absolute optical magnitudes, Hbeta luminosities (or equivalently star formation rates), stellar masses, mid-infrared colours, and oxygen abundances. Low-metallicity and XMD galaxies strongly deviate to lower metallicities in L-Z, L(Hbeta)-Z and Mstar-Z diagrams than in relations obtained for large samples of low-redshift, star-forming galaxies with non-restricted metallicities. These less chemically evolved galaxies with stellar masses ~10^6-10^8Msun, Hbeta luminosities ~10^38-10^41 erg/s, SFR~0.01-1.0Msun/yr, and sSFR~50 Gyr^-1 have physical conditions which may be characteristic of high-redshift low-mass star-forming galaxies which are still awaiting discovery.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

We present 65 Sloan Digital Sky Survey (SDSS) spectra of 62 star-forming galaxies with oxygen abundances 12 + logO/H ~ 7.5-8.4. Redshifts of selected galaxies are in the range z~0.36-0.70. This allows us to detect the redshifted MgII 2797,2803 emission lines. Our aim is to use these lines for the magnesium abundance determination. The MgII emission was detected in ~2/3 of the galaxies. We find that the MgII 2797 emission-line intensity follows a trend with the excitation parameter x= O^{2+}/O that is similar to that predicted by CLOUDY photoionised HII region models, suggesting a nebular origin of MgII emission. The Mg/O abundance ratio is lower by a factor ~2 than the solar ratio. This is probably the combined effect of interstellar MgII absorption and depletion of Mg onto dust. However, the effect of dust depletion in selected galaxies, if present, is small, by a factor of ~2 lower than that of iron.