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Register a new userJ2229+2725: an extremely low-metallicity dwarf compact star-forming galaxy with an exceptionally high [OIII]5007/[OII]3727 flux ratio of 53
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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 (SFG) in the local Universe, J2229+2725. This galaxy with a redshift z=0.0762 was selected from the Data Release 16 (DR16) 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 a SFG. With its very low metallicity, an absolute magnitude Mg=-16.39 mag, a low stellar mass Mstar=9.1x10^6 Msun and a very low mass-to-light ratio Mstar/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([OIII]5007)/I([OII]3727)~53, 2) an equivalent width of the Hbeta emission line EW(Hbeta) of 577A, 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<7.3.
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We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of eight compact star-forming galaxies at redshifts z=0.02811-0.06540, with low oxygen abundances 12+log(O/H)=7.43-7.82 and extremely high emission-line flux ratios O32=[OIII]5007/[OII]3727~22-39, aiming to study the properties of Ly-alpha emission in such conditions. We find a diversity in Ly-alpha properties. In five galaxies Ly-alpha emission line is strong, with equivalent width (EW) in the range 45-190A. In the remaining galaxies, weak Ly-alpha emission with EW(Ly-alpha)~2-7A is superposed on a broad Ly-alpha absorption line, indicating a high neutral hydrogen column density N(HI)~(1-3)x10^21 cm^-2. We examine the relation between the Ly-alpha escape fraction fesc(Ly-alpha) and the Lyman continuum escape fraction fesc(LyC), using direct measures of the latter in eleven low-redshift LyC leakers, to verify whether fesc(Ly-alpha) can be an indirect measure of escaping LyC radiation. The usefulness of O32, of the Ly-alpha equivalent width EW(Ly-alpha) and of the Ly-alpha peak separation Vsep as indirect indicators of Ly-alpha leakage is also discussed. It is shown that there is no correlation between O32 and fesc(Ly-alpha). We find an increase of fesc(Ly-alpha) with increasing EW(Ly-alpha) for EW(Ly-alpha)<100A, but for higher EW(Ly-alpha)>150A the fesc(Ly-alpha) is nearly constant attaining the value of ~0.25. We find an anticorrelation between fesc(Ly-alpha) and Vsep, though not as tight as the one found earlier between fesc(LyC) and Vsep. This finding makes Vsep a promising indirect indicator of both the Ly-alpha and ionizing radiation 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.
We have obtained optical spectroscopy of one of the most metal-poor dwarf star-forming galaxies (SFG) in the local Universe, J1234+3901, with the Large Binocular Telescope (LBT)/Multi-Object Dual Spectrograph (MODS). This blue compact dwarf (BCD) galaxy with a redshift z=0.133 was selected from the Data Release 14 (DR14) of the Sloan Digital Sky Survey (SDSS). Its properties are extreme in many ways. Its oxygen abundance 12 + log O/H = 7.035+/-0.026 is among the lowest ever observed for a SFG. Its absolute magnitude Mg = -17.35 mag makes it the brightest galaxy among the known BCDs with 12 + log O/H < 7.3. With its low metallicity, low stellar mass M* = 10^7.13 Msun and very low mass-to-light ratio M*/Lg ~ 0.01 (in solar units), it deviates strongly from the mass-metallicity and luminosity-metallicity relations defined by the bulk of the SFGs in SDSS DR14. J1234+3901 has a very high specific star-formation rate sSFR ~ 100 Gyr^-1, indicating very active ongoing star-formation. Its spectrum shows a strong HeII 4686 emission line, with a flux ~ 2.4 per cent that of the Hbeta emission line. The most probable source of ionizing radiation for producing such a strong line is fast radiative shocks. J1234+3901 has a ratio O32 = [OIII]5007/[OII]3727 ~ 15, the highest among the lowest-metallicity SFGs, and is thus likely leaking Lyman continuum radiation. It is a good candidate for being a young dwarf galaxy, with a large fraction of its stars formed recently. As such, it is probably one of the best local counterparts of dwarf primeval galaxies responsible for the reionization of the early Universe.
(abridged) We present new Spitzer, UKIRT and MMT observations of the blue compact dwarf galaxy (BCD) Mrk 996, with an oxygen abundance of 12+log(O/H)=8.0. This galaxy has the peculiarity of possessing an extraordinarily dense nuclear star-forming region, with a central density of ~10^6 cm^{-3}. The nuclear region of Mrk 996 is characterized by several unusual properties: a very red color J-K = 1.8, broad and narrow emission-line components, and ionizing radiation as hard as 54.9 eV, as implied by the presence of the OIV 25.89 micron line. The nucleus is located within an exponential disk with colors consistent with a single stellar population of age >1 Gyr. The infrared morphology of Mrk 996 changes with wavelength. The IRS spectrum shows strong narrow Polycyclic Aromatic Hydrocarbon (PAH) emission, with narrow line widths and equivalent widths that are high for the metallicity of Mrk 996. Gaseous nebular fine-structure lines are also seen. A CLOUDY model requires that they originate in two distinct HII regions: a very dense HII region of radius ~580 pc with densities declining from ~10^6 at the center to a few hundreds cm^{-3} at the outer radius, where most of the optical lines arise; and a HII region with a density of ~300 cm^{-3} that is hidden in the optical but seen in the MIR. We suggest that the infrared lines arise mainly in the optically obscured HII region while they are strongly suppressed by collisional deexcitation in the optically visible one. The hard ionizing radiation needed to account for the OIV 25.89 micron line is most likely due to fast radiative shocks propagating in an interstellar medium. A hidden population of Wolf-Rayet stars of type WNE-w or a hidden AGN as sources of hard ionizing radiation are less likely possibilities.
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.
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