Properties of the most metal-poor gas-rich LSB dwarf galaxies SDSS J0015+0104 and J2354-0005 residing in the Eridanus void

SDSS J0015+0104 is the lowest metallicity low surface brightness dwarf (LSBD) galaxy known. The oxygen abundance in its HII region SDSS J001520.70+010436.9 (at ~1.5 kpc from the galaxy centre) is 12+log(O/H)=7.07 (Guseva et al.). This galaxy, at the distance of 28.4 Mpc, appears to reside deeply in the volume devoid of luminous massive galaxies, known as the Eridanus void. SDSS J235437.29-000501.6 is another Eridanus void LSBD galaxy, with parameter 12+log(O/H)=7.36 (also Guseva et al.). We present the results of their HI observations with the Nancay Radio Telescope revealing their high ratios of M(HI)/L_B ~2.3. Based on the Sloan Digital Sky Survey images, we derived for both galaxies their radial surface brightness profiles and the main photometric parameters. Their colours and total magnitudes are used to estimate the galaxy stellar mass and ages. The related gas mass-fractions, f_g ~0.98 and ~0.97, and the extremely low metallicities (much lower than for their more typical counterparts with the same luminosity) indicate their unevolved status. We compare these Eridanus void LSBDs with several extreme LSBD galaxies residing in the nearby Lynx-Cancer void. Based on the combination of all their unusual properties, the two discussed LSBD galaxies are similar to the unusual LSBDs residing in the closer void. This finding presents additional evidence for the existence in voids of a4 figures, sizable fraction of low-mass unevolved galaxies. Their dedicated search might result in the substantial increase of the number of such objects in the local Universe and in the advancement of understanding their nature.Comment: 8 pages, 4 figures, 2 tables, accepted to MNRAS 02.04.2013. arXiv admin note: text overlap with arXiv:0909.134

Discovery of a massive variable star with Z=Zo/36 in the galaxy DDO 68

The Local Volume dwarf galaxy DDO 68, from the spectroscopy of its two brightest HII regions (Knots 1 and 2) was designated as the second most metal-poor star-forming galaxy [12+log(O/H)=7.14]. In the repeated spectral observations in 2008 January with the 6-m telescope (BTA) of the HII region Knot 3 [having 12+log(O/H)=7.10+-0.06], we find a strong evidence of a transient event related to a massive star evolution. From the follow-up observation with the higher spectral resolution in 2008 February, we confirm this phenomenon, and give parameters of its emission-line spectrum comprising of Balmer HI and HeI lines. The luminosities of the strongest transient lines (Ha, Hb) are of a few 10^36 erg s^-1. We also detected an additional continuum component in the new spectrum of Knot 3, which displays the spectral energy distribution raising to ultraviolet. The estimate of the flux of this continuum leads us to its absolute V-band magnitude of ~-7.1. Based on the spectral properties of this transient component, we suggest that it is related to an evolved massive star of luminous blue variable type with Z=Zo/36. We briefly discuss observational constraints on parameters of this unique (in the aspect of the record low metallicity of the progenitor massive star) event and propose several lines of its study.Comment: 6 pages, 5 Postscript figures, to appear in MNRAS Letters in June 2008 issu