Study of galaxies in the Lynx-Cancer void - III. New extreme low surface brightness dwarf galaxies

We present the results of a complex study of the low surface brightness dwarf (LSBD) gas-rich galaxies J0723+3621, J0737+4724 and J0852+1350, which reside in the nearby LynxCancer void. Their ratios M(H i)/LB, according to H i data obtained with the Nancay Radio Telescope (NRT), are respectively similar to 3.9, similar to 2 and similar to 2.6. For the two latter galaxies, we derived an oxygen abundance corresponding to the value of 12+log (O/H) 7.3, using spectra from the Russian 6-m telescope (BTA) and the Sloan Digital Sky Survey (SDSS) data base. We found two additional blue LSBDs, J0723+3622 and J0852+1351, which appear to be physical companions of J0723+3621 and J0852+1350 situated at projected distances of similar to 1213 kpc. The companion relative velocities, derived from the BTA spectra, are ?V=+89 km s-1 and +30 km s-1 respectively. The geometry and relative orientation of orbits and spins in these pairs indicate, respectively, prograde and polar encounters for J0723+3621 and J0852+1350. The NRT H i profiles of J0723+3621 and J0723+3622 indicate a sizeable gas flow in this system. The SDSS u, g, r, i images of the five dwarfs are used to derive photometric parameters and exponential or Sersic disc model fits. For three of them, the (u-g), (g-r), (r-i) colours of the outer parts, when compared with PEGASE evolutionary tracks, provide evidence for the dominance of old stellar populations with ages of T similar to (810) +/- 3 Gyr. For J0723+3622 and J0737+4724 the outer region colours appear rather blue, implying ages of the oldest visible stars of Gyr. The new LSB galaxies complement the list of known most metal-poor and unevolved dwarfs in this void, including DDO 68, SDSS J0812+4836, SDSS J0926+3343 and SAO 0822+3545. This unique concentration of unevolved dwarf galaxies in a small cell of the nearby Universe implies a physical relationship between slow galaxy evolution and a void-type global environment. We also compare the baryonic content of these LSBDs with predictions from the most updated cosmological simulations.