Paper detail

Observations of dwarfs in nearby voids: implications for galaxy formation and evolution

The intermediate results of the ongoing study of deep samples of ~200 galaxies residing in nearby voids, are presented. Their properties are probed via optical spectroscopy, ugri surface photometry, and HI 21-cm line measurements, with emphasis on their evolutionary status. We derive directly the hydrogen mass M(HI), the ratio M(HI)/L_B and the evolutionary parameter gas-phase O/H. Their luminosities and integrated colours are used to derive stellar mass M(*) and the second evolutionary parameter -- gas mass-fraction f_g. The colours of the outer parts, typically representative of the galaxy oldest stellar population, are used to estimate the upper limits on time since the beginning of the main SF episode. We compare properties of void galaxies with those of the similar late-type galaxies in denser environments. Most of void galaxies show smaller O/H for their luminosity, in average by ~30%, indicating slower evolution. Besides, the fraction of ~10% of the whole void sample or ~30% of the least luminous void LSB dwarfs show the oxygen deficiency by a factor of 2--5. The majority of this group appear very gas-rich, with f_g ~(95-99)%, while their outer parts appear rather blue, indicating the time of onset of the main star-formation episode of less than 1-4 Gyr. Such unevolved LSBD galaxies appear not rare among the smallest void objects, but turned out practically missed to date due to the strong observational selection effects. Our results evidense for unusual evolutionary properties of the sizable fraction of void galaxies, and thus, pose the task of better modelling of dwarf galaxy formation and evolution in voids.