Starburst in HS 0822+3542 induced by the very blue LSB dwarf SAO 0822+3545

One of the most metal-deficient blue compact galaxies (BCGs) HS 0822+3542 (Z = 1/34 Z ⊙), is also one of the nearest such objects (D ∼ 11 Mpc). It is in addition well isolated from known bright galaxies. A trigger mechanism for its current star-formation (SF) burst has thus remained unclear. We report the discovery of a very blue ((B - V) tot 0 = 0.08 and (V - R) tot 0 = 0.14) low surface brightness (LSB) (μ B 0 ≳ 23 m.4 arcsec -2) dwarf irregular (dIrr) galaxy, which we have named SAO 0822+3545. Its small relative velocity and projected distance of only ∼11 kpc from the BCG imply their physical association. For this LSB dIrr galaxy, we present spectroscopic results, total B, V, R magnitudes, and the effective radii and surface brightness (SB), and we describe its morphological properties. We compare the very blue colours of this dwarf with PEGASE.2 models of the colour evolution of a Z = 1/20 Z ⊙ stellar population, and combine this analysis with the data on the LSBD EW(Hα) values. The models best describing all available observational data depend on the relative fraction of massive stars in the IMF used. For a Salpeter IMF with M up = 120 M ⊙, the best model includes a "young" single stellar population (SSP) with an age of ∼10 Myr and an "old" SSP with the age of ∼0.2-10 Gyr. The mass ratio of the old to young components should be in the range of 10 to 30. If the age of the old component is more than ∼1 Gyr, an additional coeval component of "intermediate" age (∼100 Myr) with a mass comparable to that of the "young" population, although not required, provided a good fit to the current data. For the two options of a model IMF biased toward the low-mass end, the best match of the observed BVR and EW(Hα) is for continuous star-formation rate (SFR) single-component models, with SF durations in the range of ∼0.1 to ∼1 Gyr. However, only a longer time-scale SF gives the stellar mass, compatible with the LSB galaxy mass estimates. Nevertheless, such a scenario would be inconsistent with the recent encounter of these two dwarfs. The role of interaction between the LSBD and BCG HS 0822+3542 in triggering their major SF episodes during the last ∼ 100-200 Myr is emphasized and discussed. For the BCG, based on the results of new spectroscopy with the Russian 6 m telescope, we estimate the physical parameters of its SF region and present the first evidence of an ionized gas supershell. This pair of dwarfs lies deep within the nearby Lynx-Cancer void, with the nearest bright (L > L *) galaxies at distances >3 Mpc. This is probably one of the main factors responsible for the unevolved state of HS 0822+3542.