Abstract. The work studies of the environment of low-mass galaxies with active star formation (SF) and a possible trigger of SF bursts due to gravitational interaction. Following the study by Taylor et al. (1995), we extend the search for possible disturbing galaxies of various masses to a much larger sample of 86 BCGs from the sky region of the Second Byurakan survey (SBS). The BCG magnitudes and radial velocities are revised and up-dated. The sample under study is separated by the criteria: EW ([Oiii] λ5007 ) > 45Å and V h < 6 000 km sand should be representative of all low-mass galaxies which experience SF bursts. We argue that the moderate tidal disturbers should be taken into account, and incorporate the respective range of distances in the search for disturbing neighbours. The majority of the neighbours in the vicinity of the studied BCGs are found through the study of their environment among UZC (Falco et al. 1999) galaxies, and the follow-up careful search of the fainter galaxies in the NED database. For the remaining BCGs, the neighbouring galaxies are found based on the results of the SAO 6 m telescope spectroscopy. By studing the data on the radial velocities of galaxies in the vicinity of BCGs we found: 1) 33 of the studied BCGs (∼38.5%) are associated with significantly brighter galaxies (∆B ≥ 1.5 m ); 2) 23 BCGs (∼26.5%) have neighbours either of comparable or significantly lower brightness; 3) 14 of the studied BCGs (16%) with no evident associated galaxy are either certain, or probable, mergers. Summarizing, we conclude that in ∼80% (or more) BCGs from the studied sample, the SF bursts are triggered either by tidal action of various strengths from other galaxies, or due to mergers of low-mass galaxies. We briefly discuss the implications of our main conclusion for evolutionary links of BCGs to other types of low-mass galaxies. Part of our sample falls into the volume belonging to the Local Supercluster. Therefore we formulate the results separately on the "Local Supercluster volume" and "general field region". The total fractions of BCGs likely triggered by interaction with other galaxy are respectively, ∼84.5% and 80% for the nearby volume and for the general field. The fractions of BCGs with significantly brighter disturbers in these two groups are seemingly different (∼54±14% vs. ∼31.5±7%, respectively). Among the so called "isolated" BCGs (that is, without a bright neighbouring galaxy) in both the Local Supercluster volume and in general field, ∼43± 10% are probably disturbed by dwarf galaxies and ∼26± 8% have a merger morphology. In the Appendix we present the results of the spectroscopy with the SAO 6 m telescope of 27 galaxies in an attempt to find possible disturbing galaxies in the vicinity of some of the sample BCGs.

Abstract. We present the results of high S/N long-slit spectroscopy with the Multiple Mirror and the SAO 6-m telescopes, optical imaging with the Wise 1-m telescope and H  observations with the Nançay Radio Telescope of the very metal-deficient (12 + log (O/H) = 7.64) luminous (M B = −18. m 1) blue compact galaxy (BCG) HS 0837+4717. The blue bump near λ4650, characteristic of WR stars is detected in the central supergiant H  region, as well as the barely seen red bump at λ5808. The derived number of WR stars in the region of the current starburst is ∼1000. Evidence for fast motions in this region is also seen as broad, low-contrast components in the Hα, Hβ and strong [O ] lines λλ4959,5007. While the extinction of the narrow emission lines from the supergiant H  region is low, the very large Balmer decrement of the broad components suggests that the part of current starburst is highly obscured by dust. Abundance ratios X/O for X = Ne, Ar, S, Fe and Cl in the supergiant H  region are in good agreement with the mean values of other very metal-deficient BCGs. Nitrogen, however, is overabundant by a factor of ∼6. This implies an unusually efficient N enrichment in HS 0837+4717, and probably, a non-typical evolution scenario. The Hα-line position-velocity (P−V) diagrams for directions approximately along the major and minor axes reveal disturbed motions of the ionized gas, mainly in peripheral regions. The SW part of the major axis P-V diagram looks like a rotation curve, with the velocity amplitude V rot ∼ 50−70 km s −1 at r = 4.3 kpc. Its NE part displays, however, strong deviations, indicating either counter-rotation, or a strong outflow/supershell. If it is considered as indicating a shell-like feature its velocity amplitude of ∼70 km s −1 (relative to the extrapolated rotation curve), and the apparent extent of ∼4 (3.3 kpc) imply a dynamical age of ∼14 Myr and the full energetic equivalent of ∼2.3 × 10 4 SNe. The latter indicates continuing starbursts during at least this time interval. The long-slit spectra reveal a complex morphology for this galaxy. It consists of two compact regions at a distance of ∼2 kpc. Their continuum flux differs by a factor of four. The brightest one is related to the current starburst with the age of ∼3.7 Myr. The slightly redder fainter component could be an older starburst (∼25 Myr). The Wise 1-m telescope UBVR integrated photometry reveals a high optical luminosity for this BCG, and the unusual (B − V) and (V − R) colours. The morphology of HS 0837+4717 is highly disturbed, with two small tails emerging to NNW and SSE. Such a disturbed overall morphology, a "double-nucleus" structure, significantly disturbed velocities of ionized gas, together with the very high power of the starburst suggests a possible explanation of the object as a recent merger. We compare the properties of this BCG and of similar objects known in the literature, and conclude that their high nitrogen excess is most probably related to the short phase of a powerful starburst when many WR stars contrib...

Abstract.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) 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.