The counter-rotation phenomenon in disc galaxies directly indicates a complex galaxy assembly history which is crucial for our understanding of galaxy physics. Here we present the complex data analysis for a lenticular galaxy NGC 448, which has been recently suspected to host a counter-rotating stellar component. We collected deep long-slit spectroscopic observations using the Russian 6-m telescope and performed the photometric decomposition of Sloan Digital Sky Survey (SDSS) archival images. We exploited (i) a non-parametric approach in order to recover stellar line-of-sight velocity distributions and (ii) a parametric spectral decomposition technique in order to disentangle stellar population properties of both main and counter-rotating stellar discs. Our spectral decomposition stays in perfect agreement with the photometric analysis. The counter-rotating component contributes ≈30 per cent to the total galaxy light. We estimated its stellar mass to be 9.0 +2.7 −1.8 · 10 9 M . The radial scale length of counter-rotating disc is ≈3 times smaller than that of the main disc. Both discs harbour old stars but the counter-rotating components reveals a detectable negative age gradient that might suggest an extended inside-out formation during 3 . . . 4 Gyrs. The counter-rotating disc hosts more metal-rich stars and possesses a shallower metallicity gradient with respect to the main disc. Our findings rule out cosmological filaments as a source of external accretion which is considered as a potential mechanism of the counter-rotating component formation in NGC 448, and favour the satellite merger event with the consequent slow gas accretion.

We present a photometric and spectroscopic study of the unique isolated nearby dwarf spheroidal (dSph) galaxy KKR 25. The galaxy was resolved into stars with Hubble Space Telescope/Wide Field Planetary Camera 2 including old red giant branch and red clump. We have constructed a model of the resolved stellar populations and measured the star formation rate and metallicity as a function of time. The main star formation activity period occurred about 12.6–13.7 Gyr ago. These stars are mostly metal poor, with a mean metallicity [Fe/H] ∼ −1 to −1.6 dex. About 60 per cent of the total stellar mass was formed during this event. There are indications of intermediate‐age star formation in KKR 25 between 1 and 4 Gyr with no significant signs of metal enrichment for these stars. Long‐slit spectroscopy was carried out using the Russian 6‐m telescope of the integrated starlight and bright individual objects in the galaxy. We have discovered a planetary nebula (PN) in KKR 25. This is the first known PN in a dSph galaxy outside the Local Group. We have measured its oxygen abundance 12 + log (O/H) = 7.60 ± 0.07 dex and a radial velocity Vh = −79 km s−1. We have analysed the stellar density distribution in the galaxy body. The galaxy has an exponential surface brightness profile with a central light depression. We discuss the evolutionary status of KKR 25, which belongs to a rare class of very isolated dwarf galaxies with spheroidal morphology.

The formation scenario for giant low surface brightness (gLSB) galaxies with discs as large as 100 kpc still remains unclear. These stellar systems are rare and very hard to observe, therefore a detailed insight on every additional object helps to understand their nature. Here we present a detailed observational study of the gLSB UGC 1922 performed using deep optical imaging and spectroscopic observations combined with archival ultraviolet data. We derived spatially resolved properties of stellar and ionized gas kinematics and characteristics of stellar populations and interstellar medium. We reveal the presence of a kinematically decoupled central component, which counter rotates with respect to the main disc of UGC 1922. The radial metallicity gradient of the ionised gas is in agreement with that found for moderate-size LSB galaxies. At the same time, a slowly rotating and dynamically hot central region of the galaxy hosts a large number of old metal-rich stars, which creates an appearance of a giant elliptical galaxy, that grew an enormous star forming disc. We reproduce most of the observed features of UGC 1922 in N-body/hydrodynamical simulations of an in-plane merger of giant Sa and Sd galaxies. We also discuss alternative formation scenarios of this unusual system.

We report a list of groups consisting of dwarf galaxies only. The sample contains 126 objects, mainly combined in pairs. The most populated group contains six dwarf galaxies. The majority of systems considered reside in the low-density regions and evolve unaffected by massive galaxies. The characteristic sizes and velocity dispersions of groups are 30 kpc and 11 km/s, respectively. They resemble the associations of dwarf galaxies, but are more compact. On the whole, groups and associations form a continuous sequence. Alike the associations, our groups possess high mass-to-luminosity ratios, what is indicative of a large amount of dark matter present in these systems.

We studied the nearby edge-on galaxy NGC4656 and its dwarf low surface brightness companion with the enhanced UV brightness, NGC4656UV, belonging to the interacting system NGC4631/56. Regular photometric structure and relatively big size of NGC4656UV allows to consider this dwarf galaxy as a separate group member rather than a tidal dwarf. Spectral long-slit observations were used to obtain the kinematical parameters and gas-phase metallicity of NGC4656UV and NGC4656. Our rough estimate of the total dynamical mass of NGC4656UV allowed us to conclude that this galaxy is the dark-matter dominated LSB dwarf or ultra diffuse galaxy. Young stellar population of NGC4656UV, as well as strong local noncircular gas motions in NGC4656 and the low oxygen gas abundance in the region of this galaxy adjacent to its dwarf companion, give evidence in favour of the accretion of metalpoor gas onto the discs of both galaxies.