Roman I. Uklein

Distances to dwarf galaxies of the Canes Venatici I cloud

We determined the spatial structure of the scattered concentration of galaxies in the Canes Venatici constellation. We redefined the distances for 30 galaxies of this region using the deep images from the Hubble Space Telescope archive with the WFPC2 and ACS cameras.We carried out a high-precision stellar photometry of the resolved stars in these galaxies, and determined the photometric distances by the tip of the red giant branch (TRGB) using an advanced technique and modern calibrations. High accuracy of the results allows us to distinguish the zone of chaotic motions around the center of the system. A group of galaxies around M94 is characterized by the median velocity VLG = 287 km/s, distance D = 4.28 Mpc, internal velocity dispersion σ = 51 km/s and total luminosity LB = 1.61 × 1010L⊗. The projection mass of the system amounts to Mp = 2.56 × 1012M⊗, which corresponds to the mass-luminosity ratio of (M/L)p = 159 (M/L)⊗. The estimate of the mass-luminosity ratio is significantly higher than the typical ratio M/LB ∼ 30 for the nearby groups of galaxies. The CVn I cloud of galaxies contains 4–5 times less luminous matter compared with the well-known nearby groups, like the Local Group, M81 and CentaurusA. The central galaxy M94 is at least 1m fainter than any other central galaxy of these groups. However, the concentration of galaxies in the Canes Venatici may have a comparable total mass.

A list of groups of dwarf galaxies in the local supercluster

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.

Interacting galaxy NGC4656 and its unusual dwarf companion

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 non-circular 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 metal-poor gas onto the discs of both galaxies.

The study of two barred galaxies with curious kinematical features

We performed long-slit spectral observations of two SB-type galaxies: NGC 5347, UGC 1344. They were previously suspected as the galaxies with unusually low mass-to-light ratios (on the ground of mass estimates from the Hi linewidths), which are in conflict with their observed colours. The observations were conducted at the Russian 6-m telescope. The aim of the study was to clarify the kinematics and structure, as well as the properties of stellar populations of the galaxies. The results of observations disproved the peculiarly low mass-to-light ratios of both galaxies. The most probable reasons of underestimation of their masses are discussed. We tried to reproduce the main observed features of kinematical profiles of the galaxies in the N-body simulations of barred galaxies. We found that both galaxies possess central components of different structure. Indeed, the age and velocity dispersion of stellar population in NGC 5347 are low in its innermost part in comparison to that of the bulge or a bar, which agrees with the presence of nuclear kinematically decoupled disc. It probably was formed due to the bar which supplied the inner region with gas. The kinematical profiles of the second galaxy UGC 1344 give evidences in favour of the central peanut-shaped bulge. In spite of the different luminosities of the two galaxies, they possess nearly equal (close to solar) central stellar abundance and the flattening of the stellar metallicity gradient in the bar regions. However, in the less luminous NGC 5347 the mean stellar age is younger than that in UGC 1344.

SCORPIO-2 guiding and calibration system in the prime focus of the 6-m telescope

We describe a device (adapter) for off-axis guiding and photometric calibration of wide-angle spectrographs operating in the prime focus of the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. To compensate coma in off-axis star images an achromatic lens corrector is used, which ensures maintaining image quality (FWHM) at a level of about 1″ within 15′ from the optical axis. The device has two 54″-diameter movable guiding fields, which can move in 10′ × 4.′5 rectangular areas. The device can perform automatic search for guiding stars, use them to control the variations of atmospheric transmittance, and focus the telescope during exposure. The limiting magnitude of potential guiding stars is mR ~ 17m. The calibration path whose optical arrangement meets the telecentrism condition allows the spectrograph to be illuminated both by a source of line spectrum (a He–Ne–Ar filled lamp) and by a source of continuum spectrum. The latter is usually represented either by a halogen lamp or a set of light-emitting diodes, which provide illumination of approximately uniform intensity over the wavelength interval from 350 to 900 nm. The adapter is used for observations with SCORPIO-2 multimode focal reducer.

A Malin 1 ‘cousin’ with counter-rotation: internal dynamics and stellar content of the giant low surface brightness galaxy UGC 1922

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.

Rendez-Vous of Dwarfs

We present observations of multiple system of dwarf galaxies at the Russian 6-m telescope and the GMRT (Giant Metrewave Radio Telescope). The optical observations are a part of the programme Study of Groups of Dwarf Galaxies in the Local Supercluster. The group of galaxies under consideration looks like filament of 5 dwarfs. Two faint galaxies show peculiar structure. Long slit spectrum reveals inner motions about 150 km s−1 in one of them. It suggests that the galaxy is on stage of ongoing interaction. Probably, we see the group in moment of its formation.