V. L. Afanasiev

Kinematics and stellar populations of the dwarf elliptical galaxy IC 3653

We present the first 3D observations of a diffuse elliptical galaxy (dE). The good quality data (S/N up to 40) reveal the kinematical signature of an embedded stellar disc, reminiscent of what is commonly observed in elliptical galaxies, though similarity of their origins is questionable. Colour map built from Hubble Space Telescope Advanced Camera for Surveys (ACS) images confirms the presence of this disc. Its characteristic scale (about 3 arcsec =250 pc) is about a half of galaxys effective radius, and its metallicity is 0.1-0.2 dex larger than the underlying population. Fitting the spectra with synthetic single stellar populations (SSP), we found an SSP-equivalent age of 5 Gyr and nearly solar metallicity [Fe/H] = -0.06 dex. We checked that these determinations are consistent with those based on Lick indices, but have smaller error bars. The kinematical discovery of a stellar disc in dE gives additional support to an evolutionary link from dwarf irregular galaxies due to stripping of the gas against the intracluster medium.

The extraordinarily bright optical afterglow of GRB 991208 and its host galaxy

Broad-band optical observations of the extraordi- narily bright optical afterglow of the intense gamma-ray burst GRB 991208 started � 2.1 days after the event and continued until 4 Apr 2000. The flux decay constant of the optical after- glow in the R-band is 2.30 ± 0.07 up to � 5 days, which

Spectroscopy of optical counterparts of ultraluminous X-ray sources

Here we present the results of panoramic and long-slit observations of eight ULX nebular counterparts performed with the 6m SAO telescope. In two ULX nebulae (ULXNe) we detected for the first time signatures of high excitation ([O III]λ5007 / Hβ > 5). Two of the ULXs were identified with young (T ∼ 5–10 Myr) massive star clusters. Four of the eight ULXNe show bright high-excitation lines. This requires existence of luminous (∼ 1038 ÷ 1040 erg s−1) UV/EUV sources coinciding with the X-ray sources. The other 4 ULXNe require shock excitation of the gas with shock velocities of 20–100 km s−1. However, all the studied ULXNe spectra show signatures of shock excitation, but even those ULXNe where the shocks are prevailing show presence of a hard ionizing source with a luminosity of at least ∼ 1038 erg s−1. Most likely shock waves, X-ray and EUV ionization act simultaneously in all the ULXNe, but they may be roughly separated in two groups: shock-dominated and photoionization-dominated ULXNe. The ULXs have to produce strong winds and/or jets (∼ 1039 erg s−1) for powering their nebulae. Both the wind/jet activity and the existence of a bright UV source are consistent with the suggestion that ULXs are high-mass X-ray binaries with supercritical accretion disks of the SS433 type.

Integral field spectroscopy of the ultraluminous X-ray source Holmberg II X-1

We present optical integral field observations of the H II region containing the ultraluminous X-ray source Holmberg II X-1. We confirm the existence of an X-ray ionized nebula as the counterpart of the source owing to the detec- tion of an extended He II λ4686 region (21 × 47 pc) at the Chandra ACIS-S position. An extended blue object with a size of 11 × 14 pc is coincident with the X-ray/He II λ4686 region, which could indicate that it is either a young stellar complex or a cluster. We have derived an X-ray to optical luminosity ratio of LX/LB ≥ 170, and presumable it is LX/LB ∼ 300−400 using the recent HST ACS data. We find a complex velocity dispersion at the position of the ULX. In addition, there is a radial velocity variation in the X-ray ionized region found in the He II emission of ±50 km s −1 on spatial scales of 2-3 �� . We believe that the pu- tative black hole not only ionizes the surrounding HII gas, but also perturbs it dynamically (via jets or the accretion disk wind). The spatial analysis of the public Chandra ACIS-S data reveals a point-like X-ray source and gives marginal indication of an extended component (� 15% of the total flux). The XMM-Newton EPIC-PN spectrum of HoII X-1 is best fitted with an absorbed power law in addition to either a thermal thick plasma or a thermal thin plasma or a multi-colour disk black body (MCD). In all cases, the thermal component shows a relatively low temperature (kT ∼ 0.14−0.22 keV). Finally we discuss the optical/X-ray properties of HoII X-1 with regards to the possible nature of the source. The existence of an X-ray ionized nebula coincident with the ULX and the soft X-ray component with a cool accretion disk favours the interpretation as an intermediate-mass black hole (IMBH). However, the complex velocity behaviour at the position of the ULX indicates a dynamical influence of the black hole on the local HII gas.

Inner Polar Rings in Regular Lenticular Galaxies

We have investigated a sample of S0 galaxies, mostly with circumnuclear dust lanes orthogonal to their major axes, chosen from Hubble Space Telescope Wide Field Planetary Camera 2 images. Two-dimensional spectroscopy undertaken with the Multipupil Fiber Spectrograph of the 6 m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences has revealed that indeed the ionized gas in the centers of these eight lenticular galaxies rotate in the planes nearly orthogonal to the rotation (and symmetry) planes of their central stellar components. Although almost all the galaxies are located in dense environments, an external origin of this rotation plane tilt is not obvious because all the galaxies but one are known to have extended H I disks, and in two cases where the angular resolution of H I observations allows, we find orthogonality of the external H I and inner ionized gas disks. We discuss a possible relation of the inner gas polar rings to a triaxiality of galactic potential. The stellar populations in the nuclei of all but two galaxies are very old, which excludes recent star formation bursts and proves that the polar orbits of the circumnuclear gas are rather stable. In the nuclei of NGC 2655 and NGC 4111, we have found signatures of star formation bursts some 1.5–2 Gyr ago. This finding can be related to very central gas in NGC 2655, which is coplanar to the circumnuclear stellar disk and to radial gas inflow in NGC 4111; just these gas reservoirs and not the polar rings may be responsible for fueling nuclear star formation.

The Leo I Cloud: Secular Nuclear Evolution of NGC 3379, NGC 3384, and NGC 3368?

The central regions of the three brightest members of the Leo I galaxy group—NGC 3368, NGC 3379, and NGC 3384—are investigated by means of two-dimensional spectroscopy. In all three galaxies we have found separate circumnuclear stellar and gaseous subsystems—more probably, disks—whose spatial orientations and spins are connected to the spatial orientation of the supergiant intergalactic H I ring reported previously by Schneider et al. and Schneider. In NGC 3368 the global gaseous disk seems also to be inclined to the symmetry plane of the stellar body, being probably of external origin. Although the rather young mean stellar age and spatial orientations of the circumnuclear disks in NGC 3379, NGC 3384, and NGC 3368 could imply their recent formation from material of the intergalactic H I cloud, the timescale of these secondary formation events, on the order of 3 Gyr, does not support the collision scenario of Rood & Williams but is rather in line with the ideas of Schneider regarding tidal interactions of the galaxies with the H I cloud on timescales of the intergroup orbital motions.

Large scale nested stellar discs in NGC 7217

NGC 7217 is an unbarred early-type spiral galaxy having a multisegment exponential light profile and a system of star-forming rings of the unknown origin; it also possesses a circumnuclear gaseous polar disc. We analysed new long-slit spectroscopic data for NGC 7217 and derived the radial distributions of its stellar population parameters, and stellar and gaseous kinematics up to the radius of r ≈ 100 arcsec (~8 kpc). We performed the dynamical analysis of the galaxy by recovering its velocity ellipsoid at different radii and estimated the scale-heights of its two exponential discs. The inner exponential stellar disc of NGC 7217 appears to be thin and harbours intermediate-age stars (t SSP ≈ 5 Gyr). The outer stellar disc seen between the radii of 4 and 7 kpc is very thick (z 0 = 1,..., 3 kpc), metal-poor, [Fe/H] < -0.4 dex, and has predominantly young stars, t SSP = 2 Gyr. The remnants of minor mergers of gas-rich satellites with an early-type giant disc galaxy available in the GalMer data base well resemble different structural components of NGC 7217, suggesting two minor merger events in the past responsible for the formation of the inner polar gaseous disc and large outer star-forming ring. Another possibility to form the outer ring is the re-accretion of the tidal streams created by the first minor merger.

The orientation parameters and rotation curves of 15 spiral galaxies

We analyzed ionized gas motion and disk orientation parameters for 15 spiral galaxies. Their velocity fields were measured with the Ha emission line by using the Fabry-Perot interferometer at the 6 m telescope of SAO RAS. Special attention is paid to the problem of estimating the position angle of the major axis (PA 0 ) and the inclination (i) of a disk, which strongly affect the derived circular rotation velocity. We discuss and compare different methods of obtaining these parameters from kinematic and photometric observations, taking into account the presence of regular velocity (brightness) perturbations caused by spiral density waves. It is shown that the commonly used method of tilted rings may lead to systematic errors in the estimation of orientation parameters (and hence of circular velocity) being applied to galaxies with an ordered spiral structure. Instead we recommend using an assumption of constancy of i and PA 0 along a radius, to estimate these parameters. For each galaxy of our sample we present monochromatic He- and continuum maps, velocity fields of ionized gas, and the mean rotation curves in the frame of a model of pure circular gas motion. Significant deviations from circular motion with amplitudes of several tens of km s -1 (or higher) are found in almost all galaxies. The character and possible nature of the non-circular motion are briefly discussed.

Technique of polarimetric observations of faint objects at the 6-m BTA telescope

We describe the technique of spectropolarimetric observations allowing for the measurements of the Stokes parameters in one of the observational modes of the SCORPIO focal reducer of the 6-m BTA telescope of the SAO RAS. The characteristics of the instrument in the spectropolarimetric mode of observations are given. We present the algorithm of observational data reduction. The capabilities of the SCORPIO spectropolarimetric mode are demonstrated on the examples of observations of various astronomical objects.

Young Stellar Nuclei in Lenticular Galaxies: NGC 5574 and NGC 7457*

The stellar population properties in the centers of NGC 5574 and NGC 7457 (two lenticular galaxies previously shown to have radially homogeneous blue optical-band colors) are studied by means of integral-field spectroscopy. The compact nuclei of the galaxies appear to be chemically distinct: in NGC 5574 the nucleus is distinguished by a higher than solar iron-to-magnesium ratio, and in NGC 7457 a drop of mean stellar metallicity by a factor of 2 exists between the nucleus and the bulge. Both galaxies demonstrate a rather young mean luminosity-weighted age of the stellar populations in the bulges, not older than 5-7 Gyr; however, the chemically distinct nuclei are still younger, 2-2.5 Gyr old. The kinematics of the stars in the center of NGC 5574 are probably affected by the influence of its global bar. The compact core of NGC 7457, with a radius of about 15, shows a visible counterrotation; a combined analysis of the photometric and kinematical two-dimensional maps allows us to conclude that the core rotates axisymmetrically, but its rotation plane is inclined to the main symmetry plane of the galaxy. The stellar velocity dispersion in the centers of both galaxies is anomalously low: 60-80 km s-1.