Alexander S. Gusev

Oxygen and nitrogen abundances of H ii regions in six spiral galaxies

Spectroscopic observations of 63 Hii regions in six spiral galaxies (NGC 628, NGC 783, NGC 2336, NGC 6217, NGC 7331, and NGC 7678) were carried out with the 6-meter telescope (BTA) of Russian Special Astrophysical Observatory with the Spectral Camera attached to the focal reducer SCORPIO in the multislit mode with a dispersion of 2.1A/pixel and a spectral resolution of 10A. These observations were used to estimate the oxygen and nitrogen abundances and the electron temperatures in Hii regions through the recent variant of the strong line method (NS calibration). The parameters of the radial distribution (the extrapolated central intercept value and the gradient) of the oxyge n and nitrogen abundances in the disks of spiral galaxies NGC 628, NGC 783, NGC 2336, NGC 7331, and NGC 7678 have been determined. The abundances in the NGC 783, NGC 2336, NGC6217, and NGC 7678 are measured for the first time. Galaxies from our sample foll ow well the general trend in the luminosity ‐ central metallicity diagram for spiral and irr egular galaxies.

Parameters of the brightest star formation regions in the two principal spiral arms of NGC 628

We study photometric properties, chemical abundances and sizes of star formation regions in the two principal arms of the galaxy NGC 628 (M74). The GALEXultraviolet, optical U BVRI, and Hsurface photometry data are used, including those obtained with the 1.5-m telescope of the Maidanak Observatory. The thirty brightest star formation regions in ultraviolet light lo- cated in the spiral arms of NGC 628 are identified and studied. We find that the star formation regions in one (longer) arm are systematically brighter and larger than the regions in the other (shorter) arm. However, both luminosity and size distribution functions have approximately the same slopes for the samples of star formation regions in both arms. The star formation regions in the longer arm have a higher star formation rate density than the regions in the shorter arm. The regions in the shorter arm show higher N/O ratio at a higher oxygen abun- dance, but they have lower ultraviolet and Hluminosities. These findings can be explained if we assume that star formation regions in the shorter arm had higher star formation rate in the past, but now it is lower than for those in the opposite arm. Results of stellar evolutionary synthesis show that the brightest regions in the longer arm are slightly younger than the ones in the shorter arm (3.5± 2.2 Myr versus 6.0± 1.1 Myr). Our results demonstrate that there is a difference in the inner structures and parameters of the interst ellar medium between the spiral arms of NGC 628, one of which is long and hosts a regular chain of bright star formation complexes and the other, shorter one does not.

Hierarchy and size distribution function of star formation regions in the spiral galaxy NGC 628

Hierarchical structures and size distribution of star form ation regions in the nearby spiral galaxy NGC 628 are studied over a range of scale from 50 to 1000 pc using optical images obtained with 1.5 m telescope of the Maidanak Observatory. We found hierarchically structured concentrations of star formation regions in the galax y, smaller regions with a higher surface brightness are located inside larger complexes having a lower surface brightness. We illustrate this hierarchy by dendrogram, or structure tree of the detected star formation regions, which demonstrates that most of these regions are combined into larger structures over several levels. We found three characteristic sizes of young star gr oups:≈ 65 pc (OB associations), ≈ 240 pc (stellar aggregates) and≈ 600 pc (star complexes). The cumulative size distribution function of star formation regions is found to be a power law with a slope of approximately −1.5 on scales appropriate to diameters of associations, aggregates and complexes. This slope is close to the slope which was found earlier by B. Elmegreen et al. for star formation regions in the galaxy on scales from 2 to 100 pc.

Spectroscopy of H II regions in the late-type spiral galaxy NGC 6946

We present the results of spectroscopy of 39 H II regions in the spiral galaxy NGC 6946. The spectral observations were carried out at the 6-m BTA telescope of the SAO RAS with the SCORPIO focal reducer in the multi-slit mode with the dispersion of 2.1 Å/px and spectral resolution of 10 Å. The absorption estimates for 39 H II regions were obtained. Using the “strong line” method (NS-calibration) we determined the electron temperature, and the abundances of oxygen and nitrogen for 30 H II regions. The radial gradients of O/H and N/H were constructed.

Star-forming regions in the ring galaxies NGC 5585 and IC 1525

We use UBVRI CCD photometry to study star-forming regions (SFRs) in the galaxies NGC 5585 and IC 1525. The observations were acquired with the 1.5-m telescope of the Mt. Maidanak Observatory of the Astronomical Institute of the Uzbek Academy of Sciences (Uzbekistan), with seeing of 0.8″–1.8″. We identified 47 SFRs in NGC 5585 and 16 SFRs in IC 1525. We estimated the ages and internal extinctions of the SFRs using the PEGASE2 evolution models. The sizes of the SFRs were also determined. We discuss in detail the techniques applied to evaluate the SFR parameters from photometric analysis. The age range for the studied SFRs is (2–40) × 106 yrs, and the internal extinctions are A(V ) ≤ 1.5m. The age distributions of the SFRs in both galaxies are typical of stellar systems with intense, extended star formation. The internal extinction in the SFRs decreases with distance from the galactic centers: A(V ) ∝ −r. For both galaxies, the scale length for the decrease of the dust surface density, estimated from the A(V )−r relation for SFRs, is close to the scale length for the disk brightness decrease in the V and R bands. Relatively larger and older SFRs are observed in the galaxies’ rings, while such SFRs are not found in the spiral arms. We detected different SFR parameters for different spiral arms of NGC 5585.

Nuclear activity of the Seyfert galaxy NGC 7469 in 1990–2006: Observations from the Mt. Maidanak Observatory

We present the results of UBV RI observations of the nucleus of the Seyfert 1 galaxy NGC 7469 performed in 1990–2006 at the Mt. Maidanak Observatory of the Ulug Beg Astronomical Institute in Uzbekistan. Light curves for various apertures are plotted for all the filters. Our analysis of these light curves indicates the presence of short-time-scale (from several days to several weeks) and long-term (∼8–9 years) variability of the nucleus. The character of the brightness variations in all the optical filters is the same, with the relative amplitude of the variations decreasing from U to V, but being higher in R than in V. A comparison with earlier observations (prior to 1990) indicates an increase of the time scale for the long-term variability, from three to six years to eight to nine years, providing evidence for changing emission conditions in the accretion disk. We present the results of a statistical analysis of the light curves and an analysis of the color characteristics during different activity periods. Color-index measurements using various apertures demonstrate that the color becomes bluer towards the galaxy nucleus.

A spectral and photometric study of 102 star-forming regions in seven spiral galaxies

We present a study of complexes of young massive star clusters (YMCs), embedded in extragalactic giant HII regions, based on the coupling of spectroscopic with photometric and spectrophotometric observations of about 100 star forming regions in seven spiral galaxies (NGC 628, NGC 783, NGC 2336, NGC 6217, NGC 6946, NGC 7331, and NGC 7678). The complete observational database has been observed and accumulated within the framework of our comprehensive study of extragalactic star forming regions. The current paper presents the last part of either unpublished or refreshed photometric and spectrophotometric observations of the galaxies NGC 6217, NGC 6946, NGC 7331, and NGC 7678. We derive extinctions, chemical abundances, continuum and line emissions of ionised gas, ages and masses for cluster complexes. We find the young massive cluster complexes to have ages no greater than 10 Myr and masses between 10^4Msol and 10^7Msol, and the extinctions A(V) vary between ~ 0 and 3 mag, while the impact of the nebular emission on integrated broadband photometry mainly is not greater than 40% of the total flux and is comparable with accuracies of dereddened photometric quantities.We also find evidence of differential extinction of stellar and gas emissions in some clusters, which hinders the photometric determination of ages and masses in these cases. Finally, we show that young massive cluster complexes in the studied galaxies and open clusters in the Milky Way form a continuous sequence of luminosities/masses and colour/ages.

Studies of the Stellar Populations of Galaxies Using Two-Color Diagrams

Variations in the photometric parameters of stellar systems as a function of their evolution and the stellar populations comprising them are investigated. A set of seven evolutionary models with an exponential decrease in the star-formation rate and 672 models with a secondary burst of star formation are considered. The occurrence of a secondary burst of star formation can shift the position of a stellar system on two-color diagrams to the right or left of the normal color sequence for galaxies and the absorption line. This makes it possible to estimate the composition of the stellar population of a galaxy with a nonmonotonic star-formation history from its position on two-color diagrams. Surface photometry in both the optical (UBV RI) and near-IR (JHK) is used to study the stellar populations and star-formation histories in the structural components (nucleus, bulge, disk, spiral arms, bar, ring) of 26 galaxies of various morphological types (from S0 to Sd). Components (nucleus, bulge, bar) with color characteristics corresponding to stellar systems with secondary bursts of star formation are indicated in 10 of the 26 galaxies. The parameters of these secondary bursts are estimated. Five of the 10 galaxies with complex star-formation histories display clear structural perturbations. Appreciable differences in the photometric characteristics of relatively red early-type galaxies (S0—Sb) and relatively blue later-type galaxies (Sb—Sd) have been found. Galaxies of both early and late types are encountered among the Sb galaxies. Lenticular galaxies do not display different photometric characteristics from early-type spiral galaxies.

Star formation rates and the kinematics of gas in the spiral arms of NGC 628

The relations between star formation rates along the spiral arms and the velocities of gas inflow into the arms in the grand-design galaxy NGC 628 were studied. We found that the radial distribution of the average star formation rate in individual star formation regions in regular spiral arms correlates with the velocity of gas inflow into the spiral arms. Both distributions have maxima at a galactocentric distance of 4.5–5 kpc. There are no correlations between the radial distributions of the average star formation rate in star formation regions in spiral arms and outside spiral arms in the main disc. We also did not find a correlation between the radial distribution of the average star formation rate in star formation regions in spiral arms and the H I column density. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

UBV RI CCD photometry of the nearby low-surface-brightness galaxy NGC 5585

The structure and composition of the stellar population in the low-surface-brightness galaxy NGC 5585 is studied using UBV RI CCD photometry. The observations were obtained on the 1.5-m telescope of the Maidanak Observatory of the Astronomical Institute of the Academy of Sciences of Uzbekistan under conditions with seeing 1.2″–1.8″. A two-dimensional decomposition of the galaxy emission into bulge and disk components is carried out. Both components have low surface brightnessess. The Sersic parameter for the bulge is n = 1.2–1.6. The effective radius of the bulge in R and I is equal to the scale length for the brightness decrease in the disk, and comprises 30″–40″ (0.8–1.1 kpc). The spiral arms seem to form a bar, but the centers of the bar and ring do not coincide with the center of NGC 5585. A powerful star-forming region is observed 3.2″ (100 pc) from the galactic center, whose radiation swamps the nucleus in the U and B filters. Based on the positions of the various components of the galaxy in two-color diagrams, it is concluded that NGC 5585 has a complex star-forming history that may be different at different distances from the center.