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We have selected candidate massive stars in the galaxy IC 342 based on archival images from the Hubble Space Telescope and images from the 2 m telescope at the Natioal Astronomical Observatory Rozhen, Bulgaria. Spectral observations of 24 out of 27 selected stars are carried out with the 6 m BTA telescope at the SAO RAS and with the 3.5 m Apache Point Observatory telescope (USA) as part of the program for searching bright massive stars in galaxies outside the Local Group. Our analysis reveals that 12 objects have spectra lacking prominent features, except for the emission lines of the surrounding nebulae and are identified as single supergiants of classes O9 to F5 or spatially unresolved young compact clusters. One source with an absorption spectrum probably belongs to our Galaxy. The spectra of seven other objects show features typical of Wolf-Rayet stars or compact clusters containing Wolf-Rayet stars. Another source is a compact supernova remnant. Two other objects are tentatively classified as cold LBV candidates, and one object is classified as a B[e]-supergiant candidate.

We construct a multiparametric Tully-Fisher (TF) relation for a large sample of edge-on galaxies from the Revised Flat Galaxy Catalog using HI data from the EDD database and parameters from the EGIS catalog. We incorporate a variety of additional parameters including structural parameters of edge-on galaxies in different bandpasses. Besides the rotation curve maximum, only the HI-to-optical luminosity ratio and optical colours play a statistically significant role in the multiparametric TF relation. We are able to decrease the standard deviation of the multiparametric TF relation down to 0.32 mag, which is at the level of best modern samples of galaxies used for studies of the matter motion in the Universe via the TF-relation., Comment: 8 pages, accepted for publication in MNRAS

A cold high-velocity (HV, $\sim$ 200 km/s) peak was first reported in several Galactic bulge fields based on the APOGEE commissioning observations. Both the existence and the nature of the high-velocity peak are still under debate. Here we revisit this feature with the latest APOGEE DR13 data. We find that most of the low latitude bulge fields display a skewed Gaussian distribution with a HV shoulder. However, only 3 out of 53 fields show distinct high-velocity peaks around 200 km/s. The velocity distribution can be well described by Gauss-Hermite polynomials, except the three fields showing clear HV peaks. We find that the correlation between the skewness parameter ($h_{3}$) and the mean velocity ($\bar{v}$), instead of a distinctive HV peak, is a strong indicator of the bar. It was recently suggested that the HV peak is composed of preferentially young stars. We choose three fields showing clear HV peaks to test this hypothesis using the metallicity, [$\alpha$/M] and [C/N] as age proxies. We find that both young and old stars show HV features. The similarity between the chemical abundances of stars in the HV peaks and the main component indicates that they are not systematically different in terms of chemical abundance or age. In contrast, there are clear differences in chemical space between stars in the Sagittarius dwarf and the bulge stars. The strong HV peaks off-plane are still to be explained properly, and could be different in nature., Comment: 13 pages, 10 figures, published in ApJ. Updated to match the final ApJ published version. Minor revisions to the text and Figure 4

We present a detailed photometric study of a sample of 22 edge-on galaxies with clearly visible X-shaped structures. We propose a novel method to derive geometrical parameters of these features, along with the parameters of their host galaxies based on the multi-component photometric decomposition of galactic images. To include the X-shaped structure into our photometric model, we use the IMFIT package, in which we implement a new component describing the X-shaped structure. This method is applied for a sample of galaxies with available SDSS and Spitzer IRAC 3.6 $\mu$m observations. In order to explain our results, we perform realistic $N$-body simulations of a Milky Way-type galaxy and compare the observed and the model X-shaped structures. Our main conclusions are as follows: (1) galaxies with strong X-shaped structures reside in approximately the same local environments as field galaxies; (2) the characteristic size of the X-shaped structures is about 2/3 of the bar size; (3) there is a correlation between the X-shaped structure size and its observed flatness: the larger structures are more flattened; (4) our $N$-body simulations qualitatively confirm the observational results and support the bar-driven scenario for the X-shaped structure formation., Comment: 13 pages, 10 figures, 2 tables

We study the properties of galaxies with very thin discs using a sample of 85 objects whose stellar disc radial-to-vertical scale ratio determined from photometric decomposition, exceeds nine. We present evidences of similarities between the very thin disc galaxies (VTD galaxies) and low surface brightness (LSB) disc galaxies, and conclude that both small and giant LSB galaxies may reveal themselves as VTD, edge-on galaxies. Our VTD galaxies are mostly bulgeless, and those with large radial scale length tend to have redder colors. We performed spectral observations of 22 VTD galaxies with the Dual Imaging Spectrograph on the 3.5m telescope at the Apache Point Observatory. The spectra with good resolution (R ~ 5000) allow us to determine the distance and the ionized gas rotation curve maximum for the galaxies. Our VTD galaxies have low dust content, in contrast to regular disc galaxies. Apparently, VTD galaxies reside in specific cosmological low-density environments and tend to have less connection with filaments. Comparing a toy model that assumes marginally low star formation in galactic discs with obtained gas kinematics data, we conclude that there is a threshold central surface density of about 88 Mo/pc**2, which we observe in the case of very thin, rotationally supported galactic discs., Comment: 11 pages, 11 figures. Will be published in MNRAS

Based on SDSS data, we have selected a sample of nine edge-on spiral galaxies with bulges whose major axes show a high inclination to the disk plane. Such objects are called polar-bulge galaxies. They are similar in their morphology to polar-ring galaxies, but the central objects in them have small size and low luminosity. We have performed a photometric analysis of the galaxies in the g and r bands and determined the main characteristics of their bulges and disks. We show that the disks of such galaxies are typical for the disks of spiral galaxies of late morphological types. The integrated characteristics of their bulges are similar to the parameters of normal bulges. The stellar disks of polar-bulge galaxies often show large-scale warps, which can be explained by their interaction with neighboring galaxies or external accretion from outside., Comment: 8 pages, 3 figures

We analyze the dependence of the stellar disc flatness on the galaxy morphological type using 2D decomposition of galaxies from the reliable subsample of the Edge-on Galaxies in SDSS (EGIS) catalogue. Combining these data with the retrieved models of the edge-on galaxies from the Two Micron All Sky Survey (2MASS) and the Spitzer Survey of Stellar Structure in Galaxies (S$^4$G) catalogue, we make the following conclusions: (1) The disc relative thickness $z_0/h$ in the near- and mid-infrared passbands correlates weakly with morphological type and does not correlate with the bulge-to-total luminosity ratio $B/T$ in all studied bands. (2) Applying an 1D photometric profile analysis overestimates the disc thickness in galaxies with large bulges making an illusion of the relationship between the disc flattening and the ratio $B/T$. (3) In our sample the early-type disc galaxies (S0/a) have both flat and "puffed" discs. The early spirals and intermediate-type galaxies have a large scatter of the disc flatness, which can be caused by the presence of a bar: barred galaxies have thicker stellar discs, on average. On the other hand, the late-type spirals are mostly thin galaxies, whereas irregular galaxies have puffed stellar discs., Comment: 17 pages, 17 figures, accepted for publication in MNRAS

We present a catalog of true edge-on disk galaxies automatically selected from the Seventh Data Release (DR7) of the Sloan Digital Sky Survey. A visual inspection of the $g$, $r$ and $i$ images of about 15000 galaxies allowed us to split the initial sample of edge-on galaxy candidates into 4768 (31.8% of the initial sample) genuine edge-on galaxies, 8350 (55.7%) non-edge-ons, and 1865 (12.5%) edge-on galaxies not suitable for simple automatic analysis because these objects show signs of interaction, warps, or nearby bright stars project on it. We added more candidate galaxies from RFGC, EFIGI, RC3, and Galaxy Zoo catalogs found in the SDSS footprints. Our final sample consists of 5747 genuine edge-on galaxies. We estimate the structural parameters of the stellar disks (the stellar disk thickness, radial scale length, and central surface brightness) in the galaxies by analyzing photometric profiles in each of the g, r, and i images. We also perform simplified 3-D modeling of the light distribution in the stellar disks of edge-on galaxies from our sample. Our large sample is intended to be used for studying scaling relations in the stellar disks and bulges and for estimating parameters of the thick disks in different types of galaxies via the image stacking. In this paper we present the sample selection procedure and general description of the sample., Comment: Accepted for publication in ApJ

We discuss the results of a mass decomposition of two spiral galaxies, NGC 6824 and UGC 11919. In a previous analysis of the Hyperleda catalog, the galaxies were identified as having a peculiar dynamical $M/L$. The aim of this study is to confirm or disprove the preliminary findings, indicating a non-standard stellar initial mass function (IMF) for the galaxies. The surface photometry in B, V, and R bands was carried out with the Apache Point 0.5-m telescope and the \ion{H}{I} data cubes were obtained with the Westerbork Synthesis Radio Telescope (WSRT). Photometric profiles were decomposed into bulge and exponential disk components. Using the obtained \ion{H}{I} data cubes, rotation curves of both galaxies were constructed. Employing the photometric profiles, the mass distribution of the galaxies was decomposed into mass components: bulge, stellar disk, gas, and pseudo-isothermal dark halo. We conclude that NGC 6824 possesses a stellar disk with mass-to-light ratio $(M/L_B)_{\rm disk} = 2.5$, in agreement with its color $(B-V)_0$. On the contrary, UGC 11919 appears to have a very lightweight disk. Its dynamically estimated mass corresponds to a low stellar disk mass-to-light ratio $(M/L_B)_{\rm disk} \approx 0.5$. Under standard assumptions, this ratio does not agree with the relatively red color of the disk, while a bottom light stellar initial mass function is needed to explain the observations., Comment: 14 pages, 14 figures, accepted for publication in Astronomy and Astrophysics

Using $N$-body simulations ($N\sim 10^6 - 10^7$), we examine how a non-axisymmetric dark halo affects the dynamical evolution of the structure in collisionless (stellar) discs. We demonstrate how the model parameters such as mass of the halo, initial conditions in the disc and the halo axes ratio affect morphology and kinematics of the stellar discs. We show that a non-axisymmetric halo can generate a large-scale spiral density pattern in the embedded stellar disc. The pattern is observed in the disc for many periods of its revolution, even if the disc is gravitationally over-stable. The growth of the spiral arms is not accompanied by significant dynamical heating of the disc, irrelevant to its initial parameters. We also investigate transformation of the dark halo's shape driven by the long-lived spiral pattern in the disc . We show that the analysis of the velocity field in the stellar disc and in the spiral pattern gives us a possibility to figure out the spatial orientation of the triaxial-shaped dark halo and to measure the triaxiality., Comment: 11 pages, 13 figures, accepted to MNRAS

We performed the photometric B, V and R observations of nine disk galaxies that were suspected in having abnormally low total mass-to-light (M/L) ratios for their observed color indices. We use our surface photometry data to analyze the possible reasons for the anomalous M/L. We infer that in most cases this is a result of errors in photometry or rotational velocity, however for some galaxies we cannot exclude the real peculiarities of the galactic stellar population. The comparison of the photometric and dynamical mass estimates in the disk shows that the low M/L values for a given color of disks are probably real for a few our galaxies: NGC 4826 (Sab), NGC 5347 (Sab), and NGC 6814 (Sb). The small number of such galaxies suggests that the stellar initial mass function is indeed universal, and that only a small fraction of galaxies may have a non-typical low-mass star depleted initial mass function. Such galaxies require more careful studies for understanding their star formation history., Comment: 23 pages, 7 figures. This is a slightly shortened version of the paper published in Astronomy Letters

Spectroscopic long-slit observations of the dwarf Irr galaxy IC 10 were conducted at the 6-m Special Astrophysical Observatory telescope with the SCORPIO focal reducer. The ionized-gas emission spectra in the regions of intense current star formation were obtained for a large number of regions in IC 10. The relative abundances of oxygen, N+, and S+ in about twenty HII regions and in the synchrotron superbubble were estimated. We found that the galaxy-averaged oxygen abundance is 12 + log(O/H) = 8.17 +- 0.35 and the metallicity is Z = 0.18 +- 0.14 Z_sun. Our abundances estimated from the strong emission lines are found to be more reliable than those obtained by comparing diagnostic diagrams with photoionization models., Comment: Abridged; accepted in Astronomy Letters

In this review we consider various ring structures that are observed in galaxies. Formation and evolution of the rings are interesting problems in studies of galactic morphology. They are related to such fundamental aspects of galactic evolution and dynamics as the nature and distribution of the dark matter in galaxies, galactic interactions and internal secular evolution of galactic substructures. A significant fraction of galactic rings forms in the disks due to gravitational torques from bar-like patterns. In contrast to this internally driven origin, the phenomenon of the polar-ring galaxies is closely connected with the processes of intergalactic interactions and merging. A rare class of collisional rings reveals the density waves triggered in the stellar and gaseous disks after a strong head-on collision with a companion. We briefly review the status of studies of gas kinematics in the rings of different origin. We stress that velocity fields of the ionized gas obtained with the Fabry-Perot interferometers provide a very important information for these studies., Comment: 13 pages, 4 figures, New Astronomy Reviews (proceeding of 7th SCSLSA), in press

Propagating star formation in a collisional ring galaxy Arp10 is investigated by a complex approach, which includes the broad- and narrow-band photometry, long- slit spectroscopy, and scanning Fabry-Perot spectroscopy. The ionized gas velocity field obtained with best spatial resolution to date indicates a non- isotropic expansion of the outer ring with a maximum velocity 110km/s. Strong vertical and non-circular motions are also seen in the vicinity of the inner ring. Our kinematic data suggest that Arp10 has a small inclination i=22\degr and high total (luminous plus dark matter) mass of about $10^{12} M_{\odot}$ within a 50 kpc radius.The abundance of oxygen 12 + log(O/H) in both star- forming rings is about 8.6.The analysis of spectral indices provides an estimate on the propagation velocities of both rings and metallicity of the pre-collision stellar population.A small "knot" near the nucleus of Arp10,which was previously suspected as a possible candidate for collision, is now unambiguously identified as the "intruder" of at least 1/4 of the total mass of Arp~10.We use a simplified two-dimensional hydrodynamic modeling of galaxy collisions to test a collisional origin of Arp10. We confirm that the sizes of the inner and outer rings, maximum expansion velocity of the outer ring, and radial profile of the gas circular velocity can be reproduced by a near-central collision with the intruder galaxy, which occurred approximately 85Myr ago. We acknowledge that an apparent crescent- shaped distribution of H$\alpha$ emission in the outer ring is caused by a star formation threshold in the gas disk of Arp10., Comment: 17 pages, 17 figures, accepted by ApJ

We present results of Fabry-Perot and long-slit spectroscopy of the peculiar galaxy Arp 10. The ionized gas velocity field shows evidence for significant radial motions in both outer and inner galactic rings. Long-slit spectroscopy reveals gradients of age and metallicity of stellar population in agreement with the propagating nature of star formation in the galaxy. We present strong evidence that a small ``knot'' at 5 arcsec from the center of Arp 10 is its dwarf elliptic satellite, the most probable ``intruder'' responsible for triggering the expanding rings in Arp 10., Comment: 6 pages, 5 figures, poster contribution on American Astronomical Society Meeting 205, #26.05

We present results of numerical N-body simulations of a galactic stellar disk embedded into a spherical dark halo. The non-linear dynamics of bending instabilities developing in the disk is studied. The bending modes, axisymmetric and not, are considered as main factors increasing the disk thickness. The relation between the disk vertical scale height and the halo+bulge-to-disk mass ratio is inferred. The method of estimation of the spherical-to-disk mass ratio for edge-on spiral galaxies based on this relation is studied and applied to constrain the spherical subsystem mass and the mass of dark halos (M_h/M_d) in seven edge-on galaxies. The values of M_h/M_d are of order 1 for our galaxies., Comment: 17 pages including 20 PostScript figures. Oral talk presented in International Workshop "Progress in Study of Astrophysical Disks: Collective and Stochastic Phenomena and Computational Tools", Volgograd, September 9-11, 2003

The relationship between the geometrical properties of stellar disks (a flatness and truncation radius) and the disk kinematics are considered for edge-on galaxies. It is shown that the observed thickness of the disks and the approximate constancy of their thickness along the radius well agrees with the condition of their marginal local gravitational stability. As a consequence, those galaxies whose disks are thinner should harbor more massive dark haloes. The correlation between the de-projeced central brightness of the disks and their flatness is found (the low surface brightness disks tend to be the thinniest ones). We also show that positions of observed photometrically determined truncation radii $R_{cut}$ for the stellar disks support the idea of marginal local gravitational stability of gaseous protodisks at $R =R_{cut}$, and hence the steepening of photometric profiles may be a result of too inefficient star formation beyond $R_{cut}$., Comment: 10 pages including 6 figures. Shortened version to appear in proceedings of the JENAM-2002 meeting, Porto, Portugal, 2002

We consider a thickness of stellar disks of late-type galaxies by analyzing the R and K_s band photometric profiles for two independent samples of edge-on galaxies. The main goal is to verify a hypotesis that a thickness of old stellar disks is related to the relative masses of the spherical and disk components of galaxies. We confirm that the radial-to-vertical scale length ratio for galactic disks increases (the disks become thinner) with the increasing of total mass-to-light ratio of the galaxies, which characterize the contribution of dark halo to the total mass, and with the decreasing of central deprojected disk brightness (surface density). Our results are in good agreement with numerical models of collisionless disks evolved from subcritical velocity dispersion state to a marginally stable equilibrium state. This suggests that in most galaxies the vertical stellar velocity dispersion, which determine the equilibrium disk thickness, is close to the minimum value, that ensures disk stability. The thinnest edge-on disks appear to be low brightness galaxies (after deprojection) in which a dark halo mass far exceeds a mass of the stellar disk., Comment: 13 pages. To be Published in Astronomy Letters, v.28(2002)

Photometric parameters of stellar disks and bulges for several edge-on galaxies from the Catalog of Flat Galaxies (FGC) were determined. We discuss a difference between photometric parameters of LSB and HSB galaxies from our sample. Also we present results of R CCD photometry of edge-on galaxy RFGC 3647. Deprojecting this galaxy we show that it has thin LSB disk (central surface brightness $22^m.2$ in R) and high ratio of radial to vertical scale lengths. It is shown that initially gaseous disk of the galaxy was unstable and its velocity dispersion was low. Stellar disk of this LSB galaxy was not heated significantly since that time., Comment: 11 pages, 5 postscript figures. Was presented as a poster at JENAM-2000 meeting (29 May - 3 June, 2000, Moscow). Some corrections has been done

We used three indicators of massive star formation: $H_{\alpha}$, FIR and non-thermal radio luminosities, to compare estimations of Star Formation Rate (SFR) for the sample of 34 spiral galaxies. To adjust SFR values, obtained from different indicators, we considered the slope $\alpha$ and/or upper mass limit $M_{up}$ of Initial Mass Function (IMF) as free parameters. The best agreement between these indicators reaches for $M_{up} \approx 60 - 100 ~M_{\odot}$ and $\alpha \approx -3.1 $ in the high mass end of IMF ($M > 10 ~M_{\odot}$). Parallel with SFR we also estimated FIR excess $X_{FIR}$ defined as a fraction of the observed FIR which is not related to young massive stars directly. $X_{FIR}$ is found to be well correlated with types of spiral galaxies and their colors (B-V): the redder a galaxy the higher is its FIR excess. We conclude that for any parameters of IMF the observed FIR flux of early type spiral galaxies needs the additional source of energy but the massive star radiation., Comment: 7 pages including 3 PostScript figures. To be published in "The Evolution of galaxies on Cosmological timescales", Ap. & Space Science, Proceedings of the meeting held at Puerto de la Cruz (Spain) on November 30-December 5, 1998