N. Ya. Sotnikova

An iterative method for constructing equilibrium phase models of stellar systems

We present a new method for constructing equilibrium phase models for stellar systems, which we call the iterative method. It relies on constrained, or guided evolution, so that the equilibrium solution has a number of desired parameters and/or constraints. This method is very powerful, to a large extent due to its simplicity. It can be used for mass distributions with an arbitrary geometry and a large variety of kinematical constraints. We present several examples illustrating it. Applications of this method include the creation of initial conditions for N-body simulations and the modelling of galaxies from their photometric and kinematic observations.

2MASS photometry of edge-on spiral galaxies – I. Sample and general results

A sample of edge-on spiral galaxies aimed at a thorough study of the main structural and photometric parameters of edge-on galaxies, both of early- and late-types, is presented. The data were taken from the Two Micron All Sky Survey (2MASS) in the J, H and Ks filters. The sources were selected according to their angular size mainly on the basis of the 2MASS-selected Flat Galaxy Catalog (2MFGC). The sample consists of 175 galaxies in the Ks filter, 169 galaxies in the H filter and 165 galaxies in the J filter. We present bulge and disc decompositions of each galaxy image. All galaxies have been modelled with a Sersic bulge and exponential disc with the budda v2.1 package. Bulge and disc sizes, profile shapes, surface brightnesses are provided. Our sample is the biggest up-to-date sample of edge-on galaxies with derived structural parameters for discs and bulges. In this paper, we present the general results of the study of this sample. We determine several scaling relations for bulges and discs which indicate a tight link between their formation and evolution. We show that galaxies with bulges fitted by the Sersic index n≲ 2 have quite different distributions of their structural parameters than galaxies with n≳ 2 bulges. At a first approximation the Sersic index threshold n≃ 2 can be used to identify pseudobulges and classical bulges. Thus, the difference in parameter distributions and scaling relations for these subsamples suggests that two or more processes are responsible for disc galaxy formation. The main conclusions of our general statistical analysis of the sample are as follows. (i)  The distribution of the apparent bulge axis ratio qb for the subsample with n≲ 2 can be attributed to triaxial, nearly prolate bulges that are seen from different projections, while n≳ 2 bulges seem to be oblate spheroids with moderate flattening. Triaxiality of late-type bulges may be due to the presence of a bar that thickened in the vertical direction during its secular evolution. (ii)  For the sample galaxies, the effective radius of the bulge re,b, the disc scalelength h and the disc scaleheight z0 are well correlated. However, there is a clear trend for the ratio re,b/h to increase with n. As n is an indicator of the Hubble type, such a trend unambiguously rules out the widely discussed hypothesis of a scale-free Hubble sequence. The found correlation between z0 and re,b is new and was not described earlier. (iii)  There is a hint that the fundamental planes of discs, which links only disc parameters and the maximum rotational velocity of gas, are different for galaxies with different bulges. This may indicate a real difference of discs in galaxies with low- and high-concentration bulges. (iv)  The most surprising result arises from the investigation of the photometric plane of sample bulges. It turns that the plane is not flat and has a prominent curvature towards small values of n. For bulges, this fact was not noted earlier. (v)  The clear relation between the flattening of stellar discs h/z0 and the relative mass of a spherical component, including a dark halo, is confirmed not for bulgeless galaxies but for galaxies with massive bulges. Many of our results are in good agreement with the results of other authors, several ones are new. Thus, our sample is very useful for further detailed studying and modelling of the edge-on spiral galaxies.

THE CATALOG OF EDGE-ON DISK GALAXIES FROM SDSS. I. THE CATALOG AND THE STRUCTURAL PARAMETERS OF STELLAR DISKS

We present a catalog of true edge-on disk galaxies automatically selected from the Seventh Data Release of the Sloan Digital Sky Survey (SDSS). A visual inspection of the g, r, and i images of about 15,000 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-on galaxies, and 1865 (12.5%) edge-on galaxies not suitable for simple automatic analysis because these objects either show signs of interaction and 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 three-dimensional 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.

Estimating the dark halo mass from the relative thickness of stellar disks

We analyze the relationship between the mass of a spherical component and the minimum possible thickness of stable stellar disks. This relationship for real galaxies allows the lower limit on the dark halo mass to be estimated (the thinner the stable stellar disk is, the more massive the dark halo must be). In our analysis, we use both theoretical relations and numerical N-body simulations of the dynamical evolution of thin disks in the presence of spherical components with different density profiles and different masses. We conclude that the theoretical relationship between the thickness of disk galaxies and the mass of their spherical components is a lower envelope for the model data points. We recommend using this theoretical relationship to estimate the lower limit for the dark halo mass in galaxies. The estimate obtained turns out to be weak. Even for the thinnest galaxies, the dark halo mass within four exponential disk scale lengths must be more than one stellar disk mass.

Optimal choice of the softening length and time step in N-body simulations

A criterion for the choice of optimal softening length

Bending instability of stellar disks: The stabilizing effect of a compact bulge

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Mechanisms of the vertical secular heating of a stellar disk

and time-step

Hydrodynamic processes in young binaries as a source of cyclic variations in circumstellar extinction

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The Polar Stellar Ring and Dark Halo of NGC 5907

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Brightness oscillations in models of young binary systems with low-mass secondary components

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