M. A. Smirnov

Star formation and the kinematics of gas in the disk of NGC 628

The radial profile of the star-formation rate (SFR) in the galaxy NGC 628 is shown to be modulated by a spiral-density wave. The radial profile of the velocity of gas inflow into the spiral arm is similar to the radial distribution of the surface density of the SFR. The position of the corotation resonance is determined along with other parameters of the spiral-density wave via a Fourier analysis of the azimuthal distribution of the observed radial velocities in annular zones of the disk of NGC 628. The radial profile of the surface density of the SFR is determined using the empirical SFR—linear size relation for star-formation complexes (giant HII regions) and measurements of the coordinates, Hα fluxes, and the sizes of HII regions in NGC 628.

Comparison of U BV R photometry of giant HII regions in the galaxy NGC 628 with a detailed grid of evolutionary models for star clusters

We present the results of U BV RI CCD photometry of giant HII regions in the spiral galaxy NGC 628, acquired with the 1.5 m telescope of the Mt. Maidanak Observatory (Uzbekistan) with an angular resolution better than 1″. We estimate the ages and interstellar extinctions of these regions and identify the acting star-formation mode by comparing the observed color indices with a detailed grid of evolutionary models covering the entire range of parameters of the initial mass function and of ages of the young star-formation complexes, and taking into account two star-formation modes. We find a radial gradient of the interstellar extinction in NGC 628, which is consistent with the radial abundance gradient found earlier by other authors from independent spectrophotometry. Our age estimates agree with abundance estimates from independent observations.

Star formation in extragalactic HII regions. Determination of parameters of the initial mass function

A method for using the colors of star-forming complexes to derive the slope and upper mass limit of the initial mass function (IMF) and the age of the complex is proposed in the framework of synthetic evolutionary models of star-cluster populations. The star-formation parameters of 105 complexes in 20 spiral and irregular galaxies are determined. The IMF slopes in different star-forming complexes differ appreciably, and their dependence on the luminosities and masses of the complexes is derived. The duration of the star-formation period increases with the luminosity of the complex, and complexes with longer star-formation periods are richer in metals. The slope of the integrated IMF in a Galaxy depends on the mass spectrum of its complexes, and the upper mass limit of the IMF is lower in early-type spirals.

Numerical simulations of the initial mass function and star-formation history using a grid of evolutionary models for star clusters

Numerical simulations are used to investigate the possibility of determining the physical properties of young star-forming complexes by finding the global minimum of a so-called deviation functional, which assigns each evolutionary model for a stellar population a number characterizing the deviation of the observed photometric properties from their model values. The deviation functional is calculated using a grid of evolutionary models computed at the Institute of Astronomy of the Russian Academy of Sciences. The parameters of the initial mass function (IMF) and the age corresponding to the global minimum of the deviation functional are strongly correlated with the IMF and age of the test model. The accuracy of the parameters of the IMF and the age are related to the random errors of the colors of the test models and the number of input parameters. A special series of numerical simulations is used to demonstrate the possibility of using the deviation functional to determine the interstellar extinction and the fraction of Lyman photons that do not contribute to the ionization of gas in a star-forming complex. The simulation results can be used to assess the accuracy of the IMF parameters and ages of young star-forming complexes based on the observational data available.

Comparison of UBVR photometry of giant HII regions in NGC 628 with a detailed grid of evolution models of star clusters

We present our UBV R photometry observations of giant HII regions in NGC 628 in comparison with a detailed grid of evolutionary models of stellar clusters to interpret the observed colours of young stellar populations in terms of IMFs and SFRs parameters and dust content. Observed colours are complicate functions of searched IMF, age, regime of star formation as well as chemical abudances and interstellar reddening. A content of database of observed properties is crucial to get significant results. To exclude age-reddening and age-metallicity degenerations we carried out UBV R photometry of 127 giant HII regions are star formation complexes (SFCs) with independently measured chemical abundancies Z and interstellar reddening in NGC 628 by Belley & Roy (1992).

The initial mass function and history of the star-formation rate in star-forming complexes in galaxies

The positions of star-forming complexes (SFCs) in color-luminosity, color-color, and chemical composition-luminosity diagrams are determined by the star-formation regime (history). Taking into account the fraction of Lyman continuum photons that are not absorbed by hydrogen, we find a strong correlation between the observed color indices and the total Lyman continuum flux from the stars in SFCs. The distribution of extragalactic SFCs in a plot of the slope of the initial mass function (IMF) versus the density of stars cannot be distinguished from this distribution for clusters in the Galaxy and the Large Magellanic Cloud, where the IMF slopes were derived directly from star counts.