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Astron. Astrophys. 335, 449-462 (1998)

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1. Introduction

To understand the evolution of galaxies quantitative information is needed for individual galactic populations. Traditionally dynamical models of galaxies are constructed with the aim to determine the overall mass distribution of galaxies. Yet galaxies consist of populations of different properties which also differ in their mass distribution. For this reason in most mass distribution models several populations are considered, most frequently the bulge and disk, and in recent models also a massive dark matter (DM) population (Kent 1987, Simien 1988, Shaw & Gilmore 1990, Prieto et al. 1992, de Jong 1996; for review see e.g. Capaccioli & Caon 1992). Only for our own Galaxy more detailed population structure is studied (see for example Gilmore et al. 1990, van der Kruit & Gilmore 1995, Ng et al. 1997, Méra et al. 1998).

A direct determination of parameters characterizing their structure is difficult as from observations it is possible to derive integrated properties of galaxies summed over all populations. To overcome this difficulty modelling of galactic populations is needed. This is not a trivial task. Principles of modelling galactic populations were elaborated by Einasto (1967a, 1967b, 1969a), and applied to determine population parameters for the Andromeda galaxy (Einasto 1969b, Einasto & Rümmel 1972) and Galaxy (Einasto 1969a, Einasto 1970, Einasto & Einasto 1972b). This study showed the presence of a mass paradox: photometric data indicate the end of the visible galaxy at a distance from galaxy center [FORMULA]  kpc, whereas kinematic data suggest a much larger size of the galaxy. This mass paradox can be solved by adding a massive and large dark matter population to galactic models (Einasto 1972).

In these early models population parameters were determined by a trial-and-error procedure using graphs for galactic descriptive functions (Einasto & Einasto 1972a). Later Haud (1985) elaborated a method to determine population parameters by an automated computer algorithm. This method was described by Einasto & Haud (1989, Paper I) and applied to our Galaxy (Haud & Einasto 1989, Paper II), M 87 (Tenjes et al. 1991, Paper III), and M 31 (Tenjes et al. 1994, Paper IV).

In the present paper we investigate the structure of the nearby SA(s)ab galaxy M 81 and determine the parameters of its main stellar populations. We try to estimate the accuracy of these parameters and their sensitivity to a different kind of observational data. The decomposition of a galaxy into subsystems is complicated and often uncertain. Due to the large number of free parameters there may not be a unique solution to the problem. But when using large samples of observational data and comparing the values of the parameters during the modelling also with the models of chemical evolution it is possible to avoid obviously unphysical solutions. The uncertainty can be diminished by confining ourselves to well-observed galaxies. Its relatively large size in the sky makes the galaxy M 81 suitable for detailed studies of stellar content and for discrimination of stellar ensembles by their spatial distribution, chemical composition, kinematics and age.

M 81 is an example of galaxies with a falling rotation curve. In connection with this we have to answer a question, whether there is any dark matter associated with M 81. The two-component model consisting of a bulge and a disk calculated by Rohlfs & Kreitschmann (1980) included no DM. Also Kent (1987) has found that in his sample of 16 galaxies the galaxy M 81 may be the only one where, in addition to the luminous components, it is not necessary to include the DM component. Both these models are based on the surface photometry and on the rotation curve observations.

An additional and independent basis for mass determination is provided by the kinematics of the satellite galaxies surrounding M 81. In this case we need to decide what galaxies are gravitationally bound with M 81. Different samples of satellite galaxies may influence the quantitative mass estimates but the first glance at the highly different radial velocities of four neighbouring galaxies M 81, M 82 , NGC 3077 , and NGC 2976 hints that the problem of the existence of the DM component in M 81 is worth reconsideration.

The models taking into account the kinematics of satellite galaxies have been constructed by Einasto et al. (1980a, 1980b). However, the amount of different observational data has considerably increased, which made it necessary to review our earlier models.

Sect.  2 describes the observational data used in the modelling process. In Sect. 3 we characterize the subsystems in M 81: the nucleus, the metal-rich core, the bulge, the metal-poor halo, the old disk, the extreme flat subsystem, and the invisible massive corona, and emphasize which parameters were kept fixed during the final best-approximation fit for a particular subsystem. Sect. 4 and Appendix A are devoted to the modelling process. In Sect. 5 we give the parameters and mass distribution functions of our final model and in Sect. 6 the discussion of the model is presented.

Throughout this paper all luminosities and colour indices have been corrected for absorption in our Galaxy according to Burstein & Heiles (1984) and Freedman et al. (1994): [FORMULA] 0.14. The distance to M 81 has been taken 3.6 Mpc (Freedman et al. 1994), the position angle of the major axis PA = [FORMULA], the angle of inclination to the line of sight [FORMULA] 32o (Garcia-Gomez & Athanassoula 1991).

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© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998