2. Analysis of observational data
By now a detailed surface photometry of M 81 is available in UBVRI colours. Table 1 presents references, the faintest observed isophotes (), corresponding distances along the major axis () and colour system used. In addition, the surface brightness distribution of M 81 has been obtained by Simkin (1967) but it is difficult to convert the coordinate system used in this paper to the major or minor axis and we have not used these data.
Table 1. Photometrical data
The composite surface brightness profiles in the UBVRI colours along the major and minor axes were derived by averaging the results of different authors. Isophotes were well approximated by ellipses with different eccentricities. The observations for which the "Faintest isophote" value in Table 1 is in parenthesis were made without absolute calibration. They were calibrated with the help of other calibrated profiles in the same colour. For I-colour we have no absolute calibration and this profile entered our initial data set in arbitrary units. For this reason we do not have the I-colour luminosities of galactic populations in our final model. All the surface brightness profiles obtained in this way (in UBVRI along the major axis and in UBVR along the minor axis) belong to the initial data set of our model construction. Here we present the surface brightness distribution in V (Fig. 1), (B-V), (U-B), and (V-R) colour indices (Fig. 2), for which the absolute calibration is available, and the axial ratios (the ratio of the minor axis to the major axis of an isophote) (Fig. 3) as functions of the galactocentric distance.
The rotation curve we used for the modelling is based on gas velocities. For the inner 1.7 kpc of the galaxy the [NII], [SII] and radial velocities were obtained and rotation curve was constructed by Goad (1976). The radial velocities of CO clumps obtained by Sage & Westphal (1991) give us information on the rotation law at the distances of 1.4 - 3.3 kpc from the centre. High-resolution HI observations at and resolution by Rots (1975), Visser (1980) and resolution by Gottesman & Weliachew (1975) cover the region of 3.8 - 22 kpc. The outer parts of M 81 were mapped in HI with a resolution of 9- () by Roberts & Rots (1973) and by Rohlfs & Kreitschmann (1980). Recently a comprehensive velocity map of M 81 and nearby regions was made with VLA. Adler & Westphal (1996) mapped inner regions from 4 kpc to 20 kpc with angular resolution , Yun et al. (1994) observed velocity field with lower angular resolution () but mapped also outer regions. The averaged rotation velocities for a region 22 kpc are given in Fig. 4 by open circles. For outer parts (distances of 25 - 30 kpc) Yun mentioned that the rotation curve becomes flat at the level of about 170 km/s (their Fig. 2). This is designated in Fig. 4 by open diamonds. Two outermost velocity measurements by Rohlfs & Kreitschmann (1980) are marked by filled diamonds. Beyond kpc the HI kinematics will be influenced by tidal effects from neighbouring NGC 3034 and NGC 3077 galaxies (Yun et al. 1994).
Line-of-sight velocity dispersion profile in good seeing conditions for the central regions 0.015 kpc) was obtained by Keel (1989). The innermost measured dispersion at 0.003 kpc is from HST observations with post-COSTAR FOS by Bower et al. (1996). In the intermediate distance interval (R = 0.02 - 1.6 kpc) dispersions have been measured by Illingworth (1980), Delisle & Hardy (1992), Carter & Jenkins (1993), and Bender et al. (1994). We averaged the dispersions at various distance intervals with weights depending on seeing conditions and velocity resolution and derived the dispersion curve presented in Fig. 5 by open circles. Mean dispersion value within inner 170 km/s in in good agreement with the McElroy (1995) calibration (his Table 4).
The observed distributions and kinematics of individual objects (satellite galaxies, globular clusters, young stars, gas etc.) which we have used for the modelling will be referred to and analysed in Sects. 3.3, 3.4 and 3.6.
© European Southern Observatory (ESO) 1998
Online publication: June 18, 1998