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


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Galactic models with massive coronae

V. The spiral Sab galaxy M 81

P. Tenjes 1, 2, U. Haud 2 and J. Einasto 2

1 Institute of Theoretical Physics, Tartu University, Tähe 4, EE-2400 Tartu, Estonia
2 Tartu Observatory, EE-2444 Tõravere, Estonia (tenjes@aai.ee, urmas@aai.ee, einasto@aai.ee)

Received 3 March 1998 / Accepted 20 April 1998

Abstract

Stellar populations in spiral galaxy M 81 are studied by using modelling. The galaxy is assumed to be stationary and consisting of a superposition of several subsystems. Each subsystem corresponds to a certain stellar/gas/dark matter population with a certain density distribution, chemical composition and kinematical characteristics. We presume that equidensity surfaces of the galactic populations are similar concentric ellipsoids or can be represented as sums of such ellipsoids. The input observational data base consists of surface photometry along the minor and major axis in UBVRI colours, rotation velocities of gas, stellar velocity dispersions, distribution and kinematics of globular clusters, distribution of the young stellar component and gas, kinematics of M 81 satellite galaxies. These data are used to decompose the galaxy into a central nucleus, a metal-rich core and a bulge, a metal-poor halo, an old stellar disk, a young gaseous-stellar disk and a massive dark matter component. Each population is characterized by its ellipticity, radius, mass, luminosity, structure parameter and colour indices. These population parameters are found using the least-squares algorithm. The algorithm minimizes the sum of squares of relative deviations of the model from observations. The values of the parameters were calculated in several steps from a preliminary crude model to the final model. The sensitivity of the population parameters to various observational data is analysed. Particular attention is devoted to the dark matter problem. In the final model the mean mass-to-luminosity ratio of the optically visible parts of the galaxy is found to be [FORMULA], and the ratio of the total mass to the visible one [FORMULA] In the inner regions the best fit with observations is obtained when a central point mass [FORMULA] is added to the nucleus.

Key words: galaxies: spiral – galaxies: structure – galaxies: individual: M 81 – galaxies: nuclei – cosmology: dark matter

Send offprint requests to: P. Tenjes (second address)

© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998

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