Viscous models for slowly evolving galactic disks.

Details Viscous models for slowly evolving galactic disks. Viscous models for slowly evolving galactic disks.

Apr 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...308..403f&link_type=abstract

Astronomy and Astrophysics, v.308, p.403-414

Astronomy and Astrophysics

Astrophysics

48

Galaxies: Kinematics And Dynamics, Galaxies: Evolution, Galaxies: Formation, Galaxies: Ism

Scientific paper

We present a basic family of galactic disk evolutionary models bearing in mind three fundamental questions: i) to what extent does the present cosmological frame offer the natural initial conditions for galactic formation and evolution, ii) what is the relevance of gas hydrodynamics in determining the present physical conditions of disk galaxies and iii) how does the wide variety of star formation (SF) histories displayed by late type galaxies find an explanation within the present frame of galactic evolution. These models describe the global evolution of a disk galaxy by integrating the physics of previously published local models (Firmani and Tutukov 1992, 1994 hereafter FT1 and FT2) into a hydrodynamical framework dominated by gas turbulent dissipation. In an effort to avoid ad hoc assumptions we introduce a physical approach to the study of the gas and star disks which provides a reasonable estimate of the turbulent viscosity and energy dissipation of gas as well as of the SF rate (SFR). This approach offers the advantage of leading to models free of arbitrary parameters, the evolution of which depends on the initial conditions concerning gas and dark matter and on gas accretion. The initial conditions where introduced in consistency with a cold dark matter (CDM) cosmology with a scale invariant fluctuation spectrum normalized to COBE. Our results show that for models representative of the Galaxy a very effective gas transfer due to the turbulent viscosity takes place independent of accretion regime. This process leads to a radial exponential decay of the final stellar surface density and a flat rotation curve, in accord with a "conspiracy" behind an indistinguishable transition between visible and dark matter contributions. A star formation rate consistent with a surface Schmidt's law with a power smaller than or equal to two is also obtained.

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