Evolution Of Galactic Disks Research Articles

Thresholds and the chemical evolution of galactic discs - II. Radial flows

We introduce radial flows into the chemical evolution models of Chamcham, Pitts & Tayler. In this work, the authors used a stability criterion in disc galaxies to define a threshold for star formation and to deduce a self-consistent star formation rate by combining an energy equation for the gas with the standard equations for the chemical evolution of galaxies. These models reproduced the general observable features of spiral galaxies and of the solar neighbourhood, but failed to reproduce the metallicity gradients observed in most galaxies. A biased infall rate increased the gradient, but not sufficiently to correspond with some observations. If the disc is formed out of centrifugal equilibrium, the resulting radial flows will also enhance the composition gradient, as is shown in the present paper

The Galactic Disc Evolution from (V, B–V) Counts at the Galactic Pole

We test a model of stellar evolution synthesis by comparison with (V, B–V) counts at the pole. The history of the stellar birthrate and dynamical evolution in the disc is explicitly taken into account. The data span a large range of magnitudes from V = 7 to 22, and allow us to put new constraints on the evolution of the galactic disc.

Thresholds and the chemical evolution of galactic discs

Observations by Kennicutt suggest that significant star formation in galactic discs only occurs when a criterion for disc instability is satisfied and that, when star formation is occurring, departure from the marginal state is not great. We have studied galactic disc models in which there is no star formation until the disc has a threshold surface density at which an instability criterion is satisfied. We assume that the rate of the subsequent star formation is just such as to keep the criterion marginally satisfied. Our models contain many approximations and free parameters, but they do have properties which resemble those of real galactic discs. These include a slow variation of the star formation rate with time and a slow change in both space and time of the gas random velocity. There is good general agreement with the observed stellar age/metallicity relation. After a time, at any radial position, the rate of star formation is close to a power law in the gas surface density. Because of the threshold, no stars are produced until some time after the disc starts forming and, if the model were valid, there would therefore be no luminous discs at very high red shift. Moreover, there should not be a large density of hidden matter in the form of elementary particles in the solar neighbourhood. The threshold model cannot readily explain the G-dwarf distribution in the solar neighbourhood, and it is necessary to suppose that the disc starts with a non-zero initial metallicity. The model in which gas infall proceeds on the same time-scale at all radii does not produce abundance gradients as large as those observed. There is some improvement if the infall is biased, occurring more slowly at larger radii.

On the central surface brightness of galactic disks

Abstract In the framework of the Lin-Pringle model of the galactic disk evolution, the observed approximate constancy of the central surface brightness Io of an exponential disk is discussed. The disk is assumed to be embedded in a massive halo of the cold dark matter. We suppose that a protogalaxy acquired its angular momentum from tidal torques by interaction with surroundings at early stages of its formation. A self-similar solution for the stellar component evolution and the condition of total disk angular momentum conservation allow to estimate the disk central surface density (and luminosity) as a function of halo parameters. These parameters are determined by the amplitude and spectral index (n) of the density fluctuations from which the galaxy has been formed. It is shown that if n Io-2 at the galactic scale (CDM-model), I 0 is constant and independent of other galactic parameters. The predicted values of I 0 are in a good agreement with the observed ones if galaxies have been formed at rather high peaks of cosmological density fluctuations.

Dynamical and chemical evolution of viscous, star-forming galactic discs

Models of the formation and subsequent viscous evolution of galactic discs are examined. The discs are assumed to form as described by the cooling flow solutions of Sommer-Larsen. The star formation rate is assumed to be given by the prescription of Wyse & Silk, thereby relaxing the requirement of proportionality between |$t_\ast$| and tv used in previous works. The resulting stellar and gas profiles are found to resemble well the observed profiles of disc galaxies. Gradients in gas and star metallicities are also calculated. Values around – 0.05 dex kpc–l are obtained for the gas metallicity, in fine agreement with the observed [O/H] gradient in our Galaxy. The exact magnitude of these gradients depends only weakly on the model parameters.

Molecular Gas in Galaxies

Molecular gas is critical in determining both the morphology and evolution of galactic disks. It is within the giant molecular clouds that interstellar gas is cycled into the next generation of stars, and the most massive of these young stars produce a major part of the galactic luminosity. In addition, the dense interstellar medium, as it is highly dissipative, probably plays a fundamental part in determining the outcome of galactic interactions. Over the last two decades, CO observations have been used to probe the molecular component of hundreds of galaxies from the Local Group to the Virgo cluster, and in luminous galaxies with recession velocities up to cz = 45,000 km s^(-1).

On the cause of metallicity gradients in spiral galaxies

We investigate particular models for the chemical evolution of galactic discs in an attempt to account for the observed behaviour of metallicity with surface density, and hence explain abundance gradients. The models include simple Schmidt star formation rate laws, models in which the formation of massive stars is suppressed below a threshold interstellar gas density, and models with self-regulation of star formation

Viscous evolution of self-gravitating galactic disks within a dark halo

Viscous evolution of thin and star-forming galactic disks within a massive dark halo is investigated by taking into account the self- gravity. We assume that the well-known gravitational instability of a self-gravitating disk generates turbulence (as well as star formation). The disk is assumed to be in nearly centrifugal equilibrium under the action of viscosity, just like an accretion disk. Such a disk is found to be unstable to spatial oscillations of physical variables owing to the action of viscosity and self-gravity in the inner part of the disk where rotation is nearly uniform. Suppressing the instability by taking into account the turbulent pressure, we have numerically followed the viscous evolution of star- forming self-gravitating disks until most of the gas has been converted to stars. Exponential distributions of the surface density of the stars are produced as a result of viscous evolution with star formation. Moreover, it is found that the viscous evolution creates flat rotation curves extending from the outer part where dark halo dominates to the inner part where self-gravity dominates, just as observations indicate. Observed variety in the shape of the rotation curves near the center among galaxies seems to be reproduced by changing the efficiency of star formation or initial angular momentum of the disks.

Star formation rates and abundance gradients in disk galaxies

Analytic models for the evolution of disk galaxies are presented, placing special emphasis on the radial properties. These models are straightforward extensions of the original Schmidt (1959, 1963) models, with a dependence of star formation rate on gas density. The models provide successful descriptions of several measures of galactic disk evolution, including solar neighborhood chemical evolution, the presence and amplitude of metallicity and color gradients in disk galaxies, and the global rates of star formation in disk galaxies, and aid in the understanding of the apparent connection between young and old stellar populations in spiral galaxies.

Chemical and dynamical evolution of galactic discs

The relative roles of star formation and viscosly-induced radial flows in galactic discs are discussed. It is shown that the present-day distributions of stars, gas, and metals in galactic spirals need not reflect initial conditions but may instead indicate a cooperation between star forming and viscous processes over the disc lifetime.

Interstellar Molecular Clouds

The interstellar medium in our galaxy contains matter in a variety of states ranging from hot plasma to cold and dusty molecular gas. The molecular phase consists of giant clouds, which are the largest gravitationally bound objects in the galaxy, the primary reservoir of material for the ongoing birth of new stars, and the medium regulating the evolution of galactic disks.

A Model for a Chemical Evolution of Galactic Discs.

In this paper we discuss a simple model for the chemical evolution of galactic disks, aiming at understanding the observed metallicity gradients in spiral galaxies. The model in corporates both the recent results of Talbot and Arnett (1975), who proposed a rate of star formation driven by the surface mass density, and those of Lynden-Bell (1975), who investigated the consequences of significant infall of gas into the system. A similar attempt was made by Chiosi (1977) for the solar neighbourhood. The result was that the metallicity distribution of nearby stars can be easily interpreted if the time scale for accretion of gas into the solar pool, or in other words the duration of the disk formation phase, is of the or der of 2-3 109ys. Such a result is qualitatively in agreement with the models of Larson (1972), Ostricker and Thuan (1975), and also is implicit in the collapse models for the formation of disk galaxies, Larson (1976).