Modeling the Evolution of Disk Galaxies. I. The Chemodynamical Method and the Galaxy Model

Abstract

Here we present our two-dimensional chemodynamical code CoDEx, which we developed for the purpose of modeling the evolution of galaxies in a self-consistent manner. The code solves the hydrodynamical and momentum equations for three stellar components and the multiphase interstellar medium (clouds and intercloud medium), including star formation, Type I and Type II supernovae, planetary nebulae, stellar winds, evaporation and condensation, drag, cloud collisions, heating and cooling, and stellar nucleosynthesis. These processes are treated simultaneously, coupling a large range in temporal and spatial scales, to account for feedback and self-regulation processes, which play an extraordinarily important role in the galactic evolution. The evolution of galaxies of di †erent masses and angular momenta is followed through all stages from the initial protogalactic clouds until now. In this Ðrst paper we present a representative model of the Milky Way and compare it with observations. The capability of chemodynamical models is convincingly proved by the excellent agreement with various observations. In addition, well-known problems (the G-dwarf problem, the discrepancy between local e †ective yields, etc.), which so far could be only explained by artiÐcial constraints, are also solved in the global scenario. Starting from a rotating protogalactic gas cloud in virial equilibrium, which collapses owing to dissipative cloud-cloud collisions, we can follow the galactic evolution in detail. Owing to the collapse, the gas density increases, stars are forming, and the Ðrst Type II supernovae explode. The collapse time is 1 order of magnitude longer than the dynamical free-fall time because of the energy release by Type II supernovae. The supernovae also drive hot metal-rich gas ejected from massive stars into the halo, and as a consequence, the clouds in the star-forming regions have lower metallicities than the clouds in the halo. The observed negative metallicity gradients do not form before t \ 6 ] 109 yr. These outward gas Ñows prevent any clear correlation between local star formation rate and enrichment and also prevent a unique age-metallicity relation. The situation, however, is even more complicated, because the mass return of intermediate-mass stars (Type I supernovae and planetary nebulae) is delayed depending on the type of precursor. Since our chemodynamical model includes all these processes, we can calculate, e.g., the [O/H] distribution of stars and Ðnd good agreement everywhere in bulge, disk, and halo. From the galactic oxygen to iron ratio, we can determine the supernovae ([II ] Ib]/Ia) ratio for di †erent types of Type Ia supernovae (such as carbon deÑagration or sub-Chandrasekhar models) and Ðnd that the ratio should be in the range 1.0È3.8. The chemodynamical model also traces other chemical elements (e.g., N ] C), density distributions, gas Ñows, velocity dispersions of the stars and clouds, star formation, planetary nebula rates, cloud collision, condensation and evaporation rates, and the coo...

Planetary Nebulae: Abundances and Abundance Gradients

“…Recent classical models predict similar gradients as observed in the framework of an inside-out scenario, showing some flattening near the outer Galaxy and a time steepening (Chiappini et al 1997, Matteucci andChiappini 1999). Chemodynamical models (Samland et al 1997, Hensler 1999) also predict some flattening at large R, and time steepening gradients are predicted by theoretical models by Götz and Köppen (1992) and Tosi (1988). On the other hand, multiphase models (Ferrini et al 1994, Mollá et al 1997 suggest just the opposite behaviour, as also models developed by Prantzos and Silk (1998), Boissier and Prantzos (1999) and Allen et al (1998), so that new independent observations and estimates of the gradients are needed to settle this question.…”

The Supernova – ISM/Star-formation interplay

“…Strickland et al 2004), and starbursts (Melioli & de Gouveia Dal Pino 2004), they are yet too simplistic and mostly spatially poorly resolved to account for quantitative results. Thornton et al derived an efficiency SN of 0.1 from 1D SN simulations as already applied by chemo-dynamical galaxy models (Samland, Hensler & Theis 1997), while unity is also used in some galaxy models (see sect. 3), but seems far too large.…”

“…Recently, Stinson et al (2013) envoked indeed the necessity of short-term SF feedback which is naturely implied by massive stellar radiation and winds but was already included in the chemo-dynamical prescription (see e.g. Samland, Hensler & Theis 1997). It must be explored, however, how effectively this energy is transferred into the ISM as turbulent and finally as thermal energy.…”

Supernova Feedback on the Interstellar Medium and Star Formation

“…Comparison of the thermal energy (red line) and the accumulated supernova typeII energy (middle panel) shows that the thermal energy is almost not enhanced by SNeII. Thornton et al derived an efficiency SN of 0.1 from 1D SN simulations as already applied by chemo-dynamical galaxy models (Samland, Hensler & Theis 1997), while unity is also used in some galaxy models (see sect. 3), but seems far too large.…”