Modelling galaxies with a 3d multi-phase ISM

Abstract

We present a new particle code for modelling the evolution of galaxies. The code is based on a multi-phase description for the interstellar medium (ISM). We include star formation (SF), stellar feedback by massive stars and planetary nebulae, phase transitions, and interactions between gas clouds and ambient diffuse gas, namely condensation, evaporation, drag, and energy dissipation. The last is realised by radiative cooling and inelastic cloud-cloud collisions. We present new schemes for SF and stellar feedback that include a consistent calculation of the star-formation efficiency (SFE) based on ISM properties, as well as a detailed redistribution of the feedback energy into the different ISM phases. As a first test we show a model of the evolution of a present day Milky-Way-type galaxy. Though the model exhibits a quasi-stationary behaviour in global properties like mass fractions or surface densities, the evolution of the ISM is strongly variable locally depending on the local SF and stellar feedback. We start only with two distinct phases, but a three-phase ISM is formed soon and consists of cold molecular clouds, a warm gas disk, and a hot gaseous halo. Hot gas is also found in bubbles in the disk accompanied by type II supernovae explosions. The volume-filling factor of the hot gas in the disk is ∼35%. The mass spectrum of the clouds follows a power-law with an index of α ≈ −2. The star-formation rate (SFR) is ∼1.6 M yr −1 on average, decreasing slowly with time due to gas consumption. In order to maintain a constant SFR, gas replenishment, e.g. by infall, of the order 1 M yr −1 is required. Our model is in fair agreement with Kennicutt's (1998, ApJ, 498, 541) SF law including the cut-off at ∼10 M pc −2 . Models with a constant SFE, i.e. no feedback on the SF, fail to reproduce Kennicutt's law. We performed a parameter study varying the particle resolution, feedback energy, cloud radius, SF time scale, and metallicity. In most these cases the evolution of the model galaxy was not significantly different to our reference model. Increasing the feedback energy by a factor of 4−5 lowers the SF rate by ∼0.5 M yr −1 , while decreasing the metallicity by a factor of ∼100 increases the mass fraction of the hot gas from about 10% to 30%.

A study of major mergers using a multi‐phase ISM code

“…Since studying the evolution of star formation is the main aim of this work, the next section will describe its implementation in more detail. For a thorough description of all other processes we refer to Harfst et al (2006).…”

“…The star formation scheme according to Harfst et al (2006). First the molecular cloud is inactive for a period τia (t0).…”

“…Following Harfst et al (2006) the initial galaxies were constructed by firstly generating a disk/bulge/halo -system using the method of Kuijken & Dubinski (1995). In the next step, one fifth of the stellar disk particles were transformed into molecular clouds and finally the diffuse ISM was added as an initially slowly rotating homogeneous sphere, which will collapse in the first 200 Myr forming a warm disk.…”

“…Our code incorporates a 3-phase ISM similar to the proposition by McKee & Ostriker (1977). The code was developed in order to study Milky Way-like galaxies accounting for several components, namely stars, dark matter, and the three phases of the ISM (Harfst et al 2006). The warm and hot phases are treated by smoothed particle hydrodynamics implemented similar to Hernquist & Katz (1989).…”

Starbursts in isolated galaxies

“…The simulations were done with the SPH part of the code described in Harfst et al (2006). Two sets of constants were derived: set 1 (C P = 1.91, C j = 0.15, C fric = 2.06) gives a good fit to the early evolution, whereas set 2 (C P = 1.97, C j = 0.11, C fric = 0.88) describes the late evolution (Fig.…”