J. Schroyen

Simulations of the formation and evolution of isolated dwarf galaxies – II. Angular momentum as a second parameter

We show results based on a large suite of N-body/smoothed particle hydrodynamics simulations of isolated, flat dwarf galaxies, both rotating and non-rotating. The main goal is to investigate possible mechanisms to explain the observed dichotomy in radial stellar metallicity profiles of dwarf galaxies: dwarf irregulars (dIrrs) and flat, rotating dwarf ellipticals (dEs) generally possess flat metallicity profiles, while rounder and non-rotating dEs show strong negative metallicity gradients. These simulations show that flattening by rotation is key to reproducing the observed characteristics of flat dwarf galaxies, proving particularly efficient in erasing metallicity gradients. We propose a ‘centrifugal barrier mechanism’ as an alternative to the previously suggested ‘fountain mechanism’ for explaining the flat metallicity profiles of dIrrs and flat, rotating dEs. While only flattening the dark matter halo has little influence, the addition of angular momentum slows down the infall of gas, so that star formation (SF) and the ensuing feedback are less centrally concentrated, occurring galaxy-wide. Additionally, this leads to more continuous star formation histories by preventing large-scale oscillations in the star formation rate (‘breathing’), and creates low-density holes in the interstellar medium, in agreement with observations of dIrrs. Our general conclusion is that rotation has a significant influence on the evolution and appearance of dwarf galaxies, and we suggest angular momentum as a second parameter (after galaxy mass as the dominant parameter) in dwarf galaxy evolution. Angular momentum differentiates between SF modes, making our fast rotating models qualitatively resemble dIrrs, which does not seem possible without rotation.

Stellar orbits and the survival of metallicity gradients in simulated dwarf galaxies

We present a detailed analysis of the formation, evolution and possible longevity of metallicity gradients in simulated dwarf galaxies. Specifically, we investigate the role of potentially orbit-changing processes such as radial stellar migration and dynamical heating in shaping or destroying these gradients. We also consider the influence of the star formation scheme, investigating both the low-density star formation threshold of 0.1 a.m.u. cm-3, which has been in general use in the field, and the much higher threshold of 100 cm-3, which, together with an extension of the cooling curves below 104 K and increase of the feedback efficiency, has been argued to represent a much more realistic description of star-forming regions. The N-body-SPH models that we use to self-consistently form and evolve dwarf galaxies in isolation show that, in the absence of significant angular momentum, metallicity gradients are gradually built up during the evolution of the dwarf galaxy, by ever more centrally concentrated star formation adding to the overall gradient. Once formed, they are robust and can easily survive in the absence of external disturbances, with their strength hardly declining over several Gyr, and they agree well with observed metallicity gradients of dwarf galaxies in the Local Group. The underlying orbital displacement of stars is quite limited in our models, being of the order of only fractions of the half-light radius over time-spans of 5-10 Gyr in all star formation schemes. This is contrary to the strong radial migration found in massive disc galaxies, which is caused by scattering of stars off the corotation resonance of large-scale spiral structures. In the dwarf regime, the stellar body only seems to undergo mild dynamical heating, due to the lack of long-lived spiral structures and/or discs. The density threshold, while having profound influences on the star formation mode of the models, has only an minor influence on the evolution of metallicity gradients. Increasing the threshold 1000-fold causes comparatively stronger dynamical heating of the stellar body due to the increased turbulent gas motions and the scattering of stars off dense gas clouds, but the effect remains very limited in absolute terms.

Gaseous infall triggering starbursts in simulated dwarf galaxies

Using computer simulations, we explored gaseous infall as a possible explanation for the starburst phase in Blue Compact Dwarf galaxies. We simulate a cloud impact by merging a spherical gas cloud into an isolated dwarf galaxy. We investigated which conditions were favourable for triggering a burst and found that the orbit and the mass of the gas cloud play an important role. We discuss the metallicity, the kinematical properties, the internal dynamics and the gas, stellar and dark matter distribution of the simulations during a starburst. We find that these are in good agreement with observations and depending on the set-up (e.g. rotation of the host galaxy, radius of the gas cloud), our bursting galaxies can have qualitatively very different properties. Our simulations offer insight in how starbursts start and evolve. Based on this, we propose what postburst dwarf galaxies will look like.

The degeneracy between star formation parameters in dwarf galaxy simulations and the Mstar–Mhalo relation

We present results based on a set of N-body/smoothed particle hydrodynamics simulations of isolated dwarf galaxies. The simulations take into account star formation, stellar feedback, radiative cooling and metal enrichment. The dark matter halo initially has a cusped profile, but, at least in these simulations, starting from idealized, spherically symmetric initial conditions, a natural conversion to a core is observed due to gas dynamics and stellar feedback. A degeneracy between the efficiency with which the interstellar medium absorbs energy feedback from supernovae and stellar winds on the one hand, and the density threshold for star formation on the other, is found. We performed a parameter survey to determine, with the aid of the observed kinematic and photometric scaling relations, which combinations of these two parameters produce simulated galaxies that are in agreement with the observations. With the implemented physics we are unable to reproduce the relation between the stellar mass and the halo mass as determined by Guo et al.; however, we do reproduce the slope of this relation.

Numerical simulations of dwarf galaxy merger trees

We investigate the evolution of dwarf galaxies using N-body/smoothed particle hydrodynamics simulations that incorporate their formation histories through merger trees constructed using the extended Press-Schechter formalism. The simulations are computationally cheap and have high spatial resolution. We compare the properties of galaxies with equal final mass but with different merger histories with each other and with those of observed dwarf spheroidals and irregulars. We show that the merger history influences many observable dwarf galaxy properties. We identify two extreme cases that make this influence stand out most clearly: (i) merger trees with one massive progenitor that grows through relatively few mergers and (ii) merger trees with many small progenitors that merge only quite late. At a fixed halo mass, a type (i) tree tends to produce galaxies with larger stellar masses, larger half-light radii, lower central surface brightness and, since fewer potentially angular momentum cancelling mergers are required to build up the final galaxy, a higher specific angular momentum, compared with a type (ii) tree. We do not perform full-fledged cosmological simulations and therefore cannot hope to reproduce all observed properties of dwarf galaxies. However, we show that the simulated dwarfs are similar to real ones.

New composition-dependent cooling and heating curves for galaxy evolution simulations

In this paper, we present a new calculation of composition-dependent radiative cooling and heating curves of low-density gas, intended primarily for use in numerical simulations of galaxy formation and evolution. These curves depend on only five parameters: temperature, density, redshift, [Fe/H] and [Mg/Fe]. They are easily tabulated and can be efficiently interpolated during a simulation. The ionization equilibrium of 14 key elements is determined for temperatures between 10 K and 10(9) K and densities up to 100 amu cm(-3) taking into account collisional and radiative ionization, by the cosmic UV background and an interstellar radiation field, and by charge-transfer reactions. These elements, ranging from H to Ni, are the ones most abundantly produced and/or released by SNIa, SNII and intermediate-mass stars. Self-shielding of the gas at high densities by neutral hydrogen is taken into account in an approximate way by exponentially suppressing the H-ionizing part of the cosmic UV background for H i densities above a threshold density of n(HI, crit) 0.007 cm(-3). We discuss how the ionization equilibrium, and the cooling and heating curves, depends on the physical properties of the gas. The main advantage of the work presented here is that, within the confines of a well-defined chemical evolution model and adopting the ionization equilibrium approximation, it provides accurate cooling and heating curves for a wide range of physical and chemical gas properties, including the effects of self-shielding. The latter is key to resolving the formation of cold, neutral, high-density clouds suitable for star formation in galaxy simulations.

On the origin of bursts in blue compact dwarf galaxies: clues from kinematics and stellar populations

Blue compact dwarf galaxies (BCDs) form stars at, for their sizes, extraordinarily high rates. In this paper, we study what triggers this starburst and what is the fate of the galaxy once its gas fuel is exhausted. We select four BCDs with smooth outer regions, indicating them as possible progenitors of dwarf elliptical galaxies. We have obtained photometric and spectroscopic data with the FORS and ISAAC instruments on the VLT. We analyse their infrared spectra using a full spectrum fitting technique, which yields the kinematics of their stars and ionized gas together with their stellar population characteristics. We find that the stellar velocity to velocity dispersion ratio ((nu/sigma)(star)) of our BCDs is of the order of 1.5, similar to that of dwarf elliptical galaxies. Thus, those objects do not require significant (if any) loss of angular momentum to fade into early-type dwarfs. This finding is in discordance with previous studies, which however compared the stellar kinematics of dwarf elliptical galaxies with the gaseous kinematics of star-forming dwarfs. The stellar velocity fields of our objects are very disturbed and the star formation regions are often kinematically decoupled from the rest of the galaxy. These regions can be more or less metal rich with respect to the galactic body and sometimes they are long lived. These characteristics prevent us from pinpointing a unique trigger of the star formation, even within the same galaxy. Gas impacts, mergers, and in-spiraling gas clumps are all possible star formation igniters for our targets.