Dark influences

Abstract

Context. It has been proposed that mergers induce starbursts and lead to important morphological changes in galaxies. Most studies so far have focused on large galaxies, but dwarfs might also experience such events, since the halo mass function is scale-free in the concordance cosmological model. Notably, because of their low mass, most of their interactions will be with dark satellites. Aims. In this paper we follow the evolution of gas-rich disky dwarf galaxies as they experience a minor merger with a dark satellite. We aim to characterize the effects of such an interaction on the dwarf's star formation, morphology, and kinematical properties. Methods. We performed a suite of carefully set-up hydrodynamical simulations of dwarf galaxies that include dark matter, gas, and stars merging with a satellite consisting solely of dark matter. For the host system we vary the gas fraction, disk size and thickness, halo mass, and concentration, while we explore different masses, concentrations, and orbits for the satellite. Results. We find that the interactions cause strong starbursts of both short and long duration in the dwarfs. Their star formation rates increase by factors of a few to 10 or more. They are strongest for systems with extended gas disks and high gas fractions merging with a high-concentration satellite on a planar, radial orbit. In contrast to analogous simulations of Milky Way-mass galaxies, many of the systems experience strong morphological changes and become spheroidal even in the presence of significant amounts of gas. Conclusions. The simulated systems compare remarkably well with the observational properties of a large selection of irregular dwarf galaxies and blue compact dwarfs. This implies that mergers with dark satellites might well be happening but not be fully evident, and may thus play a role in the diversity of the dwarf galaxy population.

Dark influences

“…One of the most direct imprints in gas-rich dwarfs is a vast increase in star formation: both in short bursts (during close passages of the satellite) as well as sustained high star formation rates lasting several Gyrs. In Starkenburg et al (2016) we show that the general properties of our simulated dwarf systems compare very well to a large sample of dwarf irregular galaxies and blue compact dwarfs from the literature. In this paper we provide a quantitative characterization of the morphological and kinematic properties of the dwarf systems during the minor merger events and thereby facilitate a more detailed comparison to observations.…”

“…In Starkenburg & Helmi (2015) and Starkenburg et al (2016), we show that such minor mergers can significantly alter the morphological properties of dwarf galaxies. The disturbances induced by dark objects are much more dramatic on this scale because of the lower galaxy formation efficiency (i.e.…”

“…We analyse the structural properties of dwarf galaxies during minor mergers with dark satellites for a suite of hydrodynamical simulations recently presented in Starkenburg et al (2016). The simulations span a range of initial conditions for the dwarf galaxy, its satellite and a variety of orbital configurations for the interaction.…”

“…We will focus mostly on one of the simulated dwarfs, which we refer to as model-A (Starkenburg et al 2016) for which we explore ranges in the extend of the gas disk with respect to the stellar disk, the satellite mass, and the satellite orbit. We will also report results for smaller mass systems with a range of disk thickness and halo concentrations, models B, C, and D in Table 1 (see Starkenburg et al (2016) for more detailed information).…”

The no-spin zone: rotation versus dispersion support in observed and simulated dwarf galaxies

“…Dwarf-dwarf mergers can create dSphs (Moore et al 1996;Kazantzidis et al 2011b), and the mechanism is satisfyingly similar to models proposed for transforming massive disks into giant ellipticals (Icke 1985). Starkenburg, Helmi & Sales (2016) propose that the spheroidal shapes of dSphs can be reproduced by mergers between dwarf galaxies and lower-mass dark halos, but do not discuss rotation support. Another model, "resonant stripping", posits that a fly-by between a dwarf and a galaxy 100 times its mass can instigate resonances in the smaller dwarf that preferentially strip the stellar material (D'Onghia et al 2009).…”