The Formation and Evolution of Dwarf Spheroidal Galaxies

Introduction

The Milky Way (MW) is surrounded by a multitude of dwarf galaxies. Most of these dwarf galaxies are part of the very faint class of spherical galaxies called dwarf spheroidal galaxies (dSph).

Radial velocity measurements reveal very high velocity dispersions inside these faint objects and assuming that these dwarfs are in virial equilibrium points to the fact that they must be highly dark matter (DM) dominated, exhibiting mass-to-light (M/L) ratios of several tens to hundreds. With the newly found ultra-faint dSph galaxies these values rise up to more than 1000.

The standard cosmological theory, ΛCDM, predicts that smaller haloes merge and build up larger haloes. The same should be true for the luminous components of these haloes. This implies that there has to be a class of objects which should be the basic building blocks of larger galaxies. The dSph galaxies we see today are possibly the left-overs of these building blocks. This makes the understanding of the formation and evolution of dSph galaxies an important task in understanding the general topic of galaxy evolution.

Most theories about the formation of dSph galaxies imply that they are born as somewhat larger dwarf disc galaxies and are transformed by tidal interactions and/or ram-pressure stripping into the objects we see today. The interactions happen either with mayor galaxies like the MW or between two or more of the dwarfs themselves.

Those theories have the problem to explain isolated dSph galaxies, which still contain gas and are forming stars like, e.g. Leo T.

The Merging Star Cluster Scenario

We propose a new scenario to explain the formation of dSph galaxies, combining two very standard theories. The first ingredient is the ΛCDM formation scenario of structure formation in the Universe. In this scenario, as seen for example in the Millennium II simulation of Boylan-Kolchin et al. (2009), we see that small dark matter (DM) haloes form first and later merge into larger entities. As dSph galaxies are supposedly residing in the smallest haloes, which were able to retain gas and form stars, they are regarded the basic building blocks of larger galaxies.

At same time it became clear, that stars do not form evenly distributed over a galaxy (e.g. Lada and Lada 2003), but in clumpy, hierarchical structures spanning from associations, open clusters all the way to globular clusters. If the star formation efficiency is low then these newly formed star clusters are not able to survive - they dissolve and spread their stars inside the galaxy.

In our models we combine the two approaches and simulate dissolving star clusters inside a DM halo. With our models we are able to explain many of the unusual features presented with dSph galaxies.

Results

We show that our models provide easy explanations for various observations found with dSph galaxies around the MW. Our models explain distorted contours, density centres which are not exactly located in the geometrical centre of the dwarf, multiple centres of densities as well as elongations and tails. On the dynamical side we can explain with our models both raising and falling velocity dispersions in the centre and the same for the outer parts. Due to effects what we call fossil remnants of the formation process we are able to explain bumps and wiggles in the velocity dispersion profile, which were until now regarded as errors in the measurements and we see effects which could mimic a velocity gradient throughout the dwarf without the need of a real tidal field, destroying the satellite.

In Assmann et al. (2013a) we describe our fiducial model. It shows a surface brightness distribution similar to the ones found with the classical dSph. The long-term evolution has erased most of the sub-structure from the dissolving star clusters (SCs). The scale-length is about 500 pc and the profile is close to exponential. It also shows a flat velocity dispersion profile, with an over-all dispersion of about 9 km/s. To achieve these values we used an NFW-halo with a somewhat lower mass than deduced from observations of real dSph. While those have about 10^7 M_sun within 300 pc, our model has 10^7 M_sun within 500 pc. We introduce a measure named delta = Delta(v_mean,max) / sigma_500pc to quantify the velocity 'anomalies' stemming from the dissolved SCs (dubbed fossil remnants).

In Assmann et al. (2013b) we publish a parameter study and show how our initial parameters influence the results of the simulations. We see that the scale-length of the halo mainly influences the central brightness of our models, while the scale-length of the SC distribution influences the scale-length of the luminous component. We explore in which part of parameter space we see the largest velocity anomalies and identify which parameters lead to SCs surviving in the central areas of the luminous component.

The Star Formation Histories of dSph galaxies

In a new extension of this project we now include the star formation histories (SFH) into our dSph models. So far all SCs are formed at the same time and inserted into our simulations. While for many dSph galaxies, which only show an old population stemming from a single star burst, this may be OK, we wanted to show that our models also work when other SFHs are taken into account.

We see in these new models that we may be able to break the cusp/core problem of dSph galaxies. We see that cored halo models suppor the survival of SCs and have longer destruction times, especially in the centres of the dSph galaxies. While SCs surviving in the outer parts may have formed with high SFEs, SCs in the central regions survive more likely in cored haloes. As most dSph galaxies do not show SCs associated with them (with the exception of Fornax and Sagittarius), this could point to the fact that the haloes are cusped or at least were cusped in the past.

Models with Angular Momentum

Our group is now embarking on a new project to take our results to a larger level and explore more massive haloes and include angular momentum into the distribution of SCs. We want to investigate the transition between fully dispersion dominated object into dwarf disc galaxies. Please stay tuned for more results soon.


This work is/was supported by the following grants:

© Theory & Star Formation Group 2017