Models For dSph galaxies without Dark Matter

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.

Some theories imply that these galaxies are not born inside a DM halo but instead are formed during tidal interaction between two mayor galaxies. These interactions produce tidal tails and within these tidal tails new second-generation dwarf galaxies called tidal dwarf galaxies are formed, which later orbit their host galaxy.

Our model

In collaboration with the observational group in Cambridge we got information about new measurements of the velocity dispersion and their peculiarities of several of the ultra-faint dwarfs. For example the velocity dispersion of Bootes~I is now reduced from 7.2 km/s down to below 4 km/s. With new models we could show that the conclusion of Fellhauer et al.\ (2008), that the Bootes dSph satellite has to be dark matter dominated, no longer holds. It is possible to obtain models without dark matter which result into an object like Bootes today. The observational paper is published (Koposov et al. 2011) but the observational co-authors voted against the theoretical sub-section inside their paper.

M. Fellhauer was also part of the spectroscopic confirmation of the new dSph galaxy Bootes~II (Koch et al. 2009).

Recent observational data (deep images) shows that there seems no real connection between Ursa Major II and the Orphan Stream. Also measurements of the velocity dispersion of UMa II revealed a strong velocity gradient inside the dwarf galaxy. This could be a sign of rotation or a signal for tidal disruption. If it is the latter case this gives us new constraints for the orbit of the dwarf galaxy and new models without dark matter are needed to show if we can still reproduce the properties of the dwarf galaxy today. We find new sets of possible orbits, abandoning the possible connection to the Orphan Stream, Fellhauer et al. (2007) used in their models. Our model does reproduce the high velocity dispersion and the velocity gradient inside of UMa II. Furthermore, we match the shape and luminosity of the observed dwarf. We focused on a dark matter free progenitor as already Fellhauer et al. 2007 pointed out that with a dwarf embedded in an dark matter halo we loose all restricting parameters for the orbit. In such a case the elongation of UMa II must be due to intrinsic rotation and is not necessarily aligned with the orbit of the dwarf. We investigated further if our results are just possible because of a very lucky instant we see the dwarf or if it is a more general feature. We show in (Smith et al. 2013c) that there is regular boosting of the velocity dispersion around the apo-galacticon. A first version of the paper was submitted earlier but then withdrawn in the light of new results.

M. Blana ('topico project') investigated the minimal possible mass required to have Leo IV and Leo V forming a bound pair. Our results show that the minimum mass required for the pair to be bound is rather high - ranging from 2 x 10^9 M_sun to 4.5 x 10^10 M_sun (within the virial radius). Computing the mass in dark matter within the standard optical radius of 300 pc shows that our models are well within the predicted range of dark matter content for satellites that faint. We therefore conclude that it is indeed possible that the two galaxies form a bound pair (M. Blana et al.\ 2012).

M. Blana ('titulo' project) investigated models for the Hercules dSph galaxies using the orbit determined by Martin and Jin (2010). Finally, we found a suitable model which fits total magnitude, surface brightness, scale-length, dispersion and velocity gradient and have submitted the manuscript (Blana et al. 2013). This paper for the first time investigates a larger parameter space and shows how the different parameter can be fitted by different regions in parameter space.

M. Fellhauer was part of an observational campaign to investigate the elongation of the Hercules dwarf (Deason et al. 2012a) together with the observational group in Cambridge, UK.

Meanwhile our group has presented results not only for Hercules, Ursa Mayor II and Bootes but also for Segue 1, Coma Berenice and Canes Venaticii I.

We have show that at least some of the ultra-faint dSph galaxies are not necessarily DM dominated.


This work is/was supported by the following grants:

© Theory & Star Formation Group 2017