The vast polar structure – VPOS – of satellite objects around the Milky Way

28 April 2012 by Marcel S. Pawlowski, posted in General

After the worrisome news for dark matter in the last weeks, we have to add another today, based on our research (and there is more to come very soon). We show that the disc of satellite galaxies is only a part of a bigger structure: a vast polar structure (VPOS) of diverse satellite objects surrounds the Milky Way, unexpected from cosmological models. The work was done at the University of Bonn, largely through the support of the German Research Foundation (DFG) via its priority programme 1177 "Witnesses of Cosmic History: Formation and Evolution of Black Holes, Galaxies and their Environment" and partially with support from the Bonn-Cologne Graduate School of Physics and Astronomy (BCGS).

With the increasing resolution of cosmological simulations of structure and galaxy formation, it became possible to make predictions on smaller scales. In particular, it became apparent that the dark matter subhalos, typically identified as the sites of luminous satellite galaxies around a host galaxy, are more abundant than observed galaxies. This has been termed the Missing Satellites Problem. (The name, by the way, is an interesting contortion of perspective: taking the model for granted, it blames the observed number of satellites to be too low. One could have termed it the “Over-Abundance of Subhalos Problem”, or something similar.)

But the Missing satellites problem, its suggested solutions and the problems appearing with them are not the topic of today. Instead, let's focus on a different, more roboust test for cosmological models: the spatial distribution of subhalos / satellite galaxies. More roboust, because baryonic physics should not have an effect on scales of tens to hundreds of kpc, which are the typical observed distances between the satellites of the Milky Way.

The characteristic prediction of cold-dark-matter models is well illustrated in the following video, showing a dark matter halo similar to that assumed for the Milky Way. The 'camera' zooms in to the center of the halo, the size of the area shown is given in kpc the upper left corner.

 

 

It visualizes thedark matter density resulting from the Via Lactea-2 simulation (via lactea project, Jürg Diemand). The bright spots show the positions of dark matter subhalos, which might host luminous satellite galaxies. Not only does the number of predicted subhalos (> 1000) not match the observed number (currently 24, probably a few more), they are distributed rather evenly around the center, where the Milky Way would be situated. Simply put, there are subhalos in every direction.

 

The distribution of satellite galaxies

The MW satellites are distributed differently: they trace a disk of satellites (DoS), a planar distribution that is perpendicular to the Milky Way disc. With a radius of up to 250 kpc, this planar structure has a thickness of only 50-60 kpc.  That it is incompatible with the expectations from the standard cosmological model has been pointed out for the first time by Kroupa, Theis & Boily (2005).

It has been verified for the 11 'classical' satellites (Metz et al. 2007) that they are distributed in a thin (40 kpc) planar structure that it oriented perpendicular to the Milky Way disc. This study was complemented by Metz et al (2009) with the inclusion of several fainter Galactic satellites, which led to the same orientation of the 'disc of satellites'. Including a few additional faint satellite galaxies, we have shown in our 2010 paper (Kroupa et al. 2010) that, if you only look at the 13 faint satellite galaxies detected in the SDSS, they even independently describe the same planar orientation as the 11 classical ones.

But not only are the satellite galaxies distributed in this plane, they also move within it, as Metz et al. (2008) have shown. They looked at the 8 satellite galaxies for which the proper motion is known (that is the direction of motion on the sky, in addition to the radial velocity). Out of these 8, 7 are in agreement with moving within the plane described by all the satellites' positions.

All of this work was done exclusively by members of the Stellar Populations and Dynamics Research (SPODYR) group at the University of Bonn.

 

A new idea, adding more orbit-information: streams of stars and gas

In our most recent work, we now added more different objects to this polar structure. The idea came to my mind when I (Marcel) was reading a paper about several newly discovered stellar streams (Grillmair 2009). When an object (a star cluster or dwarf galaxy) orbits around the Milky Way it looses stars. These stars will either be slightly faster or slightly slower than the object, and therefore take over or fall back along the orbit of the object. Some streams can be clearly assigned to an object, in other cases the object has been completely torn apart and only the stream is left. In both cases, the stars in the stream move more or less into the same direction as the object they came from. The streams, therefore, are situated in the same orbital plane at the progenitor object. They tell us about the object's path around the Milky Way.

If the distribution of satellite objects around the Milky Way is stable, then the objects should move within this plane. So the orbital planes, traced by the streams of stars (or gas in some cases), should preferentially align with the satellite galaxies distributed in the DoS.

After the idea was born, we developed a method for determining the orientation of a stream. The results of the first few streams looked very promising, and so I searched the literature in order to collect data for as many streams around the Milky Way as possible. In the end, we were able to use 14 streams.

Half of them turned out to be well aligned with the DoS. If they would have been drawn from an isotropic distribution (a zeroth-order approximation of the distribution expected from cosmological models) the likelihood to find that many streams this close to the DoS is only 0.3 per cent.

Since now we know that the satellite galaxies and half of all streams align in the same structure, we began referring to this structure as the 'vast polar structure' (VPOS) of the Milky Way.

 

A different class of objects in the VPOS

The streams, as mentioned before, not only originate from dwarf galaxies, but also from globular clusters. When streams stemming from globular clusters are also located in the VPOS, shouldn't these dense stellar systems be distributed within it, too? We checked this idea in the paper.

To do this, one has to know that globular clusters (GCs) can be classified in different groups, which are thought to have different origins (e.g. Mackey & van den Bergh 2005). There are GCs that lie in the disc of the Milky Way, so their distribution must be in the Galactic plane and not perpendicular to it like the VPOS. There are so-called old halo GCs. Their very old ages tall us that they have formed together with the Milky Way. They should not show signs of the VPOS. And there are 'young' halo GCs, which exhibit similarities to GCs associated with satellite galaxies, and must have a different origin than the other two groups.

When analysing the distributions of these different groups, we were stunned how well our expectations were met. The first two groups of GCs turned out to be completely unrelated to the VPOS. But the young halo globular clusters in fact define the same vast polar structure as the satellite galaxies, their motions and the streams. Even if you look only at the near young GCs (within 20 kpc) and the far ones (beyond 20 kpc) individually, they follow virtually the same structure. The likelihood of this happening in a random distribution is only 0.1 per cent.

The video below illustrated the distribution of all the different objects forming the VPOS, extending from 10 out to 250 kpc around the Milky Way. Note that the streams are magnified by a factor of three for better visibility.

 

 

 

The vast polar structure - VPOS - about the MW in Cartesian coordinates. The movie rotates the view over 360 degree, adding different objects around the Milky Way galaxy. The y-axis points towards the Galactic north pole. The 11 classical satellites are shown as yellow dots, the 13 new satellites are represented by the smaller green dots, young halo globular clusters are plotted as blue squares. The red curves connect the anchor points of streams of stars and gas, the (light-red) shaded regions illustrating the planes defined by these and the Galactic centre. Note that the stream coordinates are magnified by a factor of 3 to ease the comparison. The obscuration-region of 10 degree around the Milky Way disc is given by the horizontal grey areas. In the centre, the Milky Way disc orientation (edge-on) is shown by a short horizontal cyan line. One can clearly see when the view is edge-on to the VPOS: The extend of all types of objects becomes minimal, also the streams align preferentially with this structure. From standard dark matter cosmology, a much more spheroidal distribution of objects around the Milky Way is expected. We therefore propose the satellite galaxies of the Milky Way to be Tidal Dwarf Galaxies. Feel free to download the movie to be used in talks.

 

Suggested solutions (and why they do not work)

Within the standard cold dark matter cosmological model, such a strongly correlated structure was not predicted. This is why there have been several attempts to explain planar distributions of satellites after it became known. The list below motivates why none of these are satisfying:

  • Chance alignment: This is one of the initial and most simple ideas. If the structure is made up of only a few objects, it might just be bad luck that they all fall into a planar distribution right now. But the addition of more and more objects has reduced the likelihood of a chance alignment. A chance-alignment of the positions of the 11 classical satellites alone can be excluded at a 99.5 confidence level (Metz et al. 2007). Including the correlated motions of these or adding more objects makes this statement even more stringent, such that a chance alignment must be excluded. In addition, the preferred motion of the objects within the plane (from proper motions and stream orientations) shows that the structure is stable over time.
  • Group infall: Maybe a number of satellite galaxies were accreted by the Milky Way together in a group. It is known that associations of dwarf galaxies exist, so this was a good idea put forward by Li & Helmi (2008) and D'Onghia & Lake (2008). But as Metz et al. (2009) have shown, the observed associations are much wider than the VPOS. A structure as thin as observed can not have formed this way. In addition, the increased number of satellite objects within the VPOS speaks against this scenario, because in addition to the infall of a group of a few subhalos, a more evenly distributed population of subhalos has to be around.
  • Filamentary accretion: As seen from simulations of structure formation in the universe, there is a giant 'cosmic web' of material connecting galaxies, which are formed preferentially within such filaments. Maybe small dark matter halos / dwarf galaxies are accreted preferentially along such filaments, resulting in a preferred spacial distribution? This seems not to work out because the filaments, like the groups before, are too thick and not the only source of subhalos. While there are overdensities of infall-directions at large distances (see for example Libeskind et al. 2011), no structure as well defined as the VPOS is produced. Nevertheless, the abstract of Lovell et al. (2011) claims that “Quasi-planar distributions of coherently rotating satellites, such as those inferred in the Milky Way and other galaxies, arise naturally in simulations of a ΛCDM universe”. In a few days we will show in detail why this claim is unjustified. UPDATE: The preprint of our paper "Can filamentary accretion explain the orbital poles of the Milky Way satellites?" is now available on the arXiv. We will blog about it soon. UPDATE: Here is our blog post on this paper.

 

A radically different scenario

In our paper, we propose a radically different alternative: the VPOS has been formed from the debris of a collision of two galaxies. The satellite galaxies would then not be dark-matter dominated objects, but tidal dwarf galaxies that formed within the tidal debris stripped from another galaxy. It is noteworthy that this had already been hinted at by the early stellar-dynamical work by Kroupa (1997)  who showed that the satellite galaxies may not require dark matter but that they may appear as if they had dark matter. Tidal dwarf galaxies are observed to form and naturally align in the plane of the interaction.

As we have shown in our paper “Making counter-orbiting tidal debris. The origin of the Milky Way disc of satellites?”, Pawlowski et al. (2011), a number of features of the satellite galaxy population of the Milky Way are consistently explained if they stem from tidal debris. In addition, Pavel has laid out more reasons in favor of a tidal origin in his recent paper (Kroupa 2012).

 

More Information

The papers reporting problems for dark matter keep coming in more frequently lately. Be prepared for another one from our side next week, discussing filamentary accretion as a possible origin of the VPOS. UPDATE: preprint available here, discussed in the blog here.

The paper this post is based on: “The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way”, by Marcel S. Pawlowski, J. Pflamm-Altenburg and Pavel Kroupa. It has been accepted for publication in MNRAS and a preprint can be found on the arXiv.

The Royal Astronomical Society has published a press release on this topic, too: “Do the Milky Way’s companions spell trouble for dark matter?

It was picked up by a number of news sites, a selection of which we list below:

Blog posts:

 

By Pavel Kroupa and Marcel Pawlowski  (28.04.2012): "The vast polar structure - VPOS - of satellite objects around the Milky Way" on SciLogs. See the overview of topics in  The Dark Matter Crisis.

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