Andromeda’s satellites behave as expected … if they are tidal dwarf galaxies

3 January 2013 by Marcel S. Pawlowski, posted in Satellite Galaxies

Today's issue of Nature contains a very exciting study by Rodrigo Ibata et al. which might be a game-changer in the research areas of galaxy formation and near-field cosmology. It is titled “A vast, thin plane of corotating dwarf galaxies orbiting the Andromeda galaxy” and already now should be seen as a candidate for the most-exciting paper of 2013.

UPDATE Jan. 4th: The article is now also available on the arXiv.

Pavel Kroupa and I have been waiting for this paper to appear for quite some time. Several months ago we've heard the first rumors that Ibata from the University of Strasbourg has detected, with great significance, a plane of satellite galaxies around our neighboring spiral galaxy Andromeda (M31). My curiosity even made me look into available data, which supported what we had heard. Chatting with Rodrigo during a recent N-body meeting in Bonn (after his paper was accepted) finally confirmed these rumors. Seldom have I been looking forward to a paper this curiously, while at the same time being aware of its essential content already.


The Facts

So, what is it all about? Ibata and his collaborators have performed the Pan-Andromeda Archeological Survey (PandAS, lead by Alan McConnachie), an extensive observational campaign of the region around the Andromeda galaxy. This survey has unveiled many of Andromeda's satellite galaxies and allowed the team to measure the distances to these satellite galaxies in a homogeneous manner (Conn et al. 2012). They then looked at the spacial distribution of the satellite galaxies around their host, motivated by the distribution of satellite galaxies of our own Galaxy. Around the Milky Way, the satellites are distributed and orbit in a thin plane, which we recently termed a vast polar structure (VPOS, Pawlowski et al. 2012a). In fact, the satellite objects are correlated to a degree which is at odds with cosmologically motivated expectations.

Now Ibata et al. find that out of the 27 satellite galaxies in their sample, 15 lie in a common plane. They report that this plane has a thickness of only 13 kpc (40,000 light years), while it has a diameter of at least 400 kpc (1.3 million light years), possibly reaching further out beyond the PAndAS survey region. They can rule out that a chance-alignment is responsible for this configuration with very high confidence, the likelihood that such a well-pronounced structure appears at random is only 0.13 per cent.


An illustration of the Andromeda satellite galaxies which belong to the co-orbiting satellite plane. The top-right vie shows the satellites plane edge-on, as seen from the Milky Way, while the bottom left shows the plane rotated by 90 degrees (the orientations of these two views are indicated in the lower right). The top-left is a optical picture of the Andromeda galaxy. Image Credit: Ibata et al.


But it is not only the existence of this plane which is stunning. The plane is aligned perfectly with the Milky Way, in a way such that we see it edge-on. This fortunate orientation allowed Ibata et al. to also look at a kinematical coherence. We can measure the radial velocities of the satellite galaxies, which lie within the plane due to the planes orientation. This reveals that 13 out of the 15 satellite galaxies in the plane show a common sense of rotation. This, again, is similar to the VPOS around the Milky Way, in which at least 8 satellites orbit in the same sense, while at least one is counter-orbiting in the same plane (Pawlowski 2012). The authors state that including this kinematic information into their analysis increases the significance of the satellite plane to 99.998 per cent. This is just amazing.

Here you can find a very nice video animation illustrating the structure's orientation with respect to the Milky Way.

Unfortunately the letter itself is behind Nature's pay-wall, so you can only access it if you have a Nature subscription. I'll update this blog post if a freely accessible arXiv version becomes available. For the meantime, please be referred to the accompanying press releases. UPDATE Jan. 4th: The arXiv version of the article can be found here.


The Interpretation

In my opinion, the importance of this discovery can not be over-stated, which is in line with Nature publishing a comment on the discovery in the same issue (“Astronomy: Andromeda's extended disk of dwarfs” by R. Brent Tully) and even making the letter its cover story. The about-the-cover text already hints at the study's importance:

“Recent studies of the dwarf galaxies of the Milky Way have lead some astronomers to suspect that their orbits are not randomly distributed. This suspicion, which challenges current theories of galaxy formation, is now bolstered by the discovery of a plane of dwarf galaxies corotating as a coherent pancake-like structure around the Andromeda galaxy”

I suppose that due to the restrictive space constraints set by Nature (4 pages, 30 references), the letter is short and does not discuss the study's implications in extensive detail. In their letter, Ibata et al. mention two broad ideas which might lead to an explanation for the structure's existence.

  • Either all the satellites in the plane were accreted together, which is unlikely because the very small thickness of the satellite plane restricts the size of an accreted group to less than 14 kpc. Such groups are not observed in the universe.
  • Or the satellites within the plane were formed in place around Andromeda, for example as tidal dwarf galaxies.

Overall, the authors prefer not to make any strong conclusions, instead stating that “the formation of this structure around M31 poses a puzzle”, which is also the prevailing tone of the press release. This is why I would like to share some of my thoughts on the discovery and also highlight some very relevant publications that obviously did not make it into the letter.


Filamentary Accretion?

The letter by Ibata et al, but also the comment by Tully, discusses that the accretion of dwarf galaxies along cosmic filaments might be responsible for the structure. However, there are several reasons why this idea does not work. First of all, the filaments found in cosmological simulations are too thick. They would need to be as thin as the observed structures (< 14 kpc) to have a chance to explain the planes, but their size typically is on the order of 500-1,000 kpc. This is supported by studies like Vera-Ciro et al (2011), who, analyzing the behavior of dark matter particles in cosmological simulations, conclude that

“[...] at later times the cross-section of the filaments becomes larger than the typical size of Milky Way like haloes and, as a result, accretion turns more isotropic [...]”.

Consequently, the satellite structure can not be both: of filamentary origin and young, which contradicts the argument in Tully's comment.

In Pawlowski et al. (2012b) we have also shown that even in case of the VPOS of the Milky Way satellites, a filamentary accretion origin can be ruled out because the coherence of the orbital poles of the sub-halos in cosmological high-resolution simulations is not strong enough to explain the alignment of the MW satellite orbits. The filament might initially lead to a preferred direction of infall, but does not produce a thin, co-rotation plane of sub-halos but a prolate distribution. And now the Andromeda satellite disc is even thinner and more coherent than the VPOS. For more details, please have a look at my blog post on filamentary accretion.


Tidal Dwarf Galaxies

In contrast to the often mentioned accretion along cosmic filaments, the tidal dwarf galaxy scenario is a much more natural explanation for co-orbiting discs of satellite galaxies. In this scenario, two galaxies interact, such that the tidal forces rip out matter from the galactic discs, which form spectacular tidal tails. Within this tidal debris new galaxies (tidal dwarf galaxies or TDGs) form, a process which is observed to happen in the universe and also reproduced by simulations. As the TDGs form from a common tidal tail, they share a common orbital direction and are generally found in a thin plane. Just as it is observed around the Milky Way and now Andromeda.

In fact, this TDG scenario can also explain the existence of counter-orbiting satellites, of which there seem to be two in the Andromeda disc and at least one around the Milky Way (Pawlowski et al. 2011). There is even a study proposing that Andromeda experienced such an galaxy encounter (Hammer et al. 2010), during which TDGs have been formed. These might even be responsible for the VPOS of the Milky Way (Fouquet et al. 2012), in which case the Milky Way should lie within the satellite plane around Andromeda ... which is indeed the case. Unfortunately, all these very relevant papers did not make it into the short Nature letter.

All this is also why I have to disagree with a sentence in R. Brent Tully's discussion of the letter (which of course got picked up by the media ...). He states that

“No theorist of galaxy formation would have dared to predict such a situation”.

This is not quite true. I would also argue that the authors of Fouquet et al. (2012) have been expecting such a situation in their tidal dwarf galaxy scenario and that most researchers working on tidal dwarf galaxies would probably predict such an orientation for TDGs. Even I wrote about this in my 2012 paper on the Milky Way VPOS:

“The M31 satellites are preferentially distributed in a structure extending approximately from north to south in Galactic coordinates, just as the MW VPOS extends in the north–south direction. A common direction of the satellite distributions of both galaxies is expected in a tidal scenario that formed both satellite populations together, as TDGs form in a plane defined by the orbit of the interaction.”

There is one major argument against the tidal dwarf galaxy scenario: tidal dwarfs do not contain a significant amount of dark matter, while some of the observed satellite galaxies seem to be completely dark matter dominated. This argument is based on two major assumptions which, however, might both be questioned:

  1. The dwarf galaxies are dynamically relaxed, gravitationally bound systems. If they are not and do not contain dark matter, high mass to light ratios might be derived from their velocity dispersion by mistake (e.g. Kroupa 1997, Klessen & Kroupa 1997).
  2. The underlying gravity law is Newtonian. If the gravity law is modified, e.g. In the low acceleration regime, most satellite galaxies would not need dark matter (e.g. Famaey & McGaugh 2012).



Because of the new study we now know that both satellite galaxy systems for which we have full three-dimensional positions available show strong planar alignments. This coherence is also supported by the available kinematic data: the objects in the VPOS around the Milky Way and in the disc of satellites around Andromeda mostly co-orbit in the same direction.

Such a phase-space coherence is expected if the satellite galaxies were born as tidal dwarf galaxies, but completely at odds with all current cosmological simulations in which the satellites are assumed to be represented by dark matter dominated sub-halos. Therefore, the discovery by Ibata and collaborators, in my opinion, supports the tidal dwarf galaxy scenario and will contribute to a paradigm shift in the field of galaxy formation. We might have to re-consider what we know about near-field cosmology and will have to develop a new understanding of the origins of dwarf satellite galaxies. In the end, this publication might even have an impact on our understanding of the laws of gravity.

The cosmological implications of VPOS-like structures are discussed at length in our paper Kroupa et al. (2010) "Local-Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation" and in the review by Kroupa (2012) "The dark matter crisis: falsification of the current standard model of cosmology".



By Marcel Pawlowski and Pavel Kroupa  (03.01.2013): "Andromeda's satellites behave as expected ... if they are tidal dwarf galaxies" on SciLogs. See the overview of topics in  The Dark Matter Crisis.

3 Responses to “Andromeda’s satellites behave as expected … if they are tidal dwarf galaxies”

  1. Ben Reply | Permalink

    Excellent post! Just to complement it, as you know, tidal dwarf galaxies are indeed not supposed to harbour large amounts of non-baryonic dark matter in the cold dark matter context *BUT* young TDGs for which the dynamical mass has been observationally measured do harbour substantial amounts of dark matter anyway: ... And they are on the baryonic Tully-Fisher relation, which has a very strong (tasteful or distateful depending on one's own prejudice) MOND aroma ;)

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