In the previous post we discussed the VPOS, the vast polar structure of satellite objects around the Milky Way. One of the suggested origins within the cosmological cold dark matter paradigm is that the satellites have been preferentially accreted along large, cosmic filaments. These are long, thread-like structures which arise naturally during the formation of structure in the cosmos. The movie below shows how they come about:
One work suggesting that filamentary accretion can solve the VPOS-problem is Lovell et al. (2011). Its abstract claims that:
“All [six] haloes [of the Aquarius simulations] possess a population of subhaloes that rotates in the same direction as the main halo and three of them possess, in addition, a population that rotates in the opposite direction. These configurations arise from the filamentary accretion of subhaloes. 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.“
Note the part we marked in bold face. This statement of theirs suggests that a structure like the VPOS is a natural outcome of cosmological simulations, which arises due to the filaments around a dark matter halo. That filaments can lead to anisotropies in the direction from which sub-halos are accreted onto larger halos is obvious, and it was a good idea that this might be a way to form anisotropic distributions of subhalos. However, there are several reasons to doubt this scenario.
First of all, the filaments are way too thick. For example, in Vera-Cirro et al (2011) it is shown that the filaments in the Aquarius simulations are very wide, of the order of 0.5 – 1 Mpc (see figure below). The VPOS has a thickness of only about 50 kpc. There is no way it can have been formed out of a much bigger filament. The filaments are in fact larger than the halo of the main galaxy (virial radius 200-250 kpc). Vera-Cirro et al (2011) write:
“[...] when the surrounding filament is sufficiently wide, i.e. of comparable or larger cross-section than the virial radius of the halo, the infalling particles will appear to be more isotropically distributed on the sky [...]. We have argued [...] that [this] case is characteristic of the late stages of mass assembly in 10^12 Msun objects.”
This statement is contrary to the Lovell et al. (2011) one. In the simulations Vera-Cirro et al. (2011) discuss, the filaments are much wider than the central halo for most of the time of the simulation (from about 5 Gyr on). Thus, for about the past 9 Gyr, the accretion must have been more isotropically. Interestingly, the Vera-Cirro et al. (2011) work is based on the Aquarius simulations, the same set of cosmological simulations as the Lovell et al. (2011) paper. And it has been accepted for publication before the Lovell et al. (2011) paper.
Caption: Part of figure 4 of Vera-Ciro et al. (2011). It illustrates the size of a cosmic filament around a Milky Way like halo. The virial radius of the central halo is shown by the white ellipse. The thickness of the VPOS is less than one 10th of the white line at the bottom giving the scale of about 700 kpc.
In addition to this, the orientation of the preferred direction of orbits of subhalos is at odds with the expectations. Lovell et al. (2011) show that there is a slight over-abundance of subhalos orbiting in the same direction as the main halo (and in some cases also in the opposite direction). However, the galaxies forming in the main halos preferentially spin in the same direction as the main halo, so the sub-halo over abundance lies in the same plane as the galactic disc. In the case of the VPOS around the MW, the orientation is perpendicular.
Finally, Lovell et al. (2011) did not test the rather strongly worded statement of their abstract quantitatively. From their figures, it is already obvious that the before mentioned over-abundance of co-orbiting subhalos is small, only a factor of about 2 compared to the isotropic case in the bin closest to the main halo spin. The majority of subhalo orbital directions is distributed more evenly around the main halo.
Caption: The directions of angular momentum vectors, called orbital poles, of sub-halos coming from a cosmological cold-dark matter simulation (upper) and satellite galaxies of the Milky Way (lower). The question we addressed in our recent paper was: how likely is it that a distribution like the observed (lower) one can arise when drawing from the modeled (upper) one.
To allow a fair comparison, we have developed a method to test this claim. It is described in our recent paper “Can filamentary accretion explain the orbital poles of the Milky Way satellites?” (by Marcel S. Pawlowski, Pavel Kroupa, Garry Angus, Klaas S. de Boer, Benoit Famaey and Gerhard Hensler). In it, we determine how likely it is to find sets of angular momenta in model data (e.g. upper plot in the figure above) which are as concentrated and as close to a polar orientation as is observed for the MW satellite orbital poles (lower plot in the figure above). We have applied the method to both cosmological simulation data as well as models of galaxy collisions resulting in polar distributions of tidal debris.
The results are clear. They unambiguously disfavor the cold dark matter models.
Caption: A part of Fig. 3 of our paper, illustrating the results of one of our criteria. The plot shows how likely it is that orbital poles derived from models can be at least as concentrated as the observed value of 35.4 degree. The integral below the curves within the shaded region give the probability that randomly drawn orbital poles from the model are as concentrated as is observed. The two cosmological simulations (Aquarius D2 and Via Lactea 1, upper panels) show curves which are very similar to that for an isotropic distribution of satellite galaxies (thin line), it is unlikely that they fall into the shaded area. The lower panel shows the results for tidal debris of a galaxy collision, which is much more concentrated towards the left. In this latter case, it is most likely to draw orbital poles as concentrated as observed.
Using data from high-resolution cosmological simulations of halos that should host Milky-Way-like galaxies, we were able to show that the sub-halo orbits do not naturally produce the observed properties. In contrast, models in which the satellite galaxies are formed as tidal dwarfs from the debris of a galaxy-collision can easily reproduce the observed distribution of orbital poles. The claim that cosmological ΛCDM simulations naturally produce satellite distributions as inferred in the Milky Way has therefore been falsified. At the same time, this shows that the tidal scenario passes the test.
For more details, please read our paper (accepted by MNRAS). It is available as a preprint.
By Pavel Kroupa and Marcel Pawlowski (15.05.2012): "Does filamentary accretion of dark matter sub-halos naturally produce a VPOS-like structure?" on SciLogs. See the overview of topics in The Dark Matter Crisis.