The OC stream in deforming dark matter halos


The Milky Way (MW) is a rich testbed for investigating predictions about and constraining the nature of dark matter. One of the predictions from simulations is that the MW’s dark matter halo is deforming due to the infall of the Large Magellanic Cloud (LMC). The LMC has a significant effect on stellar streams, such as the Orphan-Chenab (OC) stream. In this project, we investigate whether Chenab is sensitive to the deformation of the dark matter halos of the MW and the LMC using state of the art stream modelling techniques and simulations of the MW-LMC interaction in collaboration with Mike Petersen and Jorge Peñarrubia.

What is the OC stream? What are stellar streams?

OC is a stellar stream. Stellar streams result from the disruption of globular clusters or dwarf galaxies in the gravitational field of another galaxy, such as the MW. In the MW, over 70 stellar streams have been observed so far, many with high-quality, six-dimensional data (observed with Gaia and followed up with spectroscopic surveys such as the Southern Stellar Stream Spectroscopic Survey).

Stellar streams are used to reconstruct the history of the MW, and to constrain dark matter properties of the MW, such as the mass and shape of the halo. In the halo, streams move on orbits similar to their progenitor. Modelling the streams (either as orbits or as more detailed stream models) constrains mass and shape of the halo in the region the modelled stream covers.

OC is a very long stream over the Northern and Southern part of the sky. Its progenitor is a dwarf galaxy that either has completely dissolved or has not been found yet. Due to the missing progenitor, it was named Orphan. Chenab was originally found in the Dark Energy Survey footprint as an individual stream but later confirmed to be a part of Orphan. In the Southern region, the OC stream is significantly perturbed by the presence of the LMC and its direct effect on the Milky Way (see next paragraph), while it is less affected in the Northern part. Based on these results, we expect OC to be the stellar stream to show the most apparent effects of dark matter deformation.

Why is the Milky Way dark matter halo deforming?

Recent studies show that the LMC mass is between 10% and 30% of the MW mass. Two direct effects of this massive LMC on the MW were observed already: the reflex motion of the MW disc and over- and underdensities in the stellar halo. The reflex motion is a downward pull on the stellar disc towards the orbit of the LMC. The observable effect of this is that the halo moves upwards compared to us (as we are part of the disc). The overdensities are along the past orbit of the LMC due to dynamical friction and as a more global response in the Northern sky. These effects, particularly the reflex motion, should be accounted for in observations and modelling of most kinematic systems (such as stellar streams).

Simulations of the interaction of the massive LMC and the MW suggest that the dark matter halos of both galaxies are deforming as a reaction to the gravitational pull of the other galaxy. Since dark matter is not visible to us and only measurable with kinematic tracers (i.e. its gravitational effects), it is much harder to find ways to observe these deformations.

Can we measure these deformations?

One day, hopefully, we can measure the dark matter deformations directly. The goal of my PhD (or even after) is to find ways to measure the deformations using stellar streams. Currently, the computational methods to find these possible observables are being developed, and observations are not good enough yet. We are just at the beginning of solving this puzzle.

The first step is to find an effect of the deformation in our models and understand from these findings how the observables of streams are affected. Is there a clear signature we can immediately extract (highly unlikely), or do we have to disentangle the many different effects stellar streams experience? Another question we aim to answer in my next project is whether current models are biased by not including the dark matter deformation. Tackling these questions step-by-step will give us a good understanding of how reliable streams measure the dark matter halo and how sensitive they are to deformations.

If you are interested in learning more about this project, please contact me via the contact form.

Sophia Lilleengen
Sophia Lilleengen
Postdoctoral Researcher

I am interested how stellar streams are affected by time-dependent dark matter halos.