The ΛCDM cosmological framework, which is the prevalent “creation theory of the heavens”, is a great success at explaining many of the observed phenomena in the Universe we inhabit. Intriguingly, in addition to many other factors, its success also hinges on the existence of an elusive substance – dark matter – that makes up more than 80% of the total mass budget of the cosmos. Since dark matter does not, or only negligibly, interact with light it has so-far remained hidden from our observations. Nevertheless, being a dominant source of gravity in the Universe (under the considered framework), dark matter must be a prominent driver of most of the cosmic activities (e.g. structure formation and the evolution of galaxies) and therefore, constraining its nature and its distribution naturally comes as a first step in comprehending other cosmic processes.

Both ΛCDM cosmological simulations and observations have long advocated that galaxies, such as the Milky Way, are luminous systems (collections of stars, interstellar gas and dust) that are embedded in independent, gigantic halos of dark matter. In principle, these halos could be oblate, spherical, prolate or triaxial in shape. The lack of knowledge regarding the Milky Way halo’s shape has been a long standing enigma of astrophysics. Knowing the Milky Way’s dark matter distribution and its total mass, for instance, would be useful in a cosmological context. A better understanding of the Galactic density distribution of dark matter may aid in better understanding the properties of dark matter itself, which could assist experiments dedicated towards its discovery.

Dark matter has mass and hence interacts gravitationally with all other masses, including luminous objects. This implies that studying the dynamics of the luminous tracers in the Milky Way (e.g. phase-space distribution functions of stars) can help one constrain the overall gravitational potential of the Galaxy, which will inform us about the underlying matter distribution. Various methods have been proposed and employed for such an endeavour and some recent studies have turned to using “tidal stellar streams’’ as dynamical probes of the dark matter distribution. Star streams are formed by the tidal disruption of star clusters or satellite galaxies as they orbit in the gravitational potential of the host galaxy. Streams from low-mass disrupted progenitors have great promise as probes of gravity because they closely delineate orbits, and orbits provide a tremendously powerful means to probe and constrain the underlying Galactic potential.

Our Milky Way galaxy with known halo streams of stars shown above and below.
Overview of the stellar streams found up till now in Gaia DR2 using the STREAMFINDER algortihm. The structures are color coded according to their distances (Malhan et al. 2018).

In our recent paper, we probe the Milky Way’s potential by dynamically analysing one of the known stream structures in our Galaxy and adapting the orbit-fitting technique. This technique integrates orbits in a realistic family of Galaxy models and iterately compares them to stream data until the best Galactic potential model is found. For this work we use the popular GD-1 stellar stream. Originally discovered in the maps of the SDSS survey in 2006, GD-1 is a long (>70 deg) pencil line structure that is situated at an intermediate Galactocentric distance of ~14 kpc. We first recovered the GD-1 stellar stream members using our STREAMFINDER density map (an algorithm built to find stellar stream structures) that we obtained from processing the ESA/Gaia DR2 catalogue. This GD-1 dataset was then cross-matched with the SEGUE and LAMOST data to acquire the stellar line-of-sight velocity information that was missing from Gaia DR2. 


The gravitational potential model we studied possessed a cosmologically motivated dark matter halo profile, together with reasonable models for the baryonic components (to account for the Milky Way’s stellar bulge and the stellar disk). The parameterization of this model was done in terms of the a) circular velocity at the Solar radius, a physical observable that serves as a proxy for the total inner mass of the Galaxy, and b) the flattening of the dark matter halo. By varying this Galactic potential model in terms of these two parameters, we then fitted orbits to GD-1 data for each model. By doing this we found that the orbital solutions for GD-1 require the circular velocity at the Solar radius to be 244+/-4 kilometers per second, and also that the density flattening of the dark halo is 0.82+0.25−0.13 (a mildly oblate halo). This best fitted Galactic mass model further suggests that the mass of the Milky Way in the inner 20 kpc is 2.5±0.2 hundred billion solar masses. General agreement between these values and some of the previous studies suggests that the mass in the inner regions of the Milky Way’s halo is beginning to be understood, although the shape of the halo is still quite uncertain, ranging between mildly oblate to mildly prolate.

In the paper we analysed just one Milky Way stream and obtained reasonable constraints on the dark matter halo parameters. With Gaia’s unprecedentedly accurate proper motions, performing similar analyses with an ensemble of streams should provide improved constraints on the underlying potential and shape of the dark halo for the Milky Way.