Gaia data release 2 contains proper motions for more than 1.3 billion sources, and provides a wealth of information to perform new studies on the kinematics of the Milky Way. A group of researchers lead by Tobias Fritz from the Instituto de Astrofysica de Canarias in Tenerife, Spain used the Gaia data for a new study on dwarf galaxies of the Milky Way, focusing on those dwarfs that have been spectroscopically observed in literature.

Their paper shows the power of Gaia data release 2 in a long awaited field: the determination of the tangential motions of satellite systems and, consequently, the inference of their orbital properties around the Milky Way. Such determinations for the brightest of the Milky Way satellite galaxies were the subject of one of the Gaia Collaboration papers 'Gaia Data Release 2: The kinematics of globular clusters and dwarf galaxies around the Milky Way'. With this work the sample is expanded and includes 39 dwarf galaxies out to very large distances from the Milky Way (up to 420 kiloparsecs), adding 29 galaxies, compared to previous work.

After taking into account careful selection of the sources, the systemic proper motions of the dwarf galaxies in the sample were derived. The proper motions were converted into tangential velocities in the heliocentric reference frame, after which these heliocentric tangential velocities were used together with the line-of-sight velocities to determine the total velocities in the Galactocentric reference frame, as shown in Figure 2.

Figure 2: Total velocities of all galaxies in the sample. The curves show the escape velocity for the two potentials used for this research: MWPotential14 with a NFW halo of virial mass 0.8 x 10¹² Solar masses indicated with the black line, and a more massive variant with virial mass of 1.6 x 10¹² Solar masses indicated with the red line. Image credit: Fritz et al. 2018.

Subsequently the orbital poles were calculated and other orbital parameters were deduced. The orbits of all galaxies were computed for two different Milky Way potential models: the standard MWPotential14 model, based on a spherical bulge with a disc and a NFW halo (Bovy 2015) combined with a light halo with virial mass of 0.8 x 10¹² Solar masses, or combined with a heavier model with virial mass of 1.6 x 10¹² Solar masses. This lead to the determination of the eccentricity, pericentre and apocentre of all galaxies for both models. Monte Carlo realizations of the orbit integrations were used to estimate the errors on the orbital parameters.

The image shown in Figure 1, featured here as the image of the week, together with the images shown in Figure 3, show the all-sky view of the orbital poles for the objects in the sample. These images focus on the objects within 200 kiloparsecs. A comparison is made of their location on this plane with the vast polar structure (VPOS) of satellites (Pawlowski et al. 2012).

Figure 3: All-sky view of orbital poles for the objects in the sample with Galactocentric distances between 0 and 50 kiloparsecs (top) and between 50 and 100 kiloparsecs (bottom). The circles indicate the median of the 2000 Monte Carlo simulations, while the small dots around each object plot the orbital poles from the individual simulations. The magenta circles indicate a region within 10% of the assumed VPOS pole, and is indicated with an "X" for the co-orbiting direction and with a "+" for the counter-orbiting direction. Image credit: Fritz et al. 2018.

The results of this research impacts several areas of knowledge of our own galaxy and its system of satellites. The mass of the Milky Way, including its dark matter halo, is still debated and can vary with more than a factor of 2 between various estimates. The motions analysed in this work suggest that the mass of the Milky Way is likely relatively high.

Furthermore, basic physics tells us that satellites should spend more time close to the apocentre of their orbits. Using this expectation, the observed distribution of orbital properties of the population of satellites suggests that there must be several dwarf galaxies not yet discovered, hiding at large distances from the Milky Way centre.

Several of the dwarf galaxy satellites of the Milky Way are found to have orbits that bring them close to the inner regions of our Galaxy, making them likely to be tidally disturbed (like they are stretched to a stream). This explains the peculiar characteristics that were observed for some of these objects. On the other hand, new questions arise, because there are satellites that do show features likely due to tidal disturbance by a large mass but that do not have orbits that seem to put them at risk of being tidally disturbed by the Milky Way.

Finally, the satellite galaxies of the Milky Way, M31, and Centaurus A appear to be preferentially arranged within thin and vast planar structures, the origin of which is still to be understood but that appear to challenge cosmological models of galaxy formation. Many of the galaxies in the sample analysed move within this planar structure. This physical property can be used by models to help explain the nature of these structures.

The paper "Gaia DR2 proper motions of dwarf galaxies within 420 kpc - Orbits, Milky Way mass, tidal influences, planar alignments, and group infall" by T.K. Fritz et al. was published today in A&A.

For this research only part of the power of Gaia Data Release 2 was used for the determination of the systemic proper motions of dwarf galaxies (given the selection of the sample was based on the requirement of having spectroscopic observations in literature for all objects). It is expected that the precisions could be improved by adding stars without existing spectroscopic measurements. Also, given that the precision in proper motion determinations by Gaia grows with the power of 1.5 of the time-baseline, it is expected that the proper motions of Gaia will be 4.5 times more accurate after the nominal mission and possibly a factor 12 more accurate for a nominal mission plus mission extension of 5 years.

An animation was created by A. Villalobos, G. Battaglia, T. Fritz making use of the NASA Milky Way rendition. This animation is available here for full interaction. Below you can find the video impression of this animation.

The video shows the movement of tiny galaxies surrounding the Milky Way as described in the paper: "Gaia DR2 proper motions of dwarf galaxies within 420 kpc - Orbits, Milky Way mass, tidal influences, planar alignments, and group infall" by T.K. Fritz et al. Credits: A. Villalobos, G.Battaglia, T. Fritz (animation); NASA: Milky Way rendition. Based on Fritz et al. 2018, A&A, 619, 103

See also: Press release by the Instituto de Astrofísica de Canarias - IAC

Credits: ESA/Gaia/DPAC, T. Fritz, G. Battaglia

[Published: 13/11/2018 | Updated with the animation on 29/12/2018]