Probing Axial Symmetry Breaking in the Galaxy with Gaia Data Release 2

Abstract

We study a set of solar neighborhood (d < 3 kpc) stars from Gaia Data Release 2 to determine azimuthal star count differences, i.e., left and right of the line from the Galactic center through the sun -and compare these differences north and south. In this companion paper to Gardner et al. (2020), we delineate our procedures to remove false asymmetries from sampling effects, incompleteness, and/or interloper populations, as this is crucial to tests of axisymmetry. Particularly, we have taken care to make appropriate selections of magnitude, color, in-plane Galactocentric radius and Galactic |b| and |z|. We find that requiring parallax determinations of high precision induces sampling biases, so that we eschew such requirements and exclude, e.g., regions around the lines of sight to the Magellanic clouds, along with their mirror-image lines of sight, to ensure well-matched data sets. After making conservative cuts, we demonstrate the existence of azimuthal asymmetries, and find differences in those, north and south. These asymmetries give key insights into the nature and origins of the perturbations on Galactic matter, allowing us to assess the relative influence of the Magellanic Clouds (LMC & SMC), the Galactic bar, and other masses on the Galactic mass distribution, as described in Gardner et al. (2020). The asymmetry's radial dependence reveals variations that we attribute to the Galactic bar, and it changes sign at a radius of (0.95 ± 0.03)R 0 , with R 0 the Sun-Galactic-Center (GC) distance, to give us the first direct assessment of the outer Lindblad resonant radius.

Dark matter local density determination based on recent observations

“…However, it is not yet clear to what extent the cause of local disturbances comes from the Galactic bar [25,111], spiral structures in the disc [26,113,114], or the tidal interaction with the Sagittarius dwarf and past Galactic mergers [10,101,107,115]. The asymmetry seen in [116], for example, would require the effect of both the Galactic bar and the large and small Magellanic clouds.…”

The Milky Way, Coming into Focus: Precision Astrometry Probes its Evolution, and its Dark Matter

“…In the case of north-south symmetry breaking, we note that a study of north-south and axial symmetry breaking can framed to show concretely that the MW is not in steady state, following the theorems we have discussed in Sec. 2 [44,29]. The particular origins of these various symmetry-breaking and phase-space correlation effects are not well-established, but we review various proposed explanationsall of which involve external perturbations.…”

“…It is also pertinent to the assessment of other parameters, such as the pattern speed of the Galactic bar. For example, using the assessment of the Outer Lindblad Resonance (OLR) location from a distinct Gaia DR2 data sample [29], the rotation curve information determines the bar pattern speed [205]. It is worth comparing the pattern speeds that result from different rotation curve information.…”

“…Of direct concern for improving our understanding of the MW is the growing appreciation that the LMC can directly and significantly influence the local phase space of DM particles [224]. This is possible due to both the high relative velocity of the centers of mass of the MW and the LMC as well as the reflex motion of particles in the solar neighborhood to the influence of the LMC, as can be probed through studies of axial symmetry breaking in the MW [44,29]. Thus, our understanding of the MW is now sufficiently precise that further refining this understanding necessarily requires accounting for our largest satellites.…”

“…Astrometric parallaxes give an enormous improvement in the quality of distance assessments over ground-based surveys, which are largely forced to rely on photometric methods, and the precision of the distance assessments [25], and the completeness of the stellar samples [28], with Gaia DR2 are extremely high. It is possible, e.g., to select a data sample of some 14 million stars within 3 kpc of the Sun's location with an average relative parallax error of less than 10% [29]. As a separate example, we compare two distance assessment methods to local dust clouds -one uses Gaia parallaxes and the other uses parallaxes of masers measured by the VLBIand show the result in Fig.…”