Scientific Seminars

Testing the general relativistic nature of the Milky Way Rotation Curve with Gaia DR2

Mariateresa Crosta
INAF - OATo

2020-10-29    11:00    Merate - SALA VIRTUALE - https://meet.google.com/cdt-xkdr-nmq

By routinely scanning individual sources throughout the whole sky, Gaia directly measures the kinematics of the stellar component of the Milky Way with the goal to create the largest, most precise three-dimensional map of the Milky Way. The very core of the Gaia data analysis and processing involves general relativity to guarantee accurate scientific products. Nowadays, thanks to Gaia we witness the advent of Relativistic Astrometry. Nevertheless, once a relativistic model for the data reduction is in place, any Galactic model should be developed consistently with the relativistic-compliant kinematics delivered by Gaia. In this respect, I will present the first attempt to investigate a relativistic Galactic rotation curve (RC) with DR2 products. The results are published in MNRAS (Issue 496, 2, 2020) by M.Crosta, M.Giammaria, M.Lattanzi and E. Poggio. Two theoretical models, one GR and a DM-based analogue, were fit to unprecedented Galactic data (parallaxes to better than 20%, the complete set of proper motions and line-of-sight velocities, and the compliment of 2MASS photometry), a careful selection of a very homogenous sample of 5602 disk stars (5277 UMS stars and 325 classical type I Cepheids) that reconstructs the kinematics of the axisymmetric part of the Galactic potential over a large range (from 5 to 16 kpc) of galactocentric distances. The relativistic velocity profile results statistically indistinguishable from its state-of-the-art DM-based analogue. This supports the ansatz that a gravitational dragging effect could drive the stellar velocities in the plane of our Galaxy far away from its center. While DM is supposed to reside mostly in the Galactic halo, the relativistic dragging velocity in the disc can effectively compensate for its absence. Furthermore, one of Einstein’s equations provides the necessary baryonic matter density in a self-consistent way. Then, no additional extra matter is required to close the observed gap with respect to the expected Newtonian velocities. Indeed, it was crucial to correctly frame the velocity measurement made by the Gaia observer. Contrary to classical astronomy, the measurement process in GR depends on the underlying geometry and such a velocity turns out to be proportional to the off-diagonal term of the metric that has no Newtonian counterpart, i.e., is purely relativistic. The GR model that allowed us to successfully put to test the above first principles was based on a (simplified) general relativistic dust model, axisymmetric and stationary. For such a case an appropriate velocity profile was solved by Balasin and Grummiller (2008). Despite some inadequacies and incompleteness (e.g., it cannot be extended, as is, to the inner regions of the MW), this model has proven to be quite useful for demonstrating the relativistic nature of the Gaia-mapped RC from within our MW, allowing also to estimate the (external) radial size of the Galactic bulge and the disc thickness at radial distances R>4 kpc. In the context of Local Cosmology, our findings push on the fully use of Einstein’s theory and state the need to develop more complex relativistic galactic “geometries” that take into account the various structures of the Galaxy in concomitance of the incoming and increasingly accurate Gaia data releases and with other Galactic observations targeting the Galactic center.