Testing dark matter and geometry sustained circular velocities in the Milky Way with Gaia DR2

Flat rotation curves in disk galaxies represent the main evidence for large amounts of surrounding dark matter. Despite of the difficulty in identifying the dark matter contribution to the total mass density in our Galaxy, stellar kinematics, as tracer of gravitational potential, is the most reliable observable for gauging different matter components. This work tests the flatness of the MW rotation curve with a simple general relativistic model suitable to represent the geometry of a disk as a stationary axisymmetric dust metric at a sufficiently large distance from a central body. Circular velocities of unprecedented accuracy were derived from the Gaia DR2 data for a carefully selected sample of disk stars. We then fit these velocities to both the classical, i.e. including a dark matter halo, rotation curve model and a relativistic analogue, as derived form the solution of Einstein's equation. The GR-compliant MW rotational curve model results statistically indistinguishable from its state-of-the-art DM analogue. This supports our ansatz that a stationary and axisymmetric galaxy-scale metric could "fill the gap" in a baryons-only Milky Way, suggestive of star orbits dragged along the background geometry. We confirmed that geometry is a manifestation of gravity according to the Einstein theory, in particular the weak gravitational effect due to the off-diagonal term of the metric could mimic for a "DM-like" effect in the observed flatness of the MW rotation curve. In the context of Local Cosmology, our findings are suggestive of a Galaxy phase-space as the exterior gravitational field of a Kerr-like source (inner rotating bulge) without the need of extra-matter.Comment: Acknowledgments and references updated; 18 pages, 2 figures, improved version after referee's comment

Unconscious Bias versus Gender bias

This is a report of the gender balance activity held on Saturday morning on the occasion of the CANTATA summer school 2017. All of the participants to the school and the lecturers were present. An heterogeneous panel was a prerequisite for the discussion. Among many topics about gender balance we focused in particular on Implicit Bias, firstly introducing a dedicated presentation, then proposing two sets of questions formulated by taking into account the recent literature. In-between we made the implicit Harvard test, the Evolution of Trust test, and, finally, a role game on group diversity. A follow-up of the discussion was done on Monday evening, as requested by the stu- dents themselves, as they wanted to point out more controversial aspects on how gender unbalance/discrimination can be mitigated in STEM careers

Astrometry in the 21st century. From Hipparchus to Einstein

Astrometry is that fundamental part of astronomy which allows to determine the geometric, kinematical, and dynamical properties of celestial objects, including our own Galaxy, which is assembled and shaped by gravity. The knowledge of star positions was already important at the times of Hipparchus (190-120 BC) and his predecessors, Timocharis and Aristillos. Their cataloging (approximately 150 years earlier) of star positions enabled Hipparchus to update the observations with a precision of nearly half a degree and thus to discover the phenomenon of equinox precession. Nowadays a big jump is mandatory: positions, motions, and distances exist in the realm of the Einstein Theory and null geodesics represent our unique physical links to the stars through a curved space-time, namely a varying background geometry. Astrometry must be equipped with all of the proper tools of General Relativity to define the observables and the measurements needed for compiling astronomical catalogs at the microarcosecond accuracy and beyond. The astrometry of the 21st century, endowed with a fully relativistic framework, is fully fledged for new potential applications in astrophysics, can lead the way to forefront discoveries in fundamental physics, and is becoming the pillar of Local Cosmology. In this respect, it is more appropriate, in the 21st century, to refer to it as "Gravitational Astrometry"

Gravitational astrometry from within the solar system

Mission like Gaia (ESA, launched in 2013) requires to treat gravity properly when compiling microsecond stellar catalogues. This will open the opportunity to put in practice methods of Relativistic Astrometry mainly devoted to model the celestial sphere with the percepts of General Relativity (GR) and promotes the use of highly accurate astrometry to test locally fundamental physics. Gaia will be able to carry out general relativistic tests by means of both global and differential astrometric measurements. Global tests will be done through the full astrometric reconstruction of the celestial sphere, while the differential experiments will be implemented in the form of repeated Eddington-like measurements. After one century, Gaia will perform the largest experiment in GR ever made with astrometric methods (since 1919): a relativistic all-sky reconstruction which includes also QSO at different redshifts. Moreover, at zero redshift, dealing with local cosmology, accurate absolute motions of stars within our Galaxy will provide tests on current cosmological models via the detections of cosmological signatures in the disk and halo

The Relativistic All-Sky Analysis with GAIA

By providing an homogenous all-sky survey of high precision parallaxes, space motion (proper motions and radial velocities) and astrophysical characteriza- tion for more than one billion stars throughout the Galaxy and thanks to the depth of the volume achievable, Gaia will deliver a huge amount of astrometric, spectroscopic, and photometric data. Gaia will contribute also to the determi- nation of an optical reference frame by observing many thousands of quasars. In doing so Gaia will have a huge impact across many fields, including many branches of stellar astrophysics (details of the structure and stellar evolutionary phases), exoplanets, solar system objects, the cosmic distance ladder (through a model independent of the primary calibrators) and fundamental physics. New “accurate” distances and motions of the stars within our Galaxy will provide access to the cosmological signatures left in the disk and halo offering inde- pendent, direct and detailed comparisons the predictions of the most advanced cosmological simulations. But all the above goals will not be achieved without the correct characterization and exploitation of the “relativistic”, i.e. very high accuracy, astrometric data. Since a Gaia-like observer is positioned inside the Solar System, the measurements are performed in a weak gravitational regime which can be regarded as “strong” when one has to compare these slow varying fields with the accuracy achievable by Gaia

The Ray Tracing Analytical Solution within the RAMOD framework. The case of a Gaia-like observer

This paper presents the analytical solution of the inverse ray tracing problem for photons emitted by a star and collected by an observer located in the gravitational field of the Solar System. This solution has been conceived to suit the accuracy achievable by the ESA Gaia satellite (launched on December 19, 2013) consistently with the measurement protocol in General relativity adopted within the RAMOD framework. Aim of this study is to provide a general relativistic tool for the science exploitation of such a revolutionary mission, whose main goal is to trace back star directions from within our local curved space-time, therefore providing a three-dimensional map of our Galaxy. The results are useful for a thorough comparison and cross-checking validation of what already exists in the field of Relativistic Astrometry. Moreover, the analytical solutions presented here can be extended to model other measurements that require the same order of accuracy expected for Gaia.Comment: 29 pages, 1 figur