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Our Galaxy at full HD

Periodic Reporting for period 1 - Gal-HD (Our Galaxy at full HD)

Reporting period: 2018-04-01 to 2020-03-31

Gaia is a cornerstone mission of the European Space Agency (ESA) set to produce the largest and most precise census of positions, distances and velocities for a billion stars to revolutionize our knowledge on the evolution and formation of the Milky Way (MW). Our Galaxy at full HD (Gal-HD) is a project that exploited these data to study key processes that drive the evolution of the MW and of galaxies in general.

Objective 1) Assemblage of the halo. Several galaxies with very low luminosity have been found in the halo of the MW. The detection of more of these systems is extremely relevant for the comparison with cosmological simulations. The Sagittarius dwarf and its tidally striped stars (stream) is crucial since its long tidal tails should allow for a determination of the gravitational potential of the Galaxy. The plan of Gal-HD was to add velocities and scan the whole sky for the first time to detect dwarf galaxies in the halo.

Objective 2) Disk dynamics. The spiral arms are one the less understood aspects of the disk dynamics but affect significantly the orbits of the disk stars. Observations of the stellar disc prior to Gaia were limited to less than 1 kpc from the Sun. The plan of Gal-HD was to investigate the kinematics of an unprecedented disk extension to constraint the role of the different mechanisms ruling the disk dynamics.

Objective 3) Star formation processes. The young local associations are groups of coeval young stars moving together. Data prior to Gaia was very local and had very few stars or large velocity errors. The plan of Gal-HD was to investigate young kinematic substructures and trace back their origin.

The action aimed to contribute to the European task of answering one of the key questions in astrophysics as identified in A Science Vision for European Astronomy (ASTRONET, 2007): “How did our galaxy form?”. The goals of the action have been fully met and the action has had an enormous impact on the research field even changed the paradigm of the disk evolution.
To detect the dwarf galaxies in the halo of the MW a whole automatic pipeline that detects the substructures in proper motion space in each part of the sky has been prepared. The application to the Gaia DR2 allowed the detection of the Sagittarius dwarf and its tidal stream, obtaining the largest samples of stars (Antoja et al. 2020, A&A, 635:L3) and of RR Lyrae Ramos, Antoja, et al. 2020, A&A, 638:A104) to date and its first all-sky and continuous proper motions tracks, which do not match the existing models.

Different analysis techniques were developed to study the large-scale kinematics of the Galaxy disk for the first time. The so-called phase space spiral was discovered (Antoja et al. 2018, Nature 561:360–362) and was hypothesised to be a signature of phase mixing after a strong perturbation back to about 300-900 million years ago, pointing towards Sagittarius as the main cause. This shows that the disk of our Galaxy is highly responsive to perturbations and time-dependent, challenging one of the most used premises in galactic dynamics: the equilibrium of the disk. The moving groups were detected and characterised well beyond the Solar Neighbourhood for the first time (Ramos, Antoja, et al 2018, A&A, 619:72), establishing that some groups present constant vertical angular momentum with Galactic radius and others constant energy, from which it was hypothesised that there must be different dynamical mechanisms in place (spiral arms, Galactic bar, and external perturbations). Finally, different substructures in the outermost parts of the MW Disk (Monoceros, ACS, TriAnd) were revealed and studied, the data favouring models that explain these structures as made of disk stars perturbed by an external galaxy more than remnants of accreted galaxies as previously suggested.

A new code was prepared to integrate back in time the orbits of the Young Local Associations and find their age. The dynamical age of β Pictoris was determined (Miret et al, 2020, A&A, in press) with a precision three times better than before and that for the first time is consistent with independent determinations with other methods, thus solving a long standing puzzle. An improved list of member and some new radial velocities were also presented.

During the action coordination and preparation tasks of the WEAVE Low Resolution Disk sub-survey as well as several tasks related to the Gaia Early Data Release 3 (EDR3, 3/12/2020) have been performed (e.g. Gaia collaboration, Antoja, et al, submitted to A&A in October 2020, Fabricius et al, to be submitted in Fall 2020).

All results (list of member candidates of Sagittarius, of the outer disk structures, of β Pictoris) have been made public and are currently being considered as targets for surveys such as WEAVE to measure radial velocities and chemical abundances. The proper motions tracks of Sagittarius are publicly available to be used to fit the Galaxy’s gravitational potential and the tables of moving groups are being used for comparisons with different methods and simulations. The pipelines prepared during the action and applied successfully to DR2 will be applied to the EDR3 for the detection of fainter dwarfs in the halo and the explorations of farther disk regions. The results of the project have been presented in five accepted publications in international peer reviewed articles, and three more articles are in final stages of preparation (Ramos, Antoja, to be submitted to A&A, October-November 2020; Garcia-Conde, Roca-Fabrega, Antoja, et al., to be submitted to ApJ before end of 2020; Gaia collaboration, Antoja et al., submitted to A&A 2020), and presented in several international conferences.
The discovery of the phase space spiral entails a change of paradigm in our understanding of the evolution of Galaxy disk and of galaxies in general, whose global consequences are being (and will be for many years) evaluated (e.g. the validity of the Jeans equations for a Galaxy out of equilibrium to determine its dark matter content). The results of this action also require the existence of different mechanisms ruling the disk dynamics apart from the spiral arms and Galactic bar as previously thought. The impact of Sagittarius is shown to be very relevant for the inner part of the disk and not only the outermost parts as suggested before, establishing a new connection between the disk and the halo. Multiple comparison with models will be necessary to establish the relative importance of all these processes and finally build a dynamical framework that explains the observations. Finally, the possibility of determining ages of young local associations through alternative makes possible the link with planet formation studied since several stars in these associations host planets and circumstellar discs.

This action has contributed to the scientific harvest of the Gaia mission that is a ESA’s long-term planning in which all member states contribute. It has helped to maintain the European leadership in the field of Galactic Astronomy, and also contributed to the success of a spatial mission and of the Gaia Data Processing and Analysis Consortium. Of outermost importance is also the impact on the redefinition of the WEAVE Low Resolution disk survey after the discoveries of this action, for which the priority objectives had to be redefined to be more focussed on the effects of Sagittarius and the non-equilibrium of the disk.