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Mapping the velocity fields of external galaxies

Spiral galaxies exhibit a disc-shaped structure with a relatively small thickness compared to their radial extent. Typically, these systems maintain a stable configuration, particularly in their innermost regions, where stars and gas follow nearly circular orbits around the galactic center under the influence of the gravitational field generated by the overall matter distribution of the galaxy. However, it is well-established that both in our galaxy and in external galaxies, the motions of stars and other emitters are not solely confined to circular paths. Instead, they can exhibit additional components, such as radial or vertical motion. These non-circular motions arise due to various factors, including distortions in the mass distribution at large scales (e.g., the presence of spiral arms that represent local over-densities), deformations of the disc shape, or interactions with satellite galaxies. Such non-circular motions are prevalent and leave distinctive imprints on the velocity maps of galaxies.

While it is now feasible to directly observe and reconstruct individual stars’ circular, radial, and vertical motions in the Milky Way, studying outer galaxies presents a more intricate challenge. Only the galaxy’s projected image on the sky and the velocity along the observer’s line of sight (measurable through the Doppler effect) can be observed. Consequently, the task lies in utilizing this information to reconstruct the galaxy’s circular and radial velocity fields.

Until now, the prevailing assumption has been that the motions are predominantly circular and that radial motions are negligible. However, even with this assumption, reconstructing the circular velocity field in various galactic regions remains an unsolved problem.

Francesco Sylos Labini, Matteo Straccamore, Giordano De Marzo from the Enrico Fermi Research Centre in Rome, and Sébastien Comeròn from the Canary Islands Institute of Astrophysics have made a significant breakthrough by developing a method to reconstruct both the circular and radial velocity fields of outer galaxies using only the velocity information along the line of sight.

This innovative approach enables the reconstruction of the average circular and radial fields and their variations across different regions of the galaxy. These variations, known as angular anisotropies, can then be linked to the presence of density structures such as spiral arms, bars, or satellite galaxies. This method offers a promising avenue for gaining insights into the dynamics and structures of outer galaxies, further enhancing our understanding of these fascinating astronomical objects.

This groundbreaking method enables detailed measurements of kinematic quantities and offers crucial information for constructing dynamic and evolutionary models of galaxies when combined with the observed distribution of matter. In the study conducted by Sylos Labini and colleagues, they applied this method to a set of 25 galaxies from the THINGS catalog. This catalog comprises velocity fields obtained through highly precise observations of neutral hydrogen emission, ensuring excellent spatial resolution. By utilizing this method, the researchers could gain valuable insights into the kinematics of these galaxies and establish connections between the measured kinematic quantities and the distribution of matter within them. The findings from their work contribute significantly to the development of comprehensive models that capture the dynamic nature and evolution of galaxies.

The article, ‘Mapping non-axisymmetric velocity fields of external galaxies’, is being accepted for publication in the Monthly Notices of the Royal Astronomical Society. It can be downloaded at this link.

Figure 1

In the figure, the left side displays an image of the galaxy M51, which is renowned for its striking ‘grand design’ spiral structure. The image showcases the galaxy along with its satellite, which is visible on the left side, surrounded by luminous gas. On the right side of the figure, there are two maps. The bottom map represents the circular velocity field, while the map above it illustrates the radial velocity field.


The radial velocity field exhibits significant variations in proximity to the satellite, as well as in the outer regions that are diametrically opposite. These variations suggest notable deviations from a purely circular motion. Similarly, the circular velocity field also displays noticeable variations, further emphasizing that the rotational velocity is not uniform throughout the galaxy. These variations in both the radial and circular velocity fields provide valuable insights into the complex dynamics and structure of the galaxy M51.