Image of the Week

Mapping the Milky Way

Figure 1. A new map of part of the Milky Way, based on the combination of several different data sets such as YSO groups, Cepheids, High-Mass Star Forming Regions, OB stars, Young Open Clusters and 3D dust. Image credits: “Mapping the Milky Way: Galactic Cartography in the Age of High Precision Parallaxes” workshop.

A renaissance in Galactic studies is well underway, in no small part due to the contributions deriving from the data releases of the Gaia mission, which has become the most cited astrophysics space mission to date. This renaissance is not only thanks to Gaia, but also to astrometric and photometric surveys made at other wavelengths, from the near-infrared (NIR) to the radio, providing fundamental measurements such as the distance to the supermassive black hole SgrA* at the Galactic center, the positions and motions of high-mass stars tracing spiral structure, as seen by their radio maser emissions, and NIR surveys mapping out the bulge and bar in the inner Galaxy.

The need was recognized that experts from diverse observational traditions needed to meet to put all the pieces together, motivating the organization of a workshop dedicated to mapping the Milky Way that was hosted at the Lorentz Center in Leiden, Netherlands. The workshop, entitled “Mapping the Milky Way: Galactic Cartography in the Age of High Precision Parallaxes”, was held over five days and attended by 56 participants (including 10 online participants) from 12 countries. Topics covered included the distribution of young stellar populations, mapping the interstellar medium, the astrometry of masers, global mapping of hydrogen and carbon monoxide from radio data, the Galactic bar and the bar/spiral transition

The time together was not only spent sharing ideas and discussing recent results, but also used to put different data sets together in the same coordinate frame to allow comparative studies. Some comparisons made were the positions of star-forming HII (ionized Hydrogen) regions and the orientation of filamentary structure in the gas, young stellar objects and the dust distribution, the motions masers and gas toward the inner parts of the Galaxy, and the position of spiral arms as seen by Cepheids and hydrogen gas in the outer Galaxy. These mapping efforts led to a webpage with a three-dimensional map of the extended solar neighbourhood as one of the final products of the workshop, which can be viewed and reoriented interactively. The map contains some of the datasets that were explored during the workshop, including OB stars, Cepheids, masers, young open clusters and dust.

The workshop was also an important occasion to take stock of the current understanding of Galactic structure regarding its non-axisymmetric features, and what outstanding questions remain. With regards to its large-scale spiral structure, for example, the Milky Way continues to resist being unambiguously described with a simple model, notwithstanding important progress in the past decade from Gaia and radio astrometry of masers. These two data sets are complimentary: Gaia provides hundreds of millions of parallaxes, but is limited by extinction, while the maser data is much sparser, but can potentially reach the far side of the Galaxy.

For example, using both Gaia and maser astrometry together we now see that the nearby “Local” arm is much longer than previously thought. Thanks to Gaia, our local horizon for accurately mapping luminous young stars has reached to about 4-5 kpc, providing a detailed understanding of substructure and filling in the gaps between masers. On larger scales, the current maser data set seems to support a traditional four arm model, though it is currently limited to the part of the Milky Way that can be seen from the northern hemisphere. The community is looking forward to new maser parallaxes from the southern hemisphere that should help map another third of the Galaxy in the near future. Meanwhile information about the spiral structure in old stellar populations, which may only have a subtle effect on the Milky Way’s appearance, is still limited but is critical for understanding the mechanisms driving spiral structure in our galaxy.

One of the principle aims of the workshop was to work toward a consensus of the large-scale non-axisymmetric structure of the Milky Way. While it cannot be claimed a common consensus of important details of Galactic structure was reached, by the end of the conference there was a surprising agreement that it was, in any case, time for a new artist’s rendition of the Milky Way. Workshop participants pointed out the more obvious defects of the most common portraits of our Galaxy used to date, based on our current state of knowledge. A number of data visualization specialists and science communicators who were invited for the final discussion on the last day of the workshop also stressed the need for a full portrait image, even if parts of the image would have to be “invented” using reasonable assumptions and extrapolations.

3D animation source can be found here.

Credits: Ronald Drimmel from the Istituto Nazionale di Astrofysica di Torino (Italy) and the entire Science Organizing Committee of the workshop "Mapping the Milky Way: Galactic Cartography in the Age of High Precision Parallaxes": Mark Reid from Center of Astrophysics (Harvard & Smithsonian, USA), Catherine Zucker from Space Telescope Science Institute (USA), Ortwin Gerhard from the Max-Planck Institute for Extraterrestial Physics in Garching (Germany) and Robert Benjamin from the University of Wisconsin-Whitewater (USA)

[Published: 18/05/2023]