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A New Perspective On Star Formation and Spiral Structure in Our Home Galaxy

Periodic Reporting for period 1 - PERSEUS (A New Perspective On Star Formation and Spiral Structure in Our Home Galaxy)

Reporting period: 2018-05-01 to 2020-04-30

Giant molecular clouds (GMCs) are the principal sites of star formation. The increasing resolution and sensitivities of interferometers such as ALMA and NOEMA allows us now to resolve GMCs in nearby galaxies and expansions of these (sub)millimetre capabilities will enable sampling of smaller scales at larger distances over the next decade. However, the derivation of cloud properties are complicated by their large spread in distance along the line of sight. Consequently, one would compare parsec-sized clouds of a few hundred solar masses in the solar neighbourhood to the largest, million-solar-mass GMCs in the distant Milky Way or even in nearby galaxies. Another problem that prevents us from clearly setting the Milky Way within the context provided by other galaxies is that our Galaxy is observed from within, meaning without the clean perspective of an external observer. While in the inner parts of our Galaxy, within the solar circle, two distances are geometrically possible for any line-of-sight velocity and a clean separation of spiral arms is difficult, our project focuses on the outer Galaxy, where the distance is unique.
The main goal of the project is to produce the first map of the Perseus spiral arm in our outer Galaxy as would be seen by an external observer at fixed spatial resolution. The Perseus arm extends over one third of the Milky Way's observed span and covers different environmental conditions. 12CO data of the spiral arm are convolved to the same spatial resolution and regridded onto maps of constant linear scale.
From these Perseus spiral arm maps a fundamental benchmark sample of GMCs is extracted using a cloud-finding algorithm to determine their physical conditions and for comparison with GMCs in the inner Galaxy to study the impact of different kinds of environments.
The first common resolution map of the Perseus arm is reprojected to any desired spatial resolution, to match observations of nearby galaxies and to provide a tool for comparison with extragalactic surveys.
Over the last two years, the research fellow, Dr. Marion Wienen, conducted an innovative programme and produced the first map of the Milky Way’s Perseus spiral arm in the Outer Galaxy from the perspective of an external observer at a fixed spatial resolution. This together with an extracted, uniform GMC sample will be extremely valuable for future comparison of Galactic and extragalactic GMCs. The first sub-aim of this objective was the identification of the Perseus spiral arm in the outer Galaxy. Precise radial velocities of masers from the BeSSeL survey (Reid et al. 2014, ApJ, 783, 130) were combined with the highest angular resolution 12CO observations of the outer Galaxy provided by Dr. Chris Brunt from the University of Exeter to calibrate the arm’s distance-velocity structure. (see attached figure). The fellow gained expertise in various coordinate transformation techniques to convert to an internal reference frame of the arm for the processed spiral arm data cube.

MW implemented the convolution of 12CO data to the same spatial resolution along the Perseus arm and regridded those onto a constant linear scale map (see attached figure). A whole processing pipeline of the extremely large 3D spectral line data cubes was created together with tools that enabled consistency checks of intermediate results. This part was essential to lay the groundwork for the whole project and all further analysis. The scientific work is described in a research paper that is almost ready for publication.
To extract clouds form the remapped 12CO data cube MW collaborated with the author of the widely used and influential cloud finding algorithm SCIMES, Dr. Dario Colombo. The advancement of the technical capability of the code to the particular 12CO data set was motivated by this fellowship and its development to process, for the first time, such a large area of molecular line emission as examined within this project. This objective made use of MW’s experience in developing scripts to exploit large data sets to search for correlations between the cloud properties such as their gas mass, radius, luminosity or virial mass. A research article reporting the cloud extraction and the resulting catalogue is currently in preparation and will be submitted in the near future.

In addition to the aforementioned papers an overview of the project together with links to the remapped 12CO data cube and cloud catalogue are illustrated on a project webpage ( These results will shortly be made publicly available through the “Open Research Exeter (ORE)” repository as soon as the paper will be accepted. MW had the chance to discuss specific results of her research with other colleagues of the Astrophysics group such as Dr. Chris Brunt, Dr. Clare Dobbs and Prof. Tim Naylor. The fellow could contribute to an alternative method of the distance-velocity calibration that uses star counts from Gaia by providing her processed 12CO data set as a large test field. The results of this project were also disseminated at several national and international conferences among the galactic and extragalactic community. The presentations gave MW the opportunity to get in contact with researchers who are interested to use the remapped data cubes of the Perseus arm as a tool to create a link between research in our and nearby galaxies.
The objectives pursued within this fellowship led to fundamental results with an impact on the research field. MW used the still unexplored approach that separates the Perseus spiral arm based on its kinematics and remaps 12CO data to the same resolution along the whole spiral arm. This objective produced, for the first time, data cubes at a fixed resolution that are adapted to extragalactic surveys and can be seen as the starting point for future projects of a more detailed comparison with a larger number of external galaxies.
The spiral arm data cube was exploited to extract a uniform GMC sample. This led to catalogue of molecular clouds along a whole spiral arm without any distance bias which is thus an important repository for the characterization of cloud properties. The higher angular resolution and sensitivity of observing facilities enable now publications about GMCs in external galaxies. These together with the cloud catalogue of the Perseus spiral arm will open up the way for a direct comparison of the structure and properties of Galactic spiral arms with those in external galaxies.
The objectives of this project deliver interesting knowledge, which contributes to enhance quality of life, and therefore have an impact on universities as a whole. Beside the astrophysical community appealing images of the conducted astrophysical research inspire media and schoolchildren to pursue a career in science, thereby benefitting the society as a whole.
Position - velocity diagrams: translation of cloud complexes at longitude and Perseus arm length.
Linear 12CO mass surface density image of the outer Perseus arm integrated over +/- 7.8 km/s./