Full length articleA new view of observed galaxies through 3D modelling and visualisation
Introduction
The study of galaxy formation and evolution is a wide research field in astronomy, ranging from multi-frequency observations of the Milky Way and nearby galaxies to distant galaxy groups and clusters (Meurer et al., 2006, Gil de Paz et al., 2007, Koribalski et al., 2018), complemented by cosmological galaxy simulations from the Big Bang to the present day (McAlpine et al., 2016, Potter et al., 2017). While specific galaxies can only be observed from a single viewpoint on (or near) Earth, their intrinsic 3D shapes are well known from all-sky surveys, such as the Sloan Digital Sky Survey (SDSS) (York et al., 2000), containing millions of objects at all possible orientation angles. As an example, Fig. 1 highlights the contrasting views of two similar spiral disc galaxies seen at very different angles (represented by their inclination angle ): the barred galaxy M 83 (left) is seen nearly face-on ( degrees), while the galaxy NGC 4565 (right) is seen nearly edge-on ( degrees).
Detailed observations of galaxies over a large range of frequencies (e.g. optical, radio, infrared and ultraviolet) allow the measurement and derivation of many different properties, together enabling us to study galaxy morphology, kinematics, composition, mass, age, and formation history. Conversely, simulation of astronomical objects has been an active research topic in astronomy for many years (Peebles, 1970, Vogelsberger et al., 2020) with an aim to gain a better understanding of physical processes occurring over timescales so long, and across spatial scales so massive, that we cannot hope to observe them in real-time. In both cases, visualisation has been used extensively to help understand, analyse, and disseminate the results of such efforts, as discussed in Section 2. In the context of the study of galaxies, visualisations are typically created from multi-dimensional data fields generated by numerical simulations and used to study properties of the source data; the focus is on presenting data in a manner facilitating intuitive comprehension rather than showing a visually realistic galaxy representation. Conversely, realistic 3D spatial visualisation of astronomical objects based on observations requires physically robust reconstruction methods paired with visualisation tools that support high quality rendering.
This paper presents a novel methodology to reconstruct and visualise particle-based 3D models of nearby galaxies based on (a) observed multi-wavelength images and (b) detailed kinematic information. Our objectives are to: (1) create realistic views of galaxies including from viewpoints not otherwise possible to observe, (2) explore the validity of derived spatial 3D models of such galaxies, and (3) allow enhanced visual analysis of the 3D galaxy morphology (stars, gas and dust). The methodology is implemented within the open-source 3D scientific visualisation package Splotch1 (Dolag et al., 2008), enabling the reconstruction, visualisation, and exploration of individual, or groups of, recognisable galaxies. We expect that our pipeline may be utilised for scientific communication and outreach by generating immersive and realistic movies in 3D of known galaxies, allowing the non-expert viewer to grasp the connection between 2D observations and the real 3D structure of such objects. We also separate out components within that 3D structure, highlighting for example the large extent and warped morphology of the gaseous component that is often forgotten by both experts and non-experts alike. We hope that through physically realistic reconstruction and rendering this pipeline may also be used to compliment typical statistical analyses via objectives (2) and (3), allowing the astronomer to build better mental images of both the 3D structure of their object of study, and better comprehend spatial relationships between companion objects.
As a case study for our work, we reconstruct and visualise the nearby barred spiral galaxy known as M 83, for its designation in the Messier catalogue, or as NGC 5236 in the New General Catalogue (NGC) of Nebulae and Clusters of Stars. As one of the closest and brightest spiral galaxies in the sky, M 83 has a large collection of high resolution and multi-wavelength data publicly available, including H i 21-cm data from the Local Volume H i Survey (LVHIS; Koribalski et al., 2018). Furthermore, M 83 is particularly well suited for visualisation due to its nearly face-on inclination and a large warped H i disc, making it possible to use our sophisticated 3D reconstruction to visualise its likely edge-on appearance. The numerous dwarf companion galaxies surrounding M 83 have also been modelled, allowing us to create a 3D visualisation of the galaxy group.
The structure of this paper is as follows: Section 2 provides an overview of related work. Section 3 presents a brief introduction to the techniques for galaxy observation and kinematic modelling that underpin our galaxy reconstruction approach. Section 4 introduces a new technique for reconstructing the physical structure of observed galaxies. Section 5 describes our galaxy visualisation process, with results presented in Section 6. Section 7 aims to evaluate the extent to which the described approach achieves the goal of realism. Section 8 presents concluding remarks and suggests future directions.
Section snippets
Related work
Visualisation is an integral part of astronomical analysis, helping domain scientists to explore their data, identify problems or areas of interest for further analysis, as well as to convey important concepts and results during dissemination of their work (Hassan and Fluke, 2011, Steffen et al., 2014, Dykes et al., 2018). We first provide a brief contextual overview of visualisation approaches of simulated and mock data, and then focus on relevant research to the primary topic of the paper:
Observational structure and dynamics of galaxies
This section presents a brief overview of the astronomical approach to inferring structure and kinematics of observed galaxies, which we will exploit for 3D reconstruction. We introduce two key domain-specific concepts, relating to the varied structure of galaxies observable across the electromagnetic spectrum (Section 3.1), and the concept of tilted-ring fitting for galactic hydrogen discs in spiral galaxies (Section 3.2).
Overview
In this section we describe our approach for constructing a particle-based 3D galaxy representation based on observational data and a tilted-ring model as introduced in Section 3. Our representation consists of multiple galactic components:
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Stellar population
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Diffuse hydrogen gas
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Dust
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Galactic bulge
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Globular clusters surrounding the galaxy
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Diffuse stellar, or dark matter, halo
We first collect and pre-process a set of observational source data and a tilted-ring model that will inform the density and
3D galaxy visualisation
This section describes the visualisation process for a constructed galaxy model. We utilise the astronomical visualisation tool Splotch, which is well-suited to our purpose as it is designed for particle-based astronomy data and supports multiple species of particle, natively supporting the multi-component particle-based structure of our galaxy models. The software, with which the authors have significant development experience, is open-source which allowed us to modify the underlying rendering
Reconstruction and Visualisation of M 83 and its local group
Fig. 7 demonstrates the results of applying our reconstruction and visualisation methodology to the M 83 galaxy. The images are split into far and close (left and right respectively), showing face-on, angled, and edge-on views (top to bottom respectively). The far images include the H i extended gaseous disc, illustrating the large warped structure which is not easily discernible for an astronomer viewing the observed H i images (Fig. 4). The maximum extent of the H i region is 100 kilo-parsecs
Physical realism
Our presented methodology aims to improve the analysis of the structure of galaxies based on combining kinematic and image information for observed galaxies. In general, it is not feasible to observe galaxies from an angle other than that of an Earth-based observer, and as such impossible to be certain that our model is structurally accurate for the observed galaxy. One approach for validation is to compare to observations of galaxies seen from Earth at similar viewpoints. Fig. 12 compares a
Conclusions and future work
We have presented a novel 3D modelling and visualisation methodology for the reconstruction of nearby galaxies based on a wide variety of multi-resolution observed images and derived data. This includes optical, UV, IR and H i images along with H i kinematic models of nearby galaxies based on tilted-ring fitting. We addressed several objectives: (1) to create realistic views of galaxies from viewpoints that are not otherwise possible to observe from, (2) explore the validity of derived spatial 3D
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
KD acknowledges support by the ORIGINS cluster, funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy, EXC-2094, 390783311. MK acknowledges support by NEANIAS, funded by the EC Horizon 2020 research and innovation programme under Grant Agreement No. 863448.
Fig. 12 is based on photographic data of the National Geographic Society Palomar Observatory Sky Survey (NGS-POSS) obtained using the Oschin Telescope on Palomar Mountain. The
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