A new article entitled ‘Graph Theoretical Analysis of local ultraluminous infrared galaxies and quasars’, by ARC members Orestis Pavlou, I. Michos, V. Papadopoulou Lesta, M. Papadopoulos, E.S. Papaefthymiou and A. Efstathiou has been published at Astronomy and Computing Journal (August 2023). 

The work presents a methodological framework for studying galaxy evolution by utilizing Graph Theory and network analysis tools. It studies the evolutionary processes of local ultraluminous infrared galaxies (ULIRGs) and quasars and the underlying physical processes, such as star formation and active galactic nucleus (AGN) activity, through the application of Graph Theoretical analysis tools. We extract, process and analyze mid-infrared spectra of local (z<0.4) ULIRGs and quasars between 5-38𝜇𝑚 through internally developed Python routines, in order to generate similarity graphs, with the nodes representing ULIRGs being grouped together based on the similarity of their spectra. Additionally, the authors extract and compare physical features from the mid-IR spectra, such as the polycyclic aromatic hydrocarbons (PAHs) emission and silicate depth absorption features, as indicators of the presence of star-forming regions and obscuring dust, in order to understand the underlying physical mechanisms of each evolutionary stage of ULIRGs.

Their analysis identifies five groups of local ULIRGs based on their mid-IR spectra, which is quite consistent with the well established fork classification diagram by providing a higher level classification. The authors demonstrate how graph clustering algorithms and network analysis tools can be utilized as unsupervised learning techniques for revealing direct or indirect relations between various galaxy properties and evolutionary stages, which provides an alternative methodology to previous works for classification in galaxy evolution. Additionally, their methodology compares the output of several graph clustering algorithms in order to demonstrate the best-performing Graph Theoretical tools for studying galaxy evolution.

O. Pavlou, I. Michos, V. Papadopoulou Lesta, M. Papadopoulos, E.S. Papaefthymiou, A. Efstathiou, Graph Theoretical Analysis of local ultraluminous infrared galaxies and quasars, Astronomy and Computing, Volume 45, 2023.