Galaxy And Mass Assembly (GAMA): Self-Organizing Map Application on Nearby Galaxies

TL;DR

This paper applies Self-Organizing Maps to galaxy survey data to visualize and analyze complex population structures and sub-populations, including green valley galaxies, in a high-dimensional feature space.

Contribution

It demonstrates the effectiveness of SOMs in mapping galaxy populations and their morphological features, revealing sub-populations and interstitial groups within the green valley.

Findings

SOM effectively visualizes high-dimensional galaxy data.

Green valley galaxies occupy multiple regions in the SOM.

Morphological features like smoothness and ellipticity are well-mapped, but smaller features are not.

Abstract

Galaxy populations show bimodality in a variety of properties: stellar mass, colour, specific star-formation rate, size, and S\'ersic index. These parameters are our feature space. We use an existing sample of 7556 galaxies from the Galaxy and Mass Assembly (GAMA) survey, represented using five features and the K-means clustering technique, showed that the bimodalities are the manifestation of a more complex population structure, represented by between 2 and 6 clusters. Here we use Self Organizing Maps (SOM), an unsupervised learning technique which can be used to visualize similarity in a higher dimensional space using a 2D representation, to map these five-dimensional clusters in the feature space onto two-dimensional projections. To further analyze these clusters, using the SOM information, we agree with previous results that the sub-populations found in the feature space can be reasonably mapped onto three or five clusters. We explore where the "green valley" galaxies are mapped onto the SOM, indicating multiple interstitial populations within the green valley population. Finally, we use the projection of the SOM to verify whether morphological information provided by GalaxyZoo users, for example, if features are visible, can be mapped onto the SOM-generated map. Voting on whether galaxies are smooth, likely ellipticals, or "featured" can reasonably be separated but smaller morphological features (bar, spiral arms) can not. SOMs promise to be a useful tool to map and identify instructive sub-populations in multidimensional galaxy survey feature space, provided they are large enough.