Beyond Spheroids and Discs: Classifications of CANDELS Galaxy Structure at 1.4 < z < 2 via Principal Component Analysis

Michael A. Peth, Jennifer M. Lotz, Peter E. Freeman, Conor McPartland, S. Alireza Mortazavi, Gregory F. Snyder, Norman A. Grogin, Yicheng Guo, Shoubaneh Hemati, Jeyhan S. Karteltepe, Dale D. Kocevski, Anton M. Koekemoer, Daniel H. McIntosh, Hooshang Nayyeri, Casey Papovich, Joel R. Primack, Raymond C. Simons, Arjen van der Wel

Published 2015-04-07Version 1

To understand the processes driving galaxy morphology and star formation, we need a robust method to classify the structural elements of galaxies. Important but rare and subtle features may be missed by traditional spiral, elliptical, irregular or S\'ersic bulge/disc classifications. To overcome this limitation, we use a principal component analysis of non-parametric morphological indicators (concentration, asymmetry, Gini coefficient, $M_{20}$, multi-mode, intensity and deviation) measured at rest-frame $B$-band (corresponding to HST/WFC3 F125W at 1.4 $< z <$ 2) to trace the natural distribution of massive ($>10^{10} M_{\odot}$) galaxy morphologies. Principal component analysis (PCA) quantifies the correlations between these morphological indicators and determines the relative importance of each. The first three principal components (PCs) capture $\sim$75 per cent of the variance inherent to our sample. We interpret the first principal component (PC) as bulge strength, the second PC as dominated by concentration and the third PC as dominated by asymmetry. Both PC1 and PC2 correlate with the visual appearance of a central bulge and predict galaxy quiescence. We divide the PCA results into 10 groups using an agglomerative hierarchical clustering method. Unlike S\'ersic, this classification scheme separates quenched compact galaxies from larger, smooth proto-elliptical systems, and star-forming disc-dominated clumpy galaxies from star-forming bulge-dominated asymmetric galaxies. Distinguishing between these galaxy structural types in a quantitative manner is an important step towards understanding the connections between morphology, galaxy assembly and star-formation.