Ultra-compact galaxy catalog (10 galaxy objects)
83% of the galaxies in the full galaxy catalog are on the main sequence of star formation (Elbaz et al. 2011), so that we can compare their size to the model from Dutton et al. (2011), which is known to provide a good fit to MS galaxies at $z = 2$ (footnote 1). The Standard Horizon galaxies are larger than both the Extreme Horizon galaxies and observed main sequence galaxies. The Extreme Horizon galaxies generally lie around the observed relation and a small fraction of them have significantly smaller sizes. We define the compactness, $\mathscr{C}$, as the ratio between the radius expected from the Dutton et al. (2011) model and the actual radius. The compactness distribution for Extreme Horizon (Fig. 3) peaks at around $\mathscr{C} \simeq 1$ but it exhibits a distinct tail for $\mathscr{C} \gt 1.3$. We thus define two massive galaxy populations in Extreme Horizon: 10 ultra-compact (UC) galaxies with $\mathscr{C} \gt 1.3$ and 50 non ultra-compact (NUC) ones.
Here is an interactive (rotating/zoomable) 3D scatterplot of the full galaxy catalog with point sizes scaling with the galaxy total mass and point colors highlighting the ultra-compact ones (in red) :
