How the spectral energy distribution and morphology constrain each other, with application to morphological selection by colour
We introduce an empirical methodology to study how the spectral energy distribution (SED) and morphology constrain each other and implement this on ∼8000 galaxies from the HST CANDELS survey in the GOODS-South field. We show that, while SEDs constrain morphology to a large extent (i.e. galaxies with similar SEDs also tend to have similar morphology), the mapping is not perfect. The SED constrains morphology most strongly for relatively massive, red galaxies which have elliptical morphologies. We show that morphology constrains the SED more than vice versa - in other words, it is more likely that galaxies with similar morphology will also have similar SEDs than the other way around. There is a group of galaxies for which SED and morphology are mutually constraining. These systems tend to be disturbed or interacting i.e. systems which are experiencing processes like mergers that affect both the SED and the morphology simultaneously, likely on short timescales. We apply our methodology to explore likely upper bounds on the efficacy of morphological selection using colour. We show that under reasonable assumptions, colour selection is relatively ineffective at separating homogeneous morphologies. Use of single colours for morphological selection typically achieves average purity levels of only around 60 per cent. While the results can be improved by using multiple colours, the gains are not significant, with purity values typically remaining around 70 per cent or below.
This is a repo containing code and data to reproduce the results in the paper. The data is originally sourced from:
https://archive.stsci.edu/missions/hlsp/candels/goods-s/gs-tot/v1.0/
All the code is in the similarity.ipynb Notebook.