How the spectral energy distribution and galaxy morphology constrain each other, with application to morphological selection using galaxy colours

TL;DR

This study demonstrates that spectral energy distributions (SEDs) significantly constrain galaxy morphology, but using galaxy colours alone is limited in effectively classifying morphologies, even with multiple colours.

Contribution

The paper introduces an empirical method to quantify the link between SEDs and galaxy morphology and assesses the effectiveness of colour-based morphological selection.

Findings

SED constrains morphology, especially in massive red ellipticals.

Colour selection achieves only about 60-70% purity in morphological classification.

Using the full SED offers limited improvement over a few colours.

Abstract

We introduce an empirical methodology to study how the spectral energy distribution (SED) and galaxy morphology constrain each other and implement this on 8000 galaxies from the HST CANDELS survey in the GOODS-South field. We show that the SED does constrain morphology and present a method that quantifies the strength of the link between these two quantities. Two galaxies with very similar SEDs are around three times more likely to also be morphologically similar, with SED constraining morphology most strongly for relatively massive red ellipticals. 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. Even with the use of up to six colours for morphological selection, the average purity in the resultant morphological classes is only around 60 per cent. While the results can be improved by using the whole SED, the gains are not significant, with purity values remaining around 70 per cent or below.