Conditional Colour-Magnitude Distribution of Central Galaxies in Galaxy Formation Models

TL;DR

This study examines the conditional colour-magnitude distribution of central galaxies in galaxy formation models, revealing distinct red and blue components and their dependence on halo and galaxy properties, with implications for understanding galaxy evolution.

Contribution

It introduces a detailed analysis of the CCMD using Gaussian mixture models and machine learning, highlighting the origins of galaxy colour components and their relation to halo properties.

Findings

CCMD decomposes into red and blue components with distinct properties.

Galaxy colour correlates with halo assembly in low-mass haloes.

Machine learning improves colour prediction using halo and galaxy data.

Abstract

We investigate the conditional colour-magnitude distribution (CCMD), namely the colour-magnitude distribution at fixed halo mass, of the central galaxies in semi-analytic galaxy formation model (SAM) and hydrodynamic simulations. We analyse the CCMD of central galaxies in each halo mass bin with the Gaussian mixture model and find that it can be decomposed into red and blue components nearly orthogonal to each other, a red component narrow in colour and extended in magnitude and a blue component narrow in magnitude and extended in colour. We focus on the SAM galaxies to explore the origin of the CCMD components by studying the relation between central galaxy colour and halo or galaxy properties. Central galaxy colour is correlated with halo assembly properties for low mass haloes and independent of them for high mass haloes. Galaxy properties such as central supermassive black hole mass, cold gas mass, and gas specific angular momentum can all impact central galaxy colour. These results are corroborated by an alternative machine learning analysis in which we attempt to predict central galaxy colour with halo and galaxy properties. We find that the prediction for colours of central galaxies can be significantly improved using both halo and galaxy properties as input compared to using halo properties alone. With the halo and galaxy properties considered here, we find that subtle discrepancies remain between predicted and original colour distribution for low mass haloes and that no significant determining properties are identified in massive haloes, suggesting modulations by additional stochastic processes in galaxy formation.