Galaxy morphology and colour

The morphology of galaxies is typically characterised either by extensive visual inspection (e.g. Galaxy Zoo project, Lintott et al. 2011) or through the stellar kinematics (e.g. Emsellem et al. 2007). Both methods can be used to determine whether a galaxy is bulge- or disc-dominated or whether it appears disturbed (for example experiencing a merger). It is well established that disc-dominated galaxies tend to be rotationally supported, spheroidal galaxies dispersion supported, and that overall morphology strongly correlates with galaxy mass, with massive galaxies being mostly bulge-dominated (e.g. Sandage & Visvanathan 1978). It has also been shown that morphology correlates with galaxy colour (e.g. Larson et al. 1980; Strateva et al. 2001; Baldry et al. 2004) and that galaxies can be divided into two well- defined distinct populations, namely ‘red-sequence’ galaxies, that tend to be elliptical, bulge-dominated, older and redder than ‘blue-cloud’ galaxies, which are disc-dominated and star-forming .

Despite recent progress in numerical simulation predictions of galaxy morphologies (Snyder et al. 2015; Dubois et al. 2016; Rodriguez-Gomez et al. 2017; Bottrell et al. 2017) and increasing observational data (Willett et al. 2013; Haußler et al. 2013), the origin of the distribution of galaxy morphologies and its correlation with star- forming/quenching galaxies is still under debate. In this work (Correa et al., 2017, MNRAS Letters, 472, 1), we investigate the relation between kinematic morphology, intrinsic colour and stellar mass of galaxies in the EAGLE cosmological hydrodynamical simulation. We calculate the intrinsic u-r colours and measure the fraction of kinetic energy invested in ordered corotation of 3562 galaxies at z = 0 with stellar masses larger than 10^{10}M_{\odot}. Inspection of gri-composite images suggests that kinematic morphology is a useful proxy for visual morphology. EAGLE produces a galaxy population for which morphology is tightly correlated with the location in the colour-mass diagram, with the red sequence mostly populated by elliptical galaxies and the blue cloud by disc galaxies. Satellite galaxies are more likely to be on the red sequence than centrals, and for satellites the red sequence is morphologically more diverse. These results show that the connection between mass, intrinsic colour and morphology arises from galaxy formation models that reproduce the observed galaxy mass function and sizes.

The figure below shows from left to right galaxies with increasing stellar mass, and from bottom to top, galaxies with increasing \kappa_{co}, i.e. a larger fraction of the stellar kinetic energy invested in ordered corotation. As \kappa_{co} increases, galaxies tend to become more disky and extended. Based on visual inspection of a larger number of galaxies we pick \kappa_{co}=0.4 to separate galaxies that look disky from those that look elliptical.


Example of central galaxies taken from the Ref-L100N1504 EAGLE simulation. The panels correspond to either face- or edge-on gri-composite images of side length 40 kpc. From left to right the figure shows galaxies with increasing mass, and from bottom to top galaxies with an increasing fraction of their stellar kinetic energy invested in ordered corotation. The empty bins indicate that there are no rotation-supported massive galaxies in EAGLE.