Structural Dissonance in Galaxy Decomposition: The red sequence and evolutionary pathways in the Coma Cluster

Abstract

The structural and photometric properties of ‘red-and-dead’ early-type galaxies provide vital clues about the evolutionary pathway which lead to their formation. Here, I use deep Canada-France-Hawaii Telescope image data to explore the multi-component internal structures of red sequence galaxies in the Coma cluster, with a particular focus on disk-dominated early-type galaxies (i.e. S0s). Galaxies are investigated across a wide range of uminosities
(−17 > M_g > −22) and cluster-centric radii (0 < r_cluster < 1.3 r_200). I present the 2D structural decomposition of u, g, i imaging via GALFIT. Rigorous filtering is applied to ensure that the measured best-fit models are the most meaningful descriptions of their galaxy’s underlying
stellar structures.

A sample of Coma cluster members (N = 200) was identified as well described by an ‘archetypal’ S0 structure (central bulge + outer disk). Internal bulge and/or disk colour gradients were implemented by allowing component sizes to vary between bands. Such gradients are required for 30% of archetypal S0 galaxies. Bulges are characterised by n ~2
profiles with half-light radii, R_e ~1 kpc, remaining consistent in size for all but the brightest galaxies (M_g < −20.5). S0 disks are brighter (at fixed size, or smaller at fixed luminosity) than those of star-forming spirals. Similar colour-magnitude relations are found for both bulges and disks. The global red sequence for S0s in Coma hence results from a combination of both component trends. The average bulge − disk colour difference is 0.09 ± 0.01 mag in g − i, and 0.16 ± 0.01 mag in u − g. Using simple stellar population models, bulges are either ~2-3× older, or ~2× more metal-rich than disks. The trend towards bluer
global S0 colours observed further from Coma’s core is driven by a significant correlation of disk colour with cluster-centric radius. An equivalent trend is detected in bulge colours at a marginal significance level. An environment-mediated mechanism of disk fading is favoured as the dominant factor in S0 formation.

The decomposition analysis was then extended to encompass a wider range of structural models. This revealed a large sample of reliably-fit, symmetric multi-component galaxies in Coma (N = 478). 42±3% of Coma cluster galaxies (N = 201) are best described by a 3(+) component structure. In addition, 11% of galaxies (N = 52) feature a break in their outer profiles, indicating truncated or anti-truncated disks. Beyond the break radius, truncated disks are consistent in structure with untruncated disks, disfavouring a formation mechanism via physical truncation of exponential disks. The sizes/luminosities of bulges in antitruncated galaxies correlate strongly with galaxy luminosity, indicating a bulge-enhancement formation mechanism for anti-truncated disks. Both types of broken disk are found overwhelmingly (> 70%) in barred galaxies, suggesting that galaxy bar play an important role in formation of such structures. The wide variety of galaxy structures detected in Coma highlights the naivete of the simple ‘bulge + disk’ or ‘single spheroid’ morphological paradigm for early-type galaxies.