The bulge and disk structures of red-sequence galaxies in the Coma cluster have been char- acterised using deepu, g, and i band imaging. Extensive sample vetting (as described in Chapter 3) was carried out to isolate a sample of 200 galaxies well-described by an ‘archety- pal’ (central) bulge + (outer) exponential disk morphology. Fitting was carried out using two approaches: i) where model structure does not vary from band-to-band; ii) internal compo- nent colour gradients were permitted in one or both components.
The component colours of archetypal galaxies in Coma have been reported as functions of galaxy luminosity and clustercentric radius. In addition, variation of component colours and colour trends were investigated for visual classifications of galaxy morphology.
The main results of this chapter are as follows:
i) If either galaxy or component luminosity is fixed, the bulges of S0s are signif- icantly redder than their disks (by ∼ 0.1 mag in g − i, ∼ 0.2 mag in u − g on average).
Thus, bulge stellar populations are ∼2-3× older, and/or ∼ 2× more metal rich than those found in disks.
ii) Significant colour-magnitude slopes are detected for both the bulge and disk struc- tural components in g − i and u − g.
In either colour, the measured trend slopes for bulges and disks are consistent at a2σ level. Hence, the global red sequence in Coma is a consequence of the increasingly red colours of bulges and disks in more luminous galaxies.
Coma cluster.
Bulge component colours also become bluer withrcluster, but this trend is only margin-
ally significant (2.5σ) and more sensitive to the treatment of component colour gradi- ents. The global colour-radius trend for S0s is thus dominated by increasingly red disks in galaxies closer to the cluster core. Therefore, the environment-mediated mechanism which drives S0 formation is a ‘disk-fading’ process (i.e. quenching acts primarily on the disk).
iv) ‘E’ and ‘S0’ classifications from Dressler (1980a) represent the bulge-dominant and bulge-weak ends of the archetypal galaxy distribution rather than morpho- logically distinct galaxies.
D80 ellipticals are significantly more bulge dominated, indicating poor disk detection for faint disks (high B/T). No significant difference in component colours are noted for morphology relative to predictions from the colour-magnitude trend.
v) Galaxy Zoo classifications identify galaxies with low-level deviations from a smooth, regular profile, rather than categorising morphologically distinct samples.
Galaxy Zoo ‘spirals’ are thus fainter and more disk-dominated, but exhibit greater levels of scatter in (disk) colour measurements. Galaxy Zoo ‘ellipticals’ are primar- ily well-characterised lenticular galaxies and thus are consistent (in terms of median colours and colour trends) with the results of the entire analysis sample.
These results are taken from analysis of galaxies where internal component colour gra- dients are permitted due to the significant exaggeration of bulge − disk colour separation for fixed multi-band fitting. No significant change is noted (on average) due to either the inclusion of blue galaxies removed during the selection of the initial galaxy sample, or the removal of the a posteriori sample vetting used to select only well-fit, ‘archetypal’ galaxies. From the results of fitting ‘archetypal’ bulge + disk galaxies, the red sequence is inter- preted as a consistent shift towards redder colours for both the bulges and disks of more luminous galaxies. Significant trends towards bluer disks (but only marginally bluer bulges) further from the cluster core indicate that the colour-environment relation is caused by an environment-driven disk fading mechanism. To reconcile disk fading with more luminous
S0s (see Section 5.2.4), either the disks of S0s must be truncated in size during transforma- tion, or their progenitors were intrinsically smaller and/or fainter than today’s star-forming spirals.
In the next chapter, I will discuss the extension of the decomposition analysis to include a wider range of (2-3 component) candidate models structures. With this multi-component analysis, I will explore the range of galaxy structures present in the Coma cluster, including those galaxies which were not best described by archetypal bulge + disk structures.
Chapter 7
Multi-component Fitting, and Deviations
from the Exponential Disk Profile
7.1
Introduction
The simple exponential model (Type I; Freeman, 1970) adopted in the preceding chapters does not fully represent of the true range of S0 outer disk structures. Truncated or anti- truncated disks (Type II and III respectively; see Section 1.4 and Erwin et al., 2008) as detected in both cluster and field galaxies, result from the redistribution of stars due to evo- lutionary processes. For example, truncated (Type II) disks may be formed when stars are physically removed from a galaxy’s outer regions (e.g. during tidal interaction), while anti- truncated (Type III) disks may result from major merger events (Borlaff et al., 2014). In- vestigation of these ‘broken’ disks reveals their environmental or secular origins, and thus provides a deeper understanding of the evolutionary history of Coma cluster S0s.
In this chapter, decomposition analysis is carried out using a wider suite of candidate models (including 2- and 3-component broken disk galaxies) in order to explore the diversity of galaxy structure in the Coma cluster. Thus, I reinvestigate the structural morphologies of the ∼23 of galaxies in the initial Coma sample which are not well-described by an archetypal bulge + disk model. While a primary goal of this analysis is the investigation of Type I, II, and III disks galaxy structures, the extended range of (multi-component) models is necessary to avoid mis-classification of additional component structures (e.g. bars or rings) as surface brightness profile breaks. Bayesian model selection and sample filtering are applied to avoid
overfitting, and to ensure that best fit models are reliable representations of the underlying galaxy structures.
The structure of this chapter is as follows: first, I describe the multi-component de- composition methodology, highlighting differences from the previously-described AGONII
bulge-disk decomposition. The broken disk component model is also described in this sec- tion. Secondly, I present the broad fitting results, including investigation of morphological (model) fractions. Lastly, I summarise the findings and conclusions of this chapter.
A detailed analysis of this multi-component decomposition will be presented in Chap- ter 8. Therein, the structural properties and possible evolutionary formation scenarios of galaxies whose outer regions are not dominated by an exponential disk will be discussed.