Evaluación curricular

The aim of this study is to characterise the photometric and structural properties of the bulges and disks of ETGs. The Coma sample (as defined in Section 2), however, contains many galaxies that are poorly described by an archetypal (centrally bulge-dominated, disk- dominated at large radii) S0 morphology. It is necessary, therefore, to identify a sub-sample of galaxies which are well-fit by 2-component (S´ersic + exponential) models to ensure the applicability of a ‘bulge + disk’ interpretation.

A suitable sample of archetypal S0 models were selected for analysis via an a posteriori sample filter applied to the 2-component fitting results. This process involves a series of parameter cuts, and statistical tests which are described in detail in the following section. The overall filter is illustrated graphically in Figure 3.6, and the resulting filtered subsamples are described in Section 3.6.

3.5.1

Badly-fit galaxies

Asymmetric, contaminated, or poorly-fit galaxies (i.e. highχ2_{) were removed from the sam-}

ple. This includes cases where galaxy crowding (prominent towards the cluster core) would require additional deblending steps to accurately model. These fits do not yield reliable model parameters, and therefore cannot be considered accurate representations of the under- lying galaxy structures. Hence, we remove any 2-component model fits withχ2

ν ≥ 1.7 as

unstable. This cut was calibrated on visual inspection of fit residuals. However, galaxies with significant contamination or strong asymmetrical features may yield artificially low values ofχ2 _{(e.g. due to overzealous masking). Therefore, an upper limit to permitted values ofχ}2

is insufficient to selectively remove all unstable galaxies.

Contamination of the galaxy thumbnails was parametrised via the fraction of masked pixels (due to SExtractor-detected sources; see Section 2.4), f_mask. For an ellipse with

semi-major axis,a, and axis ratio, q, this masked fraction is calculated as,

fmask(a, q) =

nmask(a, q)

npix(a, q)

, (3.3)

masked pixels. If the fraction of masked pixels within the target ellipse (see Section 2.4), fmask(atarget, qtarget) ≥ 0.4, then the thumbnail is considered contaminated. Additionally,

since contamination at small galaxy-centric radii is more disruptive to fitting, galaxies are also removed as contaminated if the fraction within the inner quarter of the target ellipse, fmask(atarget/2, qtarget) ≥ 0.3.

Thumbnails were tested for asymmetric features using the asymmetry parameter,A (Home- ier et al., 2006).A is defined as

A = 1

2[Σ(|I − I180|) − Bcorr] 1 It

, (3.4)

whereI is the flux in a particular pixel, I180 is the flux in the same pixel after rotating the

image by 180◦ around the galaxy centre, and It is the sum of flux in all pixels. Bcorr is a

correction for uncorrelated noise (equivalent to calculatingΣ(|I − I180|) for a empty region

of sky). If a galaxy image hasA > 0.2 (as calculated in the unmasked target ellipse), then it is removed from analysis as unstable. The galaxy is also removed if the asymmetry calculated for the S´ersic + exponential model-subtracted residual,Ares> 0.3.

3.5.2

Single component galaxies

Galaxies best fit by a single-component model were excluded. These S´ersic-only galaxies were identified where the addition of the exponential disk component provided no significant improvement to the model fit. The comparative goodness-of-fit of the models was assessed using the Bayesian Information Criterion (see Section 3.4). Furthermore, model fits which yielded extreme values of B/T were considered indistinguishable from pure S´ersic systems due to high fitting parameter uncertainty (> 10%) in components contributing ≤ 10% of the total galaxy luminosity (i.e. B/T> 0.9 or B/T < 0.1).

3.5.3

Surface brightness profiles

Surface brightness profile types (see Section 1.4) were used to separate archetypal S0 galax- ies (Type 1; i.e. inner bulge, outer disk) from those with all other (atypical) morphologies. These surface brightness profiles were calculated analytically in theg band out to the edge

0.0

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b/a [disc]

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(

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(

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Scatter = 0.062

Scatter = 0.126

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Figure 3.5: The measuredg − i colour separation (bulge - disk) of two component model fits plotted against the axis ratio of the disk component (qD). ‘Archetypal’ (Type 1) bulge + disk

systems are plotted as red circles (unfilled circles indicate less reliable fits), while all other profile types (2, 3, 4, 5 and 6) are plotted as green squares. A line at(g − i)bulge= (g − i)disk

is plotted for clarity.

of the target ellipse (see Section 2.4).

Profile Types 4 (exponential-dominated at small radii, S´ersic-dominated at large radii) and 6 (exponential-dominated at small and large radii, S´ersic-dominated at intermediate radii) are inappropriate for a ‘bulge + disk’ interpretation and are removed from the sam- ple. As these profiles are centrally disk-dominated, such models may result from artificial swapping of the ‘bulge’ and ‘disk’ roles of the S´ersic and exponential components. It is possible to recover a fraction of the inverted fits by manually swapping the parameter val- ues of each component and re-fitting. When the re-fitted model yields a Type 1 profile, that galaxy is returned to the analysis sample. However, as a forced parameter swap may result in a higher value ofχ2

ν (. 0.01), galaxies recovered in this way are flagged as potentially

unreliable. Such galaxies may be better fit by a double S´ersic model.

Profile Types 2 (exponential-dominated at all radii), 3 (S´ersic-dominated at small and large radii, exponential-dominated at intermediate radii), and 5 (S´ersic-dominated at all radii) can be considered as ‘bulge + disk’ systems, however, galaxies with these types are not archetypal S0s (i.e. non-central bulges and/or non-outer disks), and hence are removed from the sample. While genuine galaxies with sub-dominant disks (Types 3 and 5) exist within the Coma sample, fits of these types can also result from the underfitting of more complex morphologies (e.g. barred or ring galaxies). Including underfit models would render a pho-

tometric analysis unreliable, as measured photometry would not necessarily cleanly probe the intended structures. An example of this issue is displayed in Figure 3.5, which shows bulge - disk (g − i) colour separation as a function of the apparent disk model axial ratio. While the majority of model bulges are ∼0.1 mag redder than their disks, a subsample of (predominantly Type 3 and Type 5) models have high ellipticity disks ∼0.1 mag redder than their bulges. Here, the exponential components are following bar structures, while large-n S´ersic components account for both the bulge and the outer disk. The introduction of a sec- ond S´ersic component to account for a bar corrects the photometric swap of the bulge and disk. This issue is investigated in detail in Chapters 7 and 8.

3.5.4

Blue core galaxies

Galaxies were removed from the sample based on their radial colour profiles, as measured for use during multi-band fitting (see Section 3.7.2). A two-component model is insuffi- cient to reproduce a galaxy colour profile which includes a significant downturn towards bluer colours in the core. Thus, if the innermost colour data point (as measured from the galaxy thumbnails) is more than 0.01 mag bluer than the adjacent measurement, that galaxy is considered unsuitable for bulge-disk decomposition analysis.

3.5.5

Flagged sample

A fraction of archetypal galaxies were highlighted as stable but lower-quality fits. Although selecting galaxies with Type 1 model profiles ensures that the S´ersic and exponential com- ponents measure inner and outer structures respectively, bars can still distort the photometry of the S´ersic component: Models in which the S´ersic component follows a bar rather than the bulge are also identified by flagging ‘bulges’ with low S´ersic indices (n < 1.0). Bar-like disks are flagged if the exponential component is smaller (Re,D/Re,B > 0.8) and narrower

(qD < qD) than the S´ersic component. A large PA misalignment between the two model

components (|PAD− PAB| > 45◦) also indicates a bar-like structure, so such galaxies are

flagged unless one or both components are circular (q > 0.9). Furthermore, a cut at χ2 ν > 1.2

is used to flag fits which suggest the presence of additional (unfit) structural components which would distort interpretation of bulge and disk component colours.

Figure 3.6: Flow chart for sample filtering following S´ersic + exponential fitting. For profile type definitions, refer to Section 1.4 and Allen et al. (2006).

3.5.6

Overview of sample filtering

To summarise, the results of bulge-disk decomposition are filtered to select a sub-sample of well-fit, archetypal 2-component galaxies from which bulge and disk properties can be measured reliably. Galaxies with high χ2

ν , asymmetry, or contamination are removed as

poorly-fit (hereafter ‘unstable sample’). Pure S´ersic galaxies are identified and removed where one component is either not a significant improvement to the model, or has significant parameter uncertainty (hereafter ‘S´ersic sample’). The remaining galaxies are all well-fit 2- component systems (hereafter ‘stable sample’). However, 2-component models which do not exhibit ‘central bulge + outer disk’ surface brightness profiles (hereafter ‘atypical sample’) are not representative of S0 morphologies, and are also removed. The remaining galaxies (hereafter ‘archetypal sample’) are subdivided into high and moderate-quality fits (hereafter ‘regular sample’ and ‘flagged sample’ respectively). The latter category includes galaxies with weak indications of additional components (e.g. bars), and fits which require manual adjustment to produce archetypal profiles.