HERRAMIENTAS Y TÉCNICAS

• 15 late-type galaxies withfracDev¶0.1 (MORPH51-60, MORPH66-70).

Finally, I split the normal galaxies based on their inclination, into face-on (b/a>0.5, MORPH41- 60) and edge-on (b/a<0.5, MORPH61-70) subsets. As the visual appearance of the individual galaxies within the sample is of particular importance to this study, in Figures 3.1 and 3.2, I present the SDSS composite false-colour images of all 70 galaxies in the sample.

3.2

The standard measures

Through automated analysis of the r-band galaxy images, I computed a range of structural parameters, including (n, C,A, S, G and M₂₀, see Section 2.4). It has been shown by previ- ous studies that these standard structural parameters provide a robust way of discriminating between early and late galaxy types, and some of them (A,G-M₂₀) are also useful for identify- ing galaxies with double nuclei undergoing a merger (see e.g. Sérsic (1963); Bershady et al. (2000); Conselice et al. (2000); Conselice (2003); Lotz et al. (2004); Hernández-Toledo et al. (2008)). In this section, I investigate the suitability of these parameters in identifying galaxies in the post-coalescence stages of a merger.

The subsets of galaxies with normal early- and late-type morphologies (objects MORPH41- 70) as well as mergers with double nuclei (objects MORPH2, MORPH8, MORPH9, MORPH11) provide a good standard for the measures of galaxy structure as they are expected to show values ofn,C,A,S,GandM20within well established ranges. Based on previous studies, the parameters tend to show the following values for the different galaxy types:

• Sérsic index,n: the values measured for late-type galaxies tend to be lower (n∼1.0), than those found for the earlier types (typically 2.0®n®4.0) as shown by e.g. Sérsic

(1963);

• Concentration index,C: correlates with the Sérsic index and usually ranges between 2.0 and 5.0 with highest values characteristic of early-type galaxies (C>4.0), decreasing toC∼3 for later types; (Bershady et al., 2000; Conselice, 2003; Hernández-Toledo et al., 2008);

• Asymmetry,A: all normal galaxies tend to have lowA, usually with values around 0.1 found for early-type galaxies, increasing towards later types to around 0.2; (Conselice et al., 2000; Conselice, 2003; Hernández-Toledo et al., 2008); galaxies with double

Figure 3.1: SDSS’ false-colour images of galaxies in the MORPH sample including: 1-8 - SB/PSB galaxies with highly disrupted morphologies; 9-20 - galaxies with highly disrupted morphologies and elongated tidal features; 21-30 - SB/PSB galaxies with moderate/low morphological disturbance; 31- 35 - SB/PSB galaxies with regular morphologies.

3.2. The standard measures

Figure 3.2: SDSS’ false-colour images of galaxies in the MORPH sample - cont. Images 36-40 show

SB/PSB galaxies with regular morphologies. Additionally, early- and late-type galaxies, identified based on the SDSS’fracdevparameter, are shown: ETG’s - 41-50, 61-65 (edge-on); LTGs - 51-60, 66-70 (edge- on).

bright nuclei tend to have higher values ofA- some studies use a thresholdA≥0.35 to identify ongoing mergers (see e.g. Lotz et al. 2008);

• Clumpiness,S: ranges from around 0.1 to 0.3 with a tendency to increase towards later morphological types (Conselice, 2003; Hernández-Toledo et al., 2008);

• Gini index,G: there is a clear separation in the values ofGfound for early- and late-type galaxies, withG∼0.6 for the former andG ∼0.4 for the latter (Abraham et al., 2003; Lotz et al., 2004);

• Moment of light, M20: anti-correlates with nandC, with low values characteristic of early-type galaxies (M20∼ −2.0) and somewhat higher (∼ −1.5) for later types; values as high as M20 ∼ −1.0 together with high G indicate presence of a double nucleus, characteristic of ongoing galaxy mergers (Lotz et al., 2004).

In Figure 3.3, I present the six standard structural parameters computed for all galaxies in the sample. The values obtained for the normal galaxies as well as those found for the ongoing mergers with double nuclei are in a general agreement with the corresponding values quoted by previous studies. Early-type galaxies appear more concentrated1 (2.0 ® n® 5.0

and 3.5 ® C ® 4.5) and have smoother light distribution (S ∼ 0.1) than the later types

(0.5®n®2.0, 2.5®C ®3.5,S∼0.2) and in both cases, the measured values ofAindicate

high structural symmetry under a rotation about 180o (0.05 ® A® 0.2). The early- and

late-type galaxies are also easily distinguishable from one another in theG−M20space, with the former showing more unequal light distributions (0.6® G ® 0.8) and the presence of a

compact bright nucleus (M₂₀< ₋2), compared with the latter (0.4® G ® 0.6, M20> −2). The separation between early and late types in theA₋S parameter space is not as strong as expected from the results of previous studies. This is likely a consequence of the relatively regular appearance and lack of prominent spiral arms in the late-type subsample (which can be seen in the images in Figure 3.2), leading to low values of both AandS. Galaxies with double nuclei are distinguishable from the rest of the sample in both G−M20 andA−M20 parameters spaces.

1_{The low values of the Sérsic index (n=0.5) found for the three outliers among early-type galaxies are a result of} a failure in the fitting process, in which case the parameter should be forced to one of the pre-defined boundary values. The reason for the failure of the fit is not clear; however, an investigation of the fitting process has revealed that if a problem is encountered, the algorithm forces the parameter values to the lower boundary, regardless of the object’s light distribution. In other words, the low values assigned to the early-type galaxies should not be interpreted as indicative of low central concentration. In fact, considering the corresponding values of the concentration parameter,C, it is clear that these galaxies are in fact highly concentrated objects.

3.2. The standard measures

Within the studied sample, galaxies with faint tidal features and morphological disturbance are not easily distinguishable from galaxies with regular morphologies, when using the six standard structural parameters (with the exception of ongoing mergers with bright double nuclei, as already mentioned). In the case of the ongoing mergers, care should be taken when interpreting the values ofnandC measured within theR_{ma x} as they correspond to the entire (still merging) system rather than its individual components. Given the presence of a double nucleus in such galaxies, the notion of the central concentration does no longer have the same meaning, compared to galaxies hosting a single nucleus.

For most parameters, the values found for the disturbed galaxies span the whole range characteristic of both early- and late-type galaxies, with a slight bias toward the former. The exceptions areGandA. The values ofGfound for the morphologically disturbed galaxies are predominantly very high, similar to those found for highly-concentrated early-type galaxies. It is important to note that this tendency for such high values ofG could be a result of the sample selection as all morphologically disturbed galaxies were selected from a parent sample of galaxies with central spectroscopic signatures of a recent starburst. This is supported by the fact that the morphologically undisturbed galaxies selected from the starburst mother sample also show high values ofG. Additionally, in morphologically disturbed galaxies the values of

G could potentially be enhanced by the presence of the faint tidal features which tend to add to the inequality in the light distributions in those galaxies (see Lotz et al. 2004). However, the fact that equally high values ofGwere found in the morphologically undisturbed galaxies with signs of a recent starburst suggests that this contribution of faint tidal features to the measured values ofGshould not be substantial. This is investigated further in Section 3.3.1.

In theC−M20parameter space, many of the galaxies with highly disrupted morphologies lie above the sequence defined by the normal galaxies. As expected (see Lotz et al. 2004), those with highest M₂₀ tend to be merging systems with double nuclei; however, this is not the case for the remaining outliers, for which lower values ofM20indicate a single nucleus. It is more likely that the presence of faint tidal features in these galaxies enhances the measured values of the concentration index, by increasing the size of the outer growth curve radius (r₈₀). As a result, the galaxies show higher values of C than expected given the extent of their brightest central regions, quantified by M20. However, it appears that not all galaxies with tidal features are affected by their presence in that way, and it remains unclear how one should relate the location of the galaxies in the C−M20 parameter-space to the level of the

Figure 3.3:Relations between the six standard morphology measures (Sersic index[n], concentration index[C], asymmetry parameter[A], clumpiness parameter[S], Gini index[G], and theM₂₀statistics) computed for the galaxies in the visually selected sample of galaxies with various morphologies. The meaning of the symbols is described in the legend where galaxies are split into subsets based on the visual image inspection and the values of the SDSSfracdevparameter.

3.2. The standard measures

Figure 3.4:Relations between the six standard measures of structure (Sersic index_[n], concentration index_[C], asymmetry parameter _[A], clumpiness parameter_[S], Gini index_[G], and the M₂₀ statis- tics) computed for the galaxies in the visually selected sample of galaxies with various morphologies, with colour coded values of the shape asymmetry parameter. As in 3.3, the red circles and blue rhom- bus correspond to early- and late- type galaxies, respectively, and the star symbols to (post-)starburst galaxies.

morphological disturbance in their outskirts. I will revisit the effect of the tidal features on the concentration parameter in Section 3.3.1 as well as in the next chapters, when discussing the results obtained for larger galaxy samples.

The difference between morphologically disturbed galaxies (MORPH1-20) and galaxies with regular morphologies (MORPH31-40) is most noticeable in the values of A. However, only 35% (7/13) of the galaxies with high morphological disturbance are well separated from the galaxies with undisturbed morphologies. Visual inspection of the images of these galaxies shows that those with highestAtend to be ongoing mergers with bright double nuclei, very well separated from the rest of the galaxies in the M20−Aparameter space. The values ofA measured for galaxies with some but not particularly high level of morphological disturbance (MORPH21-30) are comparable with those obtained for galaxies with regular morphologies (MORPH31-40).

In conclusion, as expected based on previous studies, the standard literature measures considered in this work provide a robust way of distinguishing between different types of normal galaxies as well as for identifying ongoing galaxy mergers with double nuclei. However, it appears that these measures are not suitable for recognising galaxies in the post-coalescence merger stages, where the dominant feature that distinguishes them from other galaxy types are the faint tidally-induced structures residing their outskirts. In the following section, I show how galaxies in these late merger stages can be identified using the shape asymmetry.