of ongoing star-formation and/or interactions. As expected, I found the strongest disturbance, measured byA, in galaxies with very recent starbursts. This can be seen in Figures 5.5 and 5.4, which also show declining trends in the asymmetry and clumpiness parameters with the stellar mass, for all galaxy types. In all mass ranges, but particularly atM >1010M, the values of
Ar−Ag tend to be negative (Figure 5.6), pointing to a more symmetric structure in the red, meaning that the spatial distribution of younger stellar populations is slightly more disordered than that of the old stars. A similar trend was found by Conselice (1997) and Bershady et al.
5.3. Quantitative description of morphology and structure and trends with stellar mass
(2000). In more detail:
• Control sample: star-forming galaxies
In all,g,randipassbands, bothAandSmeasured for the star forming galaxies decline as a function of stellar mass, and the values ofG point to a higher inequality in the light distribution in less massive galaxies2.
• Control sample: passive galaxies
The distributions of the values of bothAandS, measured in the g r ipassbands are sim- ilar to those found for the star-forming galaxies, and so are the values of their ‘colour asymmetry’,Ar−Ag. While it may be surprising that the levels of structural asymme- try and clumpiness in passive galaxies are not lower than those found in galaxies with on-going star-formation, it should be noted that the sample of passive galaxies is not exclusively composed of smooth ellipticals, typically associated with the red sequence, but also includes galaxies with lenticular and early-type spiral morphologies, as revealed by visual inspection (see also the population of red spirals found by e.g. Masters et al. 2010). The high values ofG, compared with the star-forming galaxies are a result of the high central concentration of the passive galaxies.
• Young (post-)starburst galaxies
The distribution of the parameters measuring the degree of structural disturbance in galaxies with young starbursts is broader and with higher median values than those found for the blue-cloud and red-sequence galaxies, in all three passbands and generally, the disturbance tends to be more prominent in the lowest-mass systems. There is no tendency for young (post-)starburst galaxies to show higher values of the clumpiness parameter compared with the star-forming or passive galaxies; in fact, their values of
S are generally lower than those found for the continuously star-forming and passive galaxies atM <1010.5M. This may be related to the increased contribution from the central region to the measurement ofS due to the recent central starburst rather than
2This seems to contradict the general sample characteristics inferred through visual inspection of the false-colour images, where the more massive galaxies showed signs of substructure more frequently than the low-mass systems. Aside from the fact that the parameters were measured in individual filters, rather than the composite false-colour images, the reason for this discrepancy could lie in the compact sizes of the galaxies with highest values ofAandS
which could make the substructures difficult to be seen by eye. To some extent, it could also be a consequence of the increasing contribution from the central galaxy parts to the measurements in the higher mass bins; the lack of prominent bulges could reveal the clumpy regions of star-formation within the low-mass galaxies. Furthermore, visual inspection revealed that the substructures found in massive galaxies tend to be more ordered and symmetric (bars, rings, arms) than those present in low mass galaxies (clumps).
the lack of clumpy substructure. The fact that a similar drop of the values of S, with respect to the star-forming and passive galaxies, is not found in the highest mass regime supports this claim: as indicated by the values ofn, C and M20, the light from the old stellar populations within the bulges of the most massive young (post-)starburst still dominates over the light from the stars formed in the burst.
• Old post-starburst galaxies
Generally, the galaxies with old post-starburst stellar populations do not tend to show structural disturbance on a level higher than that found in the star-forming and pas- sively evolving galaxies and, any disturbance measured (by means ofAandSin theg r i
passbands) is most notable in the lowest-mass regime. As discussed in the case of the continuously star-forming galaxies, this could be a result of the increasing contribution from bulges to the measurements in the higher mass bins. An interesting exception are theg-band values of the clumpiness parameter measured for post-starburst galaxies with
M > 1010.5M (Figure 5.5), which are notably higher than those found for the other galaxy types. Given their old estimated starburst ages and considering their values ofn,
CandM20, it is unlikely that the light from the central starburst could be dominating the measurement ofS, like in the case of young starbursts. This and the enhanced values of the g-band Gini index that suggest strongly unequal light distributions, could be a sign of ongoing star-formation in the off-centre regions of these galaxies.