Segundo análisis de sensibilidad-Valor empresarial

Upon the analysis of the evolutionary (post-)starburst sample, I found that typically they have structural properties characteristic of early-type disks (reflected in the values ofn,CandM₂₀), with values similar to those found for the star-forming blue-cloud galaxies and somewhat lower than those measured for the quiescent red-sequence galaxies in our control samples. The average structural properties of the sample are largely a result of the sample selection, where I considered only those galaxies with high stellar surface mass densities. More interestingly, the

phase

lack of evolution in these structural parameters with the starburst age, and in particular the values of n, C and M20 for the oldest (post-)starbursts, leads me to conclude that the (post- )starburst galaxies do not attain the highly concentrated structure typical for red-sequence galaxies within the first 0.6 Gyr following the merger. This does not necessarily imply that they are not heading toward the red sequence; rather that further structural evolution is required for the final transition to occur. A similar conclusion was found in a recent study of a subset of the (post-)starburst galaxies used in this work, by Rowlands et al. (2015), who found a substantial amount of gas remaining in these galaxies, that could sustain star formation for at least 0.5-1 Gyr after the starburst, suggesting that the post-starburst galaxies are not yet fully quenched. In the next chapter, I address a sample of galaxies with older starburst ages (t_SB>0.6 Gyr) and compare it with a more extended sample of red-sequence galaxies.

Finally, I note the following caveats in my conclusions. The galaxies in the control sample of red-sequence galaxies to which I compare the (post-)starburst galaxies, have not neces- sarily quenched their star formation recently and could have undergone some structural evo- lution while residing on the red sequence. Furthermore, present-day red-sequence galaxies that experienced their star-formation quenching earlier in the history of the Universe, when the physical conditions were significantly different to the present day, may not be representa- tive of the ‘future’ red-sequence which will form once the residual star-formation within the present-day (post-)starburst galaxies have ceased. Further study of (post-)starburst galaxies at higher redshifts will address that problem.

4.6

Summary

The quantitative analysis of the morphological evolution of local (post-)starburst galaxies with high stellar mass surface density, revealed that the fraction of (post-)starburst galaxies which show features characteristic of post-mergers declines with the starburst age. For the mor- phologically disturbed (post-) starburst galaxies, the median values of A_S decrease with the starburst age, and most galaxies with starbursts older than 0.5 Gyr haveA_Sbelow_∼0.2. These trends fit qualitatively to the galaxy merger picture, where the morphological disturbance of the merger remnant gradually fades away as the remnant evolves through the dynamically cold post-merger phase (see e.g. Lotz et al. 2008). Assuming that morphological disturbance in galaxies is an indicator of a past merger, both visual classification and the automated ap- proach (using the shape asymmetry) suggest that at least 45% of the youngest (post-) starburst

4.6. Summary

galaxies in the sample (tSB<0.1 Gyr) have originated in a merger.

The distribution of light within the (post-)starburst galaxies shows a high level of inequal- ity, as measured by the Gini index, (particularly for those with youngest starbursts), and this inequality decreases with the starburst age. This is likely a consequence of the decaying star- burst, which is an argument in favour of the evolutionary sequence of galaxies through the (post-)starburst phase. The structural analysis of the sample revealed moderate central con- centration, light profiles and spatial extent of the brightest regions characteristic of early- type disk galaxies, as expected for the sample selection, and showed no significant evolution of these properties over 0.6 Gyr following the most recent starburst. This suggests that the (post-)starburst galaxies do not attain the highly concentrated structure characteristic of the present-day red sequence galaxies during the first 0.6 Gyr after the starburst.

5

Dependence of the evolutionary path on stellar

mass

Correlations between physical properties of local galaxies and their stellar mass, as well as differences in the assembly history of low- and high- mass galaxies, suggest that the evolution- ary paths of galaxies with different masses are not the same. It is reasonable to expect for the properties of galaxies in the short-lived (post-)starburst phase to be different in different mass regimes. In this chapter, I present a study of the structure and morphology of (post-)starburst galaxies with a range of stellar masses, including an investigation of the frequency of occur- rence and prominence of merger signatures in different stellar mass ranges, with an aim to constrain how the role of galaxy mergers in inducing (post-)starburst signatures varies with galaxy mass. I also focus on the differences in the structural properties of the more and less massive (post-)starburst galaxies, which may imply divergent evolutionary pathways of galax- ies through the (post-)starburst phase in the different mass regimes. Consequently, through direct comparison with control samples of equally massive star-forming and passively evolving systems, I discuss the role of (post-)starburst galaxies in the build up of the high- and low-mass red sequence in the Local Universe.

5.1

Sample selection

To study the parallels between properties of galaxies with various masses and different star- formation activity, I selected volume-limited samples of young and old (post-)starburst as well as star-forming and passive galaxies (SB, PSB, SF and PAS, respectively), all within three stellar-mass ranges (9.5<log(M/M)<10.0, 10.0<log(M/M)<10.5, 10.5<log(M/M)< 11.0) at redshift 0.01<z<0.05. The selection was done using spectral indices that describe the shape of the stellar continuum, PC1, as well as the strength of Balmer absorption lines, PC2, (see Section 1.4.3 and Wild et al. 2007). The mass and redshift limits were chosen to ensure that the samples are complete in red sequence galaxies (defined by PC2¶PC1+0.5,

as estimated by eye) and all galaxies were selected to have a spectral per-pixel SNR>8 in the

g-band.