Shape asymmetry: a morphological indicator for automatic detection of galaxies in the post-coalescence merger stages

• Published in: Monthly notices of the Royal Astronomical Society 2016-03, Vol.456 (3), p.3032-3052
• Main Authors: Pawlik, M. M., Wild, V., Walcher, C. J., Johansson, P. H., Villforth, C., Rowlands, K., Mendez-Abreu, J., Hewlett, T.
• Format: Article
• Language: English
• Subjects: Accumulation; Age; Asymmetry; Automation; Galaxies; Indicators; Morphology; Quenching (cooling); Star & galaxy formation; Starburst galaxies; Starbursts; Universe
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stv2878

We present a new morphological indicator designed for automated recognition of galaxies with faint asymmetric tidal features suggestive of an ongoing or past merger. We use the new indicator, together with pre-existing diagnostics of galaxy structure to study the role of galaxy mergers in inducing (post-) starburst spectral signatures in local galaxies, and investigate whether (post-) starburst galaxies play a role in the build-up of the ‘red sequence’. Our morphological and structural analysis of an evolutionary sample of 335 (post-) starburst galaxies in the Sloan Digital Sky Survey DR7 with starburst ages 0 < t SB < 0.6 Gyr, shows that 45 per cent of galaxies with young starbursts (t SB < 0.1 Gyr) show signatures of an ongoing or past merger. This fraction declines with starburst age, and we find a good agreement between automated and visual classifications. The majority of the oldest (post-) starburst galaxies in our sample (t SB ∼ 0.6 Gyr) have structural properties characteristic of early-type discs and are not as highly concentrated as the fully quenched galaxies commonly found on the ‘red sequence’ in the present day Universe. This suggests that, if (post-) starburst galaxies are a transition phase between active star-formation and quiescence, they do not attain the structure of presently quenched galaxies within the first 0.6 Gyr after the starburst.