Revealing hidden merger signatures in post-starburst galaxies with HST

Most galaxies belong to one of two types: star-forming disks (SFGs) or quiescent bulge-dominated galaxies (QGs) that used to be star-forming. The reasons SFGs stop forming stars and evolve into QGs are still disputed, but one is certain: a complex interplay of different processes is required to both quench star formation and change the morphology of a galaxy. The search for the driver of galaxy evolution is made more difficult by its rapidity and the relative dearth of galaxies undergoing this transition.

Post-starburst galaxies, having just stopped forming stars, serve as an essential link between SFGs and QGs. They are a perfect laboratory to study quenching mechanisms. We study a selection of 23 post-starburst galaxies that have 1) ceased star formation in the last Gyr, 2) have ongoing energetic processes that ionize their gas, and 3) still retain significant fractions of molecular gas. The reason for quenching of star formation and the ongoing energy injection is uncertain: about half of post-starburst galaxies are morphologically disturbed in SDSS indicating a merger origin, but the other half appear not more disturbed than SFGs.

To probe deeper the link between morphology and star formation, we analyze HST and SDSS imaging of these galaxies and perform a detailed morphological analysis using Sérsic fitting with GALFIT and non-parametric methods. Compared to control galaxies, HST-SPOGs have intermediate Sérsic indices (n ~ 2), and bulge strengths similar, but slightly lower than those of QGs. In SDSS imaging, HST-SPOGs are as disturbed as SFGs; but HST imaging reveals a greater amount of underlying disturbance in HST-SPOGs, showing a violent origin of these galaxies initially missed by the lower-resolution SDSS survey. We also develop a new technique to quantify color distributions of these disturbed galaxies that does not rely on rotational symmetry, and aim to use it to break the degeneracy between color gradients due to dust obscuration and stellar populations to further understand this complex evolutionary stage.