The GECKOS survey: The formation history of a barred galaxy via structural decomposition and spatially resolved spectroscopy

Disentangling the (co-)evolution of individual galaxy structural components remains a difficult task, owing to the inability to cleanly isolate light from spatially overlapping components. In this pilot study of PGC,044931, observed as part of the GECKOS survey, we utilised a VIRCAM H-band image to decompose the galaxy into five photometric components, three of which dominate by contributing more than $50%$ of light in specific regions, namely, a main disc, a boxy-peanut bulge, and a nuclear disc. When mapping the photometric decompositions onto MUSE observations, we found remarkably good separation in stellar kinematic space. All three structures occupy unique locations in the parameter space of the ratio of the light-weighted stellar line-of-sight mean velocity and velocity dispersion (rm V _⋆/σ_⋆) and the high-order stellar skew (h_3). These clear and distinct kinematic behaviours allowed us to make inferences about the formation histories of the individual components from observations of the mean stellar ages and metallicities of the three components. A clear story emerged: the main disc was built over a sustained and extended star formation phase, possibly partly fuelled by gas from a low-metallicity reservoir. Early on, that disc formed a bar that buckled and subsequently formed a nuclear disc in multiple and enriched star-formation episodes. This result is an example of how careful photometric decompositions combined with spatially well-resolved stellar kinematic information can help separate age-metallicity relations of different components and therefore disentangle the formation history of a galaxy. The results of this pilot study can be extended to a differential study of all GECKOS survey galaxies to assert the true diversity of Milky Way-like galaxies.