The GECKOS Survey: revealing the formation history of a barred galaxy via structural decomposition and resolved spectroscopy

TL;DR

This study demonstrates that detailed photometric decomposition, when integrated with spatially resolved kinematic and population data from MUSE, can separate co-spatial galactic components and reveal their distinct formation histories. By decomposing PGC 044931 into an extended disc, a boxy/peanut bulge, and a nuclear disc, the authors show that these components occupy unique regions in the $h_{3}$ vs $V_{\star}/\sigma_{\star}$ and age–metallicity spaces, enabling a coherent assembly narrative where a disc forms, a bar buckles to produce a boxy/peanut bulge, and a nuclear disc grows via enriched star formation from bar-driven gas inflows. The extended disc remains comparatively metal-poor and fuelled by near-pristine gas, while the nuclear disc forms later and is more metal-rich, highlighting differential evolution within a single galaxy. This differential, component-based approach, extended across the GECKOS sample, promises a robust framework to map the assembly histories of Milky Way–like galaxies across cosmic time.

Abstract

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 utilise a VIRCAM $H$-band image to decompose the galaxy into five photometric components, three of which dominate by contributing $>50\%$ of light in specific regions: a main disc, a boxy/peanut bulge, and a nuclear disc. When the photometric decompositions are mapped onto MUSE observations, we find 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 ($\rm{V}_{\star}/σ_{\star}$), and the high-order stellar skew ($h_{3}$). These clear and distinct kinematic behaviours allow 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 emerges: the main disc 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 out 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.

Figures (3)

• Figure 1: Components of the image decomposition of PGC 044931. Panel a) is the sky-subtracted, $H$-band VIRCAM image (arcsinh scaled), b) is the residual map after subtraction of the five components (linear scale), c)--g) are the five components fit with IMFIT (arcsinh scale, limits altered to show maximum structure), and h) shows the spatial extent of the three dominant components. The extended disc is shown in purple, and the boxy/peanut bulge in pink. The nuclear disc (yellow) is very small and only dominates near the centre.
• Figure 2: Masking regions from Figure \ref{['Fig:decomp']}h overlaid as grey contours on light-weighted stellar kinematic (a) V$_{\star}$, (b) $h_{3}$, and population (c) age, and (d) metallicity maps of PGC 044931. White regions denote masked regions (e.g. due to foreground stars), or areas in which the spaxels did not reach the minimum continuum S/N threshold of 5. A distinctive, metal-rich 'X' shape is present in the metallicity map, seen in other galaxies gonzalez2016, and in agreement with simulations debattista2017fragkoudi2020.
• Figure 3: Separation of the photometric components of PGC 044931 in stellar kinematic and stellar population space. Left: stellar $h_{3}$ vs. $\rm{V}_{\star}/\sigma_{\star}$, with data points corresponding to Voronoi bins for which the majority of the bin is located in one of the photometric regions. The underlying extended disc, boxy/peanut bulge (bar), and nuclear disc show distinctly different behaviours in this parameter space, allowing us to confidently make inferences about their star formation histories. Right: mean light-weighted stellar metallicity vs. stellar age. The nuclear disc is clearly separated from the bar and boxy/peanut bulge and the boxy/peanut region is on average older and more metal-rich than the underlying disc. The colour saturation indicates the fraction of light attributed to the given component in each bin.