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Observational constraints to boxy/peanut bulge formation time

Author/s: Pérez, I., Martínez-Valpuesta, I., Ruiz-Lara, T., de Lorenzo-Cáceres, A., Falcón-Barroso, J., Florido, E., González Delgado, R. M., Lyubenova, M., Marino, R. A., Sánchez, S. F., Sánchez-Blázquez, P., van de Ven, G., Zurita, A.

Reference: 2017 MNRAS 470 L122 | Link

Left and middle panels: spatially resolved light- and mass-weighted Stellar Age Distribution (SAD), respectively. Blue colours imply no (or low) presence of stars of a given age (y axis) and at a given radius (x axis). The fraction of stars are normalised at every radius. The dashed vertical lines represent the analysed regions (bulge, inner and outer bar, and dis). Right panel: average mass-weighted SAD for each of the analysed regions.
Left and middle panels: spatially resolved light- and mass-weighted Stellar Age Distribution (SAD), respectively. Blue colours imply no (or low) presence of stars of a given age (y axis) and at a given radius (x axis). The fraction of stars are normalised at every radius. The dashed vertical lines represent the analysed regions (bulge, inner and outer bar, and dis). Right panel: average mass-weighted SAD for each of the analysed regions.
Baryonic (or visible) matter is assembled forming galaxies that, as human beings, are born, evolve, and develop features defining them. One of the most common features found in spiral galaxies (apart from the spiral structure they are named after) is the presence of a bar: an elongated over-density of stars that is considered one of the main agents driving the secular, slow phase of evolution after the violent merging of structures that form galaxies. Apart from these "cigar"-shape structures, some galaxies (including our Milky Way, our Galaxy) display in their centres what it is called "boxy/peanut" bulges (B/P); an accumulation of material in the shape of a box or a peanut depending on the observation line of sight. How these structures are formed? When were they form? Are they long-lived structures? The joint effort of simulations and observations might help us answer these questions. Simulations suggest that B/P bulges are linked to the bar formation. While a galaxy is evolving, material might accumulate towards its centre to form an elongated shape. This structure, under the right conditions, might grow and create a proper strong bar that can experience a "buckling" instability (a sudden vertical growth). This bar can continue growing afterwards, leaving at its centre a B/P bulge, consequence of this instability. Along all these events, star formation and redistribution happens at different locations and thus, different structures or regions might have stars with different characteristics. Observational studies on the age distribution of stars populating barred spiral galaxies are key to confirm this scenario as well as to date the formation of the bar and the "buckling" instability event. This is what we did in this work. Making use of modern Integral Field Spectroscopic data from the CALIFA survey and modern full-spectral fitting techniques we analysed the stellar age distribution of the bulge, bar, and disc of a barred spiral galaxy showing signs of hosting a B/P bulge, NGC6032. We compare our observational results with state-of-the-art simulations of barred, spiral galaxies to find clear fingerprints suggesting that the bar was formed around 10 Gyr ago, with the buckling instability happening around 8 Gyr ago. We find evidences of bars growing from disc material as well as acting as conveyor belts transferring material to the centre from the disc without forming stars within it. All these results point towards bars being long-lasting even in the presence of gas. These results are essential towards the understanding on how the bulge and bar of our Milky Way were formed.

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