Nature of the bulge

Figure: Colour-Magnitude diagrams illustrating the complexity of the inner Galactic bulge - several major structural components are evident even in photometry (McWilliam & Zoccali, 2010, ApJ, 724, 1491). Gaia-ESO will determine kinematics and elemental abundances for large samples, probing the complexity.

In simulations of galaxy formation, mergers tend to produce substantial bulges made of stars that either formed in a disc that was destroyed in a merger, or formed during a burst of star formation that accompanied the merger (Abadi et al., 2003, ApJ, 597, 21). Such “classical” bulges are kinematically distinguishable from “pseudo-bulges” that form when a disc becomes bar unstable, and the bar buckles into a peanut-shaped bulge (Peebles, 2011, Nature, 469, 305; Kormendy et al., 2010, ApJ, 723, 54 - see Figure). In common with the great majority of late-type galaxies, the Galaxy’s inner bulge appears to be a pseudo-bulge, but Cold Dark Matter (CDM) simulations suggest that it should also host a classical bulge, perhaps that observed at larger radii. By studying the kinematics and chemistry of K giants at |b| > 5° we will either confirm the classical bulge or place limits on it which will pose a challenge to CDM theory. Our large survey will quantify elements of various species and their variation across the bulge region, substantially improving our knowledge of this fundamental, yet surprisingly under-studied, stellar population.