The Milky Way Bulge: Observed Properties and a Comparison to External Galaxies

Abstract

The Milky Way bulge offers a unique opportunity to investigate in detail the role that different processes such as dynamical instabilities, hierarchical merging, and dissipational collapse may have played in the history of the Galaxy formation and evolution based on its resolved stellar population properties. Large observation programmes and surveys of the bulge are providing for the first time a look into the global view of the Milky Way bulge that can be compared with the bulges of other galaxies, and be used as a template for detailed comparison with models. The Milky Way has been shown to have a box/peanut (B/P) bulge and recent evidence seems to suggest the presence of an additional spheroidal component. In this review we summarise the global chemical abundances, kinematics and structural properties that allow us to disentangle these multiple components and provide constraints to understand their origin. The investigation of both detailed and global properties of the bulge now provide us with the opportunity to characterise the bulge as observed in models, and to place the mixed component bulge scenario in the general context of external galaxies. When writing this review, we considered the perspectives of researchers working with the Milky Way and researchers working with external galaxies. It is an attempt to approach both communities for a fruitful exchange of ideas.

Galactic orbits of globular clusters in the region of the Galactic bulge

“…It was recently established that the Galactic bulge has an X-like shape [32,33]. Two hypotheses have been proposed to explain this.…”

Globular Clusters in the Galactic Bulge

“…Metallicities and abundances from high-resolution spectroscopy. References: 11 : Origlia, Valenti, & Rich 2005a;12: Origlia & Rich 2004;13: Barbuy et al 200613: Barbuy et al , 201513: Barbuy et al , 201414: Origlia, Rich, & Castro 2002;15: Valenti et al 2015;16: Lee, Carney, & Balachandran 2004;17: Origlia et al 2011;18: Origlia, Valenti, & Rich 2008;19: Origlia et al 2005b;20: Barbuy et al 2014;21: Zoccali et al 2004;22: Carretta et al 2001;23: Meléndez et al 2003plus Origlia et al 200224: Cohen et al 1999;25: Alves-Brito et al 2006;26: Barbuy et al 2007;27: Valenti, Origlia, & Rich 2011;28: Smith & Wehlau 1985;29: Lee 2007;30: Gratton et al 2015 for BHB;31: Gratton et al 2015 for RHB;32: Peñaloza et al 2015;33 Rojas-Arriagada et al 2016 typical of the bar (see also review by Gonzalez & Gadotti 2016). Vásquez et al (2013) studied the kinematics of 454 field bulge stars located in the bright and the faint red clumps of the X-shaped bulge.…”

“…Harris 2001;Rich 2013;Gonzalez & Gadotti 2016). These reviews address also comparisons of the Galactic bulge with external galaxies.…”

Separation of stellar populations by an evolving bar: implications for the bulge of the Milky Way

“…Modern IFU spectrographs and their dedicated surveys offer a great opportunity for understanding the bulge of the Milky Way in context (Gonzalez & Gadotti 2016), by measuring the morphological aspects of other spiral galaxies and prevalence of mergers compared to secular evolution at high redshift (e.g. Gadotti et al 2015).…”