1.1. Age

The Galactic bulge is old, with no evidence for a younger stellar population, as can be verified in the Colour–Magnitude Diagrams by Zoccali et al. (Reference Zoccali2003), Clarkson et al. (Reference Clarkson2008), Clarkson et al. (Reference Clarkson2011), and Gennaro et al. (Reference Gennaro2015). Whereas Bensby et al. (Reference Bensby2013) claims from his sample of microlensed dwarfs towards the bulge, that 22% of bulge stars can reach ages as young as 5 Gyr, this is not confirmed in the CMDs. A counter-argument on this is given by Haywood et al. (2016). Nataf (Reference Nataf2016a) critically reviews age determinations in bulge stellar populations. He concludes that there is a consensus that metal-poor stars are old, whereas for metal-rich stars, there are discrepancies among authors. In particular, a higher helium abundance appears to better fit the red giant branch (RGB) bump than a younger age. Note that there are young stars in the innermost parts of the bulge, such as the Nuclear Star Cluster (NSC) (Bland-Hawthorn & Gerhard Reference Bland-Hawthorn and Gerhard2016 and references therein), and the recently revealed Cepheids identified as a young inner thin disk, that are confined in vertical extent (Dékány et al., Reference Dékány2015). These young stars can be formed from mass loss from bulge stars, or else from the dynamical evolution of the bar. However, for most bulge regions, there are no significant numbers of young stars.

Evidence available from kinematics and Metallicity Distribution Function (MDF) are discussed by Babusiaux (Reference Babusiaux2016) and Ness & Freeman (Reference Ness and Freeman2016). It is clear that metal-rich stars with [Fe/H] > −0.5 form the X-shape bulge, corresponding to the bar. There remains as an open question as to whether the more metal-poor stellar population belongs to an old spheroidal bulge population, or to the thick disk and/or halo. The fraction of stars included in different stellar populations is also different among authors: Babusiaux et al. (Reference Babusiaux2010, Reference Babusiaux2014), Hill et al. (Reference Hill2011), and Gonzalez et al. (Reference Gonzalez2015) consider that there are two stellar populations in roughly equal numbers. Half of them would be the metal-rich X-shaped structure, and the other half belonging to an old spheroid. Ness & Freeman (Reference Ness and Freeman2016) and Ness et al. (Reference Ness2013) propose a 5-population distribution, with only 5% of stars to be identified with an old spheroid or inner halo or metal-weak thick disk. From kinematics of Red Clump (RC) stars and M giants, there is evidence for cylindrical rotation, which supports bulge formation from a bar (Kunder et al., Reference Kunder2012; Ness et al., Reference Ness2013). An important caveat is that a few tracers can be biased: RC stars and M giants are characteristic of metal-rich populations, and would not trace all stellar populations in the bulge. Babusiaux et al. (Reference Babusiaux2010, Reference Babusiaux2014) instead found, from RGB stars, that the metal-poor population is compatible with an spheroid. Di Matteo (Reference Di Matteo2016) suggests that a small spheroidal component, if present, would be maximal in the innermost regions of the bulge. There are also controversial conclusions on the space distribution of RR Lyrae, that are found to have a metallicity peak at [Fe/H] ~ −1.0. Pietrukowicz et al. (Reference Pietrukowicz2015) found that they have a triaxial ellipsoid shape, compatible with a bar. Gran et al. (Reference Gran2016) found instead a centrally concentrated spheroidal distribution.

1.2. Alpha-elements

McWilliam (Reference McWilliam2016) reports the available data on chemical abundances in bulge stars. In particular, the abundances of alpha-elements Mg, Si, Ca, Ti, given by Alves-Brito et al. (Reference Alves-Brito, Meléndez, Asplund, Ramírez and Yong2010), Gonzalez et al. (Reference Gonzalez2011), Gonzalez et al. (Reference Gonzalez2015), Bensby et al. (Reference Bensby2013), Johnson et al. (Reference Johnson, Rich, Kobayashi, Kunder and Koch2014), and Ryde et al. (Reference Ryde, Schultheis, Grieco, Matteucci, Rich and Uttenthaler2016), are enhanced in all fields, showing essentially no difference between different fields. The same applies to Oxygen but there is a larger spread among different authors. The enhancement in alpha-elements indicates that the stellar population was enriched early during bulge formation, due to yields from core-collapse supernovae. A fast chemical enrichment is clearly needed to reproduce these abundances in the Galactic bulge (e.g. Grieco et al. Reference Grieco, Matteucci, Ryde, Schultheis and Uttenthaler2015). Ness & Freeman (Reference Ness and Freeman2016) and Di Matteo (Reference Di Matteo2016) point out, on the other hand, that the alpha elements in the bulge are similar to those in the (thick) disk, and that all these stellar populations could be the same one.

1.3. Morphology and simulations

The Galactic bulge has a boxy/peanut shape (Zoccali et al. Reference Zoccali2014; Zoccali & Valenti Reference Zoccali and Valenti2016). Di Matteo (Reference Di Matteo2016) explains that the formation of the bulge from the bar, accounting only for the thin disk (most common procedure in published work to date), does not reproduce the chemo-kinematic and structural properties of its components. The consideration of the thin plus thick disk in the process is needed, recalling that Snaith et al. (Reference Snaith, Haywood, Di Matteo, Lehnert, Combes, Katz and Gómez2014) proposed that the thick disk stellar population would involve as much mass as the thin disc.

1.4. Globular clusters

Bica et al. (Reference Bica, Ortolani and Barbuy2016) select a sample of globular clusters, and report their properties: metallicity, reddening, space velocity, distance, and abundances. Nataf (Reference Nataf2016b) reviews bulge reddening derivation from different indicators, which have implications on distances.

Finally, it is important to note that, according to Bland-Hawthorn & Gerhard (Reference Bland-Hawthorn and Gerhard2016), the Galactic bulge has a stellar mass of 1.4–1.7 × 10¹⁰ M_⊙, and a total mass of 1.8 × 10¹⁰ M_⊙, in the region covered by the VVV bulge survey (Saito et al. Reference Saito2012). The Milky Way is accepted to be of SBbc type, but it could be identified to an earlier type, closer to SBb.