The Galactic Bulge

Abstract

The central bulge of the galaxy is considered from the standpoint of stellar populations, ages, composition, kinematics, structure, and extragalactic context. The central bulge is a 3:1 bar viewed edge-on mass of 2 ×10¹⁰M_⊙; the major axis is oriented ∼ 20° toward the first quandrant. At | b | > 8° much of the mass appears to be in an X-shaped distribution that resembles similar structures seen in extragalactic boxy bars. The bar exhibits cylindrical rotation and has all the hallmarks of structures that evolve dynamically from massive disks. Although there is a compelling case for secular evolution, the evidence from the color-magnitude diagram is that the bulge is ∼ 10 Gyr old, with no significant difference between the age of the bulge and the metal-rich globular clusters. Near the Galactic nucleus, and to a lesser extent, within the inner 100–200 pc, there is evidence of ongoing star formation and intermediate-age stars. A “long bar” oriented at ∼ 45° and with vertical thickness comparable to that of the disk may be a separate structure or part of the main bar, and a nuclear disk or bar may be present. A young and intermediate-age stellar population is confined to the central 100 pc, predominantly toward the nucleus. The kinematics are also consistent with most of the mass in the bulge being in the bar, with < 10% of the mass being in a “classical” bulge.

The bulge outside of the inner 200 pc is old, globular cluster-aged, with [Fe/H] ∼ solar, with a range spanning − 1. 5–0.5 dex in [Fe/H] and a gradient of [Fe/H] = − 0. 6 dex kpc^{− 1} at | b | > 4°; there is at present no evidence for a gradient in [Fe/H] or [α/Fe] for | b | < 4°. The alpha elements are enhanced over the full Galactic bulge; models of chemical evolution associate the alpha enhancement and iron abundance distribution with a rapid ( < 1 Gyr) formation timescale. This is consistent with trends observed for heavy elements, which are consistent with a pure r-process enrichment pattern. Although there are hints that the bulge system has subpopulations that can be distinguished based on chemodynamical properties, the reality of population components remains a matter of debate.

The luminous mass of the bulge is not a spheroidal stellar system comparable to bright elliptical galaxies; rather, it meets the criteria to be classified as a pseudobulge according to Kormendy and Kennicutt (2004). While overwhelmingly an old stellar population, the kinematics and structure of the bar are consistent with it having evolved secularly from a massive disk.

The review concludes with an examination of the major new ground- and space-based surveys of the bulge that will be carried out in the time frame 2015–2025 and a recital of observational challenges for the next decade.