6 the Galactic Bulge

6 The Galactic Bulge R. Michael Rich Department of Physics and Astronomy, University of California, Los Angeles, CA, USA 1Introduction..................................................... 273 1.1 Overview, Scope, and Definition ......................................... 276 1.2 A Brief History ....................................................... 278 2 The Age and Population of the Galactic Bulge ........................... 283 2.1 Evidence for Minority Populations of Intermediate and Younger Age ........... 286 2.2 Microlensed Dwarfs: A Young, Metal-Rich Population? ...................... 290 2.3 The Luminosity Function ............................................... 293 2.4 Globular Clusters ..................................................... 294 3 Composition ..................................................... 295 3.1 Optical Spectroscopy .................................................. 296 3.2 Infrared Spectroscopy ................................................. 298 3.3 Composition and Comparison with Other Populations ...................... 300 3.4 Na and Al ........................................................... 302 3.5 Heavy Elements ...................................................... 304 4 Kinematics ...................................................... 306 4.1 Stellar Radial Velocity Surveys ........................................... 309 4.2 Proper-Motion Studies ................................................. 314 5 Kinematics and Composition ........................................ 317 5.1 Are There Subcomponents in the Bulge Abundance Distribution? .............. 317 6 Structure ........................................................ 319 6.1 The X-Shaped Bulge ................................................... 324 6.2 A Classical Bulge? ..................................................... 327 7 The Milky Way Bulge in an Extragalactic Context ........................ 328 8 Theories for the Formation of the Bulge ................................ 334 9 Future Surveys ................................................... 335 9.1 Ground-Based Imaging Surveys ......................................... 335 9.2 Spectroscopic Surveys ................................................. 337 T.D. Oswalt, G. Gilmore (eds.), Planets, Stars and Stellar Systems. Volume 5: Galactic Structure and Stellar Populations, DOI 10.1007/978-94-007-5612-0_6, © Springer Science+Business Media Dordrecht 2013 272 6 The Galactic Bulge 9.3 Radio Surveys ........................................................ 338 9.4 Space-Based Surveys .................................................. 338 10 Observational Challenges for the Future ................................ 339 References ............................................................ 341 The Galactic Bulge 6 273 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 × M⊙;themajoraxisisoriented∼ toward the first ○ quandrant. At ∣b∣> much of the mass appears to be in an X-shaped distribution that resem- bles 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 ∼ 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 −.–0.5 dex in [Fe/H] and a gradient of [Fe/H] =−. dex kpc at ∣b∣> ; ○ there is at present no evidence for a gradient in [Fe/H] or [α/Fe] for ∣b∣< . 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. Keywords: TK 1Introduction We must conclude, then, that in the central region of the Andromeda Nebula we have a metal poor Population II, which reaches −m for the brightest stars, and that underlying it there is a very much denser sheet of old stars, probably something like those in M67 or NGC 6752. We can be certain that these are enriched stars, because the cyanogen bands are strong, and so the metal/hydrogen ratio is very much closer to what we observe in the sun and in the present interstellar medium than to what is observed for Population II. And the process of enrichment has taken very little time. After the first generation of stars has been formed, we can hardly speak of a “generation”,because the enrichment takes place so soon, and there is probably very little time difference. So the CN giants 274 6 The Galactic Bulge that contribute most of the light in the nuclear region of the Nebula must also be called old stars; they are not young. – W. Baade in The Evolution of Galaxies and Stellar Popu- lations p. 256 (1961) Examples of all of the major stellar populations may be found within a reasonable vicinity of the Sun; the nearest globular clusters are just over 3 kpc distant, while the nearest disk and halo dwarfs can be found within 100 pc. The Galactic bulge is another matter. For the most part, the bulgepopulationis8kpcdistant,andovermuchofitsareaisobscuredbytensofmagnitudes of visual extinction. The nearest comparable population is the bulge of M31, one hundred times more distant. Until the last two decades, studies of the M31 bulge that population were limited to photometry and spectroscopy of its integrated light. These factors meant that much more time was required for an understanding of the bulge population to be achieved, compared with the disk, halo, and globular clusters. These challenges also meant that the bulge subject area has historically experienced a greater number of wrong turns and controversies; lingering issues, like the nature of the long bar, remain to the present day. Considering that more than half of the light in the local universe is found in spheroids (Fukugita et al. 1998) with the Galactic bulge being the only example with individual stars read- ily available for study, the importance of this population can hardly be overstated. >Figure 6-1 shows how dramatic the effect of reddening is on the bulge, even covering the relatively red wavelengths of the DIRBE instrument (Weiland et al. 1994). In this chapter, I will refer to the central region of the Milky Way galaxy as the “bulge” even though there is now growing evidence that most of the mass is in a bar structure. We open with Baade’s prescient words because they define the essence of our present-day concept of the bulge: a roughly solar-metallicity stellar population that is as old as the halo globular clusters and exhibits chemical signatures of rapid formation. Of course, Baade’sconclusions were largely based on intuition rather than observations; our present-day picture is grounded on far more sturdy observational evidence. The Galactic bulge and bulges of galaxiesve ha been the subject of two IAU symposia, IAU 153 (1993) and IAU 245 (2008), and have been reviewed in numerous other IAU symposia concerning the Milky Way and external galaxies (e.g., Binney 2009). On three occasions, the bulge or bulges have been specifically reviewed in Annual Reviews of Astronomy and Astrophysics (Frogel 1988;Wyseetal.1997; Kormendy and Kennicutt 2004). The structure of the bar and orbit theory is not reviewed here in detail; the interested reader may consult reviews by Sellwood and Wilkinson (1993), Gerhard (2002, 2011), Athanassoula (2008, 2012), Combes (2009), and Sellwood (2012; this volume) on that subject. We also do not review star count/population/kinematic models of the Milky Way, which are becoming increasingly capa- ble in modeling the observed population of the bulge and are giving constraints on the bar structure and orientation. The principal efforts are the Besancon galaxy

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