Observations of the stellar content of the Milky Way's bulge helps us to understand the stellar content and evolution of distant galaxies. In this brief overview I will first highlight some recent work directed towards measuring the history of star formation and the chemical composition of the central few parsecs of the Galaxy. High resolution spectroscopic observations by Ramirez et al. (1998) of luminous M stars in this region yield a near solar value for [Fe/H] from direct measurements of iron lines. Then I will present some results from an ongoing program by my colleagues and myself which has the objective of delineating the star formation and chemical enrichment histories of the central 100 parsecs of the Galaxy, the ‘inner bulge’. We have found a small increase in mean [Fe/H] from Baade's Window to the Galactic Center and deduce a near solar value for stars at the center. For radial distances greater than 1° we fail to find a measurable population of stars that are significantly younger than those in Baade's Window. Within 1° of the Galactic Center we find a number of luminous M giants that most likely are the result of a star formation episode not more than one or two Gyr ago.

Within recent years, there has been a confluence of data that favors a large age for the bulges of the Milky Way and M31. A short formation timescale is required by the similarity in ages between the bulge and the old, metal-rich globular clusters. Detailed abundances of bulge giants are consistent with a short enrichment timescale. The bulge of M31 is similarly old and even more metal-rich than the Galactic bulge. There appears to be a strong connection between the M31 bulge and the halo, as metal-rich giants are found in M31 out to great distances. The stellar populations data support a rapid bulge formation timescale, perhaps even less than 1 Gyr.

“We must conclude, then, that in the central region of the Andromeda Nebula we have a metal-poor Population II, which reaches −3m 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 ration is very much closer to what we observe in the Sun and in the present interstellar medium than to what is obwserved for Population II. And the process of enrichment probably has taken very little time. After the first generation of stars has formed, we can hardly speak of a ‘generation’, because the enrichment takes place so soon, and there is probably very little time difference.

The relevant dynamical processes for bulge formation are reviewed: collapse, accretion, bar formation, stochastic heating and external forcing. All of these processes take place at some level, but it appears hard to escape the conclusion that bulges formed quickly and early.

An analysis of stellar orbits in a doubly barred galaxy shows that the effect of a secondary bar is to destabilize the orbits, the process being accompanied by the appearance of vertical resonances which would enable stars to leave the galactic plane and move into the bulge. This phenomenon could contribute to bulge formation. Results of the orbital analysis are presented and their significance discussed.

Our new statistical study of bulges of disk galaxies reveals a frequency of almost 50% being boxy-or peanut-shaped. Therefore very common processes are required to explain this high fraction. In an analysis of a possible relation between this internal structure and the environment of galaxies with boxy/peanut-shaped bulge we find that on large scales there is no hint for a connection. However, galaxies with boxy- or peanut-shaped bulges have more companions and satellites and show more frequently interactions than a control sample. Thus we conclude that the small-scale environment is important for the existence of such bulges. The most likely reason responsible for the development of boxy/peanut-shaped bulges is a bar originating from galaxy interaction in stable disks or by an infalling satellite.

The gravitational clustering hierarchy and dissipative gas processes are both involved in the formation of bulges. Here I present a simple empirical model in which bulge material is assembled via gravitational accretion of visible companion galaxies. Assuming that merging leads to a starburst, I show that the resulting winds can be strong enough to self-regulate the accretion. A quasi-equilibrium accretion process naturally leads to the Kormendy relation between bulge density and size. Whether or not the winds are sufficiently strong and long lived to create the quasi-equilibrium must be tested with observations. To illustrate the model I use it to predict representative parameter-dependent star formation histories. The bulge building activity appears to peak around a redshift z ∼ 2, with tails to both higher and lower redshifts.

In the context of studying the properties of the mutual mass distribution of the bright and dark matter in bulges (or elliptical galaxies), the properties of the analytical phase–space distribution function (DF) of two–component spherical self–consistent stellar systems (where one density distribution follows the Hernquist profile, and the other a γ = 0 model, with different total masses and core radii [HO models]) are here summarized. A variable amount of radial Osipkov–Merritt (OM) orbital anisotropy is allowed in both components. The necessary and sufficient conditions that the model parameters must satisfy in order to correspond to a model where each one of the two distinct components has a positive DF (the so–called model consistency) are analytically derived, together with some results on the more general problem of the consistency of two–component γ1 + γ2 models. The possibility to add in a consistent way a black hole at the center of radially anisotropic γ-models is also discussed. In the particular case of HO models, it is proved that a globally isotropic Hernquist component is consistent for any mass and core radius of the superimposed γ = 0 halo. On the contrary, only a maximum value of the core radius is allowed to the γ = 0 component when a Hernquist halo is added. The combined effect of halo concentration and orbital anisotropy is successively investigated. […]

AGN are known to lie in galaxies, and both galaxies and AGN evolve similarly over cosmic time (e.g., Silk & Rees 1998). This suggests a close connection between the nuclear phenomena associated with black holes and the formation and evolution of ordinary galaxies. The host galaxies of AGN are a direct probe of the AGN-galaxy connection. Among AGN, BL Lac objects are know to reside mostly, if not systematically, in elliptical galaxies. BL Lac can therefore probe (massive) spheroids to large redshifts. Results from an HST WFPC2 survey of ∼ 100 BL Lac objects are here presented.

We present a study in B, I and H of a magnitude-limited sample of galactic bulges using WFPC2 and NICMOS. The high spatial resolution of HST allows us to study the dust contents near the center, and stellar populations in dust-free regions. We find extinction in 19/20 galaxies and infer an average central extinction of Av = 0.6 − 1.0 mag. For galactic bulges of types S0 to Sb, the tightness of the B − I vs I − H relation suggests that the age spread among bulges of early type galaxies is small, at most 2 Gyr independent of environment. Comparison with stellar population models shows that the bulges are old. Colors at 1 bulge effective-radius, where we expect extinction to be negligible, suggest that all of these bulges formed around at the same time as bright galaxies in the Coma cluster.

We present first results from an on-going survey of the stellar populations of the bulges and inner disks of spirals at various points along the Hubble sequence. In particular, we are investigating the hypotheses that bulges of early-type spirals are akin to (and may in fact originally have been) intermediate-luminosity ellipticals while bulges of late-type spirals are formed from dynamical instabilities in their disks. Absorption-line spectroscopy of the central regions of Sa–Sd spirals is combined with stellar population models to determine integrated mean ages and metallicities. These ages and metallicities are used to investigate stellar population differences both between the bulges and inner disks of these spirals and between bulges and ellipticals in an attempt to place observational constraints on the formation mechanisms of spiral bulges.

Galactic disks are thought to originate from the cooling of baryonic material inside virialized dark halos. In order for these disks to have scalelengths comparable to observed galaxies, the specific angular momentum of the baryons has to be largely conserved. Because of the spread in angular momenta of dark halos, a significant fraction of disks are expected to be too small for them to be stable, even if no angular momentum is lost. Here it is suggested that a self-regulating mechanism is at work, transforming part of the baryonic material into a bulge, such that the remainder of the baryons can settle in a stable disk component. This inside-out bulge formation scenario is coupled to the Fall & Efstathiou theory of disk formation to search for the parameters and physical processes that determine the disk-to-bulge ratio, and therefore explain to a large extent the origin of the Hubble sequence. The Tully-Fisher relation is used to normalize the fraction of baryons that forms the galaxy, and two different scenarios are investigated for how this baryonic material is accumulated in the center of the dark halo. This simple galaxy formation scenario can account for both spirals and S0s, but fails to incorporate more bulge dominated systems.

Inspecting a sample of edge-on galaxies selected from the RC3 (de Vaucouleurs et al. 1991) with D25 >2arcmin (∼1350 galaxies) on the ‘Digital Sky Survey’ we have identified a class of approximately 20 disk galaxies with prominent, large, and boxy bulges. These bulges all show irregularities and asymmetries which are suggestive of them being formed just recently and not yet dynamically settled. We will present some examples and first results from CCD follow-up observations.

While the large frequency of boxy- or peanut-shaped bulges in disk galaxies (nearly 50%) is best explained by the response of the stellar disk to a bar potential, we propose soft-merging of companions as the most likely scenario for the evolution of this new class of thick boxy bulges.

Figure rotation substantially increases the fraction of stochastic orbits in triaxial systems. This increase is most dramatic in systems with shallow cusps showing that it is not a direct consequence of scattering by a central density cusp or black hole. In a recent study of stationary triaxial potentials (Valluri & Merritt 1998) it was found that the most important elements that define the structure of phase space are the two-dimensional resonant tori. The increase in the fraction of stochastic orbits in models with figure rotation is a direct consequence of the destabilization of these resonant tori.

The presence of a large fraction of stochastic orbits in a triaxial bulge will result in the evolution of its shape from triaxial to axisymmetric. The timescales for evolution can be as short as a few crossing times in the bulges of galaxies and evolution is accelerated by figure rotation. This suggests that low luminosity ellipticals and the bulges of early type spirals are likely to be predominantly axisymmetric.