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.

The starburst phase of nuclear disk evolution may not be directly related to bulge formation, but the bulge formation event itself may have been a starburst, acting at the maximum possible rate allowed by the total virial density for a few internal crossing times. Starbursts in bulges differ from those in disks because the bulge potential is too deep to allow significant self-regulation by blow-out. The total luminosity of a bulge-forming starburst is comparable to that observed in distant galaxies, when the bulges are supposed to have formed.

Insight into the origin of bulges is sought in this review only from the properties of their stellar populations. Evidence concerning the age of the Galactic bulge stellar population is reviewed first, then the case of the bulge of M31 is discussed. The similarity of bulges and ellipticals is then illustrated, inferring that the problems of the origin of bulges and of the origin of ellipticals may well be one and the same: i.e. the origin of galactic spheroids. In this mood, the current evidence concerning the age of the dominant stellar populations of early-type galaxies is then reviewed, both for low- as well as high-redshift galaxies, and both for cluster as well as field ellipticals. All reported evidence argues for the bulk of the stars in galactic spheroids having formed at high redshift, with only minor late additions and a small dependence on environment. An attempt is made to evaluate how current formation scenarios can account for this observational evidence. The role of spheroids in the cosmic star formation and metal enrichment history is also briefly discussed. Finally, some critical questions are asked, answers to which may help our further understanding of the formation and evolution of galactic spheroids.

We present preliminary results of our ASCA observation of the Galactic bulge. We confirm the diffuse (spatially-unresolved) soft X-ray emission in the direction of the bulge. We also detect iron-L and neon-K complex lines in the spectrum. Therefore, the bulge emission undoubtedly originates from an optically thin thermal plasma. The plasma temperature is 0.4 keV. With the results, we present possible implications of the Galactic bulge emission.